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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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25
26 package java.lang.invoke;
27
28
29 import jdk.internal.HotSpotIntrinsicCandidate;
30
31 import java.util.Arrays;
32 import java.util.Objects;
33
34 import static java.lang.invoke.MethodHandleStatics.*;
35
36 /**
37 * A method handle is a typed, directly executable reference to an underlying method,
38 * constructor, field, or similar low-level operation, with optional
39 * transformations of arguments or return values.
40 * These transformations are quite general, and include such patterns as
41 * {@linkplain #asType conversion},
42 * {@linkplain #bindTo insertion},
43 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
44 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
45 *
46 * <h1>Method handle contents</h1>
47 * Method handles are dynamically and strongly typed according to their parameter and return types.
48 * They are not distinguished by the name or the defining class of their underlying methods.
49 * A method handle must be invoked using a symbolic type descriptor which matches
50 * the method handle's own {@linkplain #type() type descriptor}.
51 * <p>
52 * Every method handle reports its type descriptor via the {@link #type() type} accessor.
53 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
54 * whose structure is a series of classes, one of which is
55 * the return type of the method (or {@code void.class} if none).
56 * <p>
57 * A method handle's type controls the types of invocations it accepts,
58 * and the kinds of transformations that apply to it.
59 * <p>
60 * A method handle contains a pair of special invoker methods
61 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
62 * Both invoker methods provide direct access to the method handle's
63 * underlying method, constructor, field, or other operation,
64 * as modified by transformations of arguments and return values.
65 * Both invokers accept calls which exactly match the method handle's own type.
66 * The plain, inexact invoker also accepts a range of other call types.
67 * <p>
68 * Method handles are immutable and have no visible state.
69 * Of course, they can be bound to underlying methods or data which exhibit state.
70 * With respect to the Java Memory Model, any method handle will behave
71 * as if all of its (internal) fields are final variables. This means that any method
72 * handle made visible to the application will always be fully formed.
73 * This is true even if the method handle is published through a shared
74 * variable in a data race.
75 * <p>
76 * Method handles cannot be subclassed by the user.
77 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
78 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
79 * operation. The programmer should not draw conclusions about a method handle
80 * from its specific class, as the method handle class hierarchy (if any)
81 * may change from time to time or across implementations from different vendors.
82 *
83 * <h1>Method handle compilation</h1>
84 * A Java method call expression naming {@code invokeExact} or {@code invoke}
85 * can invoke a method handle from Java source code.
86 * From the viewpoint of source code, these methods can take any arguments
87 * and their result can be cast to any return type.
88 * Formally this is accomplished by giving the invoker methods
89 * {@code Object} return types and variable arity {@code Object} arguments,
90 * but they have an additional quality called <em>signature polymorphism</em>
91 * which connects this freedom of invocation directly to the JVM execution stack.
92 * <p>
93 * As is usual with virtual methods, source-level calls to {@code invokeExact}
94 * and {@code invoke} compile to an {@code invokevirtual} instruction.
95 * More unusually, the compiler must record the actual argument types,
96 * and may not perform method invocation conversions on the arguments.
97 * Instead, it must generate instructions that push them on the stack according
98 * to their own unconverted types. The method handle object itself is pushed on
99 * the stack before the arguments.
100 * The compiler then generates an {@code invokevirtual} instruction that invokes
101 * the method handle with a symbolic type descriptor which describes the argument
102 * and return types.
103 * <p>
104 * To issue a complete symbolic type descriptor, the compiler must also determine
105 * the return type. This is based on a cast on the method invocation expression,
106 * if there is one, or else {@code Object} if the invocation is an expression,
107 * or else {@code void} if the invocation is a statement.
108 * The cast may be to a primitive type (but not {@code void}).
109 * <p>
110 * As a corner case, an uncasted {@code null} argument is given
111 * a symbolic type descriptor of {@code java.lang.Void}.
112 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
113 * {@code Void} except the null reference.
114 *
115 * <h1>Method handle invocation</h1>
116 * The first time an {@code invokevirtual} instruction is executed
117 * it is linked by symbolically resolving the names in the instruction
118 * and verifying that the method call is statically legal.
119 * This also holds for calls to {@code invokeExact} and {@code invoke}.
120 * In this case, the symbolic type descriptor emitted by the compiler is checked for
121 * correct syntax, and names it contains are resolved.
122 * Thus, an {@code invokevirtual} instruction which invokes
123 * a method handle will always link, as long
124 * as the symbolic type descriptor is syntactically well-formed
125 * and the types exist.
126 * <p>
127 * When the {@code invokevirtual} is executed after linking,
128 * the receiving method handle's type is first checked by the JVM
129 * to ensure that it matches the symbolic type descriptor.
130 * If the type match fails, it means that the method which the
131 * caller is invoking is not present on the individual
132 * method handle being invoked.
133 * <p>
134 * In the case of {@code invokeExact}, the type descriptor of the invocation
135 * (after resolving symbolic type names) must exactly match the method type
136 * of the receiving method handle.
137 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
138 * must be a valid argument to the receiver's {@link #asType asType} method.
139 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
140 * <p>
141 * After type matching, a call to {@code invokeExact} directly
142 * and immediately invoke the method handle's underlying method
143 * (or other behavior, as the case may be).
144 * <p>
145 * A call to plain {@code invoke} works the same as a call to
146 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
147 * exactly matches the method handle's own type.
148 * If there is a type mismatch, {@code invoke} attempts
149 * to adjust the type of the receiving method handle,
150 * as if by a call to {@link #asType asType},
151 * to obtain an exactly invokable method handle {@code M2}.
152 * This allows a more powerful negotiation of method type
153 * between caller and callee.
154 * <p>
155 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
156 * and implementations are therefore not required to materialize it.)
157 *
158 * <h1>Invocation checking</h1>
159 * In typical programs, method handle type matching will usually succeed.
160 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
161 * either directly (in the case of {@code invokeExact}) or indirectly as if
162 * by a failed call to {@code asType} (in the case of {@code invoke}).
163 * <p>
164 * Thus, a method type mismatch which might show up as a linkage error
165 * in a statically typed program can show up as
166 * a dynamic {@code WrongMethodTypeException}
167 * in a program which uses method handles.
168 * <p>
169 * Because method types contain "live" {@code Class} objects,
170 * method type matching takes into account both type names and class loaders.
171 * Thus, even if a method handle {@code M} is created in one
172 * class loader {@code L1} and used in another {@code L2},
173 * method handle calls are type-safe, because the caller's symbolic type
174 * descriptor, as resolved in {@code L2},
175 * is matched against the original callee method's symbolic type descriptor,
176 * as resolved in {@code L1}.
177 * The resolution in {@code L1} happens when {@code M} is created
178 * and its type is assigned, while the resolution in {@code L2} happens
179 * when the {@code invokevirtual} instruction is linked.
180 * <p>
181 * Apart from type descriptor checks,
182 * a method handle's capability to call its underlying method is unrestricted.
183 * If a method handle is formed on a non-public method by a class
184 * that has access to that method, the resulting handle can be used
185 * in any place by any caller who receives a reference to it.
186 * <p>
187 * Unlike with the Core Reflection API, where access is checked every time
188 * a reflective method is invoked,
189 * method handle access checking is performed
190 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
191 * In the case of {@code ldc} (see below), access checking is performed as part of linking
192 * the constant pool entry underlying the constant method handle.
193 * <p>
194 * Thus, handles to non-public methods, or to methods in non-public classes,
195 * should generally be kept secret.
196 * They should not be passed to untrusted code unless their use from
197 * the untrusted code would be harmless.
198 *
199 * <h1>Method handle creation</h1>
200 * Java code can create a method handle that directly accesses
201 * any method, constructor, or field that is accessible to that code.
202 * This is done via a reflective, capability-based API called
203 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}.
204 * For example, a static method handle can be obtained
205 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
206 * There are also conversion methods from Core Reflection API objects,
207 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
208 * <p>
209 * Like classes and strings, method handles that correspond to accessible
210 * fields, methods, and constructors can also be represented directly
211 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
212 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
213 * refers directly to an associated {@code CONSTANT_Methodref},
214 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
215 * constant pool entry.
216 * (For full details on method handle constants,
217 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
218 * <p>
219 * Method handles produced by lookups or constant loads from methods or
220 * constructors with the variable arity modifier bit ({@code 0x0080})
221 * have a corresponding variable arity, as if they were defined with
222 * the help of {@link #asVarargsCollector asVarargsCollector}
223 * or {@link #withVarargs withVarargs}.
224 * <p>
225 * A method reference may refer either to a static or non-static method.
226 * In the non-static case, the method handle type includes an explicit
227 * receiver argument, prepended before any other arguments.
228 * In the method handle's type, the initial receiver argument is typed
229 * according to the class under which the method was initially requested.
230 * (E.g., if a non-static method handle is obtained via {@code ldc},
231 * the type of the receiver is the class named in the constant pool entry.)
232 * <p>
233 * Method handle constants are subject to the same link-time access checks
234 * their corresponding bytecode instructions, and the {@code ldc} instruction
235 * will throw corresponding linkage errors if the bytecode behaviors would
236 * throw such errors.
237 * <p>
238 * As a corollary of this, access to protected members is restricted
239 * to receivers only of the accessing class, or one of its subclasses,
240 * and the accessing class must in turn be a subclass (or package sibling)
241 * of the protected member's defining class.
242 * If a method reference refers to a protected non-static method or field
243 * of a class outside the current package, the receiver argument will
244 * be narrowed to the type of the accessing class.
245 * <p>
246 * When a method handle to a virtual method is invoked, the method is
247 * always looked up in the receiver (that is, the first argument).
248 * <p>
249 * A non-virtual method handle to a specific virtual method implementation
250 * can also be created. These do not perform virtual lookup based on
251 * receiver type. Such a method handle simulates the effect of
252 * an {@code invokespecial} instruction to the same method.
253 *
254 * <h1>Usage examples</h1>
255 * Here are some examples of usage:
256 * <blockquote><pre>{@code
257 Object x, y; String s; int i;
258 MethodType mt; MethodHandle mh;
259 MethodHandles.Lookup lookup = MethodHandles.lookup();
260 // mt is (char,char)String
261 mt = MethodType.methodType(String.class, char.class, char.class);
262 mh = lookup.findVirtual(String.class, "replace", mt);
263 s = (String) mh.invokeExact("daddy",'d','n');
264 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
265 assertEquals(s, "nanny");
266 // weakly typed invocation (using MHs.invoke)
267 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
268 assertEquals(s, "savvy");
269 // mt is (Object[])List
270 mt = MethodType.methodType(java.util.List.class, Object[].class);
271 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
272 assert(mh.isVarargsCollector());
273 x = mh.invoke("one", "two");
274 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
275 assertEquals(x, java.util.Arrays.asList("one","two"));
276 // mt is (Object,Object,Object)Object
277 mt = MethodType.genericMethodType(3);
278 mh = mh.asType(mt);
279 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
280 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
281 assertEquals(x, java.util.Arrays.asList(1,2,3));
282 // mt is ()int
283 mt = MethodType.methodType(int.class);
284 mh = lookup.findVirtual(java.util.List.class, "size", mt);
285 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
286 // invokeExact(Ljava/util/List;)I
287 assert(i == 3);
288 mt = MethodType.methodType(void.class, String.class);
289 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
290 mh.invokeExact(System.out, "Hello, world.");
291 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
292 * }</pre></blockquote>
293 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
294 * generates a single invokevirtual instruction with
295 * the symbolic type descriptor indicated in the following comment.
296 * In these examples, the helper method {@code assertEquals} is assumed to
297 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
298 * on its arguments, and asserts that the result is true.
299 *
300 * <h1>Exceptions</h1>
301 * The methods {@code invokeExact} and {@code invoke} are declared
302 * to throw {@link java.lang.Throwable Throwable},
303 * which is to say that there is no static restriction on what a method handle
304 * can throw. Since the JVM does not distinguish between checked
305 * and unchecked exceptions (other than by their class, of course),
306 * there is no particular effect on bytecode shape from ascribing
307 * checked exceptions to method handle invocations. But in Java source
308 * code, methods which perform method handle calls must either explicitly
309 * throw {@code Throwable}, or else must catch all
310 * throwables locally, rethrowing only those which are legal in the context,
311 * and wrapping ones which are illegal.
312 *
313 * <h1><a id="sigpoly"></a>Signature polymorphism</h1>
314 * The unusual compilation and linkage behavior of
315 * {@code invokeExact} and plain {@code invoke}
316 * is referenced by the term <em>signature polymorphism</em>.
317 * As defined in the Java Language Specification,
318 * a signature polymorphic method is one which can operate with
319 * any of a wide range of call signatures and return types.
320 * <p>
321 * In source code, a call to a signature polymorphic method will
322 * compile, regardless of the requested symbolic type descriptor.
323 * As usual, the Java compiler emits an {@code invokevirtual}
324 * instruction with the given symbolic type descriptor against the named method.
325 * The unusual part is that the symbolic type descriptor is derived from
326 * the actual argument and return types, not from the method declaration.
327 * <p>
328 * When the JVM processes bytecode containing signature polymorphic calls,
329 * it will successfully link any such call, regardless of its symbolic type descriptor.
330 * (In order to retain type safety, the JVM will guard such calls with suitable
331 * dynamic type checks, as described elsewhere.)
332 * <p>
333 * Bytecode generators, including the compiler back end, are required to emit
334 * untransformed symbolic type descriptors for these methods.
335 * Tools which determine symbolic linkage are required to accept such
336 * untransformed descriptors, without reporting linkage errors.
337 *
338 * <h1>Interoperation between method handles and the Core Reflection API</h1>
339 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
340 * any class member represented by a Core Reflection API object
341 * can be converted to a behaviorally equivalent method handle.
342 * For example, a reflective {@link java.lang.reflect.Method Method} can
343 * be converted to a method handle using
344 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
345 * The resulting method handles generally provide more direct and efficient
346 * access to the underlying class members.
347 * <p>
348 * As a special case,
349 * when the Core Reflection API is used to view the signature polymorphic
350 * methods {@code invokeExact} or plain {@code invoke} in this class,
351 * they appear as ordinary non-polymorphic methods.
352 * Their reflective appearance, as viewed by
353 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
354 * is unaffected by their special status in this API.
355 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
356 * will report exactly those modifier bits required for any similarly
357 * declared method, including in this case {@code native} and {@code varargs} bits.
358 * <p>
359 * As with any reflected method, these methods (when reflected) may be
360 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
361 * However, such reflective calls do not result in method handle invocations.
362 * Such a call, if passed the required argument
363 * (a single one, of type {@code Object[]}), will ignore the argument and
364 * will throw an {@code UnsupportedOperationException}.
365 * <p>
366 * Since {@code invokevirtual} instructions can natively
367 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
368 * with the normal presentation of these methods via bytecodes.
369 * Thus, these two native methods, when reflectively viewed by
370 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
371 * <p>
372 * In order to obtain an invoker method for a particular type descriptor,
373 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
374 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
375 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
376 * API is also able to return a method handle
377 * to call {@code invokeExact} or plain {@code invoke},
378 * for any specified type descriptor .
379 *
380 * <h1>Interoperation between method handles and Java generics</h1>
381 * A method handle can be obtained on a method, constructor, or field
382 * which is declared with Java generic types.
383 * As with the Core Reflection API, the type of the method handle
384 * will constructed from the erasure of the source-level type.
385 * When a method handle is invoked, the types of its arguments
386 * or the return value cast type may be generic types or type instances.
387 * If this occurs, the compiler will replace those
388 * types by their erasures when it constructs the symbolic type descriptor
389 * for the {@code invokevirtual} instruction.
390 * <p>
391 * Method handles do not represent
392 * their function-like types in terms of Java parameterized (generic) types,
393 * because there are three mismatches between function-like types and parameterized
394 * Java types.
395 * <ul>
396 * <li>Method types range over all possible arities,
397 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
398 * Generics are not variadic, and so cannot represent this.</li>
399 * <li>Method types can specify arguments of primitive types,
400 * which Java generic types cannot range over.</li>
401 * <li>Higher order functions over method handles (combinators) are
402 * often generic across a wide range of function types, including
403 * those of multiple arities. It is impossible to represent such
404 * genericity with a Java type parameter.</li>
405 * </ul>
406 *
407 * <h1><a id="maxarity"></a>Arity limits</h1>
408 * The JVM imposes on all methods and constructors of any kind an absolute
409 * limit of 255 stacked arguments. This limit can appear more restrictive
410 * in certain cases:
411 * <ul>
412 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
413 * <li>A non-static method consumes an extra argument for the object on which the method is called.
414 * <li>A constructor consumes an extra argument for the object which is being constructed.
415 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
416 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
417 * </ul>
418 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
419 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
420 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
421 * In particular, a method handle’s type must not have an arity of the exact maximum 255.
422 *
423 * @see MethodType
424 * @see MethodHandles
425 * @author John Rose, JSR 292 EG
426 * @since 1.7
427 */
428 public abstract class MethodHandle {
429
430 /**
431 * Internal marker interface which distinguishes (to the Java compiler)
432 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
433 */
434 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
435 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
436 @interface PolymorphicSignature { }
437
438 private final MethodType type;
439 /*private*/ final LambdaForm form;
440 // form is not private so that invokers can easily fetch it
441 /*private*/ MethodHandle asTypeCache;
442 // asTypeCache is not private so that invokers can easily fetch it
443 /*non-public*/ byte customizationCount;
444 // customizationCount should be accessible from invokers
445
446 /**
447 * Reports the type of this method handle.
448 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
449 * @return the method handle type
450 */
451 public MethodType type() {
452 return type;
453 }
454
455 /**
456 * Package-private constructor for the method handle implementation hierarchy.
457 * Method handle inheritance will be contained completely within
458 * the {@code java.lang.invoke} package.
459 */
460 // @param type type (permanently assigned) of the new method handle
461 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
462 this.type = Objects.requireNonNull(type);
463 this.form = Objects.requireNonNull(form).uncustomize();
464
465 this.form.prepare(); // TO DO: Try to delay this step until just before invocation.
466 }
467
468 /**
469 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
470 * The symbolic type descriptor at the call site of {@code invokeExact} must
471 * exactly match this method handle's {@link #type() type}.
472 * No conversions are allowed on arguments or return values.
473 * <p>
474 * When this method is observed via the Core Reflection API,
475 * it will appear as a single native method, taking an object array and returning an object.
476 * If this native method is invoked directly via
477 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
478 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
479 * it will throw an {@code UnsupportedOperationException}.
480 * @param args the signature-polymorphic parameter list, statically represented using varargs
481 * @return the signature-polymorphic result, statically represented using {@code Object}
482 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
483 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
484 */
485 @HotSpotIntrinsicCandidate
486 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
487
488 /**
489 * Invokes the method handle, allowing any caller type descriptor,
490 * and optionally performing conversions on arguments and return values.
491 * <p>
492 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type() type},
493 * the call proceeds as if by {@link #invokeExact invokeExact}.
494 * <p>
495 * Otherwise, the call proceeds as if this method handle were first
496 * adjusted by calling {@link #asType asType} to adjust this method handle
497 * to the required type, and then the call proceeds as if by
498 * {@link #invokeExact invokeExact} on the adjusted method handle.
499 * <p>
500 * There is no guarantee that the {@code asType} call is actually made.
501 * If the JVM can predict the results of making the call, it may perform
502 * adaptations directly on the caller's arguments,
503 * and call the target method handle according to its own exact type.
504 * <p>
505 * The resolved type descriptor at the call site of {@code invoke} must
506 * be a valid argument to the receivers {@code asType} method.
507 * In particular, the caller must specify the same argument arity
508 * as the callee's type,
509 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
510 * <p>
511 * When this method is observed via the Core Reflection API,
512 * it will appear as a single native method, taking an object array and returning an object.
513 * If this native method is invoked directly via
514 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
515 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
516 * it will throw an {@code UnsupportedOperationException}.
517 * @param args the signature-polymorphic parameter list, statically represented using varargs
518 * @return the signature-polymorphic result, statically represented using {@code Object}
519 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
520 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
521 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
522 */
523 @HotSpotIntrinsicCandidate
524 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
525
526 /**
527 * Private method for trusted invocation of a method handle respecting simplified signatures.
528 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
529 * <p>
530 * The caller signature is restricted to the following basic types:
531 * Object, int, long, float, double, and void return.
532 * <p>
533 * The caller is responsible for maintaining type correctness by ensuring
534 * that the each outgoing argument value is a member of the range of the corresponding
535 * callee argument type.
536 * (The caller should therefore issue appropriate casts and integer narrowing
537 * operations on outgoing argument values.)
538 * The caller can assume that the incoming result value is part of the range
539 * of the callee's return type.
540 * @param args the signature-polymorphic parameter list, statically represented using varargs
541 * @return the signature-polymorphic result, statically represented using {@code Object}
542 */
543 @HotSpotIntrinsicCandidate
544 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
545
546 /**
547 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
548 * The caller signature is restricted to basic types as with {@code invokeBasic}.
549 * The trailing (not leading) argument must be a MemberName.
550 * @param args the signature-polymorphic parameter list, statically represented using varargs
551 * @return the signature-polymorphic result, statically represented using {@code Object}
552 */
553 @HotSpotIntrinsicCandidate
554 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
555
556 /**
557 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
558 * The caller signature is restricted to basic types as with {@code invokeBasic}.
559 * The trailing (not leading) argument must be a MemberName.
560 * @param args the signature-polymorphic parameter list, statically represented using varargs
561 * @return the signature-polymorphic result, statically represented using {@code Object}
562 */
563 @HotSpotIntrinsicCandidate
564 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
565
566 /**
567 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
568 * The caller signature is restricted to basic types as with {@code invokeBasic}.
569 * The trailing (not leading) argument must be a MemberName.
570 * @param args the signature-polymorphic parameter list, statically represented using varargs
571 * @return the signature-polymorphic result, statically represented using {@code Object}
572 */
573 @HotSpotIntrinsicCandidate
574 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
575
576 /**
577 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
578 * The caller signature is restricted to basic types as with {@code invokeBasic}.
579 * The trailing (not leading) argument must be a MemberName.
580 * @param args the signature-polymorphic parameter list, statically represented using varargs
581 * @return the signature-polymorphic result, statically represented using {@code Object}
582 */
583 @HotSpotIntrinsicCandidate
584 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
585
586 /**
587 * Performs a variable arity invocation, passing the arguments in the given array
588 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
589 * which mentions only the type {@code Object}, and whose actual argument count is the length
590 * of the argument array.
591 * <p>
592 * Specifically, execution proceeds as if by the following steps,
593 * although the methods are not guaranteed to be called if the JVM
594 * can predict their effects.
595 * <ul>
596 * <li>Determine the length of the argument array as {@code N}.
597 * For a null reference, {@code N=0}. </li>
598 * <li>Collect the {@code N} elements of the array as a logical
599 * argument list, each argument statically typed as an {@code Object}. </li>
600 * <li>Determine, as {@code M}, the parameter count of the type of this
601 * method handle. </li>
602 * <li>Determine the general type {@code TN} of {@code N} arguments or
603 * {@code M} arguments, if smaller than {@code N}, as
604 * {@code TN=MethodType.genericMethodType(Math.min(N, M))}.</li>
605 * <li>If {@code N} is greater than {@code M}, perform the following
606 * checks and actions to shorten the logical argument list: <ul>
607 * <li>Check that this method handle has variable arity with a
608 * {@linkplain MethodType#lastParameterType trailing parameter}
609 * of some array type {@code A[]}. If not, fail with a
610 * {@code WrongMethodTypeException}. </li>
611 * <li>Collect the trailing elements (there are {@code N-M+1} of them)
612 * from the logical argument list into a single array of
613 * type {@code A[]}, using {@code asType} conversions to
614 * convert each trailing argument to type {@code A}. </li>
615 * <li>If any of these conversions proves impossible, fail with either
616 * a {@code ClassCastException} if any trailing element cannot be
617 * cast to {@code A} or a {@code NullPointerException} if any
618 * trailing element is {@code null} and {@code A} is not a reference
619 * type. </li>
620 * <li>Replace the logical arguments gathered into the array of
621 * type {@code A[]} with the array itself, thus shortening
622 * the argument list to length {@code M}. This final argument
623 * retains the static type {@code A[]}.</li>
624 * <li>Adjust the type {@code TN} by changing the {@code N}th
625 * parameter type from {@code Object} to {@code A[]}.
626 * </ul>
627 * <li>Force the original target method handle {@code MH0} to the
628 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
629 * <li>Spread the argument list into {@code N} separate arguments {@code A0, ...}. </li>
630 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
631 * MH1.invokeExact(A0, ...). </li>
632 * <li>Take the return value as an {@code Object} reference. </li>
633 * </ul>
634 * <p>
635 * If the target method handle has variable arity, and the argument list is longer
636 * than that arity, the excess arguments, starting at the position of the trailing
637 * array argument, will be gathered (if possible, as if by {@code asType} conversions)
638 * into an array of the appropriate type, and invocation will proceed on the
639 * shortened argument list.
640 * In this way, <em>jumbo argument lists</em> which would spread into more
641 * than 254 slots can still be processed uniformly.
642 * <p>
643 * Unlike the {@link #invoke(Object...) generic} invocation mode, which can
644 * "recycle" an array argument, passing it directly to the target method,
645 * this invocation mode <em>always</em> creates a new array parameter, even
646 * if the original array passed to {@code invokeWithArguments} would have
647 * been acceptable as a direct argument to the target method.
648 * Even if the number {@code M} of actual arguments is the arity {@code N},
649 * and the last argument is dynamically a suitable array of type {@code A[]},
650 * it will still be boxed into a new one-element array, since the call
651 * site statically types the argument as {@code Object}, not an array type.
652 * This is not a special rule for this method, but rather a regular effect
653 * of the {@linkplain #asVarargsCollector rules for variable-arity invocation}.
654 * <p>
655 * Because of the action of the {@code asType} step, the following argument
656 * conversions are applied as necessary:
657 * <ul>
658 * <li>reference casting
659 * <li>unboxing
660 * <li>widening primitive conversions
661 * <li>variable arity conversion
662 * </ul>
663 * <p>
664 * The result returned by the call is boxed if it is a primitive,
665 * or forced to null if the return type is void.
666 * <p>
667 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
668 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
669 * It can therefore be used as a bridge between native or reflective code and method handles.
670 * @apiNote
671 * This call is approximately equivalent to the following code:
672 * <blockquote><pre>{@code
673 * // for jumbo argument lists, adapt varargs explicitly:
674 * int N = (arguments == null? 0: arguments.length);
675 * int M = this.type.parameterCount();
676 * int MAX_SAFE = 127; // 127 longs require 254 slots, which is OK
677 * if (N > MAX_SAFE && N > M && this.isVarargsCollector()) {
678 * Class<?> arrayType = this.type().lastParameterType();
679 * Class<?> elemType = arrayType.getComponentType();
680 * if (elemType != null) {
681 * Object args2 = Array.newInstance(elemType, M);
682 * MethodHandle arraySetter = MethodHandles.arrayElementSetter(arrayType);
683 * for (int i = 0; i < M; i++) {
684 * arraySetter.invoke(args2, i, arguments[M-1 + i]);
685 * }
686 * arguments = Arrays.copyOf(arguments, M);
687 * arguments[M-1] = args2;
688 * return this.asFixedArity().invokeWithArguments(arguments);
689 * }
690 * } // done with explicit varargs processing
691 *
692 * // Handle fixed arity and non-jumbo variable arity invocation.
693 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
694 * Object result = invoker.invokeExact(this, arguments);
695 * }</pre></blockquote>
696 *
697 * @param arguments the arguments to pass to the target
698 * @return the result returned by the target
699 * @throws ClassCastException if an argument cannot be converted by reference casting
700 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
701 * @throws Throwable anything thrown by the target method invocation
702 * @see MethodHandles#spreadInvoker
703 */
704 public Object invokeWithArguments(Object... arguments) throws Throwable {
705 // Note: Jumbo argument lists are handled in the variable-arity subclass.
706 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length);
707 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments);
708 }
709
710 /**
711 * Performs a variable arity invocation, passing the arguments in the given list
712 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
713 * which mentions only the type {@code Object}, and whose actual argument count is the length
714 * of the argument list.
715 * <p>
716 * This method is also equivalent to the following code:
717 * <blockquote><pre>{@code
718 * invokeWithArguments(arguments.toArray())
719 * }</pre></blockquote>
720 * <p>
721 * Jumbo-sized lists are acceptable if this method handle has variable arity.
722 * See {@link #invokeWithArguments(Object[])} for details.
723 *
724 * @param arguments the arguments to pass to the target
725 * @return the result returned by the target
726 * @throws NullPointerException if {@code arguments} is a null reference
727 * @throws ClassCastException if an argument cannot be converted by reference casting
728 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
729 * @throws Throwable anything thrown by the target method invocation
730 */
731 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
732 return invokeWithArguments(arguments.toArray());
733 }
734
735 /**
736 * Produces an adapter method handle which adapts the type of the
737 * current method handle to a new type.
738 * The resulting method handle is guaranteed to report a type
739 * which is equal to the desired new type.
740 * <p>
741 * If the original type and new type are equal, returns {@code this}.
742 * <p>
743 * The new method handle, when invoked, will perform the following
744 * steps:
745 * <ul>
746 * <li>Convert the incoming argument list to match the original
747 * method handle's argument list.
748 * <li>Invoke the original method handle on the converted argument list.
749 * <li>Convert any result returned by the original method handle
750 * to the return type of new method handle.
751 * </ul>
752 * <p>
753 * This method provides the crucial behavioral difference between
754 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
755 * The two methods
756 * perform the same steps when the caller's type descriptor exactly matches
757 * the callee's, but when the types differ, plain {@link #invoke invoke}
758 * also calls {@code asType} (or some internal equivalent) in order
759 * to match up the caller's and callee's types.
760 * <p>
761 * If the current method is a variable arity method handle
762 * argument list conversion may involve the conversion and collection
763 * of several arguments into an array, as
764 * {@linkplain #asVarargsCollector described elsewhere}.
765 * In every other case, all conversions are applied <em>pairwise</em>,
766 * which means that each argument or return value is converted to
767 * exactly one argument or return value (or no return value).
768 * The applied conversions are defined by consulting
769 * the corresponding component types of the old and new
770 * method handle types.
771 * <p>
772 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
773 * or old and new return types. Specifically, for some valid index {@code i}, let
774 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
775 * Or else, going the other way for return values, let
776 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
777 * If the types are the same, the new method handle makes no change
778 * to the corresponding argument or return value (if any).
779 * Otherwise, one of the following conversions is applied
780 * if possible:
781 * <ul>
782 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
783 * (The types do not need to be related in any particular way.
784 * This is because a dynamic value of null can convert to any reference type.)
785 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
786 * conversion (JLS 5.3) is applied, if one exists.
787 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
788 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
789 * a Java casting conversion (JLS 5.5) is applied if one exists.
790 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
791 * which is then widened as needed to <em>T1</em>.)
792 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
793 * conversion will be applied at runtime, possibly followed
794 * by a Java method invocation conversion (JLS 5.3)
795 * on the primitive value. (These are the primitive widening conversions.)
796 * <em>T0</em> must be a wrapper class or a supertype of one.
797 * (In the case where <em>T0</em> is Object, these are the conversions
798 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
799 * The unboxing conversion must have a possibility of success, which means that
800 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
801 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
802 * primitive value can be widened to <em>T1</em>.
803 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
804 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
805 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
806 * a zero value is introduced.
807 * </ul>
808 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
809 * because neither corresponds specifically to the <em>dynamic type</em> of any
810 * actual argument or return value.)
811 * <p>
812 * The method handle conversion cannot be made if any one of the required
813 * pairwise conversions cannot be made.
814 * <p>
815 * At runtime, the conversions applied to reference arguments
816 * or return values may require additional runtime checks which can fail.
817 * An unboxing operation may fail because the original reference is null,
818 * causing a {@link java.lang.NullPointerException NullPointerException}.
819 * An unboxing operation or a reference cast may also fail on a reference
820 * to an object of the wrong type,
821 * causing a {@link java.lang.ClassCastException ClassCastException}.
822 * Although an unboxing operation may accept several kinds of wrappers,
823 * if none are available, a {@code ClassCastException} will be thrown.
824 *
825 * @param newType the expected type of the new method handle
826 * @return a method handle which delegates to {@code this} after performing
827 * any necessary argument conversions, and arranges for any
828 * necessary return value conversions
829 * @throws NullPointerException if {@code newType} is a null reference
830 * @throws WrongMethodTypeException if the conversion cannot be made
831 * @see MethodHandles#explicitCastArguments
832 */
833 public MethodHandle asType(MethodType newType) {
834 // Fast path alternative to a heavyweight {@code asType} call.
835 // Return 'this' if the conversion will be a no-op.
836 if (newType == type) {
837 return this;
838 }
839 // Return 'this.asTypeCache' if the conversion is already memoized.
840 MethodHandle atc = asTypeCached(newType);
841 if (atc != null) {
842 return atc;
843 }
844 return asTypeUncached(newType);
845 }
846
847 private MethodHandle asTypeCached(MethodType newType) {
848 MethodHandle atc = asTypeCache;
849 if (atc != null && newType == atc.type) {
850 return atc;
851 }
852 return null;
853 }
854
855 /** Override this to change asType behavior. */
856 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
857 if (!type.isConvertibleTo(newType))
858 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
859 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
860 }
861
862 /**
863 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
864 * and spreads its elements as positional arguments.
865 * The new method handle adapts, as its <i>target</i>,
866 * the current method handle. The type of the adapter will be
867 * the same as the type of the target, except that the final
868 * {@code arrayLength} parameters of the target's type are replaced
869 * by a single array parameter of type {@code arrayType}.
870 * <p>
871 * If the array element type differs from any of the corresponding
872 * argument types on the original target,
873 * the original target is adapted to take the array elements directly,
874 * as if by a call to {@link #asType asType}.
875 * <p>
876 * When called, the adapter replaces a trailing array argument
877 * by the array's elements, each as its own argument to the target.
878 * (The order of the arguments is preserved.)
879 * They are converted pairwise by casting and/or unboxing
880 * to the types of the trailing parameters of the target.
881 * Finally the target is called.
882 * What the target eventually returns is returned unchanged by the adapter.
883 * <p>
884 * Before calling the target, the adapter verifies that the array
885 * contains exactly enough elements to provide a correct argument count
886 * to the target method handle.
887 * (The array may also be null when zero elements are required.)
888 * <p>
889 * When the adapter is called, the length of the supplied {@code array}
890 * argument is queried as if by {@code array.length} or {@code arraylength}
891 * bytecode. If the adapter accepts a zero-length trailing array argument,
892 * the supplied {@code array} argument can either be a zero-length array or
893 * {@code null}; otherwise, the adapter will throw a {@code NullPointerException}
894 * if the array is {@code null} and throw an {@link IllegalArgumentException}
895 * if the array does not have the correct number of elements.
896 * <p>
897 * Here are some simple examples of array-spreading method handles:
898 * <blockquote><pre>{@code
899 MethodHandle equals = publicLookup()
900 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
901 assert( (boolean) equals.invokeExact("me", (Object)"me"));
902 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
903 // spread both arguments from a 2-array:
904 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
905 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
906 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
907 // try to spread from anything but a 2-array:
908 for (int n = 0; n <= 10; n++) {
909 Object[] badArityArgs = (n == 2 ? new Object[0] : new Object[n]);
910 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
911 catch (IllegalArgumentException ex) { } // OK
912 }
913 // spread both arguments from a String array:
914 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
915 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
916 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
917 // spread second arguments from a 1-array:
918 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
919 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
920 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
921 // spread no arguments from a 0-array or null:
922 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
923 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
924 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
925 // asSpreader and asCollector are approximate inverses:
926 for (int n = 0; n <= 2; n++) {
927 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
928 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
929 assert( (boolean) equals2.invokeWithArguments("me", "me"));
930 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
931 }
932 }
933 MethodHandle caToString = publicLookup()
934 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
935 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
936 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
937 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
938 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
939 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
940 * }</pre></blockquote>
941 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
942 * @param arrayLength the number of arguments to spread from an incoming array argument
943 * @return a new method handle which spreads its final array argument,
944 * before calling the original method handle
945 * @throws NullPointerException if {@code arrayType} is a null reference
946 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
947 * or if target does not have at least
948 * {@code arrayLength} parameter types,
949 * or if {@code arrayLength} is negative,
950 * or if the resulting method handle's type would have
951 * <a href="MethodHandle.html#maxarity">too many parameters</a>
952 * @throws WrongMethodTypeException if the implied {@code asType} call fails
953 * @see #asCollector
954 */
955 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
956 return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength);
957 }
958
959 /**
960 * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads
961 * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>,
962 * the current method handle. The type of the adapter will be the same as the type of the target, except that the
963 * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos},
964 * are replaced by a single array parameter of type {@code arrayType}.
965 * <p>
966 * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos}
967 * argument to indicate at which position in the parameter list the spreading should take place.
968 *
969 * @apiNote Example:
970 * <blockquote><pre>{@code
971 MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class));
972 MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2);
973 Object[] ints = new Object[]{3, 9, 7, 7};
974 Comparator<Integer> cmp = (a, b) -> a - b;
975 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0);
976 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0);
977 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0);
978 * }</pre></blockquote>
979 * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start.
980 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
981 * @param arrayLength the number of arguments to spread from an incoming array argument
982 * @return a new method handle which spreads an array argument at a given position,
983 * before calling the original method handle
984 * @throws NullPointerException if {@code arrayType} is a null reference
985 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
986 * or if target does not have at least
987 * {@code arrayLength} parameter types,
988 * or if {@code arrayLength} is negative,
989 * or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the
990 * number of arguments),
991 * or if the resulting method handle's type would have
992 * <a href="MethodHandle.html#maxarity">too many parameters</a>
993 * @throws WrongMethodTypeException if the implied {@code asType} call fails
994 *
995 * @see #asSpreader(Class, int)
996 * @since 9
997 */
998 public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) {
999 MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength);
1000 MethodHandle afterSpread = this.asType(postSpreadType);
1001 BoundMethodHandle mh = afterSpread.rebind();
1002 LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
1003 MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType);
1004 return mh.copyWith(preSpreadType, lform);
1005 }
1006
1007 /**
1008 * See if {@code asSpreader} can be validly called with the given arguments.
1009 * Return the type of the method handle call after spreading but before conversions.
1010 */
1011 private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) {
1012 spreadArrayChecks(arrayType, arrayLength);
1013 int nargs = type().parameterCount();
1014 if (nargs < arrayLength || arrayLength < 0)
1015 throw newIllegalArgumentException("bad spread array length");
1016 if (pos < 0 || pos + arrayLength > nargs) {
1017 throw newIllegalArgumentException("bad spread position");
1018 }
1019 Class<?> arrayElement = arrayType.getComponentType();
1020 MethodType mtype = type();
1021 boolean match = true, fail = false;
1022 for (int i = pos; i < pos + arrayLength; i++) {
1023 Class<?> ptype = mtype.parameterType(i);
1024 if (ptype != arrayElement) {
1025 match = false;
1026 if (!MethodType.canConvert(arrayElement, ptype)) {
1027 fail = true;
1028 break;
1029 }
1030 }
1031 }
1032 if (match) return mtype;
1033 MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength);
1034 if (!fail) return needType;
1035 // elicit an error:
1036 this.asType(needType);
1037 throw newInternalError("should not return");
1038 }
1039
1040 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
1041 Class<?> arrayElement = arrayType.getComponentType();
1042 if (arrayElement == null)
1043 throw newIllegalArgumentException("not an array type", arrayType);
1044 if ((arrayLength & 0x7F) != arrayLength) {
1045 if ((arrayLength & 0xFF) != arrayLength)
1046 throw newIllegalArgumentException("array length is not legal", arrayLength);
1047 assert(arrayLength >= 128);
1048 if (arrayElement == long.class ||
1049 arrayElement == double.class)
1050 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
1051 }
1052 }
1053 /**
1054 * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity}
1055 * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}.
1056 * If the method handle is already of the proper arity mode, it is returned
1057 * unchanged.
1058 * @apiNote
1059 * <p>This method is sometimes useful when adapting a method handle that
1060 * may be variable arity, to ensure that the resulting adapter is also
1061 * variable arity if and only if the original handle was. For example,
1062 * this code changes the first argument of a handle {@code mh} to {@code int} without
1063 * disturbing its variable arity property:
1064 * {@code mh.asType(mh.type().changeParameterType(0,int.class))
1065 * .withVarargs(mh.isVarargsCollector())}
1066 * <p>
1067 * This call is approximately equivalent to the following code:
1068 * <blockquote><pre>{@code
1069 * if (makeVarargs == isVarargsCollector())
1070 * return this;
1071 * else if (makeVarargs)
1072 * return asVarargsCollector(type().lastParameterType());
1073 * else
1074 * return return asFixedArity();
1075 * }</pre></blockquote>
1076 * @param makeVarargs true if the return method handle should have variable arity behavior
1077 * @return a method handle of the same type, with possibly adjusted variable arity behavior
1078 * @throws IllegalArgumentException if {@code makeVarargs} is true and
1079 * this method handle does not have a trailing array parameter
1080 * @since 9
1081 * @see #asVarargsCollector
1082 * @see #asFixedArity
1083 */
1084 public MethodHandle withVarargs(boolean makeVarargs) {
1085 assert(!isVarargsCollector()); // subclass responsibility
1086 if (makeVarargs) {
1087 return asVarargsCollector(type().lastParameterType());
1088 } else {
1089 return this;
1090 }
1091 }
1092
1093 /**
1094 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
1095 * positional arguments and collects them into an array argument.
1096 * The new method handle adapts, as its <i>target</i>,
1097 * the current method handle. The type of the adapter will be
1098 * the same as the type of the target, except that a single trailing
1099 * parameter (usually of type {@code arrayType}) is replaced by
1100 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1101 * <p>
1102 * If the array type differs from the final argument type on the original target,
1103 * the original target is adapted to take the array type directly,
1104 * as if by a call to {@link #asType asType}.
1105 * <p>
1106 * When called, the adapter replaces its trailing {@code arrayLength}
1107 * arguments by a single new array of type {@code arrayType}, whose elements
1108 * comprise (in order) the replaced arguments.
1109 * Finally the target is called.
1110 * What the target eventually returns is returned unchanged by the adapter.
1111 * <p>
1112 * (The array may also be a shared constant when {@code arrayLength} is zero.)
1113 * <p>
1114 * (<em>Note:</em> The {@code arrayType} is often identical to the
1115 * {@linkplain MethodType#lastParameterType last parameter type}
1116 * of the original target.
1117 * It is an explicit argument for symmetry with {@code asSpreader}, and also
1118 * to allow the target to use a simple {@code Object} as its last parameter type.)
1119 * <p>
1120 * In order to create a collecting adapter which is not restricted to a particular
1121 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector}
1122 * or {@link #withVarargs withVarargs} instead.
1123 * <p>
1124 * Here are some examples of array-collecting method handles:
1125 * <blockquote><pre>{@code
1126 MethodHandle deepToString = publicLookup()
1127 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1128 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
1129 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
1130 assertEquals(methodType(String.class, Object.class), ts1.type());
1131 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
1132 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
1133 // arrayType can be a subtype of Object[]
1134 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
1135 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
1136 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
1137 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
1138 assertEquals("[]", (String) ts0.invokeExact());
1139 // collectors can be nested, Lisp-style
1140 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
1141 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
1142 // arrayType can be any primitive array type
1143 MethodHandle bytesToString = publicLookup()
1144 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
1145 .asCollector(byte[].class, 3);
1146 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
1147 MethodHandle longsToString = publicLookup()
1148 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
1149 .asCollector(long[].class, 1);
1150 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
1151 * }</pre></blockquote>
1152 * <p>
1153 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1154 * variable-arity method handle}, even if the original target method handle was.
1155 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1156 * @param arrayLength the number of arguments to collect into a new array argument
1157 * @return a new method handle which collects some trailing argument
1158 * into an array, before calling the original method handle
1159 * @throws NullPointerException if {@code arrayType} is a null reference
1160 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1161 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
1162 * or {@code arrayLength} is not a legal array size,
1163 * or the resulting method handle's type would have
1164 * <a href="MethodHandle.html#maxarity">too many parameters</a>
1165 * @throws WrongMethodTypeException if the implied {@code asType} call fails
1166 * @see #asSpreader
1167 * @see #asVarargsCollector
1168 */
1169 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1170 return asCollector(type().parameterCount() - 1, arrayType, arrayLength);
1171 }
1172
1173 /**
1174 * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting
1175 * at a given position, and collects them into an array argument. The new method handle adapts, as its
1176 * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target,
1177 * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType})
1178 * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1179 * <p>
1180 * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code
1181 * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This
1182 * index is zero-based.
1183 *
1184 * @apiNote Examples:
1185 * <blockquote><pre>{@code
1186 StringWriter swr = new StringWriter();
1187 MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr);
1188 MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4);
1189 swWrite4.invoke('A', 'B', 'C', 'D', 1, 2);
1190 assertEquals("BC", swr.toString());
1191 swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4);
1192 assertEquals("BCPQRS", swr.toString());
1193 swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1);
1194 assertEquals("BCPQRSZ", swr.toString());
1195 * }</pre></blockquote>
1196 * <p>
1197 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1198 * variable-arity method handle}, even if the original target method handle was.
1199 * @param collectArgPos the zero-based position in the parameter list at which to start collecting.
1200 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1201 * @param arrayLength the number of arguments to collect into a new array argument
1202 * @return a new method handle which collects some arguments
1203 * into an array, before calling the original method handle
1204 * @throws NullPointerException if {@code arrayType} is a null reference
1205 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1206 * or {@code arrayType} is not assignable to this method handle's array parameter type,
1207 * or {@code arrayLength} is not a legal array size,
1208 * or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments),
1209 * or the resulting method handle's type would have
1210 * <a href="MethodHandle.html#maxarity">too many parameters</a>
1211 * @throws WrongMethodTypeException if the implied {@code asType} call fails
1212 *
1213 * @see #asCollector(Class, int)
1214 * @since 9
1215 */
1216 public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) {
1217 asCollectorChecks(arrayType, collectArgPos, arrayLength);
1218 BoundMethodHandle mh = rebind();
1219 MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength);
1220 MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
1221 LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
1222 if (lform != null) {
1223 return mh.copyWith(resultType, lform);
1224 }
1225 lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
1226 return mh.copyWithExtendL(resultType, lform, newArray);
1227 }
1228
1229 /**
1230 * See if {@code asCollector} can be validly called with the given arguments.
1231 * Return false if the last parameter is not an exact match to arrayType.
1232 */
1233 /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) {
1234 spreadArrayChecks(arrayType, arrayLength);
1235 int nargs = type().parameterCount();
1236 if (pos < 0 || pos >= nargs) {
1237 throw newIllegalArgumentException("bad collect position");
1238 }
1239 if (nargs != 0) {
1240 Class<?> param = type().parameterType(pos);
1241 if (param == arrayType) return true;
1242 if (param.isAssignableFrom(arrayType)) return false;
1243 }
1244 throw newIllegalArgumentException("array type not assignable to argument", this, arrayType);
1245 }
1246
1247 /**
1248 * Makes a <em>variable arity</em> adapter which is able to accept
1249 * any number of trailing positional arguments and collect them
1250 * into an array argument.
1251 * <p>
1252 * The type and behavior of the adapter will be the same as
1253 * the type and behavior of the target, except that certain
1254 * {@code invoke} and {@code asType} requests can lead to
1255 * trailing positional arguments being collected into target's
1256 * trailing parameter.
1257 * Also, the
1258 * {@linkplain MethodType#lastParameterType last parameter type}
1259 * of the adapter will be
1260 * {@code arrayType}, even if the target has a different
1261 * last parameter type.
1262 * <p>
1263 * This transformation may return {@code this} if the method handle is
1264 * already of variable arity and its trailing parameter type
1265 * is identical to {@code arrayType}.
1266 * <p>
1267 * When called with {@link #invokeExact invokeExact}, the adapter invokes
1268 * the target with no argument changes.
1269 * (<em>Note:</em> This behavior is different from a
1270 * {@linkplain #asCollector fixed arity collector},
1271 * since it accepts a whole array of indeterminate length,
1272 * rather than a fixed number of arguments.)
1273 * <p>
1274 * When called with plain, inexact {@link #invoke invoke}, if the caller
1275 * type is the same as the adapter, the adapter invokes the target as with
1276 * {@code invokeExact}.
1277 * (This is the normal behavior for {@code invoke} when types match.)
1278 * <p>
1279 * Otherwise, if the caller and adapter arity are the same, and the
1280 * trailing parameter type of the caller is a reference type identical to
1281 * or assignable to the trailing parameter type of the adapter,
1282 * the arguments and return values are converted pairwise,
1283 * as if by {@link #asType asType} on a fixed arity
1284 * method handle.
1285 * <p>
1286 * Otherwise, the arities differ, or the adapter's trailing parameter
1287 * type is not assignable from the corresponding caller type.
1288 * In this case, the adapter replaces all trailing arguments from
1289 * the original trailing argument position onward, by
1290 * a new array of type {@code arrayType}, whose elements
1291 * comprise (in order) the replaced arguments.
1292 * <p>
1293 * The caller type must provides as least enough arguments,
1294 * and of the correct type, to satisfy the target's requirement for
1295 * positional arguments before the trailing array argument.
1296 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1297 * where {@code N} is the arity of the target.
1298 * Also, there must exist conversions from the incoming arguments
1299 * to the target's arguments.
1300 * As with other uses of plain {@code invoke}, if these basic
1301 * requirements are not fulfilled, a {@code WrongMethodTypeException}
1302 * may be thrown.
1303 * <p>
1304 * In all cases, what the target eventually returns is returned unchanged by the adapter.
1305 * <p>
1306 * In the final case, it is exactly as if the target method handle were
1307 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1308 * to the arity required by the caller type.
1309 * (As with {@code asCollector}, if the array length is zero,
1310 * a shared constant may be used instead of a new array.
1311 * If the implied call to {@code asCollector} would throw
1312 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1313 * the call to the variable arity adapter must throw
1314 * {@code WrongMethodTypeException}.)
1315 * <p>
1316 * The behavior of {@link #asType asType} is also specialized for
1317 * variable arity adapters, to maintain the invariant that
1318 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1319 * call to adjust the target type, followed by {@code invokeExact}.
1320 * Therefore, a variable arity adapter responds
1321 * to an {@code asType} request by building a fixed arity collector,
1322 * if and only if the adapter and requested type differ either
1323 * in arity or trailing argument type.
1324 * The resulting fixed arity collector has its type further adjusted
1325 * (if necessary) to the requested type by pairwise conversion,
1326 * as if by another application of {@code asType}.
1327 * <p>
1328 * When a method handle is obtained by executing an {@code ldc} instruction
1329 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1330 * as a variable arity method (with the modifier bit {@code 0x0080}),
1331 * the method handle will accept multiple arities, as if the method handle
1332 * constant were created by means of a call to {@code asVarargsCollector}.
1333 * <p>
1334 * In order to create a collecting adapter which collects a predetermined
1335 * number of arguments, and whose type reflects this predetermined number,
1336 * use {@link #asCollector asCollector} instead.
1337 * <p>
1338 * No method handle transformations produce new method handles with
1339 * variable arity, unless they are documented as doing so.
1340 * Therefore, besides {@code asVarargsCollector} and {@code withVarargs},
1341 * all methods in {@code MethodHandle} and {@code MethodHandles}
1342 * will return a method handle with fixed arity,
1343 * except in the cases where they are specified to return their original
1344 * operand (e.g., {@code asType} of the method handle's own type).
1345 * <p>
1346 * Calling {@code asVarargsCollector} on a method handle which is already
1347 * of variable arity will produce a method handle with the same type and behavior.
1348 * It may (or may not) return the original variable arity method handle.
1349 * <p>
1350 * Here is an example, of a list-making variable arity method handle:
1351 * <blockquote><pre>{@code
1352 MethodHandle deepToString = publicLookup()
1353 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1354 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1355 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1356 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1357 assertEquals("[won]", (String) ts1.invoke( "won" ));
1358 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1359 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1360 MethodHandle asList = publicLookup()
1361 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1362 assertEquals(methodType(List.class, Object[].class), asList.type());
1363 assert(asList.isVarargsCollector());
1364 assertEquals("[]", asList.invoke().toString());
1365 assertEquals("[1]", asList.invoke(1).toString());
1366 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1367 String[] argv = { "three", "thee", "tee" };
1368 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1369 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1370 List ls = (List) asList.invoke((Object)argv);
1371 assertEquals(1, ls.size());
1372 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1373 * }</pre></blockquote>
1374 * <p style="font-size:smaller;">
1375 * <em>Discussion:</em>
1376 * These rules are designed as a dynamically-typed variation
1377 * of the Java rules for variable arity methods.
1378 * In both cases, callers to a variable arity method or method handle
1379 * can either pass zero or more positional arguments, or else pass
1380 * pre-collected arrays of any length. Users should be aware of the
1381 * special role of the final argument, and of the effect of a
1382 * type match on that final argument, which determines whether
1383 * or not a single trailing argument is interpreted as a whole
1384 * array or a single element of an array to be collected.
1385 * Note that the dynamic type of the trailing argument has no
1386 * effect on this decision, only a comparison between the symbolic
1387 * type descriptor of the call site and the type descriptor of the method handle.)
1388 *
1389 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1390 * @return a new method handle which can collect any number of trailing arguments
1391 * into an array, before calling the original method handle
1392 * @throws NullPointerException if {@code arrayType} is a null reference
1393 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1394 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1395 * @see #asCollector
1396 * @see #isVarargsCollector
1397 * @see #withVarargs
1398 * @see #asFixedArity
1399 */
1400 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1401 Objects.requireNonNull(arrayType);
1402 boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0);
1403 if (isVarargsCollector() && lastMatch)
1404 return this;
1405 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1406 }
1407
1408 /**
1409 * Determines if this method handle
1410 * supports {@linkplain #asVarargsCollector variable arity} calls.
1411 * Such method handles arise from the following sources:
1412 * <ul>
1413 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1414 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1415 * which resolves to a variable arity Java method or constructor
1416 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1417 * which resolves to a variable arity Java method or constructor
1418 * </ul>
1419 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1420 * @see #asVarargsCollector
1421 * @see #asFixedArity
1422 */
1423 public boolean isVarargsCollector() {
1424 return false;
1425 }
1426
1427 /**
1428 * Makes a <em>fixed arity</em> method handle which is otherwise
1429 * equivalent to the current method handle.
1430 * <p>
1431 * If the current method handle is not of
1432 * {@linkplain #asVarargsCollector variable arity},
1433 * the current method handle is returned.
1434 * This is true even if the current method handle
1435 * could not be a valid input to {@code asVarargsCollector}.
1436 * <p>
1437 * Otherwise, the resulting fixed-arity method handle has the same
1438 * type and behavior of the current method handle,
1439 * except that {@link #isVarargsCollector isVarargsCollector}
1440 * will be false.
1441 * The fixed-arity method handle may (or may not) be the
1442 * a previous argument to {@code asVarargsCollector}.
1443 * <p>
1444 * Here is an example, of a list-making variable arity method handle:
1445 * <blockquote><pre>{@code
1446 MethodHandle asListVar = publicLookup()
1447 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1448 .asVarargsCollector(Object[].class);
1449 MethodHandle asListFix = asListVar.asFixedArity();
1450 assertEquals("[1]", asListVar.invoke(1).toString());
1451 Exception caught = null;
1452 try { asListFix.invoke((Object)1); }
1453 catch (Exception ex) { caught = ex; }
1454 assert(caught instanceof ClassCastException);
1455 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1456 try { asListFix.invoke("two", "too"); }
1457 catch (Exception ex) { caught = ex; }
1458 assert(caught instanceof WrongMethodTypeException);
1459 Object[] argv = { "three", "thee", "tee" };
1460 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1461 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1462 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1463 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1464 * }</pre></blockquote>
1465 *
1466 * @return a new method handle which accepts only a fixed number of arguments
1467 * @see #asVarargsCollector
1468 * @see #isVarargsCollector
1469 * @see #withVarargs
1470 */
1471 public MethodHandle asFixedArity() {
1472 assert(!isVarargsCollector());
1473 return this;
1474 }
1475
1476 /**
1477 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1478 * The new method handle adapts, as its <i>target</i>,
1479 * the current method handle by binding it to the given argument.
1480 * The type of the bound handle will be
1481 * the same as the type of the target, except that a single leading
1482 * reference parameter will be omitted.
1483 * <p>
1484 * When called, the bound handle inserts the given value {@code x}
1485 * as a new leading argument to the target. The other arguments are
1486 * also passed unchanged.
1487 * What the target eventually returns is returned unchanged by the bound handle.
1488 * <p>
1489 * The reference {@code x} must be convertible to the first parameter
1490 * type of the target.
1491 * <p>
1492 * <em>Note:</em> Because method handles are immutable, the target method handle
1493 * retains its original type and behavior.
1494 * <p>
1495 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1496 * variable-arity method handle}, even if the original target method handle was.
1497 * @param x the value to bind to the first argument of the target
1498 * @return a new method handle which prepends the given value to the incoming
1499 * argument list, before calling the original method handle
1500 * @throws IllegalArgumentException if the target does not have a
1501 * leading parameter type that is a reference type
1502 * @throws ClassCastException if {@code x} cannot be converted
1503 * to the leading parameter type of the target
1504 * @see MethodHandles#insertArguments
1505 */
1506 public MethodHandle bindTo(Object x) {
1507 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed
1508 return bindArgumentL(0, x);
1509 }
1510
1511 /**
1512 * Returns a string representation of the method handle,
1513 * starting with the string {@code "MethodHandle"} and
1514 * ending with the string representation of the method handle's type.
1515 * In other words, this method returns a string equal to the value of:
1516 * <blockquote><pre>{@code
1517 * "MethodHandle" + type().toString()
1518 * }</pre></blockquote>
1519 * <p>
1520 * (<em>Note:</em> Future releases of this API may add further information
1521 * to the string representation.
1522 * Therefore, the present syntax should not be parsed by applications.)
1523 *
1524 * @return a string representation of the method handle
1525 */
1526 @Override
1527 public String toString() {
1528 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString();
1529 return standardString();
1530 }
1531 String standardString() {
1532 return "MethodHandle"+type;
1533 }
1534 /** Return a string with a several lines describing the method handle structure.
1535 * This string would be suitable for display in an IDE debugger.
1536 */
1537 String debugString() {
1538 return type+" : "+internalForm()+internalProperties();
1539 }
1540
1541 //// Implementation methods.
1542 //// Sub-classes can override these default implementations.
1543 //// All these methods assume arguments are already validated.
1544
1545 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
1546
1547 BoundMethodHandle bindArgumentL(int pos, Object value) {
1548 return rebind().bindArgumentL(pos, value);
1549 }
1550
1551 /*non-public*/
1552 MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1553 if (!member.isVarargs()) return this;
1554 try {
1555 return this.withVarargs(true);
1556 } catch (IllegalArgumentException ex) {
1557 throw member.makeAccessException("cannot make variable arity", null);
1558 }
1559 }
1560
1561 /*non-public*/
1562 MethodHandle viewAsType(MethodType newType, boolean strict) {
1563 // No actual conversions, just a new view of the same method.
1564 // Note that this operation must not produce a DirectMethodHandle,
1565 // because retyped DMHs, like any transformed MHs,
1566 // cannot be cracked into MethodHandleInfo.
1567 assert viewAsTypeChecks(newType, strict);
1568 BoundMethodHandle mh = rebind();
1569 return mh.copyWith(newType, mh.form);
1570 }
1571
1572 /*non-public*/
1573 boolean viewAsTypeChecks(MethodType newType, boolean strict) {
1574 if (strict) {
1575 assert(type().isViewableAs(newType, true))
1576 : Arrays.asList(this, newType);
1577 } else {
1578 assert(type().basicType().isViewableAs(newType.basicType(), true))
1579 : Arrays.asList(this, newType);
1580 }
1581 return true;
1582 }
1583
1584 // Decoding
1585
1586 /*non-public*/
1587 LambdaForm internalForm() {
1588 return form;
1589 }
1590
1591 /*non-public*/
1592 MemberName internalMemberName() {
1593 return null; // DMH returns DMH.member
1594 }
1595
1596 /*non-public*/
1597 Class<?> internalCallerClass() {
1598 return null; // caller-bound MH for @CallerSensitive method returns caller
1599 }
1600
1601 /*non-public*/
1602 MethodHandleImpl.Intrinsic intrinsicName() {
1603 // no special intrinsic meaning to most MHs
1604 return MethodHandleImpl.Intrinsic.NONE;
1605 }
1606
1607 /*non-public*/
1608 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
1609 if (member != null) {
1610 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
1611 } else if (internalMemberName() == null) {
1612 // The required internaMemberName is null, and this MH (like most) doesn't have one.
1613 return this;
1614 } else {
1615 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1616 MethodHandle result = rebind();
1617 assert (result.internalMemberName() == null);
1618 return result;
1619 }
1620 }
1621
1622 /*non-public*/
1623 boolean isInvokeSpecial() {
1624 return false; // DMH.Special returns true
1625 }
1626
1627 /*non-public*/
1628 Object internalValues() {
1629 return null;
1630 }
1631
1632 /*non-public*/
1633 Object internalProperties() {
1634 // Override to something to follow this.form, like "\n& FOO=bar"
1635 return "";
1636 }
1637
1638 //// Method handle implementation methods.
1639 //// Sub-classes can override these default implementations.
1640 //// All these methods assume arguments are already validated.
1641
1642 /*non-public*/
1643 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1644
1645 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
1646 * Many transforms are implemented only for BMHs.
1647 * @return a behaviorally equivalent BMH
1648 */
1649 abstract BoundMethodHandle rebind();
1650
1651 /**
1652 * Replace the old lambda form of this method handle with a new one.
1653 * The new one must be functionally equivalent to the old one.
1654 * Threads may continue running the old form indefinitely,
1655 * but it is likely that the new one will be preferred for new executions.
1656 * Use with discretion.
1657 */
1658 /*non-public*/
1659 void updateForm(LambdaForm newForm) {
1660 assert(newForm.customized == null || newForm.customized == this);
1661 if (form == newForm) return;
1662 newForm.prepare(); // as in MethodHandle.<init>
1663 UNSAFE.putObjectVolatile(this, FORM_OFFSET, newForm);
1664 }
1665
1666 /** Craft a LambdaForm customized for this particular MethodHandle */
1667 /*non-public*/
1668 void customize() {
1669 final LambdaForm form = this.form;
1670 if (form.customized == null) {
1671 LambdaForm newForm = form.customize(this);
1672 updateForm(newForm);
1673 } else {
1674 assert(form.customized == this);
1675 }
1676 }
1677
1678 private static final long FORM_OFFSET
1679 = UNSAFE.objectFieldOffset(MethodHandle.class, "form");
1680 }