1 /*
   2  * Copyright (c) 2008, 2017, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.invoke;
  27 
  28 import jdk.experimental.value.ValueType;
  29 import jdk.internal.misc.SharedSecrets;
  30 import jdk.internal.module.IllegalAccessLogger;
  31 import jdk.internal.org.objectweb.asm.ClassReader;
  32 import jdk.internal.reflect.CallerSensitive;
  33 import jdk.internal.reflect.Reflection;
  34 import jdk.internal.vm.annotation.ForceInline;
  35 import sun.invoke.util.ValueConversions;
  36 import sun.invoke.util.VerifyAccess;
  37 import sun.invoke.util.Wrapper;
  38 import sun.reflect.misc.ReflectUtil;
  39 import sun.security.util.SecurityConstants;
  40 import valhalla.shady.MinimalValueTypes_1_0;
  41 
  42 import java.lang.invoke.LambdaForm.BasicType;
  43 import java.lang.reflect.Constructor;
  44 import java.lang.reflect.Field;
  45 import java.lang.reflect.Member;
  46 import java.lang.reflect.Method;
  47 import java.lang.reflect.Modifier;
  48 import java.lang.reflect.ReflectPermission;
  49 import java.nio.ByteOrder;
  50 import java.security.AccessController;
  51 import java.security.PrivilegedAction;
  52 import java.security.ProtectionDomain;
  53 import java.util.ArrayList;
  54 import java.util.Arrays;
  55 import java.util.BitSet;
  56 import java.util.Iterator;
  57 import java.util.List;
  58 import java.util.Objects;
  59 import java.util.*;
  60 import java.util.concurrent.ConcurrentHashMap;
  61 import java.util.stream.Collectors;
  62 import java.util.stream.Stream;
  63 
  64 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  65 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  66 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  67 import static java.lang.invoke.MethodType.methodType;
  68 
  69 /**
  70  * This class consists exclusively of static methods that operate on or return
  71  * method handles. They fall into several categories:
  72  * <ul>
  73  * <li>Lookup methods which help create method handles for methods and fields.
  74  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  75  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  76  * </ul>
  77  *
  78  * @author John Rose, JSR 292 EG
  79  * @since 1.7
  80  */
  81 public class MethodHandles {
  82 
  83     private MethodHandles() { }  // do not instantiate
  84 
  85     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  86 
  87     // See IMPL_LOOKUP below.
  88 
  89     //// Method handle creation from ordinary methods.
  90 
  91     /**
  92      * Returns a {@link Lookup lookup object} with
  93      * full capabilities to emulate all supported bytecode behaviors of the caller.
  94      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  95      * Factory methods on the lookup object can create
  96      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  97      * for any member that the caller has access to via bytecodes,
  98      * including protected and private fields and methods.
  99      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 100      * Do not store it in place where untrusted code can access it.
 101      * <p>
 102      * This method is caller sensitive, which means that it may return different
 103      * values to different callers.
 104      * <p>
 105      * For any given caller class {@code C}, the lookup object returned by this call
 106      * has equivalent capabilities to any lookup object
 107      * supplied by the JVM to the bootstrap method of an
 108      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
 109      * executing in the same caller class {@code C}.
 110      * @return a lookup object for the caller of this method, with private access
 111      */
 112     @CallerSensitive
 113     @ForceInline // to ensure Reflection.getCallerClass optimization
 114     public static Lookup lookup() {
 115         return new Lookup(Reflection.getCallerClass());
 116     }
 117 
 118     /**
 119      * This reflected$lookup method is the alternate implementation of
 120      * the lookup method when being invoked by reflection.
 121      */
 122     @CallerSensitive
 123     private static Lookup reflected$lookup() {
 124         Class<?> caller = Reflection.getCallerClass();
 125         if (caller.getClassLoader() == null) {
 126             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 127         }
 128         return new Lookup(caller);
 129     }
 130 
 131     /**
 132      * Returns a {@link Lookup lookup object} which is trusted minimally.
 133      * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes.
 134      * It can only be used to create method handles to public members of
 135      * public classes in packages that are exported unconditionally.
 136      * <p>
 137      * As a matter of pure convention, the {@linkplain Lookup#lookupClass lookup class}
 138      * of this lookup object will be {@link java.lang.Object}.
 139      *
 140      * @apiNote The use of Object is conventional, and because the lookup modes are
 141      * limited, there is no special access provided to the internals of Object, its package
 142      * or its module. Consequently, the lookup context of this lookup object will be the
 143      * bootstrap class loader, which means it cannot find user classes.
 144      *
 145      * <p style="font-size:smaller;">
 146      * <em>Discussion:</em>
 147      * The lookup class can be changed to any other class {@code C} using an expression of the form
 148      * {@link Lookup#in publicLookup().in(C.class)}.
 149      * but may change the lookup context by virtue of changing the class loader.
 150      * A public lookup object is always subject to
 151      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 152      * Also, it cannot access
 153      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 154      * @return a lookup object which is trusted minimally
 155      *
 156      * @revised 9
 157      * @spec JPMS
 158      */
 159     public static Lookup publicLookup() {
 160         return Lookup.PUBLIC_LOOKUP;
 161     }
 162 
 163     /**
 164      * Returns a {@link Lookup lookup object} with full capabilities to emulate all
 165      * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">
 166      * private access</a>, on a target class.
 167      * This method checks that a caller, specified as a {@code Lookup} object, is allowed to
 168      * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing
 169      * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing
 170      * the target class, then this check ensures that
 171      * <ul>
 172      *     <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li>
 173      *     <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing
 174      *     the target class to at least {@code m1}.</li>
 175      *     <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li>
 176      * </ul>
 177      * <p>
 178      * If there is a security manager, its {@code checkPermission} method is called to
 179      * check {@code ReflectPermission("suppressAccessChecks")}.
 180      * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object
 181      * was created by code in the caller module (or derived from a lookup object originally
 182      * created by the caller). A lookup object with the {@code MODULE} lookup mode can be
 183      * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE}
 184      * access to the caller.
 185      * @param targetClass the target class
 186      * @param lookup the caller lookup object
 187      * @return a lookup object for the target class, with private access
 188      * @throws IllegalArgumentException if {@code targetClass} is a primitve type or array class
 189      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 190      * @throws IllegalAccessException if the access check specified above fails
 191      * @throws SecurityException if denied by the security manager
 192      * @since 9
 193      * @spec JPMS
 194      * @see Lookup#dropLookupMode
 195      */
 196     public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException {
 197         SecurityManager sm = System.getSecurityManager();
 198         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 199         if (targetClass.isPrimitive())
 200             throw new IllegalArgumentException(targetClass + " is a primitive class");
 201         if (targetClass.isArray())
 202             throw new IllegalArgumentException(targetClass + " is an array class");
 203         Module targetModule = targetClass.getModule();
 204         Module callerModule = lookup.lookupClass().getModule();
 205         if (!callerModule.canRead(targetModule))
 206             throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 207         if (targetModule.isNamed()) {
 208             String pn = targetClass.getPackageName();
 209             assert pn.length() > 0 : "unnamed package cannot be in named module";
 210             if (!targetModule.isOpen(pn, callerModule))
 211                 throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 212         }
 213         if ((lookup.lookupModes() & Lookup.MODULE) == 0)
 214             throw new IllegalAccessException("lookup does not have MODULE lookup mode");
 215         if (!callerModule.isNamed() && targetModule.isNamed()) {
 216             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 217             if (logger != null) {
 218                 logger.logIfOpenedByBackdoor(lookup, targetClass);
 219             }
 220         }
 221         return new Lookup(targetClass);
 222     }
 223 
 224     /**
 225      * Performs an unchecked "crack" of a
 226      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 227      * The result is as if the user had obtained a lookup object capable enough
 228      * to crack the target method handle, called
 229      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 230      * on the target to obtain its symbolic reference, and then called
 231      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 232      * to resolve the symbolic reference to a member.
 233      * <p>
 234      * If there is a security manager, its {@code checkPermission} method
 235      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 236      * @param <T> the desired type of the result, either {@link Member} or a subtype
 237      * @param target a direct method handle to crack into symbolic reference components
 238      * @param expected a class object representing the desired result type {@code T}
 239      * @return a reference to the method, constructor, or field object
 240      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 241      * @exception NullPointerException if either argument is {@code null}
 242      * @exception IllegalArgumentException if the target is not a direct method handle
 243      * @exception ClassCastException if the member is not of the expected type
 244      * @since 1.8
 245      */
 246     public static <T extends Member> T
 247     reflectAs(Class<T> expected, MethodHandle target) {
 248         SecurityManager smgr = System.getSecurityManager();
 249         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 250         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 251         return lookup.revealDirect(target).reflectAs(expected, lookup);
 252     }
 253     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 254     private static final java.security.Permission ACCESS_PERMISSION =
 255         new ReflectPermission("suppressAccessChecks");
 256 
 257     /**
 258      * A <em>lookup object</em> is a factory for creating method handles,
 259      * when the creation requires access checking.
 260      * Method handles do not perform
 261      * access checks when they are called, but rather when they are created.
 262      * Therefore, method handle access
 263      * restrictions must be enforced when a method handle is created.
 264      * The caller class against which those restrictions are enforced
 265      * is known as the {@linkplain #lookupClass lookup class}.
 266      * <p>
 267      * A lookup class which needs to create method handles will call
 268      * {@link MethodHandles#lookup MethodHandles.lookup} to create a factory for itself.
 269      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 270      * determined, and securely stored in the {@code Lookup} object.
 271      * The lookup class (or its delegates) may then use factory methods
 272      * on the {@code Lookup} object to create method handles for access-checked members.
 273      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 274      * even private ones.
 275      *
 276      * <h1><a id="lookups"></a>Lookup Factory Methods</h1>
 277      * The factory methods on a {@code Lookup} object correspond to all major
 278      * use cases for methods, constructors, and fields.
 279      * Each method handle created by a factory method is the functional
 280      * equivalent of a particular <em>bytecode behavior</em>.
 281      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 282      * Here is a summary of the correspondence between these factory methods and
 283      * the behavior of the resulting method handles:
 284      * <table border=1 cellpadding=5 summary="lookup method behaviors">
 285      * <tr>
 286      *     <th><a id="equiv"></a>lookup expression</th>
 287      *     <th>member</th>
 288      *     <th>bytecode behavior</th>
 289      * </tr>
 290      * <tr>
 291      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</td>
 292      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 293      * </tr>
 294      * <tr>
 295      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</td>
 296      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 297      * </tr>
 298      * <tr>
 299      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</td>
 300      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 301      * </tr>
 302      * <tr>
 303      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</td>
 304      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 305      * </tr>
 306      * <tr>
 307      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</td>
 308      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 309      * </tr>
 310      * <tr>
 311      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</td>
 312      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 313      * </tr>
 314      * <tr>
 315      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</td>
 316      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 317      * </tr>
 318      * <tr>
 319      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</td>
 320      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 321      * </tr>
 322      * <tr>
 323      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</td>
 324      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 325      * </tr>
 326      * <tr>
 327      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</td>
 328      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 329      * </tr>
 330      * <tr>
 331      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 332      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 333      * </tr>
 334      * <tr>
 335      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</td>
 336      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 337      * </tr>
 338      * <tr>
 339      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 340      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 341      * </tr>
 342      * <tr>
 343      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</td>
 344      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 345      * </tr>
 346      * </table>
 347      *
 348      * Here, the type {@code C} is the class or interface being searched for a member,
 349      * documented as a parameter named {@code refc} in the lookup methods.
 350      * The method type {@code MT} is composed from the return type {@code T}
 351      * and the sequence of argument types {@code A*}.
 352      * The constructor also has a sequence of argument types {@code A*} and
 353      * is deemed to return the newly-created object of type {@code C}.
 354      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 355      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 356      * if it is present, it is always the leading argument to the method handle invocation.
 357      * (In the case of some {@code protected} members, {@code this} may be
 358      * restricted in type to the lookup class; see below.)
 359      * The name {@code arg} stands for all the other method handle arguments.
 360      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 361      * stands for a null reference if the accessed method or field is static,
 362      * and {@code this} otherwise.
 363      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 364      * for reflective objects corresponding to the given members.
 365      * <p>
 366      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 367      * as if by {@code ldc CONSTANT_Class}.
 368      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 369      * <p>
 370      * In cases where the given member is of variable arity (i.e., a method or constructor)
 371      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 372      * In all other cases, the returned method handle will be of fixed arity.
 373      * <p style="font-size:smaller;">
 374      * <em>Discussion:</em>
 375      * The equivalence between looked-up method handles and underlying
 376      * class members and bytecode behaviors
 377      * can break down in a few ways:
 378      * <ul style="font-size:smaller;">
 379      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 380      * the lookup can still succeed, even when there is no equivalent
 381      * Java expression or bytecoded constant.
 382      * <li>Likewise, if {@code T} or {@code MT}
 383      * is not symbolically accessible from the lookup class's loader,
 384      * the lookup can still succeed.
 385      * For example, lookups for {@code MethodHandle.invokeExact} and
 386      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 387      * <li>If there is a security manager installed, it can forbid the lookup
 388      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 389      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 390      * constant is not subject to security manager checks.
 391      * <li>If the looked-up method has a
 392      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 393      * the method handle creation may fail, due to the method handle
 394      * type having too many parameters.
 395      * </ul>
 396      *
 397      * <h1><a id="access"></a>Access checking</h1>
 398      * Access checks are applied in the factory methods of {@code Lookup},
 399      * when a method handle is created.
 400      * This is a key difference from the Core Reflection API, since
 401      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 402      * performs access checking against every caller, on every call.
 403      * <p>
 404      * All access checks start from a {@code Lookup} object, which
 405      * compares its recorded lookup class against all requests to
 406      * create method handles.
 407      * A single {@code Lookup} object can be used to create any number
 408      * of access-checked method handles, all checked against a single
 409      * lookup class.
 410      * <p>
 411      * A {@code Lookup} object can be shared with other trusted code,
 412      * such as a metaobject protocol.
 413      * A shared {@code Lookup} object delegates the capability
 414      * to create method handles on private members of the lookup class.
 415      * Even if privileged code uses the {@code Lookup} object,
 416      * the access checking is confined to the privileges of the
 417      * original lookup class.
 418      * <p>
 419      * A lookup can fail, because
 420      * the containing class is not accessible to the lookup class, or
 421      * because the desired class member is missing, or because the
 422      * desired class member is not accessible to the lookup class, or
 423      * because the lookup object is not trusted enough to access the member.
 424      * In any of these cases, a {@code ReflectiveOperationException} will be
 425      * thrown from the attempted lookup.  The exact class will be one of
 426      * the following:
 427      * <ul>
 428      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 429      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 430      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 431      * </ul>
 432      * <p>
 433      * In general, the conditions under which a method handle may be
 434      * looked up for a method {@code M} are no more restrictive than the conditions
 435      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 436      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 437      * a method handle lookup will generally raise a corresponding
 438      * checked exception, such as {@code NoSuchMethodException}.
 439      * And the effect of invoking the method handle resulting from the lookup
 440      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 441      * to executing the compiled, verified, and resolved call to {@code M}.
 442      * The same point is true of fields and constructors.
 443      * <p style="font-size:smaller;">
 444      * <em>Discussion:</em>
 445      * Access checks only apply to named and reflected methods,
 446      * constructors, and fields.
 447      * Other method handle creation methods, such as
 448      * {@link MethodHandle#asType MethodHandle.asType},
 449      * do not require any access checks, and are used
 450      * independently of any {@code Lookup} object.
 451      * <p>
 452      * If the desired member is {@code protected}, the usual JVM rules apply,
 453      * including the requirement that the lookup class must be either be in the
 454      * same package as the desired member, or must inherit that member.
 455      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 456      * In addition, if the desired member is a non-static field or method
 457      * in a different package, the resulting method handle may only be applied
 458      * to objects of the lookup class or one of its subclasses.
 459      * This requirement is enforced by narrowing the type of the leading
 460      * {@code this} parameter from {@code C}
 461      * (which will necessarily be a superclass of the lookup class)
 462      * to the lookup class itself.
 463      * <p>
 464      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 465      * that the receiver argument must match both the resolved method <em>and</em>
 466      * the current class.  Again, this requirement is enforced by narrowing the
 467      * type of the leading parameter to the resulting method handle.
 468      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 469      * <p>
 470      * The JVM represents constructors and static initializer blocks as internal methods
 471      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 472      * The internal syntax of invocation instructions allows them to refer to such internal
 473      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 474      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 475      * <p>
 476      * In some cases, access between nested classes is obtained by the Java compiler by creating
 477      * an wrapper method to access a private method of another class
 478      * in the same top-level declaration.
 479      * For example, a nested class {@code C.D}
 480      * can access private members within other related classes such as
 481      * {@code C}, {@code C.D.E}, or {@code C.B},
 482      * but the Java compiler may need to generate wrapper methods in
 483      * those related classes.  In such cases, a {@code Lookup} object on
 484      * {@code C.E} would be unable to those private members.
 485      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 486      * which can transform a lookup on {@code C.E} into one on any of those other
 487      * classes, without special elevation of privilege.
 488      * <p>
 489      * The accesses permitted to a given lookup object may be limited,
 490      * according to its set of {@link #lookupModes lookupModes},
 491      * to a subset of members normally accessible to the lookup class.
 492      * For example, the {@link MethodHandles#publicLookup publicLookup}
 493      * method produces a lookup object which is only allowed to access
 494      * public members in public classes of exported packages.
 495      * The caller sensitive method {@link MethodHandles#lookup lookup}
 496      * produces a lookup object with full capabilities relative to
 497      * its caller class, to emulate all supported bytecode behaviors.
 498      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 499      * with fewer access modes than the original lookup object.
 500      *
 501      * <p style="font-size:smaller;">
 502      * <a id="privacc"></a>
 503      * <em>Discussion of private access:</em>
 504      * We say that a lookup has <em>private access</em>
 505      * if its {@linkplain #lookupModes lookup modes}
 506      * include the possibility of accessing {@code private} members.
 507      * As documented in the relevant methods elsewhere,
 508      * only lookups with private access possess the following capabilities:
 509      * <ul style="font-size:smaller;">
 510      * <li>access private fields, methods, and constructors of the lookup class
 511      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 512      *     such as {@code Class.forName}
 513      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 514      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 515      *     for classes accessible to the lookup class
 516      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 517      *     within the same package member
 518      * </ul>
 519      * <p style="font-size:smaller;">
 520      * Each of these permissions is a consequence of the fact that a lookup object
 521      * with private access can be securely traced back to an originating class,
 522      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 523      * can be reliably determined and emulated by method handles.
 524      *
 525      * <h1><a id="secmgr"></a>Security manager interactions</h1>
 526      * Although bytecode instructions can only refer to classes in
 527      * a related class loader, this API can search for methods in any
 528      * class, as long as a reference to its {@code Class} object is
 529      * available.  Such cross-loader references are also possible with the
 530      * Core Reflection API, and are impossible to bytecode instructions
 531      * such as {@code invokestatic} or {@code getfield}.
 532      * There is a {@linkplain java.lang.SecurityManager security manager API}
 533      * to allow applications to check such cross-loader references.
 534      * These checks apply to both the {@code MethodHandles.Lookup} API
 535      * and the Core Reflection API
 536      * (as found on {@link java.lang.Class Class}).
 537      * <p>
 538      * If a security manager is present, member and class lookups are subject to
 539      * additional checks.
 540      * From one to three calls are made to the security manager.
 541      * Any of these calls can refuse access by throwing a
 542      * {@link java.lang.SecurityException SecurityException}.
 543      * Define {@code smgr} as the security manager,
 544      * {@code lookc} as the lookup class of the current lookup object,
 545      * {@code refc} as the containing class in which the member
 546      * is being sought, and {@code defc} as the class in which the
 547      * member is actually defined.
 548      * (If a class or other type is being accessed,
 549      * the {@code refc} and {@code defc} values are the class itself.)
 550      * The value {@code lookc} is defined as <em>not present</em>
 551      * if the current lookup object does not have
 552      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 553      * The calls are made according to the following rules:
 554      * <ul>
 555      * <li><b>Step 1:</b>
 556      *     If {@code lookc} is not present, or if its class loader is not
 557      *     the same as or an ancestor of the class loader of {@code refc},
 558      *     then {@link SecurityManager#checkPackageAccess
 559      *     smgr.checkPackageAccess(refcPkg)} is called,
 560      *     where {@code refcPkg} is the package of {@code refc}.
 561      * <li><b>Step 2a:</b>
 562      *     If the retrieved member is not public and
 563      *     {@code lookc} is not present, then
 564      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 565      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 566      * <li><b>Step 2b:</b>
 567      *     If the retrieved class has a {@code null} class loader,
 568      *     and {@code lookc} is not present, then
 569      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 570      *     with {@code RuntimePermission("getClassLoader")} is called.
 571      * <li><b>Step 3:</b>
 572      *     If the retrieved member is not public,
 573      *     and if {@code lookc} is not present,
 574      *     and if {@code defc} and {@code refc} are different,
 575      *     then {@link SecurityManager#checkPackageAccess
 576      *     smgr.checkPackageAccess(defcPkg)} is called,
 577      *     where {@code defcPkg} is the package of {@code defc}.
 578      * </ul>
 579      * Security checks are performed after other access checks have passed.
 580      * Therefore, the above rules presuppose a member or class that is public,
 581      * or else that is being accessed from a lookup class that has
 582      * rights to access the member or class.
 583      *
 584      * <h1><a id="callsens"></a>Caller sensitive methods</h1>
 585      * A small number of Java methods have a special property called caller sensitivity.
 586      * A <em>caller-sensitive</em> method can behave differently depending on the
 587      * identity of its immediate caller.
 588      * <p>
 589      * If a method handle for a caller-sensitive method is requested,
 590      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 591      * but they take account of the lookup class in a special way.
 592      * The resulting method handle behaves as if it were called
 593      * from an instruction contained in the lookup class,
 594      * so that the caller-sensitive method detects the lookup class.
 595      * (By contrast, the invoker of the method handle is disregarded.)
 596      * Thus, in the case of caller-sensitive methods,
 597      * different lookup classes may give rise to
 598      * differently behaving method handles.
 599      * <p>
 600      * In cases where the lookup object is
 601      * {@link MethodHandles#publicLookup() publicLookup()},
 602      * or some other lookup object without
 603      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 604      * the lookup class is disregarded.
 605      * In such cases, no caller-sensitive method handle can be created,
 606      * access is forbidden, and the lookup fails with an
 607      * {@code IllegalAccessException}.
 608      * <p style="font-size:smaller;">
 609      * <em>Discussion:</em>
 610      * For example, the caller-sensitive method
 611      * {@link java.lang.Class#forName(String) Class.forName(x)}
 612      * can return varying classes or throw varying exceptions,
 613      * depending on the class loader of the class that calls it.
 614      * A public lookup of {@code Class.forName} will fail, because
 615      * there is no reasonable way to determine its bytecode behavior.
 616      * <p style="font-size:smaller;">
 617      * If an application caches method handles for broad sharing,
 618      * it should use {@code publicLookup()} to create them.
 619      * If there is a lookup of {@code Class.forName}, it will fail,
 620      * and the application must take appropriate action in that case.
 621      * It may be that a later lookup, perhaps during the invocation of a
 622      * bootstrap method, can incorporate the specific identity
 623      * of the caller, making the method accessible.
 624      * <p style="font-size:smaller;">
 625      * The function {@code MethodHandles.lookup} is caller sensitive
 626      * so that there can be a secure foundation for lookups.
 627      * Nearly all other methods in the JSR 292 API rely on lookup
 628      * objects to check access requests.
 629      *
 630      * @revised 9
 631      */
 632     public static final
 633     class Lookup {
 634         /** The class on behalf of whom the lookup is being performed. */
 635         private final Class<?> lookupClass;
 636 
 637         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 638         private final int allowedModes;
 639 
 640         /** A single-bit mask representing {@code public} access,
 641          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 642          *  The value, {@code 0x01}, happens to be the same as the value of the
 643          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 644          */
 645         public static final int PUBLIC = Modifier.PUBLIC;
 646 
 647         /** A single-bit mask representing {@code private} access,
 648          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 649          *  The value, {@code 0x02}, happens to be the same as the value of the
 650          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 651          */
 652         public static final int PRIVATE = Modifier.PRIVATE;
 653 
 654         /** A single-bit mask representing {@code protected} access,
 655          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 656          *  The value, {@code 0x04}, happens to be the same as the value of the
 657          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 658          */
 659         public static final int PROTECTED = Modifier.PROTECTED;
 660 
 661         /** A single-bit mask representing {@code package} access (default access),
 662          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 663          *  The value is {@code 0x08}, which does not correspond meaningfully to
 664          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 665          */
 666         public static final int PACKAGE = Modifier.STATIC;
 667 
 668         /** A single-bit mask representing {@code module} access (default access),
 669          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 670          *  The value is {@code 0x10}, which does not correspond meaningfully to
 671          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 672          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 673          *  with this lookup mode can access all public types in the module of the
 674          *  lookup class and public types in packages exported by other modules
 675          *  to the module of the lookup class.
 676          *  @since 9
 677          *  @spec JPMS
 678          */
 679         public static final int MODULE = PACKAGE << 1;
 680 
 681         /** A single-bit mask representing {@code unconditional} access
 682          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 683          *  The value is {@code 0x20}, which does not correspond meaningfully to
 684          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 685          *  A {@code Lookup} with this lookup mode assumes {@linkplain
 686          *  java.lang.Module#canRead(java.lang.Module) readability}.
 687          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 688          *  with this lookup mode can access all public members of public types
 689          *  of all modules where the type is in a package that is {@link
 690          *  java.lang.Module#isExported(String) exported unconditionally}.
 691          *  @since 9
 692          *  @spec JPMS
 693          *  @see #publicLookup()
 694          */
 695         public static final int UNCONDITIONAL = PACKAGE << 2;
 696 
 697         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL);
 698         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);
 699         private static final int TRUSTED   = -1;
 700 
 701         private static int fixmods(int mods) {
 702             mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL);
 703             return (mods != 0) ? mods : (PACKAGE | MODULE | UNCONDITIONAL);
 704         }
 705 
 706         /** Tells which class is performing the lookup.  It is this class against
 707          *  which checks are performed for visibility and access permissions.
 708          *  <p>
 709          *  The class implies a maximum level of access permission,
 710          *  but the permissions may be additionally limited by the bitmask
 711          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 712          *  can be accessed.
 713          *  @return the lookup class, on behalf of which this lookup object finds members
 714          */
 715         public Class<?> lookupClass() {
 716             return lookupClass;
 717         }
 718 
 719         // This is just for calling out to MethodHandleImpl.
 720         private Class<?> lookupClassOrNull() {
 721             return (allowedModes == TRUSTED) ? null : lookupClass;
 722         }
 723 
 724         /** Tells which access-protection classes of members this lookup object can produce.
 725          *  The result is a bit-mask of the bits
 726          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 727          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 728          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 729          *  {@linkplain #PACKAGE PACKAGE (0x08)},
 730          *  {@linkplain #MODULE MODULE (0x10)},
 731          *  and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}.
 732          *  <p>
 733          *  A freshly-created lookup object
 734          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
 735          *  all possible bits set, except {@code UNCONDITIONAL}. The lookup can be used to
 736          *  access all members of the caller's class, all public types in the caller's module,
 737          *  and all public types in packages exported by other modules to the caller's module.
 738          *  A lookup object on a new lookup class
 739          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 740          *  may have some mode bits set to zero.
 741          *  Mode bits can also be
 742          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
 743          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
 744          *  The purpose of this is to restrict access via the new lookup object,
 745          *  so that it can access only names which can be reached by the original
 746          *  lookup object, and also by the new lookup class.
 747          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 748          *  @see #in
 749          *  @see #dropLookupMode
 750          *
 751          *  @revised 9
 752          *  @spec JPMS
 753          */
 754         public int lookupModes() {
 755             return allowedModes & ALL_MODES;
 756         }
 757 
 758         /** Embody the current class (the lookupClass) as a lookup class
 759          * for method handle creation.
 760          * Must be called by from a method in this package,
 761          * which in turn is called by a method not in this package.
 762          */
 763         Lookup(Class<?> lookupClass) {
 764             this(lookupClass, FULL_POWER_MODES);
 765             // make sure we haven't accidentally picked up a privileged class:
 766             checkUnprivilegedlookupClass(lookupClass);
 767         }
 768 
 769         private Lookup(Class<?> lookupClass, int allowedModes) {
 770             this.lookupClass = lookupClass;
 771             this.allowedModes = allowedModes;
 772         }
 773 
 774         /**
 775          * Creates a lookup on the specified new lookup class.
 776          * The resulting object will report the specified
 777          * class as its own {@link #lookupClass lookupClass}.
 778          * <p>
 779          * However, the resulting {@code Lookup} object is guaranteed
 780          * to have no more access capabilities than the original.
 781          * In particular, access capabilities can be lost as follows:<ul>
 782          * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and
 783          * the new lookup class is in a different module {@code M}, then no members, not
 784          * even public members in {@code M}'s exported packages, will be accessible.
 785          * The exception to this is when this lookup is {@link #publicLookup()
 786          * publicLookup}, in which case {@code PUBLIC} access is not lost.
 787          * <li>If the old lookup class is in an unnamed module, and the new lookup class
 788          * is a different module then {@link #MODULE MODULE} access is lost.
 789          * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost.
 790          * <li>If the new lookup class is in a different package
 791          * than the old one, protected and default (package) members will not be accessible.
 792          * <li>If the new lookup class is not within the same package member
 793          * as the old one, private members will not be accessible, and protected members
 794          * will not be accessible by virtue of inheritance.
 795          * (Protected members may continue to be accessible because of package sharing.)
 796          * <li>If the new lookup class is not accessible to the old lookup class,
 797          * then no members, not even public members, will be accessible.
 798          * (In all other cases, public members will continue to be accessible.)
 799          * </ul>
 800          * <p>
 801          * The resulting lookup's capabilities for loading classes
 802          * (used during {@link #findClass} invocations)
 803          * are determined by the lookup class' loader,
 804          * which may change due to this operation.
 805          *
 806          * @param requestedLookupClass the desired lookup class for the new lookup object
 807          * @return a lookup object which reports the desired lookup class, or the same object
 808          * if there is no change
 809          * @throws NullPointerException if the argument is null
 810          *
 811          * @revised 9
 812          * @spec JPMS
 813          */
 814         public Lookup in(Class<?> requestedLookupClass) {
 815             Objects.requireNonNull(requestedLookupClass);
 816             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 817                 return new Lookup(requestedLookupClass, FULL_POWER_MODES);
 818             if (requestedLookupClass == this.lookupClass)
 819                 return this;  // keep same capabilities
 820             int newModes = (allowedModes & FULL_POWER_MODES);
 821             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 822                 // Need to drop all access when teleporting from a named module to another
 823                 // module. The exception is publicLookup where PUBLIC is not lost.
 824                 if (this.lookupClass.getModule().isNamed()
 825                     && (this.allowedModes & UNCONDITIONAL) == 0)
 826                     newModes = 0;
 827                 else
 828                     newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
 829             }
 830             if ((newModes & PACKAGE) != 0
 831                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 832                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
 833             }
 834             // Allow nestmate lookups to be created without special privilege:
 835             if ((newModes & PRIVATE) != 0
 836                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 837                 newModes &= ~(PRIVATE|PROTECTED);
 838             }
 839             if ((newModes & PUBLIC) != 0
 840                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 841                 // The requested class it not accessible from the lookup class.
 842                 // No permissions.
 843                 newModes = 0;
 844             }
 845 
 846             checkUnprivilegedlookupClass(requestedLookupClass);
 847             return new Lookup(requestedLookupClass, newModes);
 848         }
 849 
 850 
 851         /**
 852          * Creates a lookup on the same lookup class which this lookup object
 853          * finds members, but with a lookup mode that has lost the given lookup mode.
 854          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
 855          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}.
 856          * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always
 857          * dropped and so the resulting lookup mode will never have these access capabilities.
 858          * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE}
 859          * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will
 860          * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC}
 861          * is dropped then the resulting lookup has no access.
 862          * @param modeToDrop the lookup mode to drop
 863          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
 864          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
 865          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
 866          * @see MethodHandles#privateLookupIn
 867          * @since 9
 868          */
 869         public Lookup dropLookupMode(int modeToDrop) {
 870             int oldModes = lookupModes();
 871             int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL);
 872             switch (modeToDrop) {
 873                 case PUBLIC: newModes &= ~(ALL_MODES); break;
 874                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
 875                 case PACKAGE: newModes &= ~(PRIVATE); break;
 876                 case PROTECTED:
 877                 case PRIVATE:
 878                 case UNCONDITIONAL: break;
 879                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
 880             }
 881             if (newModes == oldModes) return this;  // return self if no change
 882             return new Lookup(lookupClass(), newModes);
 883         }
 884 
 885         /**
 886          * Defines a class to the same class loader and in the same runtime package and
 887          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
 888          * {@linkplain #lookupClass() lookup class}.
 889          *
 890          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
 891          * {@link #PACKAGE PACKAGE} access as default (package) members will be
 892          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
 893          * that the lookup object was created by a caller in the runtime package (or derived
 894          * from a lookup originally created by suitably privileged code to a target class in
 895          * the runtime package). </p>
 896          *
 897          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
 898          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
 899          * same package as the lookup class. </p>
 900          *
 901          * <p> This method does not run the class initializer. The class initializer may
 902          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
 903          * Specification</em>. </p>
 904          *
 905          * <p> If there is a security manager, its {@code checkPermission} method is first called
 906          * to check {@code RuntimePermission("defineClass")}. </p>
 907          *
 908          * @param bytes the class bytes
 909          * @return the {@code Class} object for the class
 910          * @throws IllegalArgumentException the bytes are for a class in a different package
 911          * to the lookup class
 912          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
 913          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
 914          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
 915          * @throws SecurityException if denied by the security manager
 916          * @throws NullPointerException if {@code bytes} is {@code null}
 917          * @since 9
 918          * @spec JPMS
 919          * @see Lookup#privateLookupIn
 920          * @see Lookup#dropLookupMode
 921          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
 922          */
 923         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
 924             SecurityManager sm = System.getSecurityManager();
 925             if (sm != null)
 926                 sm.checkPermission(new RuntimePermission("defineClass"));
 927             if ((lookupModes() & PACKAGE) == 0)
 928                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
 929             assert (lookupModes() & (MODULE|PUBLIC)) != 0;
 930 
 931             // parse class bytes to get class name (in internal form)
 932             bytes = bytes.clone();
 933             String name;
 934             try {
 935                 ClassReader reader = new ClassReader(bytes);
 936                 name = reader.getClassName();
 937             } catch (RuntimeException e) {
 938                 // ASM exceptions are poorly specified
 939                 ClassFormatError cfe = new ClassFormatError();
 940                 cfe.initCause(e);
 941                 throw cfe;
 942             }
 943 
 944             // get package and class name in binary form
 945             String cn, pn;
 946             int index = name.lastIndexOf('/');
 947             if (index == -1) {
 948                 cn = name;
 949                 pn = "";
 950             } else {
 951                 cn = name.replace('/', '.');
 952                 pn = cn.substring(0, index);
 953             }
 954             if (!pn.equals(lookupClass.getPackageName())) {
 955                 throw new IllegalArgumentException("Class not in same package as lookup class");
 956             }
 957 
 958             // invoke the class loader's defineClass method
 959             ClassLoader loader = lookupClass.getClassLoader();
 960             ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null;
 961             String source = "__Lookup_defineClass__";
 962             Class<?> clazz = SharedSecrets.getJavaLangAccess().defineClass(loader, cn, bytes, pd, source);
 963             assert clazz.getClassLoader() == lookupClass.getClassLoader()
 964                     && clazz.getPackageName().equals(lookupClass.getPackageName())
 965                     && protectionDomain(clazz) == lookupClassProtectionDomain();
 966             return clazz;
 967         }
 968 
 969         private ProtectionDomain lookupClassProtectionDomain() {
 970             ProtectionDomain pd = cachedProtectionDomain;
 971             if (pd == null) {
 972                 cachedProtectionDomain = pd = protectionDomain(lookupClass);
 973             }
 974             return pd;
 975         }
 976 
 977         private ProtectionDomain protectionDomain(Class<?> clazz) {
 978             PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain;
 979             return AccessController.doPrivileged(pa);
 980         }
 981 
 982         // cached protection domain
 983         private volatile ProtectionDomain cachedProtectionDomain;
 984 
 985 
 986         // Make sure outer class is initialized first.
 987         static { IMPL_NAMES.getClass(); }
 988 
 989         /** Package-private version of lookup which is trusted. */
 990         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 991 
 992         /** Version of lookup which is trusted minimally.
 993          *  It can only be used to create method handles to publicly accessible
 994          *  members in packages that are exported unconditionally.
 995          */
 996         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC|UNCONDITIONAL));
 997 
 998         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
 999             String name = lookupClass.getName();
1000             if (name.startsWith("java.lang.invoke."))
1001                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
1002         }
1003 
1004         /**
1005          * Displays the name of the class from which lookups are to be made.
1006          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
1007          * If there are restrictions on the access permitted to this lookup,
1008          * this is indicated by adding a suffix to the class name, consisting
1009          * of a slash and a keyword.  The keyword represents the strongest
1010          * allowed access, and is chosen as follows:
1011          * <ul>
1012          * <li>If no access is allowed, the suffix is "/noaccess".
1013          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
1014          * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup".
1015          * <li>If only public and module access are allowed, the suffix is "/module".
1016          * <li>If only public, module and package access are allowed, the suffix is "/package".
1017          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
1018          * </ul>
1019          * If none of the above cases apply, it is the case that full
1020          * access (public, module, package, private, and protected) is allowed.
1021          * In this case, no suffix is added.
1022          * This is true only of an object obtained originally from
1023          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
1024          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
1025          * always have restricted access, and will display a suffix.
1026          * <p>
1027          * (It may seem strange that protected access should be
1028          * stronger than private access.  Viewed independently from
1029          * package access, protected access is the first to be lost,
1030          * because it requires a direct subclass relationship between
1031          * caller and callee.)
1032          * @see #in
1033          *
1034          * @revised 9
1035          * @spec JPMS
1036          */
1037         @Override
1038         public String toString() {
1039             String cname = lookupClass.getName();
1040             switch (allowedModes) {
1041             case 0:  // no privileges
1042                 return cname + "/noaccess";
1043             case PUBLIC:
1044                 return cname + "/public";
1045             case PUBLIC|UNCONDITIONAL:
1046                 return cname  + "/publicLookup";
1047             case PUBLIC|MODULE:
1048                 return cname + "/module";
1049             case PUBLIC|MODULE|PACKAGE:
1050                 return cname + "/package";
1051             case FULL_POWER_MODES & ~PROTECTED:
1052                 return cname + "/private";
1053             case FULL_POWER_MODES:
1054                 return cname;
1055             case TRUSTED:
1056                 return "/trusted";  // internal only; not exported
1057             default:  // Should not happen, but it's a bitfield...
1058                 cname = cname + "/" + Integer.toHexString(allowedModes);
1059                 assert(false) : cname;
1060                 return cname;
1061             }
1062         }
1063 
1064         /**
1065          * Produces a method handle for a static method.
1066          * The type of the method handle will be that of the method.
1067          * (Since static methods do not take receivers, there is no
1068          * additional receiver argument inserted into the method handle type,
1069          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
1070          * The method and all its argument types must be accessible to the lookup object.
1071          * <p>
1072          * The returned method handle will have
1073          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1074          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1075          * <p>
1076          * If the returned method handle is invoked, the method's class will
1077          * be initialized, if it has not already been initialized.
1078          * <p><b>Example:</b>
1079          * <blockquote><pre>{@code
1080 import static java.lang.invoke.MethodHandles.*;
1081 import static java.lang.invoke.MethodType.*;
1082 ...
1083 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
1084   "asList", methodType(List.class, Object[].class));
1085 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
1086          * }</pre></blockquote>
1087          * @param refc the class from which the method is accessed
1088          * @param name the name of the method
1089          * @param type the type of the method
1090          * @return the desired method handle
1091          * @throws NoSuchMethodException if the method does not exist
1092          * @throws IllegalAccessException if access checking fails,
1093          *                                or if the method is not {@code static},
1094          *                                or if the method's variable arity modifier bit
1095          *                                is set and {@code asVarargsCollector} fails
1096          * @exception SecurityException if a security manager is present and it
1097          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1098          * @throws NullPointerException if any argument is null
1099          */
1100         public
1101         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1102             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
1103             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
1104         }
1105 
1106         /**
1107          * Produces a method handle for a virtual method.
1108          * The type of the method handle will be that of the method,
1109          * with the receiver type (usually {@code refc}) prepended.
1110          * The method and all its argument types must be accessible to the lookup object.
1111          * <p>
1112          * When called, the handle will treat the first argument as a receiver
1113          * and dispatch on the receiver's type to determine which method
1114          * implementation to enter.
1115          * (The dispatching action is identical with that performed by an
1116          * {@code invokevirtual} or {@code invokeinterface} instruction.)
1117          * <p>
1118          * The first argument will be of type {@code refc} if the lookup
1119          * class has full privileges to access the member.  Otherwise
1120          * the member must be {@code protected} and the first argument
1121          * will be restricted in type to the lookup class.
1122          * <p>
1123          * The returned method handle will have
1124          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1125          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1126          * <p>
1127          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
1128          * instructions and method handles produced by {@code findVirtual},
1129          * if the class is {@code MethodHandle} and the name string is
1130          * {@code invokeExact} or {@code invoke}, the resulting
1131          * method handle is equivalent to one produced by
1132          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
1133          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
1134          * with the same {@code type} argument.
1135          * <p>
1136          * If the class is {@code VarHandle} and the name string corresponds to
1137          * the name of a signature-polymorphic access mode method, the resulting
1138          * method handle is equivalent to one produced by
1139          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
1140          * the access mode corresponding to the name string and with the same
1141          * {@code type} arguments.
1142          * <p>
1143          * <b>Example:</b>
1144          * <blockquote><pre>{@code
1145 import static java.lang.invoke.MethodHandles.*;
1146 import static java.lang.invoke.MethodType.*;
1147 ...
1148 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
1149   "concat", methodType(String.class, String.class));
1150 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
1151   "hashCode", methodType(int.class));
1152 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
1153   "hashCode", methodType(int.class));
1154 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
1155 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
1156 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
1157 // interface method:
1158 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
1159   "subSequence", methodType(CharSequence.class, int.class, int.class));
1160 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
1161 // constructor "internal method" must be accessed differently:
1162 MethodType MT_newString = methodType(void.class); //()V for new String()
1163 try { assertEquals("impossible", lookup()
1164         .findVirtual(String.class, "<init>", MT_newString));
1165  } catch (NoSuchMethodException ex) { } // OK
1166 MethodHandle MH_newString = publicLookup()
1167   .findConstructor(String.class, MT_newString);
1168 assertEquals("", (String) MH_newString.invokeExact());
1169          * }</pre></blockquote>
1170          *
1171          * @param refc the class or interface from which the method is accessed
1172          * @param name the name of the method
1173          * @param type the type of the method, with the receiver argument omitted
1174          * @return the desired method handle
1175          * @throws NoSuchMethodException if the method does not exist
1176          * @throws IllegalAccessException if access checking fails,
1177          *                                or if the method is {@code static},
1178          *                                or if the method is {@code private} method of interface,
1179          *                                or if the method's variable arity modifier bit
1180          *                                is set and {@code asVarargsCollector} fails
1181          * @exception SecurityException if a security manager is present and it
1182          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1183          * @throws NullPointerException if any argument is null
1184          */
1185         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1186             if (refc == MethodHandle.class) {
1187                 MethodHandle mh = findVirtualForMH(name, type);
1188                 if (mh != null)  return mh;
1189             } else if (refc == VarHandle.class) {
1190                 MethodHandle mh = findVirtualForVH(name, type);
1191                 if (mh != null)  return mh;
1192             }
1193             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
1194             MemberName method = resolveOrFail(refKind, refc, name, type);
1195             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
1196         }
1197         private MethodHandle findVirtualForMH(String name, MethodType type) {
1198             // these names require special lookups because of the implicit MethodType argument
1199             if ("invoke".equals(name))
1200                 return invoker(type);
1201             if ("invokeExact".equals(name))
1202                 return exactInvoker(type);
1203             assert(!MemberName.isMethodHandleInvokeName(name));
1204             return null;
1205         }
1206         private MethodHandle findVirtualForVH(String name, MethodType type) {
1207             try {
1208                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
1209             } catch (IllegalArgumentException e) {
1210                 return null;
1211             }
1212         }
1213 
1214         /**
1215          * Produces a method handle which creates an object and initializes it, using
1216          * the constructor of the specified type.
1217          * The parameter types of the method handle will be those of the constructor,
1218          * while the return type will be a reference to the constructor's class.
1219          * The constructor and all its argument types must be accessible to the lookup object.
1220          * <p>
1221          * The requested type must have a return type of {@code void}.
1222          * (This is consistent with the JVM's treatment of constructor type descriptors.)
1223          * <p>
1224          * The returned method handle will have
1225          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1226          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1227          * <p>
1228          * If the returned method handle is invoked, the constructor's class will
1229          * be initialized, if it has not already been initialized.
1230          * <p><b>Example:</b>
1231          * <blockquote><pre>{@code
1232 import static java.lang.invoke.MethodHandles.*;
1233 import static java.lang.invoke.MethodType.*;
1234 ...
1235 MethodHandle MH_newArrayList = publicLookup().findConstructor(
1236   ArrayList.class, methodType(void.class, Collection.class));
1237 Collection orig = Arrays.asList("x", "y");
1238 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
1239 assert(orig != copy);
1240 assertEquals(orig, copy);
1241 // a variable-arity constructor:
1242 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
1243   ProcessBuilder.class, methodType(void.class, String[].class));
1244 ProcessBuilder pb = (ProcessBuilder)
1245   MH_newProcessBuilder.invoke("x", "y", "z");
1246 assertEquals("[x, y, z]", pb.command().toString());
1247          * }</pre></blockquote>
1248          * @param refc the class or interface from which the method is accessed
1249          * @param type the type of the method, with the receiver argument omitted, and a void return type
1250          * @return the desired method handle
1251          * @throws NoSuchMethodException if the constructor does not exist
1252          * @throws IllegalAccessException if access checking fails
1253          *                                or if the method's variable arity modifier bit
1254          *                                is set and {@code asVarargsCollector} fails
1255          * @exception SecurityException if a security manager is present and it
1256          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1257          * @throws NullPointerException if any argument is null
1258          */
1259         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1260             if (refc.isArray()) {
1261                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
1262             }
1263             String name = "<init>";
1264             if (MinimalValueTypes_1_0.isValueType(refc)) {
1265                 try {
1266                     //shady: The findConstructor method of Lookup will expose all accessible constructors of the original
1267                     //value-capable class, for both the Q-type and the legacy L-type. The return type of a method handle produced
1268                     //by findConstructor will be identical with the lookup class, even if it is a Q-type.
1269                     refc = MinimalValueTypes_1_0.getValueCapableClass(refc);
1270                 } catch (ClassNotFoundException ex) {
1271                     throw new NoSuchElementException(ex.getMessage());
1272                 }
1273             }
1274             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1275             return getDirectConstructor(refc, ctor);
1276         }
1277 
1278         /**
1279          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1280          * initializer of the class is not run.
1281          * <p>
1282          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1283          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1284          * load the requested class, and then determines whether the class is accessible to this lookup object.
1285          *
1286          * @param targetName the fully qualified name of the class to be looked up.
1287          * @return the requested class.
1288          * @exception SecurityException if a security manager is present and it
1289          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1290          * @throws LinkageError if the linkage fails
1291          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
1292          * @throws IllegalAccessException if the class is not accessible, using the allowed access
1293          * modes.
1294          * @exception SecurityException if a security manager is present and it
1295          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1296          * @since 9
1297          */
1298         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1299             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1300             return accessClass(targetClass);
1301         }
1302 
1303         /**
1304          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1305          * static initializer of the class is not run.
1306          * <p>
1307          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1308          * {@linkplain #lookupModes() lookup modes}.
1309          *
1310          * @param targetClass the class to be access-checked
1311          *
1312          * @return the class that has been access-checked
1313          *
1314          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1315          * modes.
1316          * @exception SecurityException if a security manager is present and it
1317          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1318          * @since 9
1319          */
1320         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1321             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1322                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1323             }
1324             checkSecurityManager(targetClass, null);
1325             return targetClass;
1326         }
1327 
1328         /**
1329          * Produces an early-bound method handle for a virtual method.
1330          * It will bypass checks for overriding methods on the receiver,
1331          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1332          * instruction from within the explicitly specified {@code specialCaller}.
1333          * The type of the method handle will be that of the method,
1334          * with a suitably restricted receiver type prepended.
1335          * (The receiver type will be {@code specialCaller} or a subtype.)
1336          * The method and all its argument types must be accessible
1337          * to the lookup object.
1338          * <p>
1339          * Before method resolution,
1340          * if the explicitly specified caller class is not identical with the
1341          * lookup class, or if this lookup object does not have
1342          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1343          * privileges, the access fails.
1344          * <p>
1345          * The returned method handle will have
1346          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1347          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1348          * <p style="font-size:smaller;">
1349          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1350          * even though the {@code invokespecial} instruction can refer to them
1351          * in special circumstances.  Use {@link #findConstructor findConstructor}
1352          * to access instance initialization methods in a safe manner.)</em>
1353          * <p><b>Example:</b>
1354          * <blockquote><pre>{@code
1355 import static java.lang.invoke.MethodHandles.*;
1356 import static java.lang.invoke.MethodType.*;
1357 ...
1358 static class Listie extends ArrayList {
1359   public String toString() { return "[wee Listie]"; }
1360   static Lookup lookup() { return MethodHandles.lookup(); }
1361 }
1362 ...
1363 // no access to constructor via invokeSpecial:
1364 MethodHandle MH_newListie = Listie.lookup()
1365   .findConstructor(Listie.class, methodType(void.class));
1366 Listie l = (Listie) MH_newListie.invokeExact();
1367 try { assertEquals("impossible", Listie.lookup().findSpecial(
1368         Listie.class, "<init>", methodType(void.class), Listie.class));
1369  } catch (NoSuchMethodException ex) { } // OK
1370 // access to super and self methods via invokeSpecial:
1371 MethodHandle MH_super = Listie.lookup().findSpecial(
1372   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1373 MethodHandle MH_this = Listie.lookup().findSpecial(
1374   Listie.class, "toString" , methodType(String.class), Listie.class);
1375 MethodHandle MH_duper = Listie.lookup().findSpecial(
1376   Object.class, "toString" , methodType(String.class), Listie.class);
1377 assertEquals("[]", (String) MH_super.invokeExact(l));
1378 assertEquals(""+l, (String) MH_this.invokeExact(l));
1379 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1380 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1381         String.class, "toString", methodType(String.class), Listie.class));
1382  } catch (IllegalAccessException ex) { } // OK
1383 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1384 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1385          * }</pre></blockquote>
1386          *
1387          * @param refc the class or interface from which the method is accessed
1388          * @param name the name of the method (which must not be "&lt;init&gt;")
1389          * @param type the type of the method, with the receiver argument omitted
1390          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1391          * @return the desired method handle
1392          * @throws NoSuchMethodException if the method does not exist
1393          * @throws IllegalAccessException if access checking fails,
1394          *                                or if the method is {@code static},
1395          *                                or if the method's variable arity modifier bit
1396          *                                is set and {@code asVarargsCollector} fails
1397          * @exception SecurityException if a security manager is present and it
1398          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1399          * @throws NullPointerException if any argument is null
1400          */
1401         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1402                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1403             checkSpecialCaller(specialCaller, refc);
1404             Lookup specialLookup = this.in(specialCaller);
1405             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1406             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1407         }
1408 
1409         /**
1410          * Produces a method handle giving read access to a non-static field.
1411          * The type of the method handle will have a return type of the field's
1412          * value type.
1413          * The method handle's single argument will be the instance containing
1414          * the field.
1415          * Access checking is performed immediately on behalf of the lookup class.
1416          * @param refc the class or interface from which the method is accessed
1417          * @param name the field's name
1418          * @param type the field's type
1419          * @return a method handle which can load values from the field
1420          * @throws NoSuchFieldException if the field does not exist
1421          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1422          * @exception SecurityException if a security manager is present and it
1423          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1424          * @throws NullPointerException if any argument is null
1425          * @see #findVarHandle(Class, String, Class)
1426          */
1427         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1428             if (MinimalValueTypes_1_0.isValueType(refc)) {
1429                 try {
1430                     return ValueType.forClass(MinimalValueTypes_1_0.getValueCapableClass(refc)).findGetter(this, name, type);
1431                 } catch (ReflectiveOperationException ex) {
1432                     throw new IllegalStateException(ex);
1433                 }
1434             } else {
1435                 MemberName field = resolveOrFail(REF_getField, refc, name, type);
1436                 return getDirectField(REF_getField, refc, field);
1437             }
1438         }
1439 
1440         /**
1441          * Produces a method handle giving write access to a non-static field.
1442          * The type of the method handle will have a void return type.
1443          * The method handle will take two arguments, the instance containing
1444          * the field, and the value to be stored.
1445          * The second argument will be of the field's value type.
1446          * Access checking is performed immediately on behalf of the lookup class.
1447          * @param refc the class or interface from which the method is accessed
1448          * @param name the field's name
1449          * @param type the field's type
1450          * @return a method handle which can store values into the field
1451          * @throws NoSuchFieldException if the field does not exist
1452          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1453          * @exception SecurityException if a security manager is present and it
1454          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1455          * @throws NullPointerException if any argument is null
1456          * @see #findVarHandle(Class, String, Class)
1457          */
1458         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1459             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1460             return getDirectField(REF_putField, refc, field);
1461         }
1462 
1463         /**
1464          * Produces a VarHandle giving access to a non-static field {@code name}
1465          * of type {@code type} declared in a class of type {@code recv}.
1466          * The VarHandle's variable type is {@code type} and it has one
1467          * coordinate type, {@code recv}.
1468          * <p>
1469          * Access checking is performed immediately on behalf of the lookup
1470          * class.
1471          * <p>
1472          * Certain access modes of the returned VarHandle are unsupported under
1473          * the following conditions:
1474          * <ul>
1475          * <li>if the field is declared {@code final}, then the write, atomic
1476          *     update, numeric atomic update, and bitwise atomic update access
1477          *     modes are unsupported.
1478          * <li>if the field type is anything other than {@code byte},
1479          *     {@code short}, {@code char}, {@code int}, {@code long},
1480          *     {@code float}, or {@code double} then numeric atomic update
1481          *     access modes are unsupported.
1482          * <li>if the field type is anything other than {@code boolean},
1483          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1484          *     {@code long} then bitwise atomic update access modes are
1485          *     unsupported.
1486          * </ul>
1487          * <p>
1488          * If the field is declared {@code volatile} then the returned VarHandle
1489          * will override access to the field (effectively ignore the
1490          * {@code volatile} declaration) in accordance to its specified
1491          * access modes.
1492          * <p>
1493          * If the field type is {@code float} or {@code double} then numeric
1494          * and atomic update access modes compare values using their bitwise
1495          * representation (see {@link Float#floatToRawIntBits} and
1496          * {@link Double#doubleToRawLongBits}, respectively).
1497          * @apiNote
1498          * Bitwise comparison of {@code float} values or {@code double} values,
1499          * as performed by the numeric and atomic update access modes, differ
1500          * from the primitive {@code ==} operator and the {@link Float#equals}
1501          * and {@link Double#equals} methods, specifically with respect to
1502          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1503          * Care should be taken when performing a compare and set or a compare
1504          * and exchange operation with such values since the operation may
1505          * unexpectedly fail.
1506          * There are many possible NaN values that are considered to be
1507          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1508          * provided by Java can distinguish between them.  Operation failure can
1509          * occur if the expected or witness value is a NaN value and it is
1510          * transformed (perhaps in a platform specific manner) into another NaN
1511          * value, and thus has a different bitwise representation (see
1512          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1513          * details).
1514          * The values {@code -0.0} and {@code +0.0} have different bitwise
1515          * representations but are considered equal when using the primitive
1516          * {@code ==} operator.  Operation failure can occur if, for example, a
1517          * numeric algorithm computes an expected value to be say {@code -0.0}
1518          * and previously computed the witness value to be say {@code +0.0}.
1519          * @param recv the receiver class, of type {@code R}, that declares the
1520          * non-static field
1521          * @param name the field's name
1522          * @param type the field's type, of type {@code T}
1523          * @return a VarHandle giving access to non-static fields.
1524          * @throws NoSuchFieldException if the field does not exist
1525          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1526          * @exception SecurityException if a security manager is present and it
1527          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1528          * @throws NullPointerException if any argument is null
1529          * @since 9
1530          */
1531         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1532             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
1533             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
1534             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
1535         }
1536 
1537         /**
1538          * Produces a method handle giving read access to a static field.
1539          * The type of the method handle will have a return type of the field's
1540          * value type.
1541          * The method handle will take no arguments.
1542          * Access checking is performed immediately on behalf of the lookup class.
1543          * <p>
1544          * If the returned method handle is invoked, the field's class will
1545          * be initialized, if it has not already been initialized.
1546          * @param refc the class or interface from which the method is accessed
1547          * @param name the field's name
1548          * @param type the field's type
1549          * @return a method handle which can load values from the field
1550          * @throws NoSuchFieldException if the field does not exist
1551          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1552          * @exception SecurityException if a security manager is present and it
1553          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1554          * @throws NullPointerException if any argument is null
1555          */
1556         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1557             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1558             return getDirectField(REF_getStatic, refc, field);
1559         }
1560 
1561         /**
1562          * Produces a method handle giving write access to a static field.
1563          * The type of the method handle will have a void return type.
1564          * The method handle will take a single
1565          * argument, of the field's value type, the value to be stored.
1566          * Access checking is performed immediately on behalf of the lookup class.
1567          * <p>
1568          * If the returned method handle is invoked, the field's class will
1569          * be initialized, if it has not already been initialized.
1570          * @param refc the class or interface from which the method is accessed
1571          * @param name the field's name
1572          * @param type the field's type
1573          * @return a method handle which can store values into the field
1574          * @throws NoSuchFieldException if the field does not exist
1575          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1576          * @exception SecurityException if a security manager is present and it
1577          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1578          * @throws NullPointerException if any argument is null
1579          */
1580         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1581             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1582             return getDirectField(REF_putStatic, refc, field);
1583         }
1584 
1585         /**
1586          * Produces a VarHandle giving access to a static field {@code name} of
1587          * type {@code type} declared in a class of type {@code decl}.
1588          * The VarHandle's variable type is {@code type} and it has no
1589          * coordinate types.
1590          * <p>
1591          * Access checking is performed immediately on behalf of the lookup
1592          * class.
1593          * <p>
1594          * If the returned VarHandle is operated on, the declaring class will be
1595          * initialized, if it has not already been initialized.
1596          * <p>
1597          * Certain access modes of the returned VarHandle are unsupported under
1598          * the following conditions:
1599          * <ul>
1600          * <li>if the field is declared {@code final}, then the write, atomic
1601          *     update, numeric atomic update, and bitwise atomic update access
1602          *     modes are unsupported.
1603          * <li>if the field type is anything other than {@code byte},
1604          *     {@code short}, {@code char}, {@code int}, {@code long},
1605          *     {@code float}, or {@code double}, then numeric atomic update
1606          *     access modes are unsupported.
1607          * <li>if the field type is anything other than {@code boolean},
1608          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1609          *     {@code long} then bitwise atomic update access modes are
1610          *     unsupported.
1611          * </ul>
1612          * <p>
1613          * If the field is declared {@code volatile} then the returned VarHandle
1614          * will override access to the field (effectively ignore the
1615          * {@code volatile} declaration) in accordance to its specified
1616          * access modes.
1617          * <p>
1618          * If the field type is {@code float} or {@code double} then numeric
1619          * and atomic update access modes compare values using their bitwise
1620          * representation (see {@link Float#floatToRawIntBits} and
1621          * {@link Double#doubleToRawLongBits}, respectively).
1622          * @apiNote
1623          * Bitwise comparison of {@code float} values or {@code double} values,
1624          * as performed by the numeric and atomic update access modes, differ
1625          * from the primitive {@code ==} operator and the {@link Float#equals}
1626          * and {@link Double#equals} methods, specifically with respect to
1627          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1628          * Care should be taken when performing a compare and set or a compare
1629          * and exchange operation with such values since the operation may
1630          * unexpectedly fail.
1631          * There are many possible NaN values that are considered to be
1632          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1633          * provided by Java can distinguish between them.  Operation failure can
1634          * occur if the expected or witness value is a NaN value and it is
1635          * transformed (perhaps in a platform specific manner) into another NaN
1636          * value, and thus has a different bitwise representation (see
1637          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1638          * details).
1639          * The values {@code -0.0} and {@code +0.0} have different bitwise
1640          * representations but are considered equal when using the primitive
1641          * {@code ==} operator.  Operation failure can occur if, for example, a
1642          * numeric algorithm computes an expected value to be say {@code -0.0}
1643          * and previously computed the witness value to be say {@code +0.0}.
1644          * @param decl the class that declares the static field
1645          * @param name the field's name
1646          * @param type the field's type, of type {@code T}
1647          * @return a VarHandle giving access to a static field
1648          * @throws NoSuchFieldException if the field does not exist
1649          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1650          * @exception SecurityException if a security manager is present and it
1651          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1652          * @throws NullPointerException if any argument is null
1653          * @since 9
1654          */
1655         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1656             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
1657             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
1658             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
1659         }
1660 
1661         /**
1662          * Produces an early-bound method handle for a non-static method.
1663          * The receiver must have a supertype {@code defc} in which a method
1664          * of the given name and type is accessible to the lookup class.
1665          * The method and all its argument types must be accessible to the lookup object.
1666          * The type of the method handle will be that of the method,
1667          * without any insertion of an additional receiver parameter.
1668          * The given receiver will be bound into the method handle,
1669          * so that every call to the method handle will invoke the
1670          * requested method on the given receiver.
1671          * <p>
1672          * The returned method handle will have
1673          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1674          * the method's variable arity modifier bit ({@code 0x0080}) is set
1675          * <em>and</em> the trailing array argument is not the only argument.
1676          * (If the trailing array argument is the only argument,
1677          * the given receiver value will be bound to it.)
1678          * <p>
1679          * This is almost equivalent to the following code, with some differences noted below:
1680          * <blockquote><pre>{@code
1681 import static java.lang.invoke.MethodHandles.*;
1682 import static java.lang.invoke.MethodType.*;
1683 ...
1684 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1685 MethodHandle mh1 = mh0.bindTo(receiver);
1686 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
1687 return mh1;
1688          * }</pre></blockquote>
1689          * where {@code defc} is either {@code receiver.getClass()} or a super
1690          * type of that class, in which the requested method is accessible
1691          * to the lookup class.
1692          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
1693          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
1694          * throw a {@code IllegalAccessException}, as in the case where the member is {@code protected} and
1695          * the receiver is restricted by {@code findVirtual} to the lookup class)
1696          * @param receiver the object from which the method is accessed
1697          * @param name the name of the method
1698          * @param type the type of the method, with the receiver argument omitted
1699          * @return the desired method handle
1700          * @throws NoSuchMethodException if the method does not exist
1701          * @throws IllegalAccessException if access checking fails
1702          *                                or if the method's variable arity modifier bit
1703          *                                is set and {@code asVarargsCollector} fails
1704          * @exception SecurityException if a security manager is present and it
1705          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1706          * @throws NullPointerException if any argument is null
1707          * @see MethodHandle#bindTo
1708          * @see #findVirtual
1709          */
1710         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1711             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1712             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1713             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerClass(method));
1714             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
1715                 throw new IllegalAccessException("The restricted defining class " +
1716                                                  mh.type().leadingReferenceParameter().getName() +
1717                                                  " is not assignable from receiver class " +
1718                                                  receiver.getClass().getName());
1719             }
1720             return mh.bindArgumentL(0, receiver).setVarargs(method);
1721         }
1722 
1723         /**
1724          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1725          * to <i>m</i>, if the lookup class has permission.
1726          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1727          * If <i>m</i> is virtual, overriding is respected on every call.
1728          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1729          * The type of the method handle will be that of the method,
1730          * with the receiver type prepended (but only if it is non-static).
1731          * If the method's {@code accessible} flag is not set,
1732          * access checking is performed immediately on behalf of the lookup class.
1733          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1734          * <p>
1735          * The returned method handle will have
1736          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1737          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1738          * <p>
1739          * If <i>m</i> is static, and
1740          * if the returned method handle is invoked, the method's class will
1741          * be initialized, if it has not already been initialized.
1742          * @param m the reflected method
1743          * @return a method handle which can invoke the reflected method
1744          * @throws IllegalAccessException if access checking fails
1745          *                                or if the method's variable arity modifier bit
1746          *                                is set and {@code asVarargsCollector} fails
1747          * @throws NullPointerException if the argument is null
1748          */
1749         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1750             if (m.getDeclaringClass() == MethodHandle.class) {
1751                 MethodHandle mh = unreflectForMH(m);
1752                 if (mh != null)  return mh;
1753             }
1754             if (m.getDeclaringClass() == VarHandle.class) {
1755                 MethodHandle mh = unreflectForVH(m);
1756                 if (mh != null)  return mh;
1757             }
1758             MemberName method = new MemberName(m);
1759             byte refKind = method.getReferenceKind();
1760             if (refKind == REF_invokeSpecial)
1761                 refKind = REF_invokeVirtual;
1762             assert(method.isMethod());
1763             @SuppressWarnings("deprecation")
1764             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1765             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1766         }
1767         private MethodHandle unreflectForMH(Method m) {
1768             // these names require special lookups because they throw UnsupportedOperationException
1769             if (MemberName.isMethodHandleInvokeName(m.getName()))
1770                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1771             return null;
1772         }
1773         private MethodHandle unreflectForVH(Method m) {
1774             // these names require special lookups because they throw UnsupportedOperationException
1775             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
1776                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
1777             return null;
1778         }
1779 
1780         /**
1781          * Produces a method handle for a reflected method.
1782          * It will bypass checks for overriding methods on the receiver,
1783          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1784          * instruction from within the explicitly specified {@code specialCaller}.
1785          * The type of the method handle will be that of the method,
1786          * with a suitably restricted receiver type prepended.
1787          * (The receiver type will be {@code specialCaller} or a subtype.)
1788          * If the method's {@code accessible} flag is not set,
1789          * access checking is performed immediately on behalf of the lookup class,
1790          * as if {@code invokespecial} instruction were being linked.
1791          * <p>
1792          * Before method resolution,
1793          * if the explicitly specified caller class is not identical with the
1794          * lookup class, or if this lookup object does not have
1795          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1796          * privileges, the access fails.
1797          * <p>
1798          * The returned method handle will have
1799          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1800          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1801          * @param m the reflected method
1802          * @param specialCaller the class nominally calling the method
1803          * @return a method handle which can invoke the reflected method
1804          * @throws IllegalAccessException if access checking fails,
1805          *                                or if the method is {@code static},
1806          *                                or if the method's variable arity modifier bit
1807          *                                is set and {@code asVarargsCollector} fails
1808          * @throws NullPointerException if any argument is null
1809          */
1810         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1811             checkSpecialCaller(specialCaller, null);
1812             Lookup specialLookup = this.in(specialCaller);
1813             MemberName method = new MemberName(m, true);
1814             assert(method.isMethod());
1815             // ignore m.isAccessible:  this is a new kind of access
1816             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1817         }
1818 
1819         /**
1820          * Produces a method handle for a reflected constructor.
1821          * The type of the method handle will be that of the constructor,
1822          * with the return type changed to the declaring class.
1823          * The method handle will perform a {@code newInstance} operation,
1824          * creating a new instance of the constructor's class on the
1825          * arguments passed to the method handle.
1826          * <p>
1827          * If the constructor's {@code accessible} flag is not set,
1828          * access checking is performed immediately on behalf of the lookup class.
1829          * <p>
1830          * The returned method handle will have
1831          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1832          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1833          * <p>
1834          * If the returned method handle is invoked, the constructor's class will
1835          * be initialized, if it has not already been initialized.
1836          * @param c the reflected constructor
1837          * @return a method handle which can invoke the reflected constructor
1838          * @throws IllegalAccessException if access checking fails
1839          *                                or if the method's variable arity modifier bit
1840          *                                is set and {@code asVarargsCollector} fails
1841          * @throws NullPointerException if the argument is null
1842          */
1843         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1844             MemberName ctor = new MemberName(c);
1845             assert(ctor.isConstructor());
1846             @SuppressWarnings("deprecation")
1847             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1848             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1849         }
1850 
1851         /**
1852          * Produces a method handle giving read access to a reflected field.
1853          * The type of the method handle will have a return type of the field's
1854          * value type.
1855          * If the field is static, the method handle will take no arguments.
1856          * Otherwise, its single argument will be the instance containing
1857          * the field.
1858          * If the field's {@code accessible} flag is not set,
1859          * access checking is performed immediately on behalf of the lookup class.
1860          * <p>
1861          * If the field is static, and
1862          * if the returned method handle is invoked, the field's class will
1863          * be initialized, if it has not already been initialized.
1864          * @param f the reflected field
1865          * @return a method handle which can load values from the reflected field
1866          * @throws IllegalAccessException if access checking fails
1867          * @throws NullPointerException if the argument is null
1868          */
1869         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1870             return unreflectField(f, false);
1871         }
1872         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1873             MemberName field = new MemberName(f, isSetter);
1874             assert(isSetter
1875                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1876                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1877             @SuppressWarnings("deprecation")
1878             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1879             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1880         }
1881 
1882         /**
1883          * Produces a method handle giving write access to a reflected field.
1884          * The type of the method handle will have a void return type.
1885          * If the field is static, the method handle will take a single
1886          * argument, of the field's value type, the value to be stored.
1887          * Otherwise, the two arguments will be the instance containing
1888          * the field, and the value to be stored.
1889          * If the field's {@code accessible} flag is not set,
1890          * access checking is performed immediately on behalf of the lookup class.
1891          * <p>
1892          * If the field is static, and
1893          * if the returned method handle is invoked, the field's class will
1894          * be initialized, if it has not already been initialized.
1895          * @param f the reflected field
1896          * @return a method handle which can store values into the reflected field
1897          * @throws IllegalAccessException if access checking fails
1898          * @throws NullPointerException if the argument is null
1899          */
1900         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1901             return unreflectField(f, true);
1902         }
1903 
1904         /**
1905          * Produces a VarHandle giving access to a reflected field {@code f}
1906          * of type {@code T} declared in a class of type {@code R}.
1907          * The VarHandle's variable type is {@code T}.
1908          * If the field is non-static the VarHandle has one coordinate type,
1909          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
1910          * coordinate types.
1911          * <p>
1912          * Access checking is performed immediately on behalf of the lookup
1913          * class, regardless of the value of the field's {@code accessible}
1914          * flag.
1915          * <p>
1916          * If the field is static, and if the returned VarHandle is operated
1917          * on, the field's declaring class will be initialized, if it has not
1918          * already been initialized.
1919          * <p>
1920          * Certain access modes of the returned VarHandle are unsupported under
1921          * the following conditions:
1922          * <ul>
1923          * <li>if the field is declared {@code final}, then the write, atomic
1924          *     update, numeric atomic update, and bitwise atomic update access
1925          *     modes are unsupported.
1926          * <li>if the field type is anything other than {@code byte},
1927          *     {@code short}, {@code char}, {@code int}, {@code long},
1928          *     {@code float}, or {@code double} then numeric atomic update
1929          *     access modes are unsupported.
1930          * <li>if the field type is anything other than {@code boolean},
1931          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1932          *     {@code long} then bitwise atomic update access modes are
1933          *     unsupported.
1934          * </ul>
1935          * <p>
1936          * If the field is declared {@code volatile} then the returned VarHandle
1937          * will override access to the field (effectively ignore the
1938          * {@code volatile} declaration) in accordance to its specified
1939          * access modes.
1940          * <p>
1941          * If the field type is {@code float} or {@code double} then numeric
1942          * and atomic update access modes compare values using their bitwise
1943          * representation (see {@link Float#floatToRawIntBits} and
1944          * {@link Double#doubleToRawLongBits}, respectively).
1945          * @apiNote
1946          * Bitwise comparison of {@code float} values or {@code double} values,
1947          * as performed by the numeric and atomic update access modes, differ
1948          * from the primitive {@code ==} operator and the {@link Float#equals}
1949          * and {@link Double#equals} methods, specifically with respect to
1950          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1951          * Care should be taken when performing a compare and set or a compare
1952          * and exchange operation with such values since the operation may
1953          * unexpectedly fail.
1954          * There are many possible NaN values that are considered to be
1955          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1956          * provided by Java can distinguish between them.  Operation failure can
1957          * occur if the expected or witness value is a NaN value and it is
1958          * transformed (perhaps in a platform specific manner) into another NaN
1959          * value, and thus has a different bitwise representation (see
1960          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1961          * details).
1962          * The values {@code -0.0} and {@code +0.0} have different bitwise
1963          * representations but are considered equal when using the primitive
1964          * {@code ==} operator.  Operation failure can occur if, for example, a
1965          * numeric algorithm computes an expected value to be say {@code -0.0}
1966          * and previously computed the witness value to be say {@code +0.0}.
1967          * @param f the reflected field, with a field of type {@code T}, and
1968          * a declaring class of type {@code R}
1969          * @return a VarHandle giving access to non-static fields or a static
1970          * field
1971          * @throws IllegalAccessException if access checking fails
1972          * @throws NullPointerException if the argument is null
1973          * @since 9
1974          */
1975         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
1976             MemberName getField = new MemberName(f, false);
1977             MemberName putField = new MemberName(f, true);
1978             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
1979                                                       f.getDeclaringClass(), getField, putField);
1980         }
1981 
1982         /**
1983          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1984          * created by this lookup object or a similar one.
1985          * Security and access checks are performed to ensure that this lookup object
1986          * is capable of reproducing the target method handle.
1987          * This means that the cracking may fail if target is a direct method handle
1988          * but was created by an unrelated lookup object.
1989          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1990          * and was created by a lookup object for a different class.
1991          * @param target a direct method handle to crack into symbolic reference components
1992          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
1993          * @exception SecurityException if a security manager is present and it
1994          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1995          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
1996          * @exception NullPointerException if the target is {@code null}
1997          * @see MethodHandleInfo
1998          * @since 1.8
1999          */
2000         public MethodHandleInfo revealDirect(MethodHandle target) {
2001             MemberName member = target.internalMemberName();
2002             if (member == null || (!member.isResolved() &&
2003                                    !member.isMethodHandleInvoke() &&
2004                                    !member.isVarHandleMethodInvoke()))
2005                 throw newIllegalArgumentException("not a direct method handle");
2006             Class<?> defc = member.getDeclaringClass();
2007             byte refKind = member.getReferenceKind();
2008             assert(MethodHandleNatives.refKindIsValid(refKind));
2009             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
2010                 // Devirtualized method invocation is usually formally virtual.
2011                 // To avoid creating extra MemberName objects for this common case,
2012                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
2013                 refKind = REF_invokeVirtual;
2014             if (refKind == REF_invokeVirtual && defc.isInterface())
2015                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
2016                 refKind = REF_invokeInterface;
2017             // Check SM permissions and member access before cracking.
2018             try {
2019                 checkAccess(refKind, defc, member);
2020                 checkSecurityManager(defc, member);
2021             } catch (IllegalAccessException ex) {
2022                 throw new IllegalArgumentException(ex);
2023             }
2024             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
2025                 Class<?> callerClass = target.internalCallerClass();
2026                 if (!hasPrivateAccess() || callerClass != lookupClass())
2027                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
2028             }
2029             // Produce the handle to the results.
2030             return new InfoFromMemberName(this, member, refKind);
2031         }
2032 
2033         /// Helper methods, all package-private.
2034 
2035         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2036             checkSymbolicClass(refc);  // do this before attempting to resolve
2037             Objects.requireNonNull(name);
2038             Objects.requireNonNull(type);
2039             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2040                                             NoSuchFieldException.class);
2041         }
2042 
2043         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2044             checkSymbolicClass(refc);  // do this before attempting to resolve
2045             Objects.requireNonNull(name);
2046             Objects.requireNonNull(type);
2047             checkMethodName(refKind, name);  // NPE check on name
2048             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2049                                             NoSuchMethodException.class);
2050         }
2051 
2052         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
2053             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
2054             Objects.requireNonNull(member.getName());
2055             Objects.requireNonNull(member.getType());
2056             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
2057                                             ReflectiveOperationException.class);
2058         }
2059 
2060         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
2061             Objects.requireNonNull(refc);
2062             Class<?> caller = lookupClassOrNull();
2063             if (caller != null && !VerifyAccess.isClassAccessible(refc, caller, allowedModes))
2064                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
2065         }
2066 
2067         /** Check name for an illegal leading "&lt;" character. */
2068         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
2069             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
2070                 throw new NoSuchMethodException("illegal method name: "+name);
2071         }
2072 
2073 
2074         /**
2075          * Find my trustable caller class if m is a caller sensitive method.
2076          * If this lookup object has private access, then the caller class is the lookupClass.
2077          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
2078          */
2079         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
2080             Class<?> callerClass = null;
2081             if (MethodHandleNatives.isCallerSensitive(m)) {
2082                 // Only lookups with private access are allowed to resolve caller-sensitive methods
2083                 if (hasPrivateAccess()) {
2084                     callerClass = lookupClass;
2085                 } else {
2086                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
2087                 }
2088             }
2089             return callerClass;
2090         }
2091 
2092         /**
2093          * Returns {@code true} if this lookup has {@code PRIVATE} access.
2094          * @return {@code true} if this lookup has {@code PRIVATE} access.
2095          * @since 9
2096          */
2097         public boolean hasPrivateAccess() {
2098             return (allowedModes & PRIVATE) != 0;
2099         }
2100 
2101         /**
2102          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
2103          * Determines a trustable caller class to compare with refc, the symbolic reference class.
2104          * If this lookup object has private access, then the caller class is the lookupClass.
2105          */
2106         void checkSecurityManager(Class<?> refc, MemberName m) {
2107             SecurityManager smgr = System.getSecurityManager();
2108             if (smgr == null)  return;
2109             if (allowedModes == TRUSTED)  return;
2110 
2111             // Step 1:
2112             boolean fullPowerLookup = hasPrivateAccess();
2113             if (!fullPowerLookup ||
2114                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
2115                 ReflectUtil.checkPackageAccess(refc);
2116             }
2117 
2118             if (m == null) {  // findClass or accessClass
2119                 // Step 2b:
2120                 if (!fullPowerLookup) {
2121                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
2122                 }
2123                 return;
2124             }
2125 
2126             // Step 2a:
2127             if (m.isPublic()) return;
2128             if (!fullPowerLookup) {
2129                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
2130             }
2131 
2132             // Step 3:
2133             Class<?> defc = m.getDeclaringClass();
2134             if (!fullPowerLookup && defc != refc) {
2135                 ReflectUtil.checkPackageAccess(defc);
2136             }
2137         }
2138 
2139         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2140             boolean wantStatic = (refKind == REF_invokeStatic);
2141             String message;
2142             if (m.isConstructor())
2143                 message = "expected a method, not a constructor";
2144             else if (!m.isMethod())
2145                 message = "expected a method";
2146             else if (wantStatic != m.isStatic())
2147                 message = wantStatic ? "expected a static method" : "expected a non-static method";
2148             else
2149                 { checkAccess(refKind, refc, m); return; }
2150             throw m.makeAccessException(message, this);
2151         }
2152 
2153         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2154             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
2155             String message;
2156             if (wantStatic != m.isStatic())
2157                 message = wantStatic ? "expected a static field" : "expected a non-static field";
2158             else
2159                 { checkAccess(refKind, refc, m); return; }
2160             throw m.makeAccessException(message, this);
2161         }
2162 
2163         /** Check public/protected/private bits on the symbolic reference class and its member. */
2164         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2165             assert(m.referenceKindIsConsistentWith(refKind) &&
2166                    MethodHandleNatives.refKindIsValid(refKind) &&
2167                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
2168             int allowedModes = this.allowedModes;
2169             if (allowedModes == TRUSTED)  return;
2170             int mods = m.getModifiers();
2171             if (Modifier.isProtected(mods) &&
2172                     refKind == REF_invokeVirtual &&
2173                     m.getDeclaringClass() == Object.class &&
2174                     m.getName().equals("clone") &&
2175                     refc.isArray()) {
2176                 // The JVM does this hack also.
2177                 // (See ClassVerifier::verify_invoke_instructions
2178                 // and LinkResolver::check_method_accessability.)
2179                 // Because the JVM does not allow separate methods on array types,
2180                 // there is no separate method for int[].clone.
2181                 // All arrays simply inherit Object.clone.
2182                 // But for access checking logic, we make Object.clone
2183                 // (normally protected) appear to be public.
2184                 // Later on, when the DirectMethodHandle is created,
2185                 // its leading argument will be restricted to the
2186                 // requested array type.
2187                 // N.B. The return type is not adjusted, because
2188                 // that is *not* the bytecode behavior.
2189                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
2190             }
2191             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
2192                 // cannot "new" a protected ctor in a different package
2193                 mods ^= Modifier.PROTECTED;
2194             }
2195             if (Modifier.isFinal(mods) &&
2196                     MethodHandleNatives.refKindIsSetter(refKind))
2197                 throw m.makeAccessException("unexpected set of a final field", this);
2198             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
2199             if ((requestedModes & allowedModes) != 0) {
2200                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
2201                                                     mods, lookupClass(), allowedModes))
2202                     return;
2203             } else {
2204                 // Protected members can also be checked as if they were package-private.
2205                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
2206                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
2207                     return;
2208             }
2209             throw m.makeAccessException(accessFailedMessage(refc, m), this);
2210         }
2211 
2212         String accessFailedMessage(Class<?> refc, MemberName m) {
2213             Class<?> defc = m.getDeclaringClass();
2214             int mods = m.getModifiers();
2215             // check the class first:
2216             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
2217                                (defc == refc ||
2218                                 Modifier.isPublic(refc.getModifiers())));
2219             if (!classOK && (allowedModes & PACKAGE) != 0) {
2220                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), FULL_POWER_MODES) &&
2221                            (defc == refc ||
2222                             VerifyAccess.isClassAccessible(refc, lookupClass(), FULL_POWER_MODES)));
2223             }
2224             if (!classOK)
2225                 return "class is not public";
2226             if (Modifier.isPublic(mods))
2227                 return "access to public member failed";  // (how?, module not readable?)
2228             if (Modifier.isPrivate(mods))
2229                 return "member is private";
2230             if (Modifier.isProtected(mods))
2231                 return "member is protected";
2232             return "member is private to package";
2233         }
2234 
2235         private static final boolean ALLOW_NESTMATE_ACCESS = false;
2236 
2237         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
2238             int allowedModes = this.allowedModes;
2239             if (allowedModes == TRUSTED)  return;
2240             if (!hasPrivateAccess()
2241                 || (specialCaller != lookupClass()
2242                        // ensure non-abstract methods in superinterfaces can be special-invoked
2243                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))
2244                     && !(ALLOW_NESTMATE_ACCESS &&
2245                          VerifyAccess.isSamePackageMember(specialCaller, lookupClass()))))
2246                 throw new MemberName(specialCaller).
2247                     makeAccessException("no private access for invokespecial", this);
2248         }
2249 
2250         private boolean restrictProtectedReceiver(MemberName method) {
2251             // The accessing class only has the right to use a protected member
2252             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
2253             if (!method.isProtected() || method.isStatic()
2254                 || allowedModes == TRUSTED
2255                 || method.getDeclaringClass() == lookupClass()
2256                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())
2257                 || (ALLOW_NESTMATE_ACCESS &&
2258                     VerifyAccess.isSamePackageMember(method.getDeclaringClass(), lookupClass())))
2259                 return false;
2260             return true;
2261         }
2262         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
2263             assert(!method.isStatic());
2264             // receiver type of mh is too wide; narrow to caller
2265             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
2266                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
2267             }
2268             MethodType rawType = mh.type();
2269             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
2270             MethodType narrowType = rawType.changeParameterType(0, caller);
2271             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
2272             assert(mh.viewAsTypeChecks(narrowType, true));
2273             return mh.copyWith(narrowType, mh.form);
2274         }
2275 
2276         /** Check access and get the requested method. */
2277         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
2278             final boolean doRestrict    = true;
2279             final boolean checkSecurity = true;
2280             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
2281         }
2282         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
2283         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
2284             final boolean doRestrict    = false;
2285             final boolean checkSecurity = true;
2286             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, callerClass);
2287         }
2288         /** Check access and get the requested method, eliding security manager checks. */
2289         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
2290             final boolean doRestrict    = true;
2291             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2292             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
2293         }
2294         /** Common code for all methods; do not call directly except from immediately above. */
2295         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
2296                                                    boolean checkSecurity,
2297                                                    boolean doRestrict, Class<?> callerClass) throws IllegalAccessException {
2298             checkMethod(refKind, refc, method);
2299             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2300             if (checkSecurity)
2301                 checkSecurityManager(refc, method);
2302             assert(!method.isMethodHandleInvoke());
2303 
2304             if (refKind == REF_invokeSpecial &&
2305                 refc != lookupClass() &&
2306                 !refc.isInterface() &&
2307                 refc != lookupClass().getSuperclass() &&
2308                 refc.isAssignableFrom(lookupClass())) {
2309                 assert(!method.getName().equals("<init>"));  // not this code path
2310                 // Per JVMS 6.5, desc. of invokespecial instruction:
2311                 // If the method is in a superclass of the LC,
2312                 // and if our original search was above LC.super,
2313                 // repeat the search (symbolic lookup) from LC.super
2314                 // and continue with the direct superclass of that class,
2315                 // and so forth, until a match is found or no further superclasses exist.
2316                 // FIXME: MemberName.resolve should handle this instead.
2317                 Class<?> refcAsSuper = lookupClass();
2318                 MemberName m2;
2319                 do {
2320                     refcAsSuper = refcAsSuper.getSuperclass();
2321                     m2 = new MemberName(refcAsSuper,
2322                                         method.getName(),
2323                                         method.getMethodType(),
2324                                         REF_invokeSpecial);
2325                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
2326                 } while (m2 == null &&         // no method is found yet
2327                          refc != refcAsSuper); // search up to refc
2328                 if (m2 == null)  throw new InternalError(method.toString());
2329                 method = m2;
2330                 refc = refcAsSuper;
2331                 // redo basic checks
2332                 checkMethod(refKind, refc, method);
2333             }
2334 
2335             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method);
2336             MethodHandle mh = dmh;
2337             // Optionally narrow the receiver argument to refc using restrictReceiver.
2338             if ((doRestrict && refKind == REF_invokeSpecial) ||
2339                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
2340                 mh = restrictReceiver(method, dmh, lookupClass());
2341             }
2342             mh = maybeBindCaller(method, mh, callerClass);
2343             mh = mh.setVarargs(method);
2344             return mh;
2345         }
2346         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
2347                                              Class<?> callerClass)
2348                                              throws IllegalAccessException {
2349             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
2350                 return mh;
2351             Class<?> hostClass = lookupClass;
2352             if (!hasPrivateAccess())  // caller must have private access
2353                 hostClass = callerClass;  // callerClass came from a security manager style stack walk
2354             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
2355             // Note: caller will apply varargs after this step happens.
2356             return cbmh;
2357         }
2358         /** Check access and get the requested field. */
2359         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2360             final boolean checkSecurity = true;
2361             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2362         }
2363         /** Check access and get the requested field, eliding security manager checks. */
2364         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2365             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2366             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2367         }
2368         /** Common code for all fields; do not call directly except from immediately above. */
2369         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
2370                                                   boolean checkSecurity) throws IllegalAccessException {
2371             checkField(refKind, refc, field);
2372             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2373             if (checkSecurity)
2374                 checkSecurityManager(refc, field);
2375             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
2376             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
2377                                     restrictProtectedReceiver(field));
2378             if (doRestrict)
2379                 return restrictReceiver(field, dmh, lookupClass());
2380             return dmh;
2381         }
2382         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
2383                                             Class<?> refc, MemberName getField, MemberName putField)
2384                 throws IllegalAccessException {
2385             final boolean checkSecurity = true;
2386             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2387         }
2388         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
2389                                                              Class<?> refc, MemberName getField, MemberName putField)
2390                 throws IllegalAccessException {
2391             final boolean checkSecurity = false;
2392             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2393         }
2394         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
2395                                                   Class<?> refc, MemberName getField, MemberName putField,
2396                                                   boolean checkSecurity) throws IllegalAccessException {
2397             assert getField.isStatic() == putField.isStatic();
2398             assert getField.isGetter() && putField.isSetter();
2399             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
2400             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
2401 
2402             checkField(getRefKind, refc, getField);
2403             if (checkSecurity)
2404                 checkSecurityManager(refc, getField);
2405 
2406             if (!putField.isFinal()) {
2407                 // A VarHandle does not support updates to final fields, any
2408                 // such VarHandle to a final field will be read-only and
2409                 // therefore the following write-based accessibility checks are
2410                 // only required for non-final fields
2411                 checkField(putRefKind, refc, putField);
2412                 if (checkSecurity)
2413                     checkSecurityManager(refc, putField);
2414             }
2415 
2416             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
2417                                   restrictProtectedReceiver(getField));
2418             if (doRestrict) {
2419                 assert !getField.isStatic();
2420                 // receiver type of VarHandle is too wide; narrow to caller
2421                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
2422                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
2423                 }
2424                 refc = lookupClass();
2425             }
2426             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED);
2427         }
2428         /** Check access and get the requested constructor. */
2429         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2430             final boolean checkSecurity = true;
2431             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2432         }
2433         /** Check access and get the requested constructor, eliding security manager checks. */
2434         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2435             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2436             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2437         }
2438         /** Common code for all constructors; do not call directly except from immediately above. */
2439         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
2440                                                   boolean checkSecurity) throws IllegalAccessException {
2441             assert(ctor.isConstructor());
2442             checkAccess(REF_newInvokeSpecial, refc, ctor);
2443             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2444             if (checkSecurity)
2445                 checkSecurityManager(refc, ctor);
2446             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
2447             return DirectMethodHandle.make(ctor).setVarargs(ctor);
2448         }
2449 
2450         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
2451          */
2452         /*non-public*/
2453         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
2454             if (!(type instanceof Class || type instanceof MethodType))
2455                 throw new InternalError("unresolved MemberName");
2456             MemberName member = new MemberName(refKind, defc, name, type);
2457             MethodHandle mh = LOOKASIDE_TABLE.get(member);
2458             if (mh != null) {
2459                 checkSymbolicClass(defc);
2460                 return mh;
2461             }
2462             // Treat MethodHandle.invoke and invokeExact specially.
2463             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
2464                 mh = findVirtualForMH(member.getName(), member.getMethodType());
2465                 if (mh != null) {
2466                     return mh;
2467                 }
2468             }
2469             MemberName resolved = resolveOrFail(refKind, member);
2470             mh = getDirectMethodForConstant(refKind, defc, resolved);
2471             if (mh instanceof DirectMethodHandle
2472                     && canBeCached(refKind, defc, resolved)) {
2473                 MemberName key = mh.internalMemberName();
2474                 if (key != null) {
2475                     key = key.asNormalOriginal();
2476                 }
2477                 if (member.equals(key)) {  // better safe than sorry
2478                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
2479                 }
2480             }
2481             return mh;
2482         }
2483         private
2484         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
2485             if (refKind == REF_invokeSpecial) {
2486                 return false;
2487             }
2488             if (!Modifier.isPublic(defc.getModifiers()) ||
2489                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
2490                     !member.isPublic() ||
2491                     member.isCallerSensitive()) {
2492                 return false;
2493             }
2494             ClassLoader loader = defc.getClassLoader();
2495             if (!jdk.internal.misc.VM.isSystemDomainLoader(loader)) {
2496                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
2497                 boolean found = false;
2498                 while (sysl != null) {
2499                     if (loader == sysl) { found = true; break; }
2500                     sysl = sysl.getParent();
2501                 }
2502                 if (!found) {
2503                     return false;
2504                 }
2505             }
2506             try {
2507                 MemberName resolved2 = publicLookup().resolveOrFail(refKind,
2508                     new MemberName(refKind, defc, member.getName(), member.getType()));
2509                 checkSecurityManager(defc, resolved2);
2510             } catch (ReflectiveOperationException | SecurityException ex) {
2511                 return false;
2512             }
2513             return true;
2514         }
2515         private
2516         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
2517                 throws ReflectiveOperationException {
2518             if (MethodHandleNatives.refKindIsField(refKind)) {
2519                 return getDirectFieldNoSecurityManager(refKind, defc, member);
2520             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
2521                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
2522             } else if (refKind == REF_newInvokeSpecial) {
2523                 return getDirectConstructorNoSecurityManager(defc, member);
2524             }
2525             // oops
2526             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
2527         }
2528 
2529         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
2530     }
2531 
2532     /**
2533      * Produces a method handle constructing arrays of a desired type.
2534      * The return type of the method handle will be the array type.
2535      * The type of its sole argument will be {@code int}, which specifies the size of the array.
2536      * @param arrayClass an array type
2537      * @return a method handle which can create arrays of the given type
2538      * @throws NullPointerException if the argument is {@code null}
2539      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
2540      * @see java.lang.reflect.Array#newInstance(Class, int)
2541      * @since 9
2542      */
2543     public static
2544     MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
2545         if (!arrayClass.isArray()) {
2546             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
2547         }
2548         ValueType<?> compValue = valueComponent(arrayClass);
2549         if (compValue != null) {
2550             return compValue.newArray();
2551         } else {
2552             MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
2553                     bindTo(arrayClass.getComponentType());
2554             return ani.asType(ani.type().changeReturnType(arrayClass));
2555         }
2556     }
2557 
2558     /**
2559      * Produces a method handle returning the length of an array.
2560      * The type of the method handle will have {@code int} as return type,
2561      * and its sole argument will be the array type.
2562      * @param arrayClass an array type
2563      * @return a method handle which can retrieve the length of an array of the given array type
2564      * @throws NullPointerException if the argument is {@code null}
2565      * @throws IllegalArgumentException if arrayClass is not an array type
2566      * @since 9
2567      */
2568     public static
2569     MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
2570         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
2571     }
2572 
2573     /**
2574      * Produces a method handle giving read access to elements of an array.
2575      * The type of the method handle will have a return type of the array's
2576      * element type.  Its first argument will be the array type,
2577      * and the second will be {@code int}.
2578      * @param arrayClass an array type
2579      * @return a method handle which can load values from the given array type
2580      * @throws NullPointerException if the argument is null
2581      * @throws  IllegalArgumentException if arrayClass is not an array type
2582      */
2583     public static
2584     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
2585         ValueType<?> compValue = valueComponent(arrayClass);
2586         return (compValue != null) ?
2587                 compValue.arrayGetter() :
2588                 MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
2589     }
2590 
2591     /**
2592      * Produces a method handle giving write access to elements of an array.
2593      * The type of the method handle will have a void return type.
2594      * Its last argument will be the array's element type.
2595      * The first and second arguments will be the array type and int.
2596      * @param arrayClass the class of an array
2597      * @return a method handle which can store values into the array type
2598      * @throws NullPointerException if the argument is null
2599      * @throws IllegalArgumentException if arrayClass is not an array type
2600      */
2601     public static
2602     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
2603         ValueType<?> compValue = valueComponent(arrayClass);
2604         return (compValue != null) ?
2605                 compValue.arraySetter() :
2606                 MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
2607     }
2608 
2609     @SuppressWarnings("unchecked")
2610     private static <Z> ValueType<Z> valueComponent(Class<Z> clazz) {
2611         Class<?> comp = clazz.getComponentType();
2612         if (MinimalValueTypes_1_0.isValueType(comp)) {
2613             try {
2614                 return (ValueType<Z>)ValueType.forClass(MinimalValueTypes_1_0.getValueCapableClass(comp));
2615             } catch (ClassNotFoundException ex) {
2616                 throw new IllegalStateException(ex);
2617             }
2618         } else {
2619             return null;
2620         }
2621     }
2622 
2623     /**
2624      * Produces a VarHandle giving access to elements of an array of type
2625      * {@code arrayClass}.  The VarHandle's variable type is the component type
2626      * of {@code arrayClass} and the list of coordinate types is
2627      * {@code (arrayClass, int)}, where the {@code int} coordinate type
2628      * corresponds to an argument that is an index into an array.
2629      * <p>
2630      * Certain access modes of the returned VarHandle are unsupported under
2631      * the following conditions:
2632      * <ul>
2633      * <li>if the component type is anything other than {@code byte},
2634      *     {@code short}, {@code char}, {@code int}, {@code long},
2635      *     {@code float}, or {@code double} then numeric atomic update access
2636      *     modes are unsupported.
2637      * <li>if the field type is anything other than {@code boolean},
2638      *     {@code byte}, {@code short}, {@code char}, {@code int} or
2639      *     {@code long} then bitwise atomic update access modes are
2640      *     unsupported.
2641      * </ul>
2642      * <p>
2643      * If the component type is {@code float} or {@code double} then numeric
2644      * and atomic update access modes compare values using their bitwise
2645      * representation (see {@link Float#floatToRawIntBits} and
2646      * {@link Double#doubleToRawLongBits}, respectively).
2647      * @apiNote
2648      * Bitwise comparison of {@code float} values or {@code double} values,
2649      * as performed by the numeric and atomic update access modes, differ
2650      * from the primitive {@code ==} operator and the {@link Float#equals}
2651      * and {@link Double#equals} methods, specifically with respect to
2652      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2653      * Care should be taken when performing a compare and set or a compare
2654      * and exchange operation with such values since the operation may
2655      * unexpectedly fail.
2656      * There are many possible NaN values that are considered to be
2657      * {@code NaN} in Java, although no IEEE 754 floating-point operation
2658      * provided by Java can distinguish between them.  Operation failure can
2659      * occur if the expected or witness value is a NaN value and it is
2660      * transformed (perhaps in a platform specific manner) into another NaN
2661      * value, and thus has a different bitwise representation (see
2662      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2663      * details).
2664      * The values {@code -0.0} and {@code +0.0} have different bitwise
2665      * representations but are considered equal when using the primitive
2666      * {@code ==} operator.  Operation failure can occur if, for example, a
2667      * numeric algorithm computes an expected value to be say {@code -0.0}
2668      * and previously computed the witness value to be say {@code +0.0}.
2669      * @param arrayClass the class of an array, of type {@code T[]}
2670      * @return a VarHandle giving access to elements of an array
2671      * @throws NullPointerException if the arrayClass is null
2672      * @throws IllegalArgumentException if arrayClass is not an array type
2673      * @since 9
2674      */
2675     public static
2676     VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
2677         return VarHandles.makeArrayElementHandle(arrayClass);
2678     }
2679 
2680     /**
2681      * Produces a VarHandle giving access to elements of a {@code byte[]} array
2682      * viewed as if it were a different primitive array type, such as
2683      * {@code int[]} or {@code long[]}.
2684      * The VarHandle's variable type is the component type of
2685      * {@code viewArrayClass} and the list of coordinate types is
2686      * {@code (byte[], int)}, where the {@code int} coordinate type
2687      * corresponds to an argument that is an index into a {@code byte[]} array.
2688      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
2689      * array, composing bytes to or from a value of the component type of
2690      * {@code viewArrayClass} according to the given endianness.
2691      * <p>
2692      * The supported component types (variables types) are {@code short},
2693      * {@code char}, {@code int}, {@code long}, {@code float} and
2694      * {@code double}.
2695      * <p>
2696      * Access of bytes at a given index will result in an
2697      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2698      * or greater than the {@code byte[]} array length minus the size (in bytes)
2699      * of {@code T}.
2700      * <p>
2701      * Access of bytes at an index may be aligned or misaligned for {@code T},
2702      * with respect to the underlying memory address, {@code A} say, associated
2703      * with the array and index.
2704      * If access is misaligned then access for anything other than the
2705      * {@code get} and {@code set} access modes will result in an
2706      * {@code IllegalStateException}.  In such cases atomic access is only
2707      * guaranteed with respect to the largest power of two that divides the GCD
2708      * of {@code A} and the size (in bytes) of {@code T}.
2709      * If access is aligned then following access modes are supported and are
2710      * guaranteed to support atomic access:
2711      * <ul>
2712      * <li>read write access modes for all {@code T}, with the exception of
2713      *     access modes {@code get} and {@code set} for {@code long} and
2714      *     {@code double} on 32-bit platforms.
2715      * <li>atomic update access modes for {@code int}, {@code long},
2716      *     {@code float} or {@code double}.
2717      *     (Future major platform releases of the JDK may support additional
2718      *     types for certain currently unsupported access modes.)
2719      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2720      *     (Future major platform releases of the JDK may support additional
2721      *     numeric types for certain currently unsupported access modes.)
2722      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2723      *     (Future major platform releases of the JDK may support additional
2724      *     numeric types for certain currently unsupported access modes.)
2725      * </ul>
2726      * <p>
2727      * Misaligned access, and therefore atomicity guarantees, may be determined
2728      * for {@code byte[]} arrays without operating on a specific array.  Given
2729      * an {@code index}, {@code T} and it's corresponding boxed type,
2730      * {@code T_BOX}, misalignment may be determined as follows:
2731      * <pre>{@code
2732      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2733      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
2734      *     alignmentOffset(0, sizeOfT);
2735      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
2736      * boolean isMisaligned = misalignedAtIndex != 0;
2737      * }</pre>
2738      * <p>
2739      * If the variable type is {@code float} or {@code double} then atomic
2740      * update access modes compare values using their bitwise representation
2741      * (see {@link Float#floatToRawIntBits} and
2742      * {@link Double#doubleToRawLongBits}, respectively).
2743      * @param viewArrayClass the view array class, with a component type of
2744      * type {@code T}
2745      * @param byteOrder the endianness of the view array elements, as
2746      * stored in the underlying {@code byte} array
2747      * @return a VarHandle giving access to elements of a {@code byte[]} array
2748      * viewed as if elements corresponding to the components type of the view
2749      * array class
2750      * @throws NullPointerException if viewArrayClass or byteOrder is null
2751      * @throws IllegalArgumentException if viewArrayClass is not an array type
2752      * @throws UnsupportedOperationException if the component type of
2753      * viewArrayClass is not supported as a variable type
2754      * @since 9
2755      */
2756     public static
2757     VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
2758                                      ByteOrder byteOrder) throws IllegalArgumentException {
2759         Objects.requireNonNull(byteOrder);
2760         return VarHandles.byteArrayViewHandle(viewArrayClass,
2761                                               byteOrder == ByteOrder.BIG_ENDIAN);
2762     }
2763 
2764     /**
2765      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
2766      * viewed as if it were an array of elements of a different primitive
2767      * component type to that of {@code byte}, such as {@code int[]} or
2768      * {@code long[]}.
2769      * The VarHandle's variable type is the component type of
2770      * {@code viewArrayClass} and the list of coordinate types is
2771      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
2772      * corresponds to an argument that is an index into a {@code byte[]} array.
2773      * The returned VarHandle accesses bytes at an index in a
2774      * {@code ByteBuffer}, composing bytes to or from a value of the component
2775      * type of {@code viewArrayClass} according to the given endianness.
2776      * <p>
2777      * The supported component types (variables types) are {@code short},
2778      * {@code char}, {@code int}, {@code long}, {@code float} and
2779      * {@code double}.
2780      * <p>
2781      * Access will result in a {@code ReadOnlyBufferException} for anything
2782      * other than the read access modes if the {@code ByteBuffer} is read-only.
2783      * <p>
2784      * Access of bytes at a given index will result in an
2785      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2786      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
2787      * {@code T}.
2788      * <p>
2789      * Access of bytes at an index may be aligned or misaligned for {@code T},
2790      * with respect to the underlying memory address, {@code A} say, associated
2791      * with the {@code ByteBuffer} and index.
2792      * If access is misaligned then access for anything other than the
2793      * {@code get} and {@code set} access modes will result in an
2794      * {@code IllegalStateException}.  In such cases atomic access is only
2795      * guaranteed with respect to the largest power of two that divides the GCD
2796      * of {@code A} and the size (in bytes) of {@code T}.
2797      * If access is aligned then following access modes are supported and are
2798      * guaranteed to support atomic access:
2799      * <ul>
2800      * <li>read write access modes for all {@code T}, with the exception of
2801      *     access modes {@code get} and {@code set} for {@code long} and
2802      *     {@code double} on 32-bit platforms.
2803      * <li>atomic update access modes for {@code int}, {@code long},
2804      *     {@code float} or {@code double}.
2805      *     (Future major platform releases of the JDK may support additional
2806      *     types for certain currently unsupported access modes.)
2807      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2808      *     (Future major platform releases of the JDK may support additional
2809      *     numeric types for certain currently unsupported access modes.)
2810      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2811      *     (Future major platform releases of the JDK may support additional
2812      *     numeric types for certain currently unsupported access modes.)
2813      * </ul>
2814      * <p>
2815      * Misaligned access, and therefore atomicity guarantees, may be determined
2816      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
2817      * {@code index}, {@code T} and it's corresponding boxed type,
2818      * {@code T_BOX}, as follows:
2819      * <pre>{@code
2820      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2821      * ByteBuffer bb = ...
2822      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
2823      * boolean isMisaligned = misalignedAtIndex != 0;
2824      * }</pre>
2825      * <p>
2826      * If the variable type is {@code float} or {@code double} then atomic
2827      * update access modes compare values using their bitwise representation
2828      * (see {@link Float#floatToRawIntBits} and
2829      * {@link Double#doubleToRawLongBits}, respectively).
2830      * @param viewArrayClass the view array class, with a component type of
2831      * type {@code T}
2832      * @param byteOrder the endianness of the view array elements, as
2833      * stored in the underlying {@code ByteBuffer} (Note this overrides the
2834      * endianness of a {@code ByteBuffer})
2835      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
2836      * viewed as if elements corresponding to the components type of the view
2837      * array class
2838      * @throws NullPointerException if viewArrayClass or byteOrder is null
2839      * @throws IllegalArgumentException if viewArrayClass is not an array type
2840      * @throws UnsupportedOperationException if the component type of
2841      * viewArrayClass is not supported as a variable type
2842      * @since 9
2843      */
2844     public static
2845     VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
2846                                       ByteOrder byteOrder) throws IllegalArgumentException {
2847         Objects.requireNonNull(byteOrder);
2848         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
2849                                                    byteOrder == ByteOrder.BIG_ENDIAN);
2850     }
2851 
2852 
2853     /// method handle invocation (reflective style)
2854 
2855     /**
2856      * Produces a method handle which will invoke any method handle of the
2857      * given {@code type}, with a given number of trailing arguments replaced by
2858      * a single trailing {@code Object[]} array.
2859      * The resulting invoker will be a method handle with the following
2860      * arguments:
2861      * <ul>
2862      * <li>a single {@code MethodHandle} target
2863      * <li>zero or more leading values (counted by {@code leadingArgCount})
2864      * <li>an {@code Object[]} array containing trailing arguments
2865      * </ul>
2866      * <p>
2867      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
2868      * the indicated {@code type}.
2869      * That is, if the target is exactly of the given {@code type}, it will behave
2870      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
2871      * is used to convert the target to the required {@code type}.
2872      * <p>
2873      * The type of the returned invoker will not be the given {@code type}, but rather
2874      * will have all parameters except the first {@code leadingArgCount}
2875      * replaced by a single array of type {@code Object[]}, which will be
2876      * the final parameter.
2877      * <p>
2878      * Before invoking its target, the invoker will spread the final array, apply
2879      * reference casts as necessary, and unbox and widen primitive arguments.
2880      * If, when the invoker is called, the supplied array argument does
2881      * not have the correct number of elements, the invoker will throw
2882      * an {@link IllegalArgumentException} instead of invoking the target.
2883      * <p>
2884      * This method is equivalent to the following code (though it may be more efficient):
2885      * <blockquote><pre>{@code
2886 MethodHandle invoker = MethodHandles.invoker(type);
2887 int spreadArgCount = type.parameterCount() - leadingArgCount;
2888 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
2889 return invoker;
2890      * }</pre></blockquote>
2891      * This method throws no reflective or security exceptions.
2892      * @param type the desired target type
2893      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
2894      * @return a method handle suitable for invoking any method handle of the given type
2895      * @throws NullPointerException if {@code type} is null
2896      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
2897      *                  the range from 0 to {@code type.parameterCount()} inclusive,
2898      *                  or if the resulting method handle's type would have
2899      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2900      */
2901     public static
2902     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
2903         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
2904             throw newIllegalArgumentException("bad argument count", leadingArgCount);
2905         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
2906         return type.invokers().spreadInvoker(leadingArgCount);
2907     }
2908 
2909     /**
2910      * Produces a special <em>invoker method handle</em> which can be used to
2911      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
2912      * The resulting invoker will have a type which is
2913      * exactly equal to the desired type, except that it will accept
2914      * an additional leading argument of type {@code MethodHandle}.
2915      * <p>
2916      * This method is equivalent to the following code (though it may be more efficient):
2917      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
2918      *
2919      * <p style="font-size:smaller;">
2920      * <em>Discussion:</em>
2921      * Invoker method handles can be useful when working with variable method handles
2922      * of unknown types.
2923      * For example, to emulate an {@code invokeExact} call to a variable method
2924      * handle {@code M}, extract its type {@code T},
2925      * look up the invoker method {@code X} for {@code T},
2926      * and call the invoker method, as {@code X.invoke(T, A...)}.
2927      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
2928      * is unknown.)
2929      * If spreading, collecting, or other argument transformations are required,
2930      * they can be applied once to the invoker {@code X} and reused on many {@code M}
2931      * method handle values, as long as they are compatible with the type of {@code X}.
2932      * <p style="font-size:smaller;">
2933      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2934      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2935      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2936      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2937      * <p>
2938      * This method throws no reflective or security exceptions.
2939      * @param type the desired target type
2940      * @return a method handle suitable for invoking any method handle of the given type
2941      * @throws IllegalArgumentException if the resulting method handle's type would have
2942      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2943      */
2944     public static
2945     MethodHandle exactInvoker(MethodType type) {
2946         return type.invokers().exactInvoker();
2947     }
2948 
2949     /**
2950      * Produces a special <em>invoker method handle</em> which can be used to
2951      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
2952      * The resulting invoker will have a type which is
2953      * exactly equal to the desired type, except that it will accept
2954      * an additional leading argument of type {@code MethodHandle}.
2955      * <p>
2956      * Before invoking its target, if the target differs from the expected type,
2957      * the invoker will apply reference casts as
2958      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
2959      * Similarly, the return value will be converted as necessary.
2960      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
2961      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
2962      * <p>
2963      * This method is equivalent to the following code (though it may be more efficient):
2964      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
2965      * <p style="font-size:smaller;">
2966      * <em>Discussion:</em>
2967      * A {@linkplain MethodType#genericMethodType general method type} is one which
2968      * mentions only {@code Object} arguments and return values.
2969      * An invoker for such a type is capable of calling any method handle
2970      * of the same arity as the general type.
2971      * <p style="font-size:smaller;">
2972      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2973      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2974      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2975      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2976      * <p>
2977      * This method throws no reflective or security exceptions.
2978      * @param type the desired target type
2979      * @return a method handle suitable for invoking any method handle convertible to the given type
2980      * @throws IllegalArgumentException if the resulting method handle's type would have
2981      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2982      */
2983     public static
2984     MethodHandle invoker(MethodType type) {
2985         return type.invokers().genericInvoker();
2986     }
2987 
2988     /**
2989      * Produces a special <em>invoker method handle</em> which can be used to
2990      * invoke a signature-polymorphic access mode method on any VarHandle whose
2991      * associated access mode type is compatible with the given type.
2992      * The resulting invoker will have a type which is exactly equal to the
2993      * desired given type, except that it will accept an additional leading
2994      * argument of type {@code VarHandle}.
2995      *
2996      * @param accessMode the VarHandle access mode
2997      * @param type the desired target type
2998      * @return a method handle suitable for invoking an access mode method of
2999      *         any VarHandle whose access mode type is of the given type.
3000      * @since 9
3001      */
3002     static public
3003     MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3004         return type.invokers().varHandleMethodExactInvoker(accessMode);
3005     }
3006 
3007     /**
3008      * Produces a special <em>invoker method handle</em> which can be used to
3009      * invoke a signature-polymorphic access mode method on any VarHandle whose
3010      * associated access mode type is compatible with the given type.
3011      * The resulting invoker will have a type which is exactly equal to the
3012      * desired given type, except that it will accept an additional leading
3013      * argument of type {@code VarHandle}.
3014      * <p>
3015      * Before invoking its target, if the access mode type differs from the
3016      * desired given type, the invoker will apply reference casts as necessary
3017      * and box, unbox, or widen primitive values, as if by
3018      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
3019      * converted as necessary.
3020      * <p>
3021      * This method is equivalent to the following code (though it may be more
3022      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
3023      *
3024      * @param accessMode the VarHandle access mode
3025      * @param type the desired target type
3026      * @return a method handle suitable for invoking an access mode method of
3027      *         any VarHandle whose access mode type is convertible to the given
3028      *         type.
3029      * @since 9
3030      */
3031     static public
3032     MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3033         return type.invokers().varHandleMethodInvoker(accessMode);
3034     }
3035 
3036     static /*non-public*/
3037     MethodHandle basicInvoker(MethodType type) {
3038         return type.invokers().basicInvoker();
3039     }
3040 
3041      /// method handle modification (creation from other method handles)
3042 
3043     /**
3044      * Produces a method handle which adapts the type of the
3045      * given method handle to a new type by pairwise argument and return type conversion.
3046      * The original type and new type must have the same number of arguments.
3047      * The resulting method handle is guaranteed to report a type
3048      * which is equal to the desired new type.
3049      * <p>
3050      * If the original type and new type are equal, returns target.
3051      * <p>
3052      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
3053      * and some additional conversions are also applied if those conversions fail.
3054      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
3055      * if possible, before or instead of any conversions done by {@code asType}:
3056      * <ul>
3057      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
3058      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
3059      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
3060      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
3061      *     the boolean is converted to a byte value, 1 for true, 0 for false.
3062      *     (This treatment follows the usage of the bytecode verifier.)
3063      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
3064      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
3065      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
3066      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
3067      *     then a Java casting conversion (JLS 5.5) is applied.
3068      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
3069      *     widening and/or narrowing.)
3070      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
3071      *     conversion will be applied at runtime, possibly followed
3072      *     by a Java casting conversion (JLS 5.5) on the primitive value,
3073      *     possibly followed by a conversion from byte to boolean by testing
3074      *     the low-order bit.
3075      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
3076      *     and if the reference is null at runtime, a zero value is introduced.
3077      * </ul>
3078      * @param target the method handle to invoke after arguments are retyped
3079      * @param newType the expected type of the new method handle
3080      * @return a method handle which delegates to the target after performing
3081      *           any necessary argument conversions, and arranges for any
3082      *           necessary return value conversions
3083      * @throws NullPointerException if either argument is null
3084      * @throws WrongMethodTypeException if the conversion cannot be made
3085      * @see MethodHandle#asType
3086      */
3087     public static
3088     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
3089         explicitCastArgumentsChecks(target, newType);
3090         // use the asTypeCache when possible:
3091         MethodType oldType = target.type();
3092         if (oldType == newType)  return target;
3093         if (oldType.explicitCastEquivalentToAsType(newType)) {
3094             return target.asFixedArity().asType(newType);
3095         }
3096         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
3097     }
3098 
3099     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
3100         if (target.type().parameterCount() != newType.parameterCount()) {
3101             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
3102         }
3103     }
3104 
3105     /**
3106      * Produces a method handle which adapts the calling sequence of the
3107      * given method handle to a new type, by reordering the arguments.
3108      * The resulting method handle is guaranteed to report a type
3109      * which is equal to the desired new type.
3110      * <p>
3111      * The given array controls the reordering.
3112      * Call {@code #I} the number of incoming parameters (the value
3113      * {@code newType.parameterCount()}, and call {@code #O} the number
3114      * of outgoing parameters (the value {@code target.type().parameterCount()}).
3115      * Then the length of the reordering array must be {@code #O},
3116      * and each element must be a non-negative number less than {@code #I}.
3117      * For every {@code N} less than {@code #O}, the {@code N}-th
3118      * outgoing argument will be taken from the {@code I}-th incoming
3119      * argument, where {@code I} is {@code reorder[N]}.
3120      * <p>
3121      * No argument or return value conversions are applied.
3122      * The type of each incoming argument, as determined by {@code newType},
3123      * must be identical to the type of the corresponding outgoing parameter
3124      * or parameters in the target method handle.
3125      * The return type of {@code newType} must be identical to the return
3126      * type of the original target.
3127      * <p>
3128      * The reordering array need not specify an actual permutation.
3129      * An incoming argument will be duplicated if its index appears
3130      * more than once in the array, and an incoming argument will be dropped
3131      * if its index does not appear in the array.
3132      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
3133      * incoming arguments which are not mentioned in the reordering array
3134      * may be of any type, as determined only by {@code newType}.
3135      * <blockquote><pre>{@code
3136 import static java.lang.invoke.MethodHandles.*;
3137 import static java.lang.invoke.MethodType.*;
3138 ...
3139 MethodType intfn1 = methodType(int.class, int.class);
3140 MethodType intfn2 = methodType(int.class, int.class, int.class);
3141 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
3142 assert(sub.type().equals(intfn2));
3143 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
3144 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
3145 assert((int)rsub.invokeExact(1, 100) == 99);
3146 MethodHandle add = ... (int x, int y) -> (x+y) ...;
3147 assert(add.type().equals(intfn2));
3148 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
3149 assert(twice.type().equals(intfn1));
3150 assert((int)twice.invokeExact(21) == 42);
3151      * }</pre></blockquote>
3152      * <p>
3153      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3154      * variable-arity method handle}, even if the original target method handle was.
3155      * @param target the method handle to invoke after arguments are reordered
3156      * @param newType the expected type of the new method handle
3157      * @param reorder an index array which controls the reordering
3158      * @return a method handle which delegates to the target after it
3159      *           drops unused arguments and moves and/or duplicates the other arguments
3160      * @throws NullPointerException if any argument is null
3161      * @throws IllegalArgumentException if the index array length is not equal to
3162      *                  the arity of the target, or if any index array element
3163      *                  not a valid index for a parameter of {@code newType},
3164      *                  or if two corresponding parameter types in
3165      *                  {@code target.type()} and {@code newType} are not identical,
3166      */
3167     public static
3168     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
3169         reorder = reorder.clone();  // get a private copy
3170         MethodType oldType = target.type();
3171         permuteArgumentChecks(reorder, newType, oldType);
3172         // first detect dropped arguments and handle them separately
3173         int[] originalReorder = reorder;
3174         BoundMethodHandle result = target.rebind();
3175         LambdaForm form = result.form;
3176         int newArity = newType.parameterCount();
3177         // Normalize the reordering into a real permutation,
3178         // by removing duplicates and adding dropped elements.
3179         // This somewhat improves lambda form caching, as well
3180         // as simplifying the transform by breaking it up into steps.
3181         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
3182             if (ddIdx > 0) {
3183                 // We found a duplicated entry at reorder[ddIdx].
3184                 // Example:  (x,y,z)->asList(x,y,z)
3185                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
3186                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
3187                 // The starred element corresponds to the argument
3188                 // deleted by the dupArgumentForm transform.
3189                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
3190                 boolean killFirst = false;
3191                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
3192                     // Set killFirst if the dup is larger than an intervening position.
3193                     // This will remove at least one inversion from the permutation.
3194                     if (dupVal > val) killFirst = true;
3195                 }
3196                 if (!killFirst) {
3197                     srcPos = dstPos;
3198                     dstPos = ddIdx;
3199                 }
3200                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
3201                 assert (reorder[srcPos] == reorder[dstPos]);
3202                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
3203                 // contract the reordering by removing the element at dstPos
3204                 int tailPos = dstPos + 1;
3205                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
3206                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
3207             } else {
3208                 int dropVal = ~ddIdx, insPos = 0;
3209                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
3210                     // Find first element of reorder larger than dropVal.
3211                     // This is where we will insert the dropVal.
3212                     insPos += 1;
3213                 }
3214                 Class<?> ptype = newType.parameterType(dropVal);
3215                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
3216                 oldType = oldType.insertParameterTypes(insPos, ptype);
3217                 // expand the reordering by inserting an element at insPos
3218                 int tailPos = insPos + 1;
3219                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
3220                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
3221                 reorder[insPos] = dropVal;
3222             }
3223             assert (permuteArgumentChecks(reorder, newType, oldType));
3224         }
3225         assert (reorder.length == newArity);  // a perfect permutation
3226         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
3227         form = form.editor().permuteArgumentsForm(1, reorder);
3228         if (newType == result.type() && form == result.internalForm())
3229             return result;
3230         return result.copyWith(newType, form);
3231     }
3232 
3233     /**
3234      * Return an indication of any duplicate or omission in reorder.
3235      * If the reorder contains a duplicate entry, return the index of the second occurrence.
3236      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
3237      * Otherwise, return zero.
3238      * If an element not in [0..newArity-1] is encountered, return reorder.length.
3239      */
3240     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
3241         final int BIT_LIMIT = 63;  // max number of bits in bit mask
3242         if (newArity < BIT_LIMIT) {
3243             long mask = 0;
3244             for (int i = 0; i < reorder.length; i++) {
3245                 int arg = reorder[i];
3246                 if (arg >= newArity) {
3247                     return reorder.length;
3248                 }
3249                 long bit = 1L << arg;
3250                 if ((mask & bit) != 0) {
3251                     return i;  // >0 indicates a dup
3252                 }
3253                 mask |= bit;
3254             }
3255             if (mask == (1L << newArity) - 1) {
3256                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
3257                 return 0;
3258             }
3259             // find first zero
3260             long zeroBit = Long.lowestOneBit(~mask);
3261             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
3262             assert(zeroPos <= newArity);
3263             if (zeroPos == newArity) {
3264                 return 0;
3265             }
3266             return ~zeroPos;
3267         } else {
3268             // same algorithm, different bit set
3269             BitSet mask = new BitSet(newArity);
3270             for (int i = 0; i < reorder.length; i++) {
3271                 int arg = reorder[i];
3272                 if (arg >= newArity) {
3273                     return reorder.length;
3274                 }
3275                 if (mask.get(arg)) {
3276                     return i;  // >0 indicates a dup
3277                 }
3278                 mask.set(arg);
3279             }
3280             int zeroPos = mask.nextClearBit(0);
3281             assert(zeroPos <= newArity);
3282             if (zeroPos == newArity) {
3283                 return 0;
3284             }
3285             return ~zeroPos;
3286         }
3287     }
3288 
3289     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
3290         if (newType.returnType() != oldType.returnType())
3291             throw newIllegalArgumentException("return types do not match",
3292                     oldType, newType);
3293         if (reorder.length == oldType.parameterCount()) {
3294             int limit = newType.parameterCount();
3295             boolean bad = false;
3296             for (int j = 0; j < reorder.length; j++) {
3297                 int i = reorder[j];
3298                 if (i < 0 || i >= limit) {
3299                     bad = true; break;
3300                 }
3301                 Class<?> src = newType.parameterType(i);
3302                 Class<?> dst = oldType.parameterType(j);
3303                 if (src != dst)
3304                     throw newIllegalArgumentException("parameter types do not match after reorder",
3305                             oldType, newType);
3306             }
3307             if (!bad)  return true;
3308         }
3309         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
3310     }
3311 
3312     /**
3313      * Produces a method handle of the requested return type which returns the given
3314      * constant value every time it is invoked.
3315      * <p>
3316      * Before the method handle is returned, the passed-in value is converted to the requested type.
3317      * If the requested type is primitive, widening primitive conversions are attempted,
3318      * else reference conversions are attempted.
3319      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
3320      * @param type the return type of the desired method handle
3321      * @param value the value to return
3322      * @return a method handle of the given return type and no arguments, which always returns the given value
3323      * @throws NullPointerException if the {@code type} argument is null
3324      * @throws ClassCastException if the value cannot be converted to the required return type
3325      * @throws IllegalArgumentException if the given type is {@code void.class}
3326      */
3327     public static
3328     MethodHandle constant(Class<?> type, Object value) {
3329         if (type.isPrimitive()) {
3330             if (type == void.class)
3331                 throw newIllegalArgumentException("void type");
3332             Wrapper w = Wrapper.forPrimitiveType(type);
3333             value = w.convert(value, type);
3334             if (w.zero().equals(value))
3335                 return zero(w, type);
3336             return insertArguments(identity(type), 0, value);
3337         } else {
3338             if (value == null)
3339                 return zero(Wrapper.OBJECT, type);
3340             return identity(type).bindTo(value);
3341         }
3342     }
3343 
3344     /**
3345      * Produces a method handle which returns its sole argument when invoked.
3346      * @param type the type of the sole parameter and return value of the desired method handle
3347      * @return a unary method handle which accepts and returns the given type
3348      * @throws NullPointerException if the argument is null
3349      * @throws IllegalArgumentException if the given type is {@code void.class}
3350      */
3351     public static
3352     MethodHandle identity(Class<?> type) {
3353         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
3354         int pos = btw.ordinal();
3355         MethodHandle ident = IDENTITY_MHS[pos];
3356         if (ident == null) {
3357             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
3358         }
3359         if (ident.type().returnType() == type)
3360             return ident;
3361         // something like identity(Foo.class); do not bother to intern these
3362         assert (btw == Wrapper.OBJECT);
3363         return makeIdentity(type);
3364     }
3365 
3366     /**
3367      * Produces a constant method handle of the requested return type which
3368      * returns the default value for that type every time it is invoked.
3369      * The resulting constant method handle will have no side effects.
3370      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
3371      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
3372      * since {@code explicitCastArguments} converts {@code null} to default values.
3373      * @param type the expected return type of the desired method handle
3374      * @return a constant method handle that takes no arguments
3375      *         and returns the default value of the given type (or void, if the type is void)
3376      * @throws NullPointerException if the argument is null
3377      * @see MethodHandles#constant
3378      * @see MethodHandles#empty
3379      * @see MethodHandles#explicitCastArguments
3380      * @since 9
3381      */
3382     public static MethodHandle zero(Class<?> type) {
3383         Objects.requireNonNull(type);
3384         if (type.isPrimitive()) {
3385             return zero(Wrapper.forPrimitiveType(type), type);
3386         } else if (MinimalValueTypes_1_0.isValueType(type)) {
3387             try {
3388                 return ValueType.forClass(MinimalValueTypes_1_0.getValueCapableClass(type)).defaultValueConstant();
3389             } catch (ClassNotFoundException ex) {
3390                 throw new IllegalStateException(ex);
3391             }
3392         } else {
3393             return zero(Wrapper.OBJECT, type);
3394         }
3395     }
3396 
3397     private static MethodHandle identityOrVoid(Class<?> type) {
3398         return type == void.class ? zero(type) : identity(type);
3399     }
3400 
3401     /**
3402      * Produces a method handle of the requested type which ignores any arguments, does nothing,
3403      * and returns a suitable default depending on the return type.
3404      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
3405      * <p>The returned method handle is equivalent to
3406      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
3407      * <p>
3408      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
3409      * {@code guardWithTest(pred, target, empty(target.type())}.
3410      * @param type the type of the desired method handle
3411      * @return a constant method handle of the given type, which returns a default value of the given return type
3412      * @throws NullPointerException if the argument is null
3413      * @see MethodHandles#zero
3414      * @see MethodHandles#constant
3415      * @since 9
3416      */
3417     public static  MethodHandle empty(MethodType type) {
3418         Objects.requireNonNull(type);
3419         return dropArguments(zero(type.returnType()), 0, type.parameterList());
3420     }
3421 
3422     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
3423     private static MethodHandle makeIdentity(Class<?> ptype) {
3424         if (!MinimalValueTypes_1_0.isValueType(ptype)) {
3425             MethodType mtype = MethodType.methodType(ptype, ptype);
3426             LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
3427             return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
3428         } else {
3429             try {
3430                 return ValueType.forClass(MinimalValueTypes_1_0.getValueCapableClass(ptype)).identity();
3431             } catch (ReflectiveOperationException ex) {
3432                 throw new IllegalStateException(ex);
3433             }
3434         }
3435     }
3436 
3437     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
3438         int pos = btw.ordinal();
3439         MethodHandle zero = ZERO_MHS[pos];
3440         if (zero == null) {
3441             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
3442         }
3443         if (zero.type().returnType() == rtype)
3444             return zero;
3445         assert(btw == Wrapper.OBJECT);
3446         return makeZero(rtype);
3447     }
3448     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
3449     private static MethodHandle makeZero(Class<?> rtype) {
3450         MethodType mtype = methodType(rtype);
3451         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
3452         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
3453     }
3454 
3455     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
3456         // Simulate a CAS, to avoid racy duplication of results.
3457         MethodHandle prev = cache[pos];
3458         if (prev != null) return prev;
3459         return cache[pos] = value;
3460     }
3461 
3462     /**
3463      * Provides a target method handle with one or more <em>bound arguments</em>
3464      * in advance of the method handle's invocation.
3465      * The formal parameters to the target corresponding to the bound
3466      * arguments are called <em>bound parameters</em>.
3467      * Returns a new method handle which saves away the bound arguments.
3468      * When it is invoked, it receives arguments for any non-bound parameters,
3469      * binds the saved arguments to their corresponding parameters,
3470      * and calls the original target.
3471      * <p>
3472      * The type of the new method handle will drop the types for the bound
3473      * parameters from the original target type, since the new method handle
3474      * will no longer require those arguments to be supplied by its callers.
3475      * <p>
3476      * Each given argument object must match the corresponding bound parameter type.
3477      * If a bound parameter type is a primitive, the argument object
3478      * must be a wrapper, and will be unboxed to produce the primitive value.
3479      * <p>
3480      * The {@code pos} argument selects which parameters are to be bound.
3481      * It may range between zero and <i>N-L</i> (inclusively),
3482      * where <i>N</i> is the arity of the target method handle
3483      * and <i>L</i> is the length of the values array.
3484      * <p>
3485      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3486      * variable-arity method handle}, even if the original target method handle was.
3487      * @param target the method handle to invoke after the argument is inserted
3488      * @param pos where to insert the argument (zero for the first)
3489      * @param values the series of arguments to insert
3490      * @return a method handle which inserts an additional argument,
3491      *         before calling the original method handle
3492      * @throws NullPointerException if the target or the {@code values} array is null
3493      * @see MethodHandle#bindTo
3494      */
3495     public static
3496     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
3497         int insCount = values.length;
3498         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
3499         if (insCount == 0)  return target;
3500         BoundMethodHandle result = target.rebind();
3501         for (int i = 0; i < insCount; i++) {
3502             Object value = values[i];
3503             Class<?> ptype = ptypes[pos+i];
3504             if (ptype.isPrimitive()) {
3505                 result = insertArgumentPrimitive(result, pos, ptype, value);
3506             } else {
3507                 value = ptype.cast(value);  // throw CCE if needed
3508                 result = result.bindArgumentL(pos, value);
3509             }
3510         }
3511         return result;
3512     }
3513 
3514     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
3515                                                              Class<?> ptype, Object value) {
3516         Wrapper w = Wrapper.forPrimitiveType(ptype);
3517         // perform unboxing and/or primitive conversion
3518         value = w.convert(value, ptype);
3519         switch (w) {
3520         case INT:     return result.bindArgumentI(pos, (int)value);
3521         case LONG:    return result.bindArgumentJ(pos, (long)value);
3522         case FLOAT:   return result.bindArgumentF(pos, (float)value);
3523         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
3524         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
3525         }
3526     }
3527 
3528     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
3529         MethodType oldType = target.type();
3530         int outargs = oldType.parameterCount();
3531         int inargs  = outargs - insCount;
3532         if (inargs < 0)
3533             throw newIllegalArgumentException("too many values to insert");
3534         if (pos < 0 || pos > inargs)
3535             throw newIllegalArgumentException("no argument type to append");
3536         return oldType.ptypes();
3537     }
3538 
3539     /**
3540      * Produces a method handle which will discard some dummy arguments
3541      * before calling some other specified <i>target</i> method handle.
3542      * The type of the new method handle will be the same as the target's type,
3543      * except it will also include the dummy argument types,
3544      * at some given position.
3545      * <p>
3546      * The {@code pos} argument may range between zero and <i>N</i>,
3547      * where <i>N</i> is the arity of the target.
3548      * If {@code pos} is zero, the dummy arguments will precede
3549      * the target's real arguments; if {@code pos} is <i>N</i>
3550      * they will come after.
3551      * <p>
3552      * <b>Example:</b>
3553      * <blockquote><pre>{@code
3554 import static java.lang.invoke.MethodHandles.*;
3555 import static java.lang.invoke.MethodType.*;
3556 ...
3557 MethodHandle cat = lookup().findVirtual(String.class,
3558   "concat", methodType(String.class, String.class));
3559 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3560 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
3561 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
3562 assertEquals(bigType, d0.type());
3563 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
3564      * }</pre></blockquote>
3565      * <p>
3566      * This method is also equivalent to the following code:
3567      * <blockquote><pre>
3568      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
3569      * </pre></blockquote>
3570      * @param target the method handle to invoke after the arguments are dropped
3571      * @param valueTypes the type(s) of the argument(s) to drop
3572      * @param pos position of first argument to drop (zero for the leftmost)
3573      * @return a method handle which drops arguments of the given types,
3574      *         before calling the original method handle
3575      * @throws NullPointerException if the target is null,
3576      *                              or if the {@code valueTypes} list or any of its elements is null
3577      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3578      *                  or if {@code pos} is negative or greater than the arity of the target,
3579      *                  or if the new method handle's type would have too many parameters
3580      */
3581     public static
3582     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3583         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
3584     }
3585 
3586     private static List<Class<?>> copyTypes(Object[] array) {
3587         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
3588     }
3589 
3590     private static
3591     MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3592         MethodType oldType = target.type();  // get NPE
3593         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
3594         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
3595         if (dropped == 0)  return target;
3596         BoundMethodHandle result = target.rebind();
3597         LambdaForm lform = result.form;
3598         int insertFormArg = 1 + pos;
3599         for (Class<?> ptype : valueTypes) {
3600             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
3601         }
3602         result = result.copyWith(newType, lform);
3603         return result;
3604     }
3605 
3606     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
3607         int dropped = valueTypes.size();
3608         MethodType.checkSlotCount(dropped);
3609         int outargs = oldType.parameterCount();
3610         int inargs  = outargs + dropped;
3611         if (pos < 0 || pos > outargs)
3612             throw newIllegalArgumentException("no argument type to remove"
3613                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
3614                     );
3615         return dropped;
3616     }
3617 
3618     /**
3619      * Produces a method handle which will discard some dummy arguments
3620      * before calling some other specified <i>target</i> method handle.
3621      * The type of the new method handle will be the same as the target's type,
3622      * except it will also include the dummy argument types,
3623      * at some given position.
3624      * <p>
3625      * The {@code pos} argument may range between zero and <i>N</i>,
3626      * where <i>N</i> is the arity of the target.
3627      * If {@code pos} is zero, the dummy arguments will precede
3628      * the target's real arguments; if {@code pos} is <i>N</i>
3629      * they will come after.
3630      * @apiNote
3631      * <blockquote><pre>{@code
3632 import static java.lang.invoke.MethodHandles.*;
3633 import static java.lang.invoke.MethodType.*;
3634 ...
3635 MethodHandle cat = lookup().findVirtual(String.class,
3636   "concat", methodType(String.class, String.class));
3637 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3638 MethodHandle d0 = dropArguments(cat, 0, String.class);
3639 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
3640 MethodHandle d1 = dropArguments(cat, 1, String.class);
3641 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
3642 MethodHandle d2 = dropArguments(cat, 2, String.class);
3643 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
3644 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
3645 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
3646      * }</pre></blockquote>
3647      * <p>
3648      * This method is also equivalent to the following code:
3649      * <blockquote><pre>
3650      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
3651      * </pre></blockquote>
3652      * @param target the method handle to invoke after the arguments are dropped
3653      * @param valueTypes the type(s) of the argument(s) to drop
3654      * @param pos position of first argument to drop (zero for the leftmost)
3655      * @return a method handle which drops arguments of the given types,
3656      *         before calling the original method handle
3657      * @throws NullPointerException if the target is null,
3658      *                              or if the {@code valueTypes} array or any of its elements is null
3659      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3660      *                  or if {@code pos} is negative or greater than the arity of the target,
3661      *                  or if the new method handle's type would have
3662      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
3663      */
3664     public static
3665     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
3666         return dropArguments0(target, pos, copyTypes(valueTypes));
3667     }
3668 
3669     // private version which allows caller some freedom with error handling
3670     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
3671                                       boolean nullOnFailure) {
3672         newTypes = copyTypes(newTypes.toArray());
3673         List<Class<?>> oldTypes = target.type().parameterList();
3674         int match = oldTypes.size();
3675         if (skip != 0) {
3676             if (skip < 0 || skip > match) {
3677                 throw newIllegalArgumentException("illegal skip", skip, target);
3678             }
3679             oldTypes = oldTypes.subList(skip, match);
3680             match -= skip;
3681         }
3682         List<Class<?>> addTypes = newTypes;
3683         int add = addTypes.size();
3684         if (pos != 0) {
3685             if (pos < 0 || pos > add) {
3686                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
3687             }
3688             addTypes = addTypes.subList(pos, add);
3689             add -= pos;
3690             assert(addTypes.size() == add);
3691         }
3692         // Do not add types which already match the existing arguments.
3693         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
3694             if (nullOnFailure) {
3695                 return null;
3696             }
3697             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
3698         }
3699         addTypes = addTypes.subList(match, add);
3700         add -= match;
3701         assert(addTypes.size() == add);
3702         // newTypes:     (   P*[pos], M*[match], A*[add] )
3703         // target: ( S*[skip],        M*[match]  )
3704         MethodHandle adapter = target;
3705         if (add > 0) {
3706             adapter = dropArguments0(adapter, skip+ match, addTypes);
3707         }
3708         // adapter: (S*[skip],        M*[match], A*[add] )
3709         if (pos > 0) {
3710             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
3711         }
3712         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
3713         return adapter;
3714     }
3715 
3716     /**
3717      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
3718      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
3719      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
3720      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
3721      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
3722      * {@link #dropArguments(MethodHandle, int, Class[])}.
3723      * <p>
3724      * The resulting handle will have the same return type as the target handle.
3725      * <p>
3726      * In more formal terms, assume these two type lists:<ul>
3727      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
3728      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
3729      * {@code newTypes}.
3730      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
3731      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
3732      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
3733      * sub-list.
3734      * </ul>
3735      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
3736      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
3737      * {@link #dropArguments(MethodHandle, int, Class[])}.
3738      * <p>
3739      * @apiNote
3740      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
3741      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
3742      * <blockquote><pre>{@code
3743 import static java.lang.invoke.MethodHandles.*;
3744 import static java.lang.invoke.MethodType.*;
3745 ...
3746 ...
3747 MethodHandle h0 = constant(boolean.class, true);
3748 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
3749 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
3750 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
3751 if (h1.type().parameterCount() < h2.type().parameterCount())
3752     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
3753 else
3754     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
3755 MethodHandle h3 = guardWithTest(h0, h1, h2);
3756 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
3757      * }</pre></blockquote>
3758      * @param target the method handle to adapt
3759      * @param skip number of targets parameters to disregard (they will be unchanged)
3760      * @param newTypes the list of types to match {@code target}'s parameter type list to
3761      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
3762      * @return a possibly adapted method handle
3763      * @throws NullPointerException if either argument is null
3764      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
3765      *         or if {@code skip} is negative or greater than the arity of the target,
3766      *         or if {@code pos} is negative or greater than the newTypes list size,
3767      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
3768      *         {@code pos}.
3769      * @since 9
3770      */
3771     public static
3772     MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
3773         Objects.requireNonNull(target);
3774         Objects.requireNonNull(newTypes);
3775         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
3776     }
3777 
3778     /**
3779      * Adapts a target method handle by pre-processing
3780      * one or more of its arguments, each with its own unary filter function,
3781      * and then calling the target with each pre-processed argument
3782      * replaced by the result of its corresponding filter function.
3783      * <p>
3784      * The pre-processing is performed by one or more method handles,
3785      * specified in the elements of the {@code filters} array.
3786      * The first element of the filter array corresponds to the {@code pos}
3787      * argument of the target, and so on in sequence.
3788      * <p>
3789      * Null arguments in the array are treated as identity functions,
3790      * and the corresponding arguments left unchanged.
3791      * (If there are no non-null elements in the array, the original target is returned.)
3792      * Each filter is applied to the corresponding argument of the adapter.
3793      * <p>
3794      * If a filter {@code F} applies to the {@code N}th argument of
3795      * the target, then {@code F} must be a method handle which
3796      * takes exactly one argument.  The type of {@code F}'s sole argument
3797      * replaces the corresponding argument type of the target
3798      * in the resulting adapted method handle.
3799      * The return type of {@code F} must be identical to the corresponding
3800      * parameter type of the target.
3801      * <p>
3802      * It is an error if there are elements of {@code filters}
3803      * (null or not)
3804      * which do not correspond to argument positions in the target.
3805      * <p><b>Example:</b>
3806      * <blockquote><pre>{@code
3807 import static java.lang.invoke.MethodHandles.*;
3808 import static java.lang.invoke.MethodType.*;
3809 ...
3810 MethodHandle cat = lookup().findVirtual(String.class,
3811   "concat", methodType(String.class, String.class));
3812 MethodHandle upcase = lookup().findVirtual(String.class,
3813   "toUpperCase", methodType(String.class));
3814 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3815 MethodHandle f0 = filterArguments(cat, 0, upcase);
3816 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
3817 MethodHandle f1 = filterArguments(cat, 1, upcase);
3818 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
3819 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
3820 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
3821      * }</pre></blockquote>
3822      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3823      * denotes the return type of both the {@code target} and resulting adapter.
3824      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
3825      * of the parameters and arguments that precede and follow the filter position
3826      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
3827      * values of the filtered parameters and arguments; they also represent the
3828      * return types of the {@code filter[i]} handles. The latter accept arguments
3829      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
3830      * the resulting adapter.
3831      * <blockquote><pre>{@code
3832      * T target(P... p, A[i]... a[i], B... b);
3833      * A[i] filter[i](V[i]);
3834      * T adapter(P... p, V[i]... v[i], B... b) {
3835      *   return target(p..., filter[i](v[i])..., b...);
3836      * }
3837      * }</pre></blockquote>
3838      * <p>
3839      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3840      * variable-arity method handle}, even if the original target method handle was.
3841      *
3842      * @param target the method handle to invoke after arguments are filtered
3843      * @param pos the position of the first argument to filter
3844      * @param filters method handles to call initially on filtered arguments
3845      * @return method handle which incorporates the specified argument filtering logic
3846      * @throws NullPointerException if the target is null
3847      *                              or if the {@code filters} array is null
3848      * @throws IllegalArgumentException if a non-null element of {@code filters}
3849      *          does not match a corresponding argument type of target as described above,
3850      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
3851      *          or if the resulting method handle's type would have
3852      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3853      */
3854     public static
3855     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
3856         filterArgumentsCheckArity(target, pos, filters);
3857         MethodHandle adapter = target;
3858         int curPos = pos-1;  // pre-incremented
3859         for (MethodHandle filter : filters) {
3860             curPos += 1;
3861             if (filter == null)  continue;  // ignore null elements of filters
3862             adapter = filterArgument(adapter, curPos, filter);
3863         }
3864         return adapter;
3865     }
3866 
3867     /*non-public*/ static
3868     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
3869         filterArgumentChecks(target, pos, filter);
3870         MethodType targetType = target.type();
3871         MethodType filterType = filter.type();
3872         BoundMethodHandle result = target.rebind();
3873         Class<?> newParamType = filterType.parameterType(0);
3874         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
3875         MethodType newType = targetType.changeParameterType(pos, newParamType);
3876         result = result.copyWithExtendL(newType, lform, filter);
3877         return result;
3878     }
3879 
3880     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
3881         MethodType targetType = target.type();
3882         int maxPos = targetType.parameterCount();
3883         if (pos + filters.length > maxPos)
3884             throw newIllegalArgumentException("too many filters");
3885     }
3886 
3887     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
3888         MethodType targetType = target.type();
3889         MethodType filterType = filter.type();
3890         if (filterType.parameterCount() != 1
3891             || filterType.returnType() != targetType.parameterType(pos))
3892             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3893     }
3894 
3895     /**
3896      * Adapts a target method handle by pre-processing
3897      * a sub-sequence of its arguments with a filter (another method handle).
3898      * The pre-processed arguments are replaced by the result (if any) of the
3899      * filter function.
3900      * The target is then called on the modified (usually shortened) argument list.
3901      * <p>
3902      * If the filter returns a value, the target must accept that value as
3903      * its argument in position {@code pos}, preceded and/or followed by
3904      * any arguments not passed to the filter.
3905      * If the filter returns void, the target must accept all arguments
3906      * not passed to the filter.
3907      * No arguments are reordered, and a result returned from the filter
3908      * replaces (in order) the whole subsequence of arguments originally
3909      * passed to the adapter.
3910      * <p>
3911      * The argument types (if any) of the filter
3912      * replace zero or one argument types of the target, at position {@code pos},
3913      * in the resulting adapted method handle.
3914      * The return type of the filter (if any) must be identical to the
3915      * argument type of the target at position {@code pos}, and that target argument
3916      * is supplied by the return value of the filter.
3917      * <p>
3918      * In all cases, {@code pos} must be greater than or equal to zero, and
3919      * {@code pos} must also be less than or equal to the target's arity.
3920      * <p><b>Example:</b>
3921      * <blockquote><pre>{@code
3922 import static java.lang.invoke.MethodHandles.*;
3923 import static java.lang.invoke.MethodType.*;
3924 ...
3925 MethodHandle deepToString = publicLookup()
3926   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
3927 
3928 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
3929 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
3930 
3931 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
3932 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
3933 
3934 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
3935 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
3936 assertEquals("[top, [up, down], strange]",
3937              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
3938 
3939 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
3940 assertEquals("[top, [up, down], [strange]]",
3941              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
3942 
3943 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
3944 assertEquals("[top, [[up, down, strange], charm], bottom]",
3945              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
3946      * }</pre></blockquote>
3947      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3948      * represents the return type of the {@code target} and resulting adapter.
3949      * {@code V}/{@code v} stand for the return type and value of the
3950      * {@code filter}, which are also found in the signature and arguments of
3951      * the {@code target}, respectively, unless {@code V} is {@code void}.
3952      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
3953      * and values preceding and following the collection position, {@code pos},
3954      * in the {@code target}'s signature. They also turn up in the resulting
3955      * adapter's signature and arguments, where they surround
3956      * {@code B}/{@code b}, which represent the parameter types and arguments
3957      * to the {@code filter} (if any).
3958      * <blockquote><pre>{@code
3959      * T target(A...,V,C...);
3960      * V filter(B...);
3961      * T adapter(A... a,B... b,C... c) {
3962      *   V v = filter(b...);
3963      *   return target(a...,v,c...);
3964      * }
3965      * // and if the filter has no arguments:
3966      * T target2(A...,V,C...);
3967      * V filter2();
3968      * T adapter2(A... a,C... c) {
3969      *   V v = filter2();
3970      *   return target2(a...,v,c...);
3971      * }
3972      * // and if the filter has a void return:
3973      * T target3(A...,C...);
3974      * void filter3(B...);
3975      * T adapter3(A... a,B... b,C... c) {
3976      *   filter3(b...);
3977      *   return target3(a...,c...);
3978      * }
3979      * }</pre></blockquote>
3980      * <p>
3981      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
3982      * one which first "folds" the affected arguments, and then drops them, in separate
3983      * steps as follows:
3984      * <blockquote><pre>{@code
3985      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
3986      * mh = MethodHandles.foldArguments(mh, coll); //step 1
3987      * }</pre></blockquote>
3988      * If the target method handle consumes no arguments besides than the result
3989      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
3990      * is equivalent to {@code filterReturnValue(coll, mh)}.
3991      * If the filter method handle {@code coll} consumes one argument and produces
3992      * a non-void result, then {@code collectArguments(mh, N, coll)}
3993      * is equivalent to {@code filterArguments(mh, N, coll)}.
3994      * Other equivalences are possible but would require argument permutation.
3995      * <p>
3996      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3997      * variable-arity method handle}, even if the original target method handle was.
3998      *
3999      * @param target the method handle to invoke after filtering the subsequence of arguments
4000      * @param pos the position of the first adapter argument to pass to the filter,
4001      *            and/or the target argument which receives the result of the filter
4002      * @param filter method handle to call on the subsequence of arguments
4003      * @return method handle which incorporates the specified argument subsequence filtering logic
4004      * @throws NullPointerException if either argument is null
4005      * @throws IllegalArgumentException if the return type of {@code filter}
4006      *          is non-void and is not the same as the {@code pos} argument of the target,
4007      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
4008      *          or if the resulting method handle's type would have
4009      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4010      * @see MethodHandles#foldArguments
4011      * @see MethodHandles#filterArguments
4012      * @see MethodHandles#filterReturnValue
4013      */
4014     public static
4015     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
4016         MethodType newType = collectArgumentsChecks(target, pos, filter);
4017         MethodType collectorType = filter.type();
4018         BoundMethodHandle result = target.rebind();
4019         LambdaForm lform;
4020         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
4021             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
4022             if (lform != null) {
4023                 return result.copyWith(newType, lform);
4024             }
4025         }
4026         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
4027         return result.copyWithExtendL(newType, lform, filter);
4028     }
4029 
4030     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
4031         MethodType targetType = target.type();
4032         MethodType filterType = filter.type();
4033         Class<?> rtype = filterType.returnType();
4034         List<Class<?>> filterArgs = filterType.parameterList();
4035         if (rtype == void.class) {
4036             return targetType.insertParameterTypes(pos, filterArgs);
4037         }
4038         if (rtype != targetType.parameterType(pos)) {
4039             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4040         }
4041         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
4042     }
4043 
4044     /**
4045      * Adapts a target method handle by post-processing
4046      * its return value (if any) with a filter (another method handle).
4047      * The result of the filter is returned from the adapter.
4048      * <p>
4049      * If the target returns a value, the filter must accept that value as
4050      * its only argument.
4051      * If the target returns void, the filter must accept no arguments.
4052      * <p>
4053      * The return type of the filter
4054      * replaces the return type of the target
4055      * in the resulting adapted method handle.
4056      * The argument type of the filter (if any) must be identical to the
4057      * return type of the target.
4058      * <p><b>Example:</b>
4059      * <blockquote><pre>{@code
4060 import static java.lang.invoke.MethodHandles.*;
4061 import static java.lang.invoke.MethodType.*;
4062 ...
4063 MethodHandle cat = lookup().findVirtual(String.class,
4064   "concat", methodType(String.class, String.class));
4065 MethodHandle length = lookup().findVirtual(String.class,
4066   "length", methodType(int.class));
4067 System.out.println((String) cat.invokeExact("x", "y")); // xy
4068 MethodHandle f0 = filterReturnValue(cat, length);
4069 System.out.println((int) f0.invokeExact("x", "y")); // 2
4070      * }</pre></blockquote>
4071      * <p>Here is pseudocode for the resulting adapter. In the code,
4072      * {@code T}/{@code t} represent the result type and value of the
4073      * {@code target}; {@code V}, the result type of the {@code filter}; and
4074      * {@code A}/{@code a}, the types and values of the parameters and arguments
4075      * of the {@code target} as well as the resulting adapter.
4076      * <blockquote><pre>{@code
4077      * T target(A...);
4078      * V filter(T);
4079      * V adapter(A... a) {
4080      *   T t = target(a...);
4081      *   return filter(t);
4082      * }
4083      * // and if the target has a void return:
4084      * void target2(A...);
4085      * V filter2();
4086      * V adapter2(A... a) {
4087      *   target2(a...);
4088      *   return filter2();
4089      * }
4090      * // and if the filter has a void return:
4091      * T target3(A...);
4092      * void filter3(V);
4093      * void adapter3(A... a) {
4094      *   T t = target3(a...);
4095      *   filter3(t);
4096      * }
4097      * }</pre></blockquote>
4098      * <p>
4099      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4100      * variable-arity method handle}, even if the original target method handle was.
4101      * @param target the method handle to invoke before filtering the return value
4102      * @param filter method handle to call on the return value
4103      * @return method handle which incorporates the specified return value filtering logic
4104      * @throws NullPointerException if either argument is null
4105      * @throws IllegalArgumentException if the argument list of {@code filter}
4106      *          does not match the return type of target as described above
4107      */
4108     public static
4109     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
4110         MethodType targetType = target.type();
4111         MethodType filterType = filter.type();
4112         filterReturnValueChecks(targetType, filterType);
4113         BoundMethodHandle result = target.rebind();
4114         BasicType rtype = BasicType.basicType(filterType.returnType());
4115         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
4116         MethodType newType = targetType.changeReturnType(filterType.returnType());
4117         result = result.copyWithExtendL(newType, lform, filter);
4118         return result;
4119     }
4120 
4121     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
4122         Class<?> rtype = targetType.returnType();
4123         int filterValues = filterType.parameterCount();
4124         if (filterValues == 0
4125                 ? (rtype != void.class)
4126                 : (rtype != filterType.parameterType(0) || filterValues != 1))
4127             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4128     }
4129 
4130     /**
4131      * Adapts a target method handle by pre-processing
4132      * some of its arguments, and then calling the target with
4133      * the result of the pre-processing, inserted into the original
4134      * sequence of arguments.
4135      * <p>
4136      * The pre-processing is performed by {@code combiner}, a second method handle.
4137      * Of the arguments passed to the adapter, the first {@code N} arguments
4138      * are copied to the combiner, which is then called.
4139      * (Here, {@code N} is defined as the parameter count of the combiner.)
4140      * After this, control passes to the target, with any result
4141      * from the combiner inserted before the original {@code N} incoming
4142      * arguments.
4143      * <p>
4144      * If the combiner returns a value, the first parameter type of the target
4145      * must be identical with the return type of the combiner, and the next
4146      * {@code N} parameter types of the target must exactly match the parameters
4147      * of the combiner.
4148      * <p>
4149      * If the combiner has a void return, no result will be inserted,
4150      * and the first {@code N} parameter types of the target
4151      * must exactly match the parameters of the combiner.
4152      * <p>
4153      * The resulting adapter is the same type as the target, except that the
4154      * first parameter type is dropped,
4155      * if it corresponds to the result of the combiner.
4156      * <p>
4157      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
4158      * that either the combiner or the target does not wish to receive.
4159      * If some of the incoming arguments are destined only for the combiner,
4160      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
4161      * arguments will not need to be live on the stack on entry to the
4162      * target.)
4163      * <p><b>Example:</b>
4164      * <blockquote><pre>{@code
4165 import static java.lang.invoke.MethodHandles.*;
4166 import static java.lang.invoke.MethodType.*;
4167 ...
4168 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4169   "println", methodType(void.class, String.class))
4170     .bindTo(System.out);
4171 MethodHandle cat = lookup().findVirtual(String.class,
4172   "concat", methodType(String.class, String.class));
4173 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4174 MethodHandle catTrace = foldArguments(cat, trace);
4175 // also prints "boo":
4176 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4177      * }</pre></blockquote>
4178      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4179      * represents the result type of the {@code target} and resulting adapter.
4180      * {@code V}/{@code v} represent the type and value of the parameter and argument
4181      * of {@code target} that precedes the folding position; {@code V} also is
4182      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4183      * types and values of the {@code N} parameters and arguments at the folding
4184      * position. {@code B}/{@code b} represent the types and values of the
4185      * {@code target} parameters and arguments that follow the folded parameters
4186      * and arguments.
4187      * <blockquote><pre>{@code
4188      * // there are N arguments in A...
4189      * T target(V, A[N]..., B...);
4190      * V combiner(A...);
4191      * T adapter(A... a, B... b) {
4192      *   V v = combiner(a...);
4193      *   return target(v, a..., b...);
4194      * }
4195      * // and if the combiner has a void return:
4196      * T target2(A[N]..., B...);
4197      * void combiner2(A...);
4198      * T adapter2(A... a, B... b) {
4199      *   combiner2(a...);
4200      *   return target2(a..., b...);
4201      * }
4202      * }</pre></blockquote>
4203      * <p>
4204      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4205      * variable-arity method handle}, even if the original target method handle was.
4206      * @param target the method handle to invoke after arguments are combined
4207      * @param combiner method handle to call initially on the incoming arguments
4208      * @return method handle which incorporates the specified argument folding logic
4209      * @throws NullPointerException if either argument is null
4210      * @throws IllegalArgumentException if {@code combiner}'s return type
4211      *          is non-void and not the same as the first argument type of
4212      *          the target, or if the initial {@code N} argument types
4213      *          of the target
4214      *          (skipping one matching the {@code combiner}'s return type)
4215      *          are not identical with the argument types of {@code combiner}
4216      */
4217     public static
4218     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
4219         return foldArguments(target, 0, combiner);
4220     }
4221 
4222     /**
4223      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
4224      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
4225      * before the folded arguments.
4226      * <p>
4227      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
4228      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
4229      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
4230      * 0.
4231      * <p>
4232      * @apiNote Example:
4233      * <blockquote><pre>{@code
4234     import static java.lang.invoke.MethodHandles.*;
4235     import static java.lang.invoke.MethodType.*;
4236     ...
4237     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4238     "println", methodType(void.class, String.class))
4239     .bindTo(System.out);
4240     MethodHandle cat = lookup().findVirtual(String.class,
4241     "concat", methodType(String.class, String.class));
4242     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4243     MethodHandle catTrace = foldArguments(cat, 1, trace);
4244     // also prints "jum":
4245     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4246      * }</pre></blockquote>
4247      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4248      * represents the result type of the {@code target} and resulting adapter.
4249      * {@code V}/{@code v} represent the type and value of the parameter and argument
4250      * of {@code target} that precedes the folding position; {@code V} also is
4251      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4252      * types and values of the {@code N} parameters and arguments at the folding
4253      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
4254      * and values of the {@code target} parameters and arguments that precede and
4255      * follow the folded parameters and arguments starting at {@code pos},
4256      * respectively.
4257      * <blockquote><pre>{@code
4258      * // there are N arguments in A...
4259      * T target(Z..., V, A[N]..., B...);
4260      * V combiner(A...);
4261      * T adapter(Z... z, A... a, B... b) {
4262      *   V v = combiner(a...);
4263      *   return target(z..., v, a..., b...);
4264      * }
4265      * // and if the combiner has a void return:
4266      * T target2(Z..., A[N]..., B...);
4267      * void combiner2(A...);
4268      * T adapter2(Z... z, A... a, B... b) {
4269      *   combiner2(a...);
4270      *   return target2(z..., a..., b...);
4271      * }
4272      * }</pre></blockquote>
4273      * <p>
4274      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4275      * variable-arity method handle}, even if the original target method handle was.
4276      *
4277      * @param target the method handle to invoke after arguments are combined
4278      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
4279      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4280      * @param combiner method handle to call initially on the incoming arguments
4281      * @return method handle which incorporates the specified argument folding logic
4282      * @throws NullPointerException if either argument is null
4283      * @throws IllegalArgumentException if either of the following two conditions holds:
4284      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
4285      *              {@code pos} of the target signature;
4286      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
4287      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
4288      *
4289      * @see #foldArguments(MethodHandle, MethodHandle)
4290      * @since 9
4291      */
4292     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
4293         MethodType targetType = target.type();
4294         MethodType combinerType = combiner.type();
4295         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
4296         BoundMethodHandle result = target.rebind();
4297         boolean dropResult = rtype == void.class;
4298         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
4299         MethodType newType = targetType;
4300         if (!dropResult) {
4301             newType = newType.dropParameterTypes(pos, pos + 1);
4302         }
4303         result = result.copyWithExtendL(newType, lform, combiner);
4304         return result;
4305     }
4306 
4307     /**
4308      * As {@see foldArguments(MethodHandle, int, MethodHandle)}, but with the
4309      * added capability of selecting the arguments from the targets parameters
4310      * to call the combiner with. This allows us to avoid some simple cases of
4311      * permutations and padding the combiner with dropArguments to select the
4312      * right argument, which may ultimately produce fewer intermediaries.
4313      */
4314     static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner, int ... argPositions) {
4315         MethodType targetType = target.type();
4316         MethodType combinerType = combiner.type();
4317         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType, argPositions);
4318         BoundMethodHandle result = target.rebind();
4319         boolean dropResult = rtype == void.class;
4320         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType(), argPositions);
4321         MethodType newType = targetType;
4322         if (!dropResult) {
4323             newType = newType.dropParameterTypes(pos, pos + 1);
4324         }
4325         result = result.copyWithExtendL(newType, lform, combiner);
4326         return result;
4327     }
4328 
4329     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
4330         int foldArgs   = combinerType.parameterCount();
4331         Class<?> rtype = combinerType.returnType();
4332         int foldVals = rtype == void.class ? 0 : 1;
4333         int afterInsertPos = foldPos + foldVals;
4334         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
4335         if (ok) {
4336             for (int i = 0; i < foldArgs; i++) {
4337                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
4338                     ok = false;
4339                     break;
4340                 }
4341             }
4342         }
4343         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
4344             ok = false;
4345         if (!ok)
4346             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4347         return rtype;
4348     }
4349 
4350     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType, int ... argPos) {
4351         int foldArgs = combinerType.parameterCount();
4352         if (argPos.length != foldArgs) {
4353             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
4354         }
4355         Class<?> rtype = combinerType.returnType();
4356         int foldVals = rtype == void.class ? 0 : 1;
4357         boolean ok = true;
4358         for (int i = 0; i < foldArgs; i++) {
4359             int arg = argPos[i];
4360             if (arg < 0 || arg > targetType.parameterCount()) {
4361                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
4362             }
4363             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
4364                 throw newIllegalArgumentException("target argument type at position " + arg
4365                         + " must match combiner argument type at index " + i + ": " + targetType
4366                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
4367             }
4368         }
4369         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos)) {
4370             ok = false;
4371         }
4372         if (!ok)
4373             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4374         return rtype;
4375     }
4376 
4377     /**
4378      * Makes a method handle which adapts a target method handle,
4379      * by guarding it with a test, a boolean-valued method handle.
4380      * If the guard fails, a fallback handle is called instead.
4381      * All three method handles must have the same corresponding
4382      * argument and return types, except that the return type
4383      * of the test must be boolean, and the test is allowed
4384      * to have fewer arguments than the other two method handles.
4385      * <p>
4386      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4387      * represents the uniform result type of the three involved handles;
4388      * {@code A}/{@code a}, the types and values of the {@code target}
4389      * parameters and arguments that are consumed by the {@code test}; and
4390      * {@code B}/{@code b}, those types and values of the {@code target}
4391      * parameters and arguments that are not consumed by the {@code test}.
4392      * <blockquote><pre>{@code
4393      * boolean test(A...);
4394      * T target(A...,B...);
4395      * T fallback(A...,B...);
4396      * T adapter(A... a,B... b) {
4397      *   if (test(a...))
4398      *     return target(a..., b...);
4399      *   else
4400      *     return fallback(a..., b...);
4401      * }
4402      * }</pre></blockquote>
4403      * Note that the test arguments ({@code a...} in the pseudocode) cannot
4404      * be modified by execution of the test, and so are passed unchanged
4405      * from the caller to the target or fallback as appropriate.
4406      * @param test method handle used for test, must return boolean
4407      * @param target method handle to call if test passes
4408      * @param fallback method handle to call if test fails
4409      * @return method handle which incorporates the specified if/then/else logic
4410      * @throws NullPointerException if any argument is null
4411      * @throws IllegalArgumentException if {@code test} does not return boolean,
4412      *          or if all three method types do not match (with the return
4413      *          type of {@code test} changed to match that of the target).
4414      */
4415     public static
4416     MethodHandle guardWithTest(MethodHandle test,
4417                                MethodHandle target,
4418                                MethodHandle fallback) {
4419         MethodType gtype = test.type();
4420         MethodType ttype = target.type();
4421         MethodType ftype = fallback.type();
4422         if (!ttype.equals(ftype))
4423             throw misMatchedTypes("target and fallback types", ttype, ftype);
4424         if (gtype.returnType() != boolean.class)
4425             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
4426         List<Class<?>> targs = ttype.parameterList();
4427         test = dropArgumentsToMatch(test, 0, targs, 0, true);
4428         if (test == null) {
4429             throw misMatchedTypes("target and test types", ttype, gtype);
4430         }
4431         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
4432     }
4433 
4434     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
4435         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
4436     }
4437 
4438     /**
4439      * Makes a method handle which adapts a target method handle,
4440      * by running it inside an exception handler.
4441      * If the target returns normally, the adapter returns that value.
4442      * If an exception matching the specified type is thrown, the fallback
4443      * handle is called instead on the exception, plus the original arguments.
4444      * <p>
4445      * The target and handler must have the same corresponding
4446      * argument and return types, except that handler may omit trailing arguments
4447      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
4448      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
4449      * <p>
4450      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4451      * represents the return type of the {@code target} and {@code handler},
4452      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
4453      * the types and values of arguments to the resulting handle consumed by
4454      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
4455      * resulting handle discarded by {@code handler}.
4456      * <blockquote><pre>{@code
4457      * T target(A..., B...);
4458      * T handler(ExType, A...);
4459      * T adapter(A... a, B... b) {
4460      *   try {
4461      *     return target(a..., b...);
4462      *   } catch (ExType ex) {
4463      *     return handler(ex, a...);
4464      *   }
4465      * }
4466      * }</pre></blockquote>
4467      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
4468      * be modified by execution of the target, and so are passed unchanged
4469      * from the caller to the handler, if the handler is invoked.
4470      * <p>
4471      * The target and handler must return the same type, even if the handler
4472      * always throws.  (This might happen, for instance, because the handler
4473      * is simulating a {@code finally} clause).
4474      * To create such a throwing handler, compose the handler creation logic
4475      * with {@link #throwException throwException},
4476      * in order to create a method handle of the correct return type.
4477      * @param target method handle to call
4478      * @param exType the type of exception which the handler will catch
4479      * @param handler method handle to call if a matching exception is thrown
4480      * @return method handle which incorporates the specified try/catch logic
4481      * @throws NullPointerException if any argument is null
4482      * @throws IllegalArgumentException if {@code handler} does not accept
4483      *          the given exception type, or if the method handle types do
4484      *          not match in their return types and their
4485      *          corresponding parameters
4486      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
4487      */
4488     public static
4489     MethodHandle catchException(MethodHandle target,
4490                                 Class<? extends Throwable> exType,
4491                                 MethodHandle handler) {
4492         MethodType ttype = target.type();
4493         MethodType htype = handler.type();
4494         if (!Throwable.class.isAssignableFrom(exType))
4495             throw new ClassCastException(exType.getName());
4496         if (htype.parameterCount() < 1 ||
4497             !htype.parameterType(0).isAssignableFrom(exType))
4498             throw newIllegalArgumentException("handler does not accept exception type "+exType);
4499         if (htype.returnType() != ttype.returnType())
4500             throw misMatchedTypes("target and handler return types", ttype, htype);
4501         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
4502         if (handler == null) {
4503             throw misMatchedTypes("target and handler types", ttype, htype);
4504         }
4505         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
4506     }
4507 
4508     /**
4509      * Produces a method handle which will throw exceptions of the given {@code exType}.
4510      * The method handle will accept a single argument of {@code exType},
4511      * and immediately throw it as an exception.
4512      * The method type will nominally specify a return of {@code returnType}.
4513      * The return type may be anything convenient:  It doesn't matter to the
4514      * method handle's behavior, since it will never return normally.
4515      * @param returnType the return type of the desired method handle
4516      * @param exType the parameter type of the desired method handle
4517      * @return method handle which can throw the given exceptions
4518      * @throws NullPointerException if either argument is null
4519      */
4520     public static
4521     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
4522         if (!Throwable.class.isAssignableFrom(exType))
4523             throw new ClassCastException(exType.getName());
4524         return MethodHandleImpl.throwException(methodType(returnType, exType));
4525     }
4526 
4527     /**
4528      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
4529      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
4530      * delivers the loop's result, which is the return value of the resulting handle.
4531      * <p>
4532      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
4533      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
4534      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
4535      * terms of method handles, each clause will specify up to four independent actions:<ul>
4536      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
4537      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
4538      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
4539      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
4540      * </ul>
4541      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
4542      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
4543      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
4544      * <p>
4545      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
4546      * this case. See below for a detailed description.
4547      * <p>
4548      * <em>Parameters optional everywhere:</em>
4549      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
4550      * As an exception, the init functions cannot take any {@code v} parameters,
4551      * because those values are not yet computed when the init functions are executed.
4552      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
4553      * In fact, any clause function may take no arguments at all.
4554      * <p>
4555      * <em>Loop parameters:</em>
4556      * A clause function may take all the iteration variable values it is entitled to, in which case
4557      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
4558      * with their types and values notated as {@code (A...)} and {@code (a...)}.
4559      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
4560      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
4561      * init function is automatically a loop parameter {@code a}.)
4562      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
4563      * These loop parameters act as loop-invariant values visible across the whole loop.
4564      * <p>
4565      * <em>Parameters visible everywhere:</em>
4566      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
4567      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
4568      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
4569      * Most clause functions will not need all of this information, but they will be formally connected to it
4570      * as if by {@link #dropArguments}.
4571      * <a id="astar"></a>
4572      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
4573      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
4574      * In that notation, the general form of an init function parameter list
4575      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
4576      * <p>
4577      * <em>Checking clause structure:</em>
4578      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
4579      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
4580      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
4581      * met by the inputs to the loop combinator.
4582      * <p>
4583      * <em>Effectively identical sequences:</em>
4584      * <a id="effid"></a>
4585      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
4586      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
4587      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
4588      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
4589      * that longest list.
4590      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
4591      * and the same is true if more sequences of the form {@code (V... A*)} are added.
4592      * <p>
4593      * <em>Step 0: Determine clause structure.</em><ol type="a">
4594      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
4595      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
4596      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
4597      * four. Padding takes place by appending elements to the array.
4598      * <li>Clauses with all {@code null}s are disregarded.
4599      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
4600      * </ol>
4601      * <p>
4602      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
4603      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
4604      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
4605      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
4606      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
4607      * iteration variable type.
4608      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
4609      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
4610      * </ol>
4611      * <p>
4612      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
4613      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
4614      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
4615      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
4616      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
4617      * (These types will checked in step 2, along with all the clause function types.)
4618      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
4619      * <li>All of the collected parameter lists must be effectively identical.
4620      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
4621      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
4622      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
4623      * the "internal parameter list".
4624      * </ul>
4625      * <p>
4626      * <em>Step 1C: Determine loop return type.</em><ol type="a">
4627      * <li>Examine fini function return types, disregarding omitted fini functions.
4628      * <li>If there are no fini functions, the loop return type is {@code void}.
4629      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
4630      * type.
4631      * </ol>
4632      * <p>
4633      * <em>Step 1D: Check other types.</em><ol type="a">
4634      * <li>There must be at least one non-omitted pred function.
4635      * <li>Every non-omitted pred function must have a {@code boolean} return type.
4636      * </ol>
4637      * <p>
4638      * <em>Step 2: Determine parameter lists.</em><ol type="a">
4639      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
4640      * <li>The parameter list for init functions will be adjusted to the external parameter list.
4641      * (Note that their parameter lists are already effectively identical to this list.)
4642      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
4643      * effectively identical to the internal parameter list {@code (V... A...)}.
4644      * </ol>
4645      * <p>
4646      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
4647      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
4648      * type.
4649      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
4650      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
4651      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
4652      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
4653      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
4654      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
4655      * loop return type.
4656      * </ol>
4657      * <p>
4658      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
4659      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
4660      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
4661      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
4662      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
4663      * pad out the end of the list.
4664      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch dropping unused trailing arguments}.
4665      * </ol>
4666      * <p>
4667      * <em>Final observations.</em><ol type="a">
4668      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
4669      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
4670      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
4671      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
4672      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
4673      * <li>Each pair of init and step functions agrees in their return type {@code V}.
4674      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
4675      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
4676      * </ol>
4677      * <p>
4678      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
4679      * <ul>
4680      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
4681      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
4682      * (Only one {@code Pn} has to be non-{@code null}.)
4683      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
4684      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
4685      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
4686      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
4687      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
4688      * the resulting loop handle's parameter types {@code (A...)}.
4689      * </ul>
4690      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
4691      * which is natural if most of the loop computation happens in the steps.  For some loops,
4692      * the burden of computation might be heaviest in the pred functions, and so the pred functions
4693      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
4694      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
4695      * where the init functions will need the extra parameters.  For such reasons, the rules for
4696      * determining these parameters are as symmetric as possible, across all clause parts.
4697      * In general, the loop parameters function as common invariant values across the whole
4698      * loop, while the iteration variables function as common variant values, or (if there is
4699      * no step function) as internal loop invariant temporaries.
4700      * <p>
4701      * <em>Loop execution.</em><ol type="a">
4702      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
4703      * every clause function. These locals are loop invariant.
4704      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
4705      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
4706      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
4707      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
4708      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
4709      * (in argument order).
4710      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
4711      * returns {@code false}.
4712      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
4713      * sequence {@code (v...)} of loop variables.
4714      * The updated value is immediately visible to all subsequent function calls.
4715      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
4716      * (of type {@code R}) is returned from the loop as a whole.
4717      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
4718      * except by throwing an exception.
4719      * </ol>
4720      * <p>
4721      * <em>Usage tips.</em>
4722      * <ul>
4723      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
4724      * sometimes a step function only needs to observe the current value of its own variable.
4725      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
4726      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
4727      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
4728      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
4729      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
4730      * <li>If some of the clause functions are virtual methods on an instance, the instance
4731      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
4732      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
4733      * will be the first iteration variable value, and it will be easy to use virtual
4734      * methods as clause parts, since all of them will take a leading instance reference matching that value.
4735      * </ul>
4736      * <p>
4737      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
4738      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
4739      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
4740      * <blockquote><pre>{@code
4741      * V... init...(A...);
4742      * boolean pred...(V..., A...);
4743      * V... step...(V..., A...);
4744      * R fini...(V..., A...);
4745      * R loop(A... a) {
4746      *   V... v... = init...(a...);
4747      *   for (;;) {
4748      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
4749      *       v = s(v..., a...);
4750      *       if (!p(v..., a...)) {
4751      *         return f(v..., a...);
4752      *       }
4753      *     }
4754      *   }
4755      * }
4756      * }</pre></blockquote>
4757      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
4758      * to their full length, even though individual clause functions may neglect to take them all.
4759      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch}.
4760      * <p>
4761      * @apiNote Example:
4762      * <blockquote><pre>{@code
4763      * // iterative implementation of the factorial function as a loop handle
4764      * static int one(int k) { return 1; }
4765      * static int inc(int i, int acc, int k) { return i + 1; }
4766      * static int mult(int i, int acc, int k) { return i * acc; }
4767      * static boolean pred(int i, int acc, int k) { return i < k; }
4768      * static int fin(int i, int acc, int k) { return acc; }
4769      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4770      * // null initializer for counter, should initialize to 0
4771      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4772      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4773      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4774      * assertEquals(120, loop.invoke(5));
4775      * }</pre></blockquote>
4776      * The same example, dropping arguments and using combinators:
4777      * <blockquote><pre>{@code
4778      * // simplified implementation of the factorial function as a loop handle
4779      * static int inc(int i) { return i + 1; } // drop acc, k
4780      * static int mult(int i, int acc) { return i * acc; } //drop k
4781      * static boolean cmp(int i, int k) { return i < k; }
4782      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
4783      * // null initializer for counter, should initialize to 0
4784      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4785      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
4786      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
4787      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4788      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4789      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4790      * assertEquals(720, loop.invoke(6));
4791      * }</pre></blockquote>
4792      * A similar example, using a helper object to hold a loop parameter:
4793      * <blockquote><pre>{@code
4794      * // instance-based implementation of the factorial function as a loop handle
4795      * static class FacLoop {
4796      *   final int k;
4797      *   FacLoop(int k) { this.k = k; }
4798      *   int inc(int i) { return i + 1; }
4799      *   int mult(int i, int acc) { return i * acc; }
4800      *   boolean pred(int i) { return i < k; }
4801      *   int fin(int i, int acc) { return acc; }
4802      * }
4803      * // assume MH_FacLoop is a handle to the constructor
4804      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4805      * // null initializer for counter, should initialize to 0
4806      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4807      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
4808      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4809      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4810      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
4811      * assertEquals(5040, loop.invoke(7));
4812      * }</pre></blockquote>
4813      *
4814      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
4815      *
4816      * @return a method handle embodying the looping behavior as defined by the arguments.
4817      *
4818      * @throws IllegalArgumentException in case any of the constraints described above is violated.
4819      *
4820      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
4821      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
4822      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
4823      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
4824      * @since 9
4825      */
4826     public static MethodHandle loop(MethodHandle[]... clauses) {
4827         // Step 0: determine clause structure.
4828         loopChecks0(clauses);
4829 
4830         List<MethodHandle> init = new ArrayList<>();
4831         List<MethodHandle> step = new ArrayList<>();
4832         List<MethodHandle> pred = new ArrayList<>();
4833         List<MethodHandle> fini = new ArrayList<>();
4834 
4835         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
4836             init.add(clause[0]); // all clauses have at least length 1
4837             step.add(clause.length <= 1 ? null : clause[1]);
4838             pred.add(clause.length <= 2 ? null : clause[2]);
4839             fini.add(clause.length <= 3 ? null : clause[3]);
4840         });
4841 
4842         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
4843         final int nclauses = init.size();
4844 
4845         // Step 1A: determine iteration variables (V...).
4846         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
4847         for (int i = 0; i < nclauses; ++i) {
4848             MethodHandle in = init.get(i);
4849             MethodHandle st = step.get(i);
4850             if (in == null && st == null) {
4851                 iterationVariableTypes.add(void.class);
4852             } else if (in != null && st != null) {
4853                 loopChecks1a(i, in, st);
4854                 iterationVariableTypes.add(in.type().returnType());
4855             } else {
4856                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
4857             }
4858         }
4859         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
4860                 collect(Collectors.toList());
4861 
4862         // Step 1B: determine loop parameters (A...).
4863         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
4864         loopChecks1b(init, commonSuffix);
4865 
4866         // Step 1C: determine loop return type.
4867         // Step 1D: check other types.
4868         final Class<?> loopReturnType = fini.stream().filter(Objects::nonNull).map(MethodHandle::type).
4869                 map(MethodType::returnType).findFirst().orElse(void.class);
4870         loopChecks1cd(pred, fini, loopReturnType);
4871 
4872         // Step 2: determine parameter lists.
4873         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
4874         commonParameterSequence.addAll(commonSuffix);
4875         loopChecks2(step, pred, fini, commonParameterSequence);
4876 
4877         // Step 3: fill in omitted functions.
4878         for (int i = 0; i < nclauses; ++i) {
4879             Class<?> t = iterationVariableTypes.get(i);
4880             if (init.get(i) == null) {
4881                 init.set(i, empty(methodType(t, commonSuffix)));
4882             }
4883             if (step.get(i) == null) {
4884                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
4885             }
4886             if (pred.get(i) == null) {
4887                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
4888             }
4889             if (fini.get(i) == null) {
4890                 fini.set(i, empty(methodType(t, commonParameterSequence)));
4891             }
4892         }
4893 
4894         // Step 4: fill in missing parameter types.
4895         // Also convert all handles to fixed-arity handles.
4896         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
4897         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
4898         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
4899         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
4900 
4901         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
4902                 allMatch(pl -> pl.equals(commonSuffix));
4903         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
4904                 allMatch(pl -> pl.equals(commonParameterSequence));
4905 
4906         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
4907     }
4908 
4909     private static void loopChecks0(MethodHandle[][] clauses) {
4910         if (clauses == null || clauses.length == 0) {
4911             throw newIllegalArgumentException("null or no clauses passed");
4912         }
4913         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
4914             throw newIllegalArgumentException("null clauses are not allowed");
4915         }
4916         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
4917             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
4918         }
4919     }
4920 
4921     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
4922         if (in.type().returnType() != st.type().returnType()) {
4923             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
4924                     st.type().returnType());
4925         }
4926     }
4927 
4928     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
4929         final List<Class<?>> empty = List.of();
4930         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
4931                 // take only those that can contribute to a common suffix because they are longer than the prefix
4932                         map(MethodHandle::type).
4933                         filter(t -> t.parameterCount() > skipSize).
4934                         map(MethodType::parameterList).
4935                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
4936         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
4937     }
4938 
4939     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
4940         final List<Class<?>> empty = List.of();
4941         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
4942     }
4943 
4944     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
4945         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
4946         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
4947         return longestParameterList(Arrays.asList(longest1, longest2));
4948     }
4949 
4950     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
4951         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
4952                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
4953             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
4954                     " (common suffix: " + commonSuffix + ")");
4955         }
4956     }
4957 
4958     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
4959         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4960                 anyMatch(t -> t != loopReturnType)) {
4961             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
4962                     loopReturnType + ")");
4963         }
4964 
4965         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
4966             throw newIllegalArgumentException("no predicate found", pred);
4967         }
4968         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4969                 anyMatch(t -> t != boolean.class)) {
4970             throw newIllegalArgumentException("predicates must have boolean return type", pred);
4971         }
4972     }
4973 
4974     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
4975         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
4976                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
4977             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
4978                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
4979         }
4980     }
4981 
4982     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
4983         return hs.stream().map(h -> {
4984             int pc = h.type().parameterCount();
4985             int tpsize = targetParams.size();
4986             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
4987         }).collect(Collectors.toList());
4988     }
4989 
4990     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
4991         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
4992     }
4993 
4994     /**
4995      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
4996      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
4997      * <p>
4998      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
4999      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
5000      * evaluates to {@code true}).
5001      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
5002      * <p>
5003      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5004      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5005      * and updated with the value returned from its invocation. The result of loop execution will be
5006      * the final value of the additional loop-local variable (if present).
5007      * <p>
5008      * The following rules hold for these argument handles:<ul>
5009      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5010      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5011      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5012      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5013      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5014      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5015      * It will constrain the parameter lists of the other loop parts.
5016      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5017      * list {@code (A...)} is called the <em>external parameter list</em>.
5018      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5019      * additional state variable of the loop.
5020      * The body must both accept and return a value of this type {@code V}.
5021      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5022      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5023      * <a href="MethodHandles.html#effid">effectively identical</a>
5024      * to the external parameter list {@code (A...)}.
5025      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5026      * {@linkplain #empty default value}.
5027      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5028      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5029      * effectively identical to the internal parameter list.
5030      * </ul>
5031      * <p>
5032      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5033      * <li>The loop handle's result type is the result type {@code V} of the body.
5034      * <li>The loop handle's parameter types are the types {@code (A...)},
5035      * from the external parameter list.
5036      * </ul>
5037      * <p>
5038      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5039      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5040      * passed to the loop.
5041      * <blockquote><pre>{@code
5042      * V init(A...);
5043      * boolean pred(V, A...);
5044      * V body(V, A...);
5045      * V whileLoop(A... a...) {
5046      *   V v = init(a...);
5047      *   while (pred(v, a...)) {
5048      *     v = body(v, a...);
5049      *   }
5050      *   return v;
5051      * }
5052      * }</pre></blockquote>
5053      * <p>
5054      * @apiNote Example:
5055      * <blockquote><pre>{@code
5056      * // implement the zip function for lists as a loop handle
5057      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
5058      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
5059      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
5060      *   zip.add(a.next());
5061      *   zip.add(b.next());
5062      *   return zip;
5063      * }
5064      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
5065      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
5066      * List<String> a = Arrays.asList("a", "b", "c", "d");
5067      * List<String> b = Arrays.asList("e", "f", "g", "h");
5068      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
5069      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
5070      * }</pre></blockquote>
5071      *
5072      * <p>
5073      * @apiNote The implementation of this method can be expressed as follows:
5074      * <blockquote><pre>{@code
5075      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5076      *     MethodHandle fini = (body.type().returnType() == void.class
5077      *                         ? null : identity(body.type().returnType()));
5078      *     MethodHandle[]
5079      *         checkExit = { null, null, pred, fini },
5080      *         varBody   = { init, body };
5081      *     return loop(checkExit, varBody);
5082      * }
5083      * }</pre></blockquote>
5084      *
5085      * @param init optional initializer, providing the initial value of the loop variable.
5086      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5087      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5088      *             above for other constraints.
5089      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5090      *             See above for other constraints.
5091      *
5092      * @return a method handle implementing the {@code while} loop as described by the arguments.
5093      * @throws IllegalArgumentException if the rules for the arguments are violated.
5094      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5095      *
5096      * @see #loop(MethodHandle[][])
5097      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
5098      * @since 9
5099      */
5100     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5101         whileLoopChecks(init, pred, body);
5102         MethodHandle fini = identityOrVoid(body.type().returnType());
5103         MethodHandle[] checkExit = { null, null, pred, fini };
5104         MethodHandle[] varBody = { init, body };
5105         return loop(checkExit, varBody);
5106     }
5107 
5108     /**
5109      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
5110      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5111      * <p>
5112      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5113      * method will, in each iteration, first execute its body and then evaluate the predicate.
5114      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
5115      * <p>
5116      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5117      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5118      * and updated with the value returned from its invocation. The result of loop execution will be
5119      * the final value of the additional loop-local variable (if present).
5120      * <p>
5121      * The following rules hold for these argument handles:<ul>
5122      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5123      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5124      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5125      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5126      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5127      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5128      * It will constrain the parameter lists of the other loop parts.
5129      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5130      * list {@code (A...)} is called the <em>external parameter list</em>.
5131      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5132      * additional state variable of the loop.
5133      * The body must both accept and return a value of this type {@code V}.
5134      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5135      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5136      * <a href="MethodHandles.html#effid">effectively identical</a>
5137      * to the external parameter list {@code (A...)}.
5138      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5139      * {@linkplain #empty default value}.
5140      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5141      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5142      * effectively identical to the internal parameter list.
5143      * </ul>
5144      * <p>
5145      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5146      * <li>The loop handle's result type is the result type {@code V} of the body.
5147      * <li>The loop handle's parameter types are the types {@code (A...)},
5148      * from the external parameter list.
5149      * </ul>
5150      * <p>
5151      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5152      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5153      * passed to the loop.
5154      * <blockquote><pre>{@code
5155      * V init(A...);
5156      * boolean pred(V, A...);
5157      * V body(V, A...);
5158      * V doWhileLoop(A... a...) {
5159      *   V v = init(a...);
5160      *   do {
5161      *     v = body(v, a...);
5162      *   } while (pred(v, a...));
5163      *   return v;
5164      * }
5165      * }</pre></blockquote>
5166      * <p>
5167      * @apiNote Example:
5168      * <blockquote><pre>{@code
5169      * // int i = 0; while (i < limit) { ++i; } return i; => limit
5170      * static int zero(int limit) { return 0; }
5171      * static int step(int i, int limit) { return i + 1; }
5172      * static boolean pred(int i, int limit) { return i < limit; }
5173      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
5174      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
5175      * assertEquals(23, loop.invoke(23));
5176      * }</pre></blockquote>
5177      *
5178      * <p>
5179      * @apiNote The implementation of this method can be expressed as follows:
5180      * <blockquote><pre>{@code
5181      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5182      *     MethodHandle fini = (body.type().returnType() == void.class
5183      *                         ? null : identity(body.type().returnType()));
5184      *     MethodHandle[] clause = { init, body, pred, fini };
5185      *     return loop(clause);
5186      * }
5187      * }</pre></blockquote>
5188      *
5189      * @param init optional initializer, providing the initial value of the loop variable.
5190      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5191      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5192      *             See above for other constraints.
5193      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5194      *             above for other constraints.
5195      *
5196      * @return a method handle implementing the {@code while} loop as described by the arguments.
5197      * @throws IllegalArgumentException if the rules for the arguments are violated.
5198      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5199      *
5200      * @see #loop(MethodHandle[][])
5201      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
5202      * @since 9
5203      */
5204     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5205         whileLoopChecks(init, pred, body);
5206         MethodHandle fini = identityOrVoid(body.type().returnType());
5207         MethodHandle[] clause = {init, body, pred, fini };
5208         return loop(clause);
5209     }
5210 
5211     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
5212         Objects.requireNonNull(pred);
5213         Objects.requireNonNull(body);
5214         MethodType bodyType = body.type();
5215         Class<?> returnType = bodyType.returnType();
5216         List<Class<?>> innerList = bodyType.parameterList();
5217         List<Class<?>> outerList = innerList;
5218         if (returnType == void.class) {
5219             // OK
5220         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
5221             // leading V argument missing => error
5222             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5223             throw misMatchedTypes("body function", bodyType, expected);
5224         } else {
5225             outerList = innerList.subList(1, innerList.size());
5226         }
5227         MethodType predType = pred.type();
5228         if (predType.returnType() != boolean.class ||
5229                 !predType.effectivelyIdenticalParameters(0, innerList)) {
5230             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
5231         }
5232         if (init != null) {
5233             MethodType initType = init.type();
5234             if (initType.returnType() != returnType ||
5235                     !initType.effectivelyIdenticalParameters(0, outerList)) {
5236                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5237             }
5238         }
5239     }
5240 
5241     /**
5242      * Constructs a loop that runs a given number of iterations.
5243      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5244      * <p>
5245      * The number of iterations is determined by the {@code iterations} handle evaluation result.
5246      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
5247      * It will be initialized to 0 and incremented by 1 in each iteration.
5248      * <p>
5249      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5250      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5251      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5252      * <p>
5253      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5254      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5255      * iteration variable.
5256      * The result of the loop handle execution will be the final {@code V} value of that variable
5257      * (or {@code void} if there is no {@code V} variable).
5258      * <p>
5259      * The following rules hold for the argument handles:<ul>
5260      * <li>The {@code iterations} handle must not be {@code null}, and must return
5261      * the type {@code int}, referred to here as {@code I} in parameter type lists.
5262      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5263      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5264      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5265      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5266      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5267      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5268      * of types called the <em>internal parameter list</em>.
5269      * It will constrain the parameter lists of the other loop parts.
5270      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5271      * with no additional {@code A} types, then the internal parameter list is extended by
5272      * the argument types {@code A...} of the {@code iterations} handle.
5273      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5274      * list {@code (A...)} is called the <em>external parameter list</em>.
5275      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5276      * additional state variable of the loop.
5277      * The body must both accept a leading parameter and return a value of this type {@code V}.
5278      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5279      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5280      * <a href="MethodHandles.html#effid">effectively identical</a>
5281      * to the external parameter list {@code (A...)}.
5282      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5283      * {@linkplain #empty default value}.
5284      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
5285      * effectively identical to the external parameter list {@code (A...)}.
5286      * </ul>
5287      * <p>
5288      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5289      * <li>The loop handle's result type is the result type {@code V} of the body.
5290      * <li>The loop handle's parameter types are the types {@code (A...)},
5291      * from the external parameter list.
5292      * </ul>
5293      * <p>
5294      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5295      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5296      * arguments passed to the loop.
5297      * <blockquote><pre>{@code
5298      * int iterations(A...);
5299      * V init(A...);
5300      * V body(V, int, A...);
5301      * V countedLoop(A... a...) {
5302      *   int end = iterations(a...);
5303      *   V v = init(a...);
5304      *   for (int i = 0; i < end; ++i) {
5305      *     v = body(v, i, a...);
5306      *   }
5307      *   return v;
5308      * }
5309      * }</pre></blockquote>
5310      * <p>
5311      * @apiNote Example with a fully conformant body method:
5312      * <blockquote><pre>{@code
5313      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5314      * // => a variation on a well known theme
5315      * static String step(String v, int counter, String init) { return "na " + v; }
5316      * // assume MH_step is a handle to the method above
5317      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
5318      * MethodHandle start = MethodHandles.identity(String.class);
5319      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
5320      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
5321      * }</pre></blockquote>
5322      * <p>
5323      * @apiNote Example with the simplest possible body method type,
5324      * and passing the number of iterations to the loop invocation:
5325      * <blockquote><pre>{@code
5326      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5327      * // => a variation on a well known theme
5328      * static String step(String v, int counter ) { return "na " + v; }
5329      * // assume MH_step is a handle to the method above
5330      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
5331      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
5332      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
5333      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
5334      * }</pre></blockquote>
5335      * <p>
5336      * @apiNote Example that treats the number of iterations, string to append to, and string to append
5337      * as loop parameters:
5338      * <blockquote><pre>{@code
5339      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5340      * // => a variation on a well known theme
5341      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
5342      * // assume MH_step is a handle to the method above
5343      * MethodHandle count = MethodHandles.identity(int.class);
5344      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
5345      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
5346      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
5347      * }</pre></blockquote>
5348      * <p>
5349      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
5350      * to enforce a loop type:
5351      * <blockquote><pre>{@code
5352      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5353      * // => a variation on a well known theme
5354      * static String step(String v, int counter, String pre) { return pre + " " + v; }
5355      * // assume MH_step is a handle to the method above
5356      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
5357      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
5358      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
5359      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
5360      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
5361      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
5362      * }</pre></blockquote>
5363      * <p>
5364      * @apiNote The implementation of this method can be expressed as follows:
5365      * <blockquote><pre>{@code
5366      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5367      *     return countedLoop(empty(iterations.type()), iterations, init, body);
5368      * }
5369      * }</pre></blockquote>
5370      *
5371      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
5372      *                   result type must be {@code int}. See above for other constraints.
5373      * @param init optional initializer, providing the initial value of the loop variable.
5374      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5375      * @param body body of the loop, which may not be {@code null}.
5376      *             It controls the loop parameters and result type in the standard case (see above for details).
5377      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5378      *             and may accept any number of additional types.
5379      *             See above for other constraints.
5380      *
5381      * @return a method handle representing the loop.
5382      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
5383      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5384      *
5385      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
5386      * @since 9
5387      */
5388     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5389         return countedLoop(empty(iterations.type()), iterations, init, body);
5390     }
5391 
5392     /**
5393      * Constructs a loop that counts over a range of numbers.
5394      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5395      * <p>
5396      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
5397      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
5398      * values of the loop counter.
5399      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
5400      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
5401      * <p>
5402      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5403      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5404      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5405      * <p>
5406      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5407      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5408      * iteration variable.
5409      * The result of the loop handle execution will be the final {@code V} value of that variable
5410      * (or {@code void} if there is no {@code V} variable).
5411      * <p>
5412      * The following rules hold for the argument handles:<ul>
5413      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
5414      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
5415      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5416      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5417      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5418      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5419      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5420      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5421      * of types called the <em>internal parameter list</em>.
5422      * It will constrain the parameter lists of the other loop parts.
5423      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5424      * with no additional {@code A} types, then the internal parameter list is extended by
5425      * the argument types {@code A...} of the {@code end} handle.
5426      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5427      * list {@code (A...)} is called the <em>external parameter list</em>.
5428      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5429      * additional state variable of the loop.
5430      * The body must both accept a leading parameter and return a value of this type {@code V}.
5431      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5432      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5433      * <a href="MethodHandles.html#effid">effectively identical</a>
5434      * to the external parameter list {@code (A...)}.
5435      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5436      * {@linkplain #empty default value}.
5437      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
5438      * effectively identical to the external parameter list {@code (A...)}.
5439      * <li>Likewise, the parameter list of {@code end} must be effectively identical
5440      * to the external parameter list.
5441      * </ul>
5442      * <p>
5443      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5444      * <li>The loop handle's result type is the result type {@code V} of the body.
5445      * <li>The loop handle's parameter types are the types {@code (A...)},
5446      * from the external parameter list.
5447      * </ul>
5448      * <p>
5449      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5450      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5451      * arguments passed to the loop.
5452      * <blockquote><pre>{@code
5453      * int start(A...);
5454      * int end(A...);
5455      * V init(A...);
5456      * V body(V, int, A...);
5457      * V countedLoop(A... a...) {
5458      *   int e = end(a...);
5459      *   int s = start(a...);
5460      *   V v = init(a...);
5461      *   for (int i = s; i < e; ++i) {
5462      *     v = body(v, i, a...);
5463      *   }
5464      *   return v;
5465      * }
5466      * }</pre></blockquote>
5467      *
5468      * <p>
5469      * @apiNote The implementation of this method can be expressed as follows:
5470      * <blockquote><pre>{@code
5471      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5472      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
5473      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
5474      *     // the following semantics:
5475      *     // MH_increment: (int limit, int counter) -> counter + 1
5476      *     // MH_predicate: (int limit, int counter) -> counter < limit
5477      *     Class<?> counterType = start.type().returnType();  // int
5478      *     Class<?> returnType = body.type().returnType();
5479      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
5480      *     if (returnType != void.class) {  // ignore the V variable
5481      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5482      *         pred = dropArguments(pred, 1, returnType);  // ditto
5483      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
5484      *     }
5485      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
5486      *     MethodHandle[]
5487      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5488      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
5489      *         indexVar   = { start, incr };           // i = start(); i = i + 1
5490      *     return loop(loopLimit, bodyClause, indexVar);
5491      * }
5492      * }</pre></blockquote>
5493      *
5494      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
5495      *              See above for other constraints.
5496      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
5497      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
5498      * @param init optional initializer, providing the initial value of the loop variable.
5499      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5500      * @param body body of the loop, which may not be {@code null}.
5501      *             It controls the loop parameters and result type in the standard case (see above for details).
5502      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5503      *             and may accept any number of additional types.
5504      *             See above for other constraints.
5505      *
5506      * @return a method handle representing the loop.
5507      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
5508      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5509      *
5510      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
5511      * @since 9
5512      */
5513     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5514         countedLoopChecks(start, end, init, body);
5515         Class<?> counterType = start.type().returnType();  // int, but who's counting?
5516         Class<?> limitType   = end.type().returnType();    // yes, int again
5517         Class<?> returnType  = body.type().returnType();
5518         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
5519         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
5520         MethodHandle retv = null;
5521         if (returnType != void.class) {
5522             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5523             pred = dropArguments(pred, 1, returnType);  // ditto
5524             retv = dropArguments(identity(returnType), 0, counterType);
5525         }
5526         body = dropArguments(body, 0, counterType);  // ignore the limit variable
5527         MethodHandle[]
5528             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5529             bodyClause = { init, body },            // v = init(); v = body(v, i)
5530             indexVar   = { start, incr };           // i = start(); i = i + 1
5531         return loop(loopLimit, bodyClause, indexVar);
5532     }
5533 
5534     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5535         Objects.requireNonNull(start);
5536         Objects.requireNonNull(end);
5537         Objects.requireNonNull(body);
5538         Class<?> counterType = start.type().returnType();
5539         if (counterType != int.class) {
5540             MethodType expected = start.type().changeReturnType(int.class);
5541             throw misMatchedTypes("start function", start.type(), expected);
5542         } else if (end.type().returnType() != counterType) {
5543             MethodType expected = end.type().changeReturnType(counterType);
5544             throw misMatchedTypes("end function", end.type(), expected);
5545         }
5546         MethodType bodyType = body.type();
5547         Class<?> returnType = bodyType.returnType();
5548         List<Class<?>> innerList = bodyType.parameterList();
5549         // strip leading V value if present
5550         int vsize = (returnType == void.class ? 0 : 1);
5551         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
5552             // argument list has no "V" => error
5553             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5554             throw misMatchedTypes("body function", bodyType, expected);
5555         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
5556             // missing I type => error
5557             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
5558             throw misMatchedTypes("body function", bodyType, expected);
5559         }
5560         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
5561         if (outerList.isEmpty()) {
5562             // special case; take lists from end handle
5563             outerList = end.type().parameterList();
5564             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
5565         }
5566         MethodType expected = methodType(counterType, outerList);
5567         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
5568             throw misMatchedTypes("start parameter types", start.type(), expected);
5569         }
5570         if (end.type() != start.type() &&
5571             !end.type().effectivelyIdenticalParameters(0, outerList)) {
5572             throw misMatchedTypes("end parameter types", end.type(), expected);
5573         }
5574         if (init != null) {
5575             MethodType initType = init.type();
5576             if (initType.returnType() != returnType ||
5577                 !initType.effectivelyIdenticalParameters(0, outerList)) {
5578                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5579             }
5580         }
5581     }
5582 
5583     /**
5584      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
5585      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5586      * <p>
5587      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
5588      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
5589      * <p>
5590      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5591      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5592      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5593      * <p>
5594      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5595      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5596      * iteration variable.
5597      * The result of the loop handle execution will be the final {@code V} value of that variable
5598      * (or {@code void} if there is no {@code V} variable).
5599      * <p>
5600      * The following rules hold for the argument handles:<ul>
5601      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5602      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
5603      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5604      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
5605      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
5606      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
5607      * of types called the <em>internal parameter list</em>.
5608      * It will constrain the parameter lists of the other loop parts.
5609      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
5610      * with no additional {@code A} types, then the internal parameter list is extended by
5611      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
5612      * single type {@code Iterable} is added and constitutes the {@code A...} list.
5613      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
5614      * list {@code (A...)} is called the <em>external parameter list</em>.
5615      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5616      * additional state variable of the loop.
5617      * The body must both accept a leading parameter and return a value of this type {@code V}.
5618      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5619      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5620      * <a href="MethodHandles.html#effid">effectively identical</a>
5621      * to the external parameter list {@code (A...)}.
5622      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5623      * {@linkplain #empty default value}.
5624      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
5625      * type {@code java.util.Iterator} or a subtype thereof.
5626      * The iterator it produces when the loop is executed will be assumed
5627      * to yield values which can be converted to type {@code T}.
5628      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
5629      * effectively identical to the external parameter list {@code (A...)}.
5630      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
5631      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
5632      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
5633      * handle parameter is adjusted to accept the leading {@code A} type, as if by
5634      * the {@link MethodHandle#asType asType} conversion method.
5635      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
5636      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
5637      * </ul>
5638      * <p>
5639      * The type {@code T} may be either a primitive or reference.
5640      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
5641      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
5642      * as if by the {@link MethodHandle#asType asType} conversion method.
5643      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
5644      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
5645      * <p>
5646      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5647      * <li>The loop handle's result type is the result type {@code V} of the body.
5648      * <li>The loop handle's parameter types are the types {@code (A...)},
5649      * from the external parameter list.
5650      * </ul>
5651      * <p>
5652      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5653      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
5654      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
5655      * <blockquote><pre>{@code
5656      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
5657      * V init(A...);
5658      * V body(V,T,A...);
5659      * V iteratedLoop(A... a...) {
5660      *   Iterator<T> it = iterator(a...);
5661      *   V v = init(a...);
5662      *   while (it.hasNext()) {
5663      *     T t = it.next();
5664      *     v = body(v, t, a...);
5665      *   }
5666      *   return v;
5667      * }
5668      * }</pre></blockquote>
5669      * <p>
5670      * @apiNote Example:
5671      * <blockquote><pre>{@code
5672      * // get an iterator from a list
5673      * static List<String> reverseStep(List<String> r, String e) {
5674      *   r.add(0, e);
5675      *   return r;
5676      * }
5677      * static List<String> newArrayList() { return new ArrayList<>(); }
5678      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
5679      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
5680      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
5681      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
5682      * assertEquals(reversedList, (List<String>) loop.invoke(list));
5683      * }</pre></blockquote>
5684      * <p>
5685      * @apiNote The implementation of this method can be expressed approximately as follows:
5686      * <blockquote><pre>{@code
5687      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5688      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
5689      *     Class<?> returnType = body.type().returnType();
5690      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5691      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
5692      *     MethodHandle retv = null, step = body, startIter = iterator;
5693      *     if (returnType != void.class) {
5694      *         // the simple thing first:  in (I V A...), drop the I to get V
5695      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
5696      *         // body type signature (V T A...), internal loop types (I V A...)
5697      *         step = swapArguments(body, 0, 1);  // swap V <-> T
5698      *     }
5699      *     if (startIter == null)  startIter = MH_getIter;
5700      *     MethodHandle[]
5701      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
5702      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
5703      *     return loop(iterVar, bodyClause);
5704      * }
5705      * }</pre></blockquote>
5706      *
5707      * @param iterator an optional handle to return the iterator to start the loop.
5708      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
5709      *                 See above for other constraints.
5710      * @param init optional initializer, providing the initial value of the loop variable.
5711      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5712      * @param body body of the loop, which may not be {@code null}.
5713      *             It controls the loop parameters and result type in the standard case (see above for details).
5714      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
5715      *             and may accept any number of additional types.
5716      *             See above for other constraints.
5717      *
5718      * @return a method handle embodying the iteration loop functionality.
5719      * @throws NullPointerException if the {@code body} handle is {@code null}.
5720      * @throws IllegalArgumentException if any argument violates the above requirements.
5721      *
5722      * @since 9
5723      */
5724     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5725         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
5726         Class<?> returnType = body.type().returnType();
5727         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
5728         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
5729         MethodHandle startIter;
5730         MethodHandle nextVal;
5731         {
5732             MethodType iteratorType;
5733             if (iterator == null) {
5734                 // derive argument type from body, if available, else use Iterable
5735                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
5736                 iteratorType = startIter.type().changeParameterType(0, iterableType);
5737             } else {
5738                 // force return type to the internal iterator class
5739                 iteratorType = iterator.type().changeReturnType(Iterator.class);
5740                 startIter = iterator;
5741             }
5742             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5743             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
5744 
5745             // perform the asType transforms under an exception transformer, as per spec.:
5746             try {
5747                 startIter = startIter.asType(iteratorType);
5748                 nextVal = nextRaw.asType(nextValType);
5749             } catch (WrongMethodTypeException ex) {
5750                 throw new IllegalArgumentException(ex);
5751             }
5752         }
5753 
5754         MethodHandle retv = null, step = body;
5755         if (returnType != void.class) {
5756             // the simple thing first:  in (I V A...), drop the I to get V
5757             retv = dropArguments(identity(returnType), 0, Iterator.class);
5758             // body type signature (V T A...), internal loop types (I V A...)
5759             step = swapArguments(body, 0, 1);  // swap V <-> T
5760         }
5761 
5762         MethodHandle[]
5763             iterVar    = { startIter, null, hasNext, retv },
5764             bodyClause = { init, filterArgument(step, 0, nextVal) };
5765         return loop(iterVar, bodyClause);
5766     }
5767 
5768     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5769         Objects.requireNonNull(body);
5770         MethodType bodyType = body.type();
5771         Class<?> returnType = bodyType.returnType();
5772         List<Class<?>> internalParamList = bodyType.parameterList();
5773         // strip leading V value if present
5774         int vsize = (returnType == void.class ? 0 : 1);
5775         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
5776             // argument list has no "V" => error
5777             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5778             throw misMatchedTypes("body function", bodyType, expected);
5779         } else if (internalParamList.size() <= vsize) {
5780             // missing T type => error
5781             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
5782             throw misMatchedTypes("body function", bodyType, expected);
5783         }
5784         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
5785         Class<?> iterableType = null;
5786         if (iterator != null) {
5787             // special case; if the body handle only declares V and T then
5788             // the external parameter list is obtained from iterator handle
5789             if (externalParamList.isEmpty()) {
5790                 externalParamList = iterator.type().parameterList();
5791             }
5792             MethodType itype = iterator.type();
5793             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
5794                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
5795             }
5796             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
5797                 MethodType expected = methodType(itype.returnType(), externalParamList);
5798                 throw misMatchedTypes("iterator parameters", itype, expected);
5799             }
5800         } else {
5801             if (externalParamList.isEmpty()) {
5802                 // special case; if the iterator handle is null and the body handle
5803                 // only declares V and T then the external parameter list consists
5804                 // of Iterable
5805                 externalParamList = Arrays.asList(Iterable.class);
5806                 iterableType = Iterable.class;
5807             } else {
5808                 // special case; if the iterator handle is null and the external
5809                 // parameter list is not empty then the first parameter must be
5810                 // assignable to Iterable
5811                 iterableType = externalParamList.get(0);
5812                 if (!Iterable.class.isAssignableFrom(iterableType)) {
5813                     throw newIllegalArgumentException(
5814                             "inferred first loop argument must inherit from Iterable: " + iterableType);
5815                 }
5816             }
5817         }
5818         if (init != null) {
5819             MethodType initType = init.type();
5820             if (initType.returnType() != returnType ||
5821                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
5822                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
5823             }
5824         }
5825         return iterableType;  // help the caller a bit
5826     }
5827 
5828     /*non-public*/ static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
5829         // there should be a better way to uncross my wires
5830         int arity = mh.type().parameterCount();
5831         int[] order = new int[arity];
5832         for (int k = 0; k < arity; k++)  order[k] = k;
5833         order[i] = j; order[j] = i;
5834         Class<?>[] types = mh.type().parameterArray();
5835         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
5836         MethodType swapType = methodType(mh.type().returnType(), types);
5837         return permuteArguments(mh, swapType, order);
5838     }
5839 
5840     /**
5841      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
5842      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
5843      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
5844      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
5845      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
5846      * {@code try-finally} handle.
5847      * <p>
5848      * The {@code cleanup} handle will be passed one or two additional leading arguments.
5849      * The first is the exception thrown during the
5850      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
5851      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
5852      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
5853      * The second argument is not present if the {@code target} handle has a {@code void} return type.
5854      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
5855      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
5856      * <p>
5857      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
5858      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
5859      * two extra leading parameters:<ul>
5860      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
5861      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
5862      * the result from the execution of the {@code target} handle.
5863      * This parameter is not present if the {@code target} returns {@code void}.
5864      * </ul>
5865      * <p>
5866      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
5867      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
5868      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
5869      * the cleanup.
5870      * <blockquote><pre>{@code
5871      * V target(A..., B...);
5872      * V cleanup(Throwable, V, A...);
5873      * V adapter(A... a, B... b) {
5874      *   V result = (zero value for V);
5875      *   Throwable throwable = null;
5876      *   try {
5877      *     result = target(a..., b...);
5878      *   } catch (Throwable t) {
5879      *     throwable = t;
5880      *     throw t;
5881      *   } finally {
5882      *     result = cleanup(throwable, result, a...);
5883      *   }
5884      *   return result;
5885      * }
5886      * }</pre></blockquote>
5887      * <p>
5888      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
5889      * be modified by execution of the target, and so are passed unchanged
5890      * from the caller to the cleanup, if it is invoked.
5891      * <p>
5892      * The target and cleanup must return the same type, even if the cleanup
5893      * always throws.
5894      * To create such a throwing cleanup, compose the cleanup logic
5895      * with {@link #throwException throwException},
5896      * in order to create a method handle of the correct return type.
5897      * <p>
5898      * Note that {@code tryFinally} never converts exceptions into normal returns.
5899      * In rare cases where exceptions must be converted in that way, first wrap
5900      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
5901      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
5902      *
5903      * @param target the handle whose execution is to be wrapped in a {@code try} block.
5904      * @param cleanup the handle that is invoked in the finally block.
5905      *
5906      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
5907      * @throws NullPointerException if any argument is null
5908      * @throws IllegalArgumentException if {@code cleanup} does not accept
5909      *          the required leading arguments, or if the method handle types do
5910      *          not match in their return types and their
5911      *          corresponding trailing parameters
5912      *
5913      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
5914      * @since 9
5915      */
5916     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
5917         List<Class<?>> targetParamTypes = target.type().parameterList();
5918         List<Class<?>> cleanupParamTypes = cleanup.type().parameterList();
5919         Class<?> rtype = target.type().returnType();
5920 
5921         tryFinallyChecks(target, cleanup);
5922 
5923         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
5924         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
5925         // target parameter list.
5926         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
5927 
5928         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
5929         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
5930     }
5931 
5932     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
5933         Class<?> rtype = target.type().returnType();
5934         if (rtype != cleanup.type().returnType()) {
5935             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
5936         }
5937         MethodType cleanupType = cleanup.type();
5938         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
5939             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
5940         }
5941         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
5942             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
5943         }
5944         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
5945         // target parameter list.
5946         int cleanupArgIndex = rtype == void.class ? 1 : 2;
5947         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
5948             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
5949                     cleanup.type(), target.type());
5950         }
5951     }
5952 
5953 }
--- EOF ---