- java.lang.Object
-
- java.lang.invoke.MethodHandles
-
public class MethodHandles extends Object
This class consists exclusively of static methods that operate on or return method handles. They fall into several categories:- Lookup methods which help create method handles for methods and fields.
- Combinator methods, which combine or transform pre-existing method handles into new ones.
- Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
IllegalArgumentException
if the created method handle's type would have too many parameters.- Since:
- 1.7
-
-
Nested Class Summary
Nested Classes Modifier and Type Class Description static class
MethodHandles.Lookup
A lookup object is a factory for creating method handles, when the creation requires access checking.
-
Method Summary
All Methods Static Methods Concrete Methods Modifier and Type Method Description static MethodHandle
arrayConstructor(Class<?> arrayClass)
Produces a method handle constructing arrays of a desired type, as if by theanewarray
bytecode.static MethodHandle
arrayElementGetter(Class<?> arrayClass)
Produces a method handle giving read access to elements of an array, as if by theaaload
bytecode.static MethodHandle
arrayElementSetter(Class<?> arrayClass)
Produces a method handle giving write access to elements of an array, as if by theastore
bytecode.static VarHandle
arrayElementVarHandle(Class<?> arrayClass)
Produces a VarHandle giving access to elements of an array of typearrayClass
.static MethodHandle
arrayLength(Class<?> arrayClass)
Produces a method handle returning the length of an array, as if by thearraylength
bytecode.static VarHandle
byteArrayViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder)
Produces a VarHandle giving access to elements of abyte[]
array viewed as if it were a different primitive array type, such asint[]
orlong[]
.static VarHandle
byteBufferViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder)
Produces a VarHandle giving access to elements of aByteBuffer
viewed as if it were an array of elements of a different primitive component type to that ofbyte
, such asint[]
orlong[]
.static MethodHandle
catchException(MethodHandle target, Class<? extends Throwable> exType, MethodHandle handler)
Makes a method handle which adapts a target method handle, by running it inside an exception handler.static MethodHandle
collectArguments(MethodHandle target, int pos, MethodHandle filter)
Adapts a target method handle by pre-processing a sub-sequence of its arguments with a filter (another method handle).static MethodHandle
constant(Class<?> type, Object value)
Produces a method handle of the requested return type which returns the given constant value every time it is invoked.static MethodHandle
countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body)
Constructs a loop that runs a given number of iterations.static MethodHandle
countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body)
Constructs a loop that counts over a range of numbers.static MethodHandle
doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred)
Constructs ado-while
loop from an initializer, a body, and a predicate.static MethodHandle
dropArguments(MethodHandle target, int pos, Class<?>... valueTypes)
Produces a method handle which will discard some dummy arguments before calling some other specified target method handle.static MethodHandle
dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes)
Produces a method handle which will discard some dummy arguments before calling some other specified target method handle.static MethodHandle
dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos)
Adapts a target method handle to match the given parameter type list.static MethodHandle
empty(MethodType type)
Produces a method handle of the requested type which ignores any arguments, does nothing, and returns a suitable default depending on the return type.static MethodHandle
exactInvoker(MethodType type)
Produces a special invoker method handle which can be used to invoke any method handle of the given type, as if byinvokeExact
.static MethodHandle
explicitCastArguments(MethodHandle target, MethodType newType)
Produces a method handle which adapts the type of the given method handle to a new type by pairwise argument and return type conversion.static MethodHandle
filterArguments(MethodHandle target, int pos, MethodHandle... filters)
Adapts a target method handle by pre-processing one or more of its arguments, each with its own unary filter function, and then calling the target with each pre-processed argument replaced by the result of its corresponding filter function.static MethodHandle
filterReturnValue(MethodHandle target, MethodHandle filter)
Adapts a target method handle by post-processing its return value (if any) with a filter (another method handle).static MethodHandle
foldArguments(MethodHandle target, int pos, MethodHandle combiner)
Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments just before the folded arguments.static MethodHandle
foldArguments(MethodHandle target, MethodHandle combiner)
Adapts a target method handle by pre-processing some of its arguments, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments.static MethodHandle
guardWithTest(MethodHandle test, MethodHandle target, MethodHandle fallback)
Makes a method handle which adapts a target method handle, by guarding it with a test, a boolean-valued method handle.static MethodHandle
identity(Class<?> type)
Produces a method handle which returns its sole argument when invoked.static MethodHandle
insertArguments(MethodHandle target, int pos, Object... values)
Provides a target method handle with one or more bound arguments in advance of the method handle's invocation.static MethodHandle
invoker(MethodType type)
Produces a special invoker method handle which can be used to invoke any method handle compatible with the given type, as if byinvoke
.static MethodHandle
iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body)
Constructs a loop that ranges over the values produced by anIterator<T>
.static MethodHandles.Lookup
lookup()
Returns alookup object
with full capabilities to emulate all supported bytecode behaviors of the caller.static MethodHandle
loop(MethodHandle[]... clauses)
Constructs a method handle representing a loop with several loop variables that are updated and checked upon each iteration.static MethodHandle
permuteArguments(MethodHandle target, MethodType newType, int... reorder)
Produces a method handle which adapts the calling sequence of the given method handle to a new type, by reordering the arguments.static MethodHandles.Lookup
privateLookupIn(Class<?> targetClass, MethodHandles.Lookup lookup)
Returns alookup object
with full capabilities to emulate all supported bytecode behaviors, including private access, on a target class.static MethodHandles.Lookup
publicLookup()
Returns alookup object
which is trusted minimally.static <T extends Member>
TreflectAs(Class<T> expected, MethodHandle target)
Performs an unchecked "crack" of a direct method handle.static MethodHandle
spreadInvoker(MethodType type, int leadingArgCount)
Produces a method handle which will invoke any method handle of the giventype
, with a given number of trailing arguments replaced by a single trailingObject[]
array.static MethodHandle
throwException(Class<?> returnType, Class<? extends Throwable> exType)
Produces a method handle which will throw exceptions of the givenexType
.static MethodHandle
tryFinally(MethodHandle target, MethodHandle cleanup)
Makes a method handle that adapts atarget
method handle by wrapping it in atry-finally
block.static MethodHandle
varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type)
Produces a special invoker method handle which can be used to invoke a signature-polymorphic access mode method on any VarHandle whose associated access mode type is compatible with the given type.static MethodHandle
varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type)
Produces a special invoker method handle which can be used to invoke a signature-polymorphic access mode method on any VarHandle whose associated access mode type is compatible with the given type.static MethodHandle
whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body)
Constructs awhile
loop from an initializer, a body, and a predicate.static MethodHandle
zero(Class<?> type)
Produces a constant method handle of the requested return type which returns the default value for that type every time it is invoked.
-
-
-
Method Detail
-
lookup
public static MethodHandles.Lookup lookup()
Returns alookup object
with full capabilities to emulate all supported bytecode behaviors of the caller. These capabilities include private access to the caller. Factory methods on the lookup object can create direct method handles for any member that the caller has access to via bytecodes, including protected and private fields and methods. This lookup object is a capability which may be delegated to trusted agents. Do not store it in place where untrusted code can access it.This method is caller sensitive, which means that it may return different values to different callers.
- Returns:
- a lookup object for the caller of this method, with private access
-
publicLookup
public static MethodHandles.Lookup publicLookup()
Returns alookup object
which is trusted minimally. The lookup has thePUBLIC
andUNCONDITIONAL
modes. It can only be used to create method handles to public members of public classes in packages that are exported unconditionally.As a matter of pure convention, the lookup class of this lookup object will be
Object
.- API Note:
- The use of Object is conventional, and because the lookup modes are
limited, there is no special access provided to the internals of Object, its package
or its module. Consequently, the lookup context of this lookup object will be the
bootstrap class loader, which means it cannot find user classes.
Discussion: The lookup class can be changed to any other class
C
using an expression of the formpublicLookup().in(C.class)
. but may change the lookup context by virtue of changing the class loader. A public lookup object is always subject to security manager checks. Also, it cannot access caller sensitive methods. - Returns:
- a lookup object which is trusted minimally
-
privateLookupIn
public static MethodHandles.Lookup privateLookupIn(Class<?> targetClass, MethodHandles.Lookup lookup) throws IllegalAccessException
Returns alookup object
with full capabilities to emulate all supported bytecode behaviors, including private access, on a target class. This method checks that a caller, specified as aLookup
object, is allowed to do deep reflection on the target class. Ifm1
is the module containing thelookup class
, andm2
is the module containing the target class, then this check ensures thatm1
reads
m2
.m2
opens
the package containing the target class to at leastm1
.- The lookup has the
MODULE
lookup mode.
If there is a security manager, its
checkPermission
method is called to checkReflectPermission("suppressAccessChecks")
.- API Note:
- The
MODULE
lookup mode serves to authenticate that the lookup object was created by code in the caller module (or derived from a lookup object originally created by the caller). A lookup object with theMODULE
lookup mode can be shared with trusted parties without giving awayPRIVATE
andPACKAGE
access to the caller. - Parameters:
targetClass
- the target classlookup
- the caller lookup object- Returns:
- a lookup object for the target class, with private access
- Throws:
IllegalArgumentException
- iftargetClass
is a primitve type or array classNullPointerException
- iftargetClass
orcaller
isnull
IllegalAccessException
- if the access check specified above failsSecurityException
- if denied by the security manager- Since:
- 9
- See Also:
MethodHandles.Lookup.dropLookupMode(int)
-
reflectAs
public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target)
Performs an unchecked "crack" of a direct method handle. The result is as if the user had obtained a lookup object capable enough to crack the target method handle, calledLookup.revealDirect
on the target to obtain its symbolic reference, and then calledMethodHandleInfo.reflectAs
to resolve the symbolic reference to a member.If there is a security manager, its
checkPermission
method is called with aReflectPermission("suppressAccessChecks")
permission.- Type Parameters:
T
- the desired type of the result, eitherMember
or a subtype- Parameters:
target
- a direct method handle to crack into symbolic reference componentsexpected
- a class object representing the desired result typeT
- Returns:
- a reference to the method, constructor, or field object
- Throws:
SecurityException
- if the caller is not privileged to callsetAccessible
NullPointerException
- if either argument isnull
IllegalArgumentException
- if the target is not a direct method handleClassCastException
- if the member is not of the expected type- Since:
- 1.8
-
arrayConstructor
public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException
Produces a method handle constructing arrays of a desired type, as if by theanewarray
bytecode. The return type of the method handle will be the array type. The type of its sole argument will beint
, which specifies the size of the array.If the returned method handle is invoked with a negative array size, a
NegativeArraySizeException
will be thrown.- Parameters:
arrayClass
- an array type- Returns:
- a method handle which can create arrays of the given type
- Throws:
NullPointerException
- if the argument isnull
IllegalArgumentException
- ifarrayClass
is not an array type- Since:
- 9
- See Also:
Array.newInstance(Class, int)
- See The Java™ Virtual Machine Specification:
- 6.5
anewarray
Instruction
-
arrayLength
public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException
Produces a method handle returning the length of an array, as if by thearraylength
bytecode. The type of the method handle will haveint
as return type, and its sole argument will be the array type.If the returned method handle is invoked with a
null
array reference, aNullPointerException
will be thrown.- Parameters:
arrayClass
- an array type- Returns:
- a method handle which can retrieve the length of an array of the given array type
- Throws:
NullPointerException
- if the argument isnull
IllegalArgumentException
- if arrayClass is not an array type- Since:
- 9
- See The Java™ Virtual Machine Specification:
- 6.5
arraylength
Instruction
-
arrayElementGetter
public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException
Produces a method handle giving read access to elements of an array, as if by theaaload
bytecode. The type of the method handle will have a return type of the array's element type. Its first argument will be the array type, and the second will beint
.When the returned method handle is invoked, the array reference and array index are checked. A
NullPointerException
will be thrown if the array reference isnull
and anArrayIndexOutOfBoundsException
will be thrown if the index is negative or if it is greater than or equal to the length of the array.- Parameters:
arrayClass
- an array type- Returns:
- a method handle which can load values from the given array type
- Throws:
NullPointerException
- if the argument is nullIllegalArgumentException
- if arrayClass is not an array type- See The Java™ Virtual Machine Specification:
- 6.5
aaload
Instruction
-
arrayElementSetter
public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException
Produces a method handle giving write access to elements of an array, as if by theastore
bytecode. The type of the method handle will have a void return type. Its last argument will be the array's element type. The first and second arguments will be the array type and int.When the returned method handle is invoked, the array reference and array index are checked. A
NullPointerException
will be thrown if the array reference isnull
and anArrayIndexOutOfBoundsException
will be thrown if the index is negative or if it is greater than or equal to the length of the array.- Parameters:
arrayClass
- the class of an array- Returns:
- a method handle which can store values into the array type
- Throws:
NullPointerException
- if the argument is nullIllegalArgumentException
- if arrayClass is not an array type- See The Java™ Virtual Machine Specification:
- 6.5
aastore
Instruction
-
arrayElementVarHandle
public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException
Produces a VarHandle giving access to elements of an array of typearrayClass
. The VarHandle's variable type is the component type ofarrayClass
and the list of coordinate types is(arrayClass, int)
, where theint
coordinate type corresponds to an argument that is an index into an array.Certain access modes of the returned VarHandle are unsupported under the following conditions:
- if the component type is anything other than
byte
,short
,char
,int
,long
,float
, ordouble
then numeric atomic update access modes are unsupported. - if the field type is anything other than
boolean
,byte
,short
,char
,int
orlong
then bitwise atomic update access modes are unsupported.
If the component type is
float
ordouble
then numeric and atomic update access modes compare values using their bitwise representation (seeFloat.floatToRawIntBits(float)
andDouble.doubleToRawLongBits(double)
, respectively).When the returned
VarHandle
is invoked, the array reference and array index are checked. ANullPointerException
will be thrown if the array reference isnull
and anArrayIndexOutOfBoundsException
will be thrown if the index is negative or if it is greater than or equal to the length of the array.- API Note:
- Bitwise comparison of
float
values ordouble
values, as performed by the numeric and atomic update access modes, differ from the primitive==
operator and theFloat.equals(java.lang.Object)
andDouble.equals(java.lang.Object)
methods, specifically with respect to comparing NaN values or comparing-0.0
with+0.0
. Care should be taken when performing a compare and set or a compare and exchange operation with such values since the operation may unexpectedly fail. There are many possible NaN values that are considered to beNaN
in Java, although no IEEE 754 floating-point operation provided by Java can distinguish between them. Operation failure can occur if the expected or witness value is a NaN value and it is transformed (perhaps in a platform specific manner) into another NaN value, and thus has a different bitwise representation (seeFloat.intBitsToFloat(int)
orDouble.longBitsToDouble(long)
for more details). The values-0.0
and+0.0
have different bitwise representations but are considered equal when using the primitive==
operator. Operation failure can occur if, for example, a numeric algorithm computes an expected value to be say-0.0
and previously computed the witness value to be say+0.0
. - Parameters:
arrayClass
- the class of an array, of typeT[]
- Returns:
- a VarHandle giving access to elements of an array
- Throws:
NullPointerException
- if the arrayClass is nullIllegalArgumentException
- if arrayClass is not an array type- Since:
- 9
- if the component type is anything other than
-
byteArrayViewVarHandle
public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder) throws IllegalArgumentException
Produces a VarHandle giving access to elements of abyte[]
array viewed as if it were a different primitive array type, such asint[]
orlong[]
. The VarHandle's variable type is the component type ofviewArrayClass
and the list of coordinate types is(byte[], int)
, where theint
coordinate type corresponds to an argument that is an index into abyte[]
array. The returned VarHandle accesses bytes at an index in abyte[]
array, composing bytes to or from a value of the component type ofviewArrayClass
according to the given endianness.The supported component types (variables types) are
short
,char
,int
,long
,float
anddouble
.Access of bytes at a given index will result in an
IndexOutOfBoundsException
if the index is less than0
or greater than thebyte[]
array length minus the size (in bytes) ofT
.Access of bytes at an index may be aligned or misaligned for
T
, with respect to the underlying memory address,A
say, associated with the array and index. If access is misaligned then access for anything other than theget
andset
access modes will result in anIllegalStateException
. In such cases atomic access is only guaranteed with respect to the largest power of two that divides the GCD ofA
and the size (in bytes) ofT
. If access is aligned then following access modes are supported and are guaranteed to support atomic access:- read write access modes for all
T
, with the exception of access modesget
andset
forlong
anddouble
on 32-bit platforms. - atomic update access modes for
int
,long
,float
ordouble
. (Future major platform releases of the JDK may support additional types for certain currently unsupported access modes.) - numeric atomic update access modes for
int
andlong
. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.) - bitwise atomic update access modes for
int
andlong
. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.)
Misaligned access, and therefore atomicity guarantees, may be determined for
byte[]
arrays without operating on a specific array. Given anindex
,T
and it's corresponding boxed type,T_BOX
, misalignment may be determined as follows:int sizeOfT = T_BOX.BYTES; // size in bytes of T int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]). alignmentOffset(0, sizeOfT); int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT; boolean isMisaligned = misalignedAtIndex != 0;
If the variable type is
float
ordouble
then atomic update access modes compare values using their bitwise representation (seeFloat.floatToRawIntBits(float)
andDouble.doubleToRawLongBits(double)
, respectively).- Parameters:
viewArrayClass
- the view array class, with a component type of typeT
byteOrder
- the endianness of the view array elements, as stored in the underlyingbyte
array- Returns:
- a VarHandle giving access to elements of a
byte[]
array viewed as if elements corresponding to the components type of the view array class - Throws:
NullPointerException
- if viewArrayClass or byteOrder is nullIllegalArgumentException
- if viewArrayClass is not an array typeUnsupportedOperationException
- if the component type of viewArrayClass is not supported as a variable type- Since:
- 9
- read write access modes for all
-
byteBufferViewVarHandle
public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder) throws IllegalArgumentException
Produces a VarHandle giving access to elements of aByteBuffer
viewed as if it were an array of elements of a different primitive component type to that ofbyte
, such asint[]
orlong[]
. The VarHandle's variable type is the component type ofviewArrayClass
and the list of coordinate types is(ByteBuffer, int)
, where theint
coordinate type corresponds to an argument that is an index into abyte[]
array. The returned VarHandle accesses bytes at an index in aByteBuffer
, composing bytes to or from a value of the component type ofviewArrayClass
according to the given endianness.The supported component types (variables types) are
short
,char
,int
,long
,float
anddouble
.Access will result in a
ReadOnlyBufferException
for anything other than the read access modes if theByteBuffer
is read-only.Access of bytes at a given index will result in an
IndexOutOfBoundsException
if the index is less than0
or greater than theByteBuffer
limit minus the size (in bytes) ofT
.Access of bytes at an index may be aligned or misaligned for
T
, with respect to the underlying memory address,A
say, associated with theByteBuffer
and index. If access is misaligned then access for anything other than theget
andset
access modes will result in anIllegalStateException
. In such cases atomic access is only guaranteed with respect to the largest power of two that divides the GCD ofA
and the size (in bytes) ofT
. If access is aligned then following access modes are supported and are guaranteed to support atomic access:- read write access modes for all
T
, with the exception of access modesget
andset
forlong
anddouble
on 32-bit platforms. - atomic update access modes for
int
,long
,float
ordouble
. (Future major platform releases of the JDK may support additional types for certain currently unsupported access modes.) - numeric atomic update access modes for
int
andlong
. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.) - bitwise atomic update access modes for
int
andlong
. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.)
Misaligned access, and therefore atomicity guarantees, may be determined for a
ByteBuffer
,bb
(direct or otherwise), anindex
,T
and it's corresponding boxed type,T_BOX
, as follows:int sizeOfT = T_BOX.BYTES; // size in bytes of T ByteBuffer bb = ... int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT); boolean isMisaligned = misalignedAtIndex != 0;
If the variable type is
float
ordouble
then atomic update access modes compare values using their bitwise representation (seeFloat.floatToRawIntBits(float)
andDouble.doubleToRawLongBits(double)
, respectively).- Parameters:
viewArrayClass
- the view array class, with a component type of typeT
byteOrder
- the endianness of the view array elements, as stored in the underlyingByteBuffer
(Note this overrides the endianness of aByteBuffer
)- Returns:
- a VarHandle giving access to elements of a
ByteBuffer
viewed as if elements corresponding to the components type of the view array class - Throws:
NullPointerException
- if viewArrayClass or byteOrder is nullIllegalArgumentException
- if viewArrayClass is not an array typeUnsupportedOperationException
- if the component type of viewArrayClass is not supported as a variable type- Since:
- 9
- read write access modes for all
-
spreadInvoker
public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount)
Produces a method handle which will invoke any method handle of the giventype
, with a given number of trailing arguments replaced by a single trailingObject[]
array. The resulting invoker will be a method handle with the following arguments:- a single
MethodHandle
target - zero or more leading values (counted by
leadingArgCount
) - an
Object[]
array containing trailing arguments
The invoker will invoke its target like a call to
invoke
with the indicatedtype
. That is, if the target is exactly of the giventype
, it will behave likeinvokeExact
; otherwise it behave as ifasType
is used to convert the target to the requiredtype
.The type of the returned invoker will not be the given
type
, but rather will have all parameters except the firstleadingArgCount
replaced by a single array of typeObject[]
, which will be the final parameter.Before invoking its target, the invoker will spread the final array, apply reference casts as necessary, and unbox and widen primitive arguments. If, when the invoker is called, the supplied array argument does not have the correct number of elements, the invoker will throw an
IllegalArgumentException
instead of invoking the target.This method is equivalent to the following code (though it may be more efficient):
This method throws no reflective or security exceptions.MethodHandle invoker = MethodHandles.invoker(type); int spreadArgCount = type.parameterCount() - leadingArgCount; invoker = invoker.asSpreader(Object[].class, spreadArgCount); return invoker;
- Parameters:
type
- the desired target typeleadingArgCount
- number of fixed arguments, to be passed unchanged to the target- Returns:
- a method handle suitable for invoking any method handle of the given type
- Throws:
NullPointerException
- iftype
is nullIllegalArgumentException
- ifleadingArgCount
is not in the range from 0 totype.parameterCount()
inclusive, or if the resulting method handle's type would have too many parameters
- a single
-
exactInvoker
public static MethodHandle exactInvoker(MethodType type)
Produces a special invoker method handle which can be used to invoke any method handle of the given type, as if byinvokeExact
. The resulting invoker will have a type which is exactly equal to the desired type, except that it will accept an additional leading argument of typeMethodHandle
.This method is equivalent to the following code (though it may be more efficient):
publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)
Discussion: Invoker method handles can be useful when working with variable method handles of unknown types. For example, to emulate an
invokeExact
call to a variable method handleM
, extract its typeT
, look up the invoker methodX
forT
, and call the invoker method, asX.invoke(T, A...)
. (It would not work to callX.invokeExact
, since the typeT
is unknown.) If spreading, collecting, or other argument transformations are required, they can be applied once to the invokerX
and reused on manyM
method handle values, as long as they are compatible with the type ofX
.(Note: The invoker method is not available via the Core Reflection API. An attempt to call java.lang.reflect.Method.invoke on the declared
invokeExact
orinvoke
method will raise anUnsupportedOperationException
.)This method throws no reflective or security exceptions.
- Parameters:
type
- the desired target type- Returns:
- a method handle suitable for invoking any method handle of the given type
- Throws:
IllegalArgumentException
- if the resulting method handle's type would have too many parameters
-
invoker
public static MethodHandle invoker(MethodType type)
Produces a special invoker method handle which can be used to invoke any method handle compatible with the given type, as if byinvoke
. The resulting invoker will have a type which is exactly equal to the desired type, except that it will accept an additional leading argument of typeMethodHandle
.Before invoking its target, if the target differs from the expected type, the invoker will apply reference casts as necessary and box, unbox, or widen primitive values, as if by
asType
. Similarly, the return value will be converted as necessary. If the target is a variable arity method handle, the required arity conversion will be made, again as if byasType
.This method is equivalent to the following code (though it may be more efficient):
publicLookup().findVirtual(MethodHandle.class, "invoke", type)
Discussion: A general method type is one which mentions only
Object
arguments and return values. An invoker for such a type is capable of calling any method handle of the same arity as the general type.(Note: The invoker method is not available via the Core Reflection API. An attempt to call java.lang.reflect.Method.invoke on the declared
invokeExact
orinvoke
method will raise anUnsupportedOperationException
.)This method throws no reflective or security exceptions.
- Parameters:
type
- the desired target type- Returns:
- a method handle suitable for invoking any method handle convertible to the given type
- Throws:
IllegalArgumentException
- if the resulting method handle's type would have too many parameters
-
varHandleExactInvoker
public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type)
Produces a special invoker method handle which can be used to invoke a signature-polymorphic access mode method on any VarHandle whose associated access mode type is compatible with the given type. The resulting invoker will have a type which is exactly equal to the desired given type, except that it will accept an additional leading argument of typeVarHandle
.- Parameters:
accessMode
- the VarHandle access modetype
- the desired target type- Returns:
- a method handle suitable for invoking an access mode method of any VarHandle whose access mode type is of the given type.
- Since:
- 9
-
varHandleInvoker
public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type)
Produces a special invoker method handle which can be used to invoke a signature-polymorphic access mode method on any VarHandle whose associated access mode type is compatible with the given type. The resulting invoker will have a type which is exactly equal to the desired given type, except that it will accept an additional leading argument of typeVarHandle
.Before invoking its target, if the access mode type differs from the desired given type, the invoker will apply reference casts as necessary and box, unbox, or widen primitive values, as if by
asType
. Similarly, the return value will be converted as necessary.This method is equivalent to the following code (though it may be more efficient):
publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)
- Parameters:
accessMode
- the VarHandle access modetype
- the desired target type- Returns:
- a method handle suitable for invoking an access mode method of any VarHandle whose access mode type is convertible to the given type.
- Since:
- 9
-
explicitCastArguments
public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType)
Produces a method handle which adapts the type of the given method handle to a new type by pairwise argument and return type conversion. The original type and new type must have the same number of arguments. The resulting method handle is guaranteed to report a type which is equal to the desired new type.If the original type and new type are equal, returns target.
The same conversions are allowed as for
MethodHandle.asType
, and some additional conversions are also applied if those conversions fail. Given types T0, T1, one of the following conversions is applied if possible, before or instead of any conversions done byasType
:- If T0 and T1 are references, and T1 is an interface type, then the value of type T0 is passed as a T1 without a cast. (This treatment of interfaces follows the usage of the bytecode verifier.)
- If T0 is boolean and T1 is another primitive, the boolean is converted to a byte value, 1 for true, 0 for false. (This treatment follows the usage of the bytecode verifier.)
- If T1 is boolean and T0 is another primitive,
T0 is converted to byte via Java casting conversion (JLS 5.5),
and the low order bit of the result is tested, as if by
(x & 1) != 0
. - If T0 and T1 are primitives other than boolean, then a Java casting conversion (JLS 5.5) is applied. (Specifically, T0 will convert to T1 by widening and/or narrowing.)
- If T0 is a reference and T1 a primitive, an unboxing conversion will be applied at runtime, possibly followed by a Java casting conversion (JLS 5.5) on the primitive value, possibly followed by a conversion from byte to boolean by testing the low-order bit.
- If T0 is a reference and T1 a primitive, and if the reference is null at runtime, a zero value is introduced.
- Parameters:
target
- the method handle to invoke after arguments are retypednewType
- the expected type of the new method handle- Returns:
- a method handle which delegates to the target after performing any necessary argument conversions, and arranges for any necessary return value conversions
- Throws:
NullPointerException
- if either argument is nullWrongMethodTypeException
- if the conversion cannot be made- See Also:
MethodHandle.asType(java.lang.invoke.MethodType)
-
permuteArguments
public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder)
Produces a method handle which adapts the calling sequence of the given method handle to a new type, by reordering the arguments. The resulting method handle is guaranteed to report a type which is equal to the desired new type.The given array controls the reordering. Call
#I
the number of incoming parameters (the valuenewType.parameterCount()
, and call#O
the number of outgoing parameters (the valuetarget.type().parameterCount()
). Then the length of the reordering array must be#O
, and each element must be a non-negative number less than#I
. For everyN
less than#O
, theN
-th outgoing argument will be taken from theI
-th incoming argument, whereI
isreorder[N]
.No argument or return value conversions are applied. The type of each incoming argument, as determined by
newType
, must be identical to the type of the corresponding outgoing parameter or parameters in the target method handle. The return type ofnewType
must be identical to the return type of the original target.The reordering array need not specify an actual permutation. An incoming argument will be duplicated if its index appears more than once in the array, and an incoming argument will be dropped if its index does not appear in the array. As in the case of
dropArguments
, incoming arguments which are not mentioned in the reordering array may be of any type, as determined only bynewType
.import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodType intfn1 = methodType(int.class, int.class); MethodType intfn2 = methodType(int.class, int.class, int.class); MethodHandle sub = ... (int x, int y) -> (x-y) ...; assert(sub.type().equals(intfn2)); MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1); MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0); assert((int)rsub.invokeExact(1, 100) == 99); MethodHandle add = ... (int x, int y) -> (x+y) ...; assert(add.type().equals(intfn2)); MethodHandle twice = permuteArguments(add, intfn1, 0, 0); assert(twice.type().equals(intfn1)); assert((int)twice.invokeExact(21) == 42);
Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after arguments are reorderednewType
- the expected type of the new method handlereorder
- an index array which controls the reordering- Returns:
- a method handle which delegates to the target after it drops unused arguments and moves and/or duplicates the other arguments
- Throws:
NullPointerException
- if any argument is nullIllegalArgumentException
- if the index array length is not equal to the arity of the target, or if any index array element not a valid index for a parameter ofnewType
, or if two corresponding parameter types intarget.type()
andnewType
are not identical,
-
constant
public static MethodHandle constant(Class<?> type, Object value)
Produces a method handle of the requested return type which returns the given constant value every time it is invoked.Before the method handle is returned, the passed-in value is converted to the requested type. If the requested type is primitive, widening primitive conversions are attempted, else reference conversions are attempted.
The returned method handle is equivalent to
identity(type).bindTo(value)
.- Parameters:
type
- the return type of the desired method handlevalue
- the value to return- Returns:
- a method handle of the given return type and no arguments, which always returns the given value
- Throws:
NullPointerException
- if thetype
argument is nullClassCastException
- if the value cannot be converted to the required return typeIllegalArgumentException
- if the given type isvoid.class
-
identity
public static MethodHandle identity(Class<?> type)
Produces a method handle which returns its sole argument when invoked.- Parameters:
type
- the type of the sole parameter and return value of the desired method handle- Returns:
- a unary method handle which accepts and returns the given type
- Throws:
NullPointerException
- if the argument is nullIllegalArgumentException
- if the given type isvoid.class
-
zero
public static MethodHandle zero(Class<?> type)
Produces a constant method handle of the requested return type which returns the default value for that type every time it is invoked. The resulting constant method handle will have no side effects.The returned method handle is equivalent to
empty(methodType(type))
. It is also equivalent toexplicitCastArguments(constant(Object.class, null), methodType(type))
, sinceexplicitCastArguments
convertsnull
to default values.- Parameters:
type
- the expected return type of the desired method handle- Returns:
- a constant method handle that takes no arguments and returns the default value of the given type (or void, if the type is void)
- Throws:
NullPointerException
- if the argument is null- Since:
- 9
- See Also:
constant(java.lang.Class<?>, java.lang.Object)
,empty(java.lang.invoke.MethodType)
,explicitCastArguments(java.lang.invoke.MethodHandle, java.lang.invoke.MethodType)
-
empty
public static MethodHandle empty(MethodType type)
Produces a method handle of the requested type which ignores any arguments, does nothing, and returns a suitable default depending on the return type. That is, it returns a zero primitive value, anull
, orvoid
.The returned method handle is equivalent to
dropArguments(zero(type.returnType()), 0, type.parameterList())
.- API Note:
- Given a predicate and target, a useful "if-then" construct can be produced as
guardWithTest(pred, target, empty(target.type())
. - Parameters:
type
- the type of the desired method handle- Returns:
- a constant method handle of the given type, which returns a default value of the given return type
- Throws:
NullPointerException
- if the argument is null- Since:
- 9
- See Also:
zero(java.lang.Class<?>)
,constant(java.lang.Class<?>, java.lang.Object)
-
insertArguments
public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values)
Provides a target method handle with one or more bound arguments in advance of the method handle's invocation. The formal parameters to the target corresponding to the bound arguments are called bound parameters. Returns a new method handle which saves away the bound arguments. When it is invoked, it receives arguments for any non-bound parameters, binds the saved arguments to their corresponding parameters, and calls the original target.The type of the new method handle will drop the types for the bound parameters from the original target type, since the new method handle will no longer require those arguments to be supplied by its callers.
Each given argument object must match the corresponding bound parameter type. If a bound parameter type is a primitive, the argument object must be a wrapper, and will be unboxed to produce the primitive value.
The
pos
argument selects which parameters are to be bound. It may range between zero and N-L (inclusively), where N is the arity of the target method handle and L is the length of the values array.Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after the argument is insertedpos
- where to insert the argument (zero for the first)values
- the series of arguments to insert- Returns:
- a method handle which inserts an additional argument, before calling the original method handle
- Throws:
NullPointerException
- if the target or thevalues
array is nullIllegalArgumentException
- if (@code pos) is less than0
or greater thanN - L
whereN
is the arity of the target method handle andL
is the length of the values array.ClassCastException
- if an argument does not match the corresponding bound parameter type.- See Also:
MethodHandle.bindTo(java.lang.Object)
-
dropArguments
public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes)
Produces a method handle which will discard some dummy arguments before calling some other specified target method handle. The type of the new method handle will be the same as the target's type, except it will also include the dummy argument types, at some given position.The
pos
argument may range between zero and N, where N is the arity of the target. Ifpos
is zero, the dummy arguments will precede the target's real arguments; ifpos
is N they will come after.Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class); MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2)); assertEquals(bigType, d0.type()); assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
This method is also equivalent to the following code:
dropArguments
(target, pos, valueTypes.toArray(new Class[0]))
- Parameters:
target
- the method handle to invoke after the arguments are droppedvalueTypes
- the type(s) of the argument(s) to droppos
- position of first argument to drop (zero for the leftmost)- Returns:
- a method handle which drops arguments of the given types, before calling the original method handle
- Throws:
NullPointerException
- if the target is null, or if thevalueTypes
list or any of its elements is nullIllegalArgumentException
- if any element ofvalueTypes
isvoid.class
, or ifpos
is negative or greater than the arity of the target, or if the new method handle's type would have too many parameters
-
dropArguments
public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes)
Produces a method handle which will discard some dummy arguments before calling some other specified target method handle. The type of the new method handle will be the same as the target's type, except it will also include the dummy argument types, at some given position.The
pos
argument may range between zero and N, where N is the arity of the target. Ifpos
is zero, the dummy arguments will precede the target's real arguments; ifpos
is N they will come after.- API Note:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodHandle d0 = dropArguments(cat, 0, String.class); assertEquals("yz", (String) d0.invokeExact("x", "y", "z")); MethodHandle d1 = dropArguments(cat, 1, String.class); assertEquals("xz", (String) d1.invokeExact("x", "y", "z")); MethodHandle d2 = dropArguments(cat, 2, String.class); assertEquals("xy", (String) d2.invokeExact("x", "y", "z")); MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class); assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
This method is also equivalent to the following code:
dropArguments
(target, pos, Arrays.asList(valueTypes))
- Parameters:
target
- the method handle to invoke after the arguments are droppedvalueTypes
- the type(s) of the argument(s) to droppos
- position of first argument to drop (zero for the leftmost)- Returns:
- a method handle which drops arguments of the given types, before calling the original method handle
- Throws:
NullPointerException
- if the target is null, or if thevalueTypes
array or any of its elements is nullIllegalArgumentException
- if any element ofvalueTypes
isvoid.class
, or ifpos
is negative or greater than the arity of the target, or if the new method handle's type would have too many parameters
-
dropArgumentsToMatch
public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos)
Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some leading parameters can be skipped before matching begins. The remaining types in thetarget
's parameter type list must be a sub-list of thenewTypes
type list at the starting positionpos
. The resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if bydropArguments(MethodHandle, int, Class[])
.The resulting handle will have the same return type as the target handle.
In more formal terms, assume these two type lists:
- The target handle has the parameter type list
S..., M...
, with as many types inS
as indicated byskip
. TheM
types are those that are supposed to match part of the given type list,newTypes
. - The
newTypes
list contains typesP..., M..., A...
, with as many types inP
as indicated bypos
. TheM
types are precisely those that theM
types in the target handle's parameter type list are supposed to match. The types inA
are additional types found after the matching sub-list.
dropArgumentsToMatch
will have the parameter type listS..., P..., M..., A...
, with theP
andA
types inserted as if bydropArguments(MethodHandle, int, Class[])
.- API Note:
- Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
mutually converted to a common type by two calls to
dropArgumentsToMatch
, as follows:import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... ... MethodHandle h0 = constant(boolean.class, true); MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class); MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList()); if (h1.type().parameterCount() < h2.type().parameterCount()) h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0); // lengthen h1 else h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0); // lengthen h2 MethodHandle h3 = guardWithTest(h0, h1, h2); assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
- Parameters:
target
- the method handle to adaptskip
- number of targets parameters to disregard (they will be unchanged)newTypes
- the list of types to matchtarget
's parameter type list topos
- place innewTypes
where the non-skipped target parameters must occur- Returns:
- a possibly adapted method handle
- Throws:
NullPointerException
- if either argument is nullIllegalArgumentException
- if any element ofnewTypes
isvoid.class
, or ifskip
is negative or greater than the arity of the target, or ifpos
is negative or greater than the newTypes list size, or ifnewTypes
does not contain thetarget
's non-skipped parameter types at positionpos
.- Since:
- 9
- The target handle has the parameter type list
-
filterArguments
public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters)
Adapts a target method handle by pre-processing one or more of its arguments, each with its own unary filter function, and then calling the target with each pre-processed argument replaced by the result of its corresponding filter function.The pre-processing is performed by one or more method handles, specified in the elements of the
filters
array. The first element of the filter array corresponds to thepos
argument of the target, and so on in sequence. The filter functions are invoked in left to right order.Null arguments in the array are treated as identity functions, and the corresponding arguments left unchanged. (If there are no non-null elements in the array, the original target is returned.) Each filter is applied to the corresponding argument of the adapter.
If a filter
F
applies to theN
th argument of the target, thenF
must be a method handle which takes exactly one argument. The type ofF
's sole argument replaces the corresponding argument type of the target in the resulting adapted method handle. The return type ofF
must be identical to the corresponding parameter type of the target.It is an error if there are elements of
filters
(null or not) which do not correspond to argument positions in the target.Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodHandle upcase = lookup().findVirtual(String.class, "toUpperCase", methodType(String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodHandle f0 = filterArguments(cat, 0, upcase); assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy MethodHandle f1 = filterArguments(cat, 1, upcase); assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY MethodHandle f2 = filterArguments(cat, 0, upcase, upcase); assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
Here is pseudocode for the resulting adapter. In the code,
T
denotes the return type of both thetarget
and resulting adapter.P
/p
andB
/b
represent the types and values of the parameters and arguments that precede and follow the filter positionpos
, respectively.A[i]
/a[i]
stand for the types and values of the filtered parameters and arguments; they also represent the return types of thefilter[i]
handles. The latter accept argumentsv[i]
of typeV[i]
, which also appear in the signature of the resulting adapter.T target(P... p, A[i]... a[i], B... b); A[i] filter[i](V[i]); T adapter(P... p, V[i]... v[i], B... b) { return target(p..., filter[i](v[i])..., b...); }
Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after arguments are filteredpos
- the position of the first argument to filterfilters
- method handles to call initially on filtered arguments- Returns:
- method handle which incorporates the specified argument filtering logic
- Throws:
NullPointerException
- if the target is null or if thefilters
array is nullIllegalArgumentException
- if a non-null element offilters
does not match a corresponding argument type of target as described above, or if thepos+filters.length
is greater thantarget.type().parameterCount()
, or if the resulting method handle's type would have too many parameters
-
collectArguments
public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter)
Adapts a target method handle by pre-processing a sub-sequence of its arguments with a filter (another method handle). The pre-processed arguments are replaced by the result (if any) of the filter function. The target is then called on the modified (usually shortened) argument list.If the filter returns a value, the target must accept that value as its argument in position
pos
, preceded and/or followed by any arguments not passed to the filter. If the filter returns void, the target must accept all arguments not passed to the filter. No arguments are reordered, and a result returned from the filter replaces (in order) the whole subsequence of arguments originally passed to the adapter.The argument types (if any) of the filter replace zero or one argument types of the target, at position
pos
, in the resulting adapted method handle. The return type of the filter (if any) must be identical to the argument type of the target at positionpos
, and that target argument is supplied by the return value of the filter.In all cases,
pos
must be greater than or equal to zero, andpos
must also be less than or equal to the target's arity.Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle deepToString = publicLookup() .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); MethodHandle ts1 = deepToString.asCollector(String[].class, 1); assertEquals("[strange]", (String) ts1.invokeExact("strange")); MethodHandle ts2 = deepToString.asCollector(String[].class, 2); assertEquals("[up, down]", (String) ts2.invokeExact("up", "down")); MethodHandle ts3 = deepToString.asCollector(String[].class, 3); MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2); assertEquals("[top, [up, down], strange]", (String) ts3_ts2.invokeExact("top", "up", "down", "strange")); MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1); assertEquals("[top, [up, down], [strange]]", (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange")); MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3); assertEquals("[top, [[up, down, strange], charm], bottom]", (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
Here is pseudocode for the resulting adapter. In the code,
T
represents the return type of thetarget
and resulting adapter.V
/v
stand for the return type and value of thefilter
, which are also found in the signature and arguments of thetarget
, respectively, unlessV
isvoid
.A
/a
andC
/c
represent the parameter types and values preceding and following the collection position,pos
, in thetarget
's signature. They also turn up in the resulting adapter's signature and arguments, where they surroundB
/b
, which represent the parameter types and arguments to thefilter
(if any).T target(A...,V,C...); V filter(B...); T adapter(A... a,B... b,C... c) { V v = filter(b...); return target(a...,v,c...); } // and if the filter has no arguments: T target2(A...,V,C...); V filter2(); T adapter2(A... a,C... c) { V v = filter2(); return target2(a...,v,c...); } // and if the filter has a void return: T target3(A...,C...); void filter3(B...); T adapter3(A... a,B... b,C... c) { filter3(b...); return target3(a...,c...); }
A collection adapter
collectArguments(mh, 0, coll)
is equivalent to one which first "folds" the affected arguments, and then drops them, in separate steps as follows:
If the target method handle consumes no arguments besides than the result (if any) of the filtermh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2 mh = MethodHandles.foldArguments(mh, coll); //step 1
coll
, thencollectArguments(mh, 0, coll)
is equivalent tofilterReturnValue(coll, mh)
. If the filter method handlecoll
consumes one argument and produces a non-void result, thencollectArguments(mh, N, coll)
is equivalent tofilterArguments(mh, N, coll)
. Other equivalences are possible but would require argument permutation.Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after filtering the subsequence of argumentspos
- the position of the first adapter argument to pass to the filter, and/or the target argument which receives the result of the filterfilter
- method handle to call on the subsequence of arguments- Returns:
- method handle which incorporates the specified argument subsequence filtering logic
- Throws:
NullPointerException
- if either argument is nullIllegalArgumentException
- if the return type offilter
is non-void and is not the same as thepos
argument of the target, or ifpos
is not between 0 and the target's arity, inclusive, or if the resulting method handle's type would have too many parameters- See Also:
foldArguments(java.lang.invoke.MethodHandle, java.lang.invoke.MethodHandle)
,filterArguments(java.lang.invoke.MethodHandle, int, java.lang.invoke.MethodHandle...)
,filterReturnValue(java.lang.invoke.MethodHandle, java.lang.invoke.MethodHandle)
-
filterReturnValue
public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter)
Adapts a target method handle by post-processing its return value (if any) with a filter (another method handle). The result of the filter is returned from the adapter.If the target returns a value, the filter must accept that value as its only argument. If the target returns void, the filter must accept no arguments.
The return type of the filter replaces the return type of the target in the resulting adapted method handle. The argument type of the filter (if any) must be identical to the return type of the target.
Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodHandle length = lookup().findVirtual(String.class, "length", methodType(int.class)); System.out.println((String) cat.invokeExact("x", "y")); // xy MethodHandle f0 = filterReturnValue(cat, length); System.out.println((int) f0.invokeExact("x", "y")); // 2
Here is pseudocode for the resulting adapter. In the code,
T
/t
represent the result type and value of thetarget
;V
, the result type of thefilter
; andA
/a
, the types and values of the parameters and arguments of thetarget
as well as the resulting adapter.T target(A...); V filter(T); V adapter(A... a) { T t = target(a...); return filter(t); } // and if the target has a void return: void target2(A...); V filter2(); V adapter2(A... a) { target2(a...); return filter2(); } // and if the filter has a void return: T target3(A...); void filter3(V); void adapter3(A... a) { T t = target3(a...); filter3(t); }
Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke before filtering the return valuefilter
- method handle to call on the return value- Returns:
- method handle which incorporates the specified return value filtering logic
- Throws:
NullPointerException
- if either argument is nullIllegalArgumentException
- if the argument list offilter
does not match the return type of target as described above
-
foldArguments
public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner)
Adapts a target method handle by pre-processing some of its arguments, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments.The pre-processing is performed by
combiner
, a second method handle. Of the arguments passed to the adapter, the firstN
arguments are copied to the combiner, which is then called. (Here,N
is defined as the parameter count of the combiner.) After this, control passes to the target, with any result from the combiner inserted before the originalN
incoming arguments.If the combiner returns a value, the first parameter type of the target must be identical with the return type of the combiner, and the next
N
parameter types of the target must exactly match the parameters of the combiner.If the combiner has a void return, no result will be inserted, and the first
N
parameter types of the target must exactly match the parameters of the combiner.The resulting adapter is the same type as the target, except that the first parameter type is dropped, if it corresponds to the result of the combiner.
(Note that
dropArguments
can be used to remove any arguments that either the combiner or the target does not wish to receive. If some of the incoming arguments are destined only for the combiner, consider usingasCollector
instead, since those arguments will not need to be live on the stack on entry to the target.)Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class, "println", methodType(void.class, String.class)) .bindTo(System.out); MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("boojum", (String) cat.invokeExact("boo", "jum")); MethodHandle catTrace = foldArguments(cat, trace); // also prints "boo": assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
Here is pseudocode for the resulting adapter. In the code,
T
represents the result type of thetarget
and resulting adapter.V
/v
represent the type and value of the parameter and argument oftarget
that precedes the folding position;V
also is the result type of thecombiner
.A
/a
denote the types and values of theN
parameters and arguments at the folding position.B
/b
represent the types and values of thetarget
parameters and arguments that follow the folded parameters and arguments.// there are N arguments in A... T target(V, A[N]..., B...); V combiner(A...); T adapter(A... a, B... b) { V v = combiner(a...); return target(v, a..., b...); } // and if the combiner has a void return: T target2(A[N]..., B...); void combiner2(A...); T adapter2(A... a, B... b) { combiner2(a...); return target2(a..., b...); }
Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after arguments are combinedcombiner
- method handle to call initially on the incoming arguments- Returns:
- method handle which incorporates the specified argument folding logic
- Throws:
NullPointerException
- if either argument is nullIllegalArgumentException
- ifcombiner
's return type is non-void and not the same as the first argument type of the target, or if the initialN
argument types of the target (skipping one matching thecombiner
's return type) are not identical with the argument types ofcombiner
-
foldArguments
public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner)
Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then calling the target with the result of the pre-processing, inserted into the original sequence of arguments just before the folded arguments.This method is closely related to
foldArguments(MethodHandle, MethodHandle)
, but allows to control the position in the parameter list at which folding takes place. The argument controlling this,pos
, is a zero-based index. The aforementioned methodfoldArguments(MethodHandle, MethodHandle)
assumes position 0.- API Note:
- Example:
import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class, "println", methodType(void.class, String.class)) .bindTo(System.out); MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("boojum", (String) cat.invokeExact("boo", "jum")); MethodHandle catTrace = foldArguments(cat, 1, trace); // also prints "jum": assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
Here is pseudocode for the resulting adapter. In the code,
T
represents the result type of thetarget
and resulting adapter.V
/v
represent the type and value of the parameter and argument oftarget
that precedes the folding position;V
also is the result type of thecombiner
.A
/a
denote the types and values of theN
parameters and arguments at the folding position.Z
/z
andB
/b
represent the types and values of thetarget
parameters and arguments that precede and follow the folded parameters and arguments starting atpos
, respectively.// there are N arguments in A... T target(Z..., V, A[N]..., B...); V combiner(A...); T adapter(Z... z, A... a, B... b) { V v = combiner(a...); return target(z..., v, a..., b...); } // and if the combiner has a void return: T target2(Z..., A[N]..., B...); void combiner2(A...); T adapter2(Z... z, A... a, B... b) { combiner2(a...); return target2(z..., a..., b...); }
Note: The resulting adapter is never a variable-arity method handle, even if the original target method handle was.
- Parameters:
target
- the method handle to invoke after arguments are combinedpos
- the position at which to start folding and at which to insert the folding result; if this is0
, the effect is the same as forfoldArguments(MethodHandle, MethodHandle)
.combiner
- method handle to call initially on the incoming arguments- Returns:
- method handle which incorporates the specified argument folding logic
- Throws:
NullPointerException
- if either argument is nullIllegalArgumentException
- if either of the following two conditions holds: (1)combiner
's return type is non-void
and not the same as the argument type at positionpos
of the target signature; (2) theN
argument types at positionpos
of the target signature (skipping one matching thecombiner
's return type) are not identical with the argument types ofcombiner
.- Since:
- 9
- See Also:
foldArguments(MethodHandle, MethodHandle)
-
guardWithTest
public static MethodHandle guardWithTest(MethodHandle test, MethodHandle target, MethodHandle fallback)
Makes a method handle which adapts a target method handle, by guarding it with a test, a boolean-valued method handle. If the guard fails, a fallback handle is called instead. All three method handles must have the same corresponding argument and return types, except that the return type of the test must be boolean, and the test is allowed to have fewer arguments than the other two method handles.Here is pseudocode for the resulting adapter. In the code,
T
represents the uniform result type of the three involved handles;A
/a
, the types and values of thetarget
parameters and arguments that are consumed by thetest
; andB
/b
, those types and values of thetarget
parameters and arguments that are not consumed by thetest
.
Note that the test arguments (boolean test(A...); T target(A...,B...); T fallback(A...,B...); T adapter(A... a,B... b) { if (test(a...)) return target(a..., b...); else return fallback(a..., b...); }
a...
in the pseudocode) cannot be modified by execution of the test, and so are passed unchanged from the caller to the target or fallback as appropriate.- Parameters:
test
- method handle used for test, must return booleantarget
- method handle to call if test passesfallback
- method handle to call if test fails- Returns:
- method handle which incorporates the specified if/then/else logic
- Throws:
NullPointerException
- if any argument is nullIllegalArgumentException
- iftest
does not return boolean, or if all three method types do not match (with the return type oftest
changed to match that of the target).
-
catchException
public static MethodHandle catchException(MethodHandle target, Class<? extends Throwable> exType, MethodHandle handler)
Makes a method handle which adapts a target method handle, by running it inside an exception handler. If the target returns normally, the adapter returns that value. If an exception matching the specified type is thrown, the fallback handle is called instead on the exception, plus the original arguments.The target and handler must have the same corresponding argument and return types, except that handler may omit trailing arguments (similarly to the predicate in
guardWithTest
). Also, the handler must have an extra leading parameter ofexType
or a supertype.Here is pseudocode for the resulting adapter. In the code,
T
represents the return type of thetarget
andhandler
, and correspondingly that of the resulting adapter;A
/a
, the types and values of arguments to the resulting handle consumed byhandler
; andB
/b
, those of arguments to the resulting handle discarded byhandler
.
Note that the saved arguments (T target(A..., B...); T handler(ExType, A...); T adapter(A... a, B... b) { try { return target(a..., b...); } catch (ExType ex) { return handler(ex, a...); } }
a...
in the pseudocode) cannot be modified by execution of the target, and so are passed unchanged from the caller to the handler, if the handler is invoked.The target and handler must return the same type, even if the handler always throws. (This might happen, for instance, because the handler is simulating a
finally
clause). To create such a throwing handler, compose the handler creation logic withthrowException
, in order to create a method handle of the correct return type.- Parameters:
target
- method handle to callexType
- the type of exception which the handler will catchhandler
- method handle to call if a matching exception is thrown- Returns:
- method handle which incorporates the specified try/catch logic
- Throws:
NullPointerException
- if any argument is nullIllegalArgumentException
- ifhandler
does not accept the given exception type, or if the method handle types do not match in their return types and their corresponding parameters- See Also:
tryFinally(MethodHandle, MethodHandle)
-
throwException
public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType)
Produces a method handle which will throw exceptions of the givenexType
. The method handle will accept a single argument ofexType
, and immediately throw it as an exception. The method type will nominally specify a return ofreturnType
. The return type may be anything convenient: It doesn't matter to the method handle's behavior, since it will never return normally.- Parameters:
returnType
- the return type of the desired method handleexType
- the parameter type of the desired method handle- Returns:
- method handle which can throw the given exceptions
- Throws:
NullPointerException
- if either argument is null
-
loop
public static MethodHandle loop(MethodHandle[]... clauses)
Constructs a method handle representing a loop with several loop variables that are updated and checked upon each iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and delivers the loop's result, which is the return value of the resulting handle.Intuitively, every loop is formed by one or more "clauses", each specifying a local iteration variable and/or a loop exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in terms of method handles, each clause will specify up to four independent actions:
- init: Before the loop executes, the initialization of an iteration variable
v
of typeV
. - step: When a clause executes, an update step for the iteration variable
v
. - pred: When a clause executes, a predicate execution to test for loop exit.
- fini: If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
(V...)
. The values themselves will be(v...)
. When we speak of "parameter lists", we will usually be referring to types, but in some contexts (describing execution) the lists will be of actual values.Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in this case. See below for a detailed description.
Parameters optional everywhere: Each clause function is allowed but not required to accept a parameter for each iteration variable
v
. As an exception, the init functions cannot take anyv
parameters, because those values are not yet computed when the init functions are executed. Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take. In fact, any clause function may take no arguments at all.Loop parameters: A clause function may take all the iteration variable values it is entitled to, in which case it may also take more trailing parameters. Such extra values are called loop parameters, with their types and values notated as
(A...)
and(a...)
. These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed. (Since init functions do not accept iteration variablesv
, any parameter to an init function is automatically a loop parametera
.) As with iteration variables, clause functions are allowed but not required to accept loop parameters. These loop parameters act as loop-invariant values visible across the whole loop.Parameters visible everywhere: Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full list
(v... a...)
of current iteration variable values and incoming loop parameters. The init functions can observe initial pre-loop state, in the form(a...)
. Most clause functions will not need all of this information, but they will be formally connected to it as if bydropArguments(java.lang.invoke.MethodHandle, int, java.util.List<java.lang.Class<?>>)
. More specifically, we shall use the notation(V*)
to express an arbitrary prefix of a full sequence(V...)
(and likewise for(v*)
,(A*)
,(a*)
). In that notation, the general form of an init function parameter list is(A*)
, and the general form of a non-init function parameter list is(V*)
or(V... A*)
.Checking clause structure: Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must" corresponds to a place where
IllegalArgumentException
will be thrown if the required constraint is not met by the inputs to the loop combinator.Effectively identical sequences: A parameter list
A
is defined to be effectively identical to another parameter listB
ifA
andB
are identical, or ifA
is shorter and is identical with a proper prefix ofB
. When speaking of an unordered set of parameter lists, we say they the set is "effectively identical" as a whole if the set contains a longest list, and all members of the set are effectively identical to that longest list. For example, any set of type sequences of the form(V*)
is effectively identical, and the same is true if more sequences of the form(V... A*)
are added.Step 0: Determine clause structure.
- The clause array (of type
MethodHandle[][]
) must be non-null
and contain at least one element. - The clause array may not contain
null
s or sub-arrays longer than four elements. - Clauses shorter than four elements are treated as if they were padded by
null
elements to length four. Padding takes place by appending elements to the array. - Clauses with all
null
s are disregarded. - Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
Step 1A: Determine iteration variable types
(V...)
.- The iteration variable type for each clause is determined using the clause's init and step return types.
- If both functions are omitted, there is no iteration variable for the corresponding clause (
void
is used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's iteration variable type. If both are given, the common return type (they must be identical) defines the clause's iteration variable type. - Form the list of return types (in clause order), omitting all occurrences of
void
. - This list of types is called the "iteration variable types" (
(V...)
).
Step 1B: Determine loop parameters
(A...)
.- Examine and collect init function parameter lists (which are of the form
(A*)
). - Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
(They must have the form
(V... A*)
; collect the(A*)
parts only.) - Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types. (These types will checked in step 2, along with all the clause function types.)
- Omitted clause functions are ignored. (Equivalently, they are deemed to have empty parameter lists.)
- All of the collected parameter lists must be effectively identical.
- The longest parameter list (which is necessarily unique) is called the "external parameter list" (
(A...)
). - If there is no such parameter list, the external parameter list is taken to be the empty sequence.
- The combined list consisting of iteration variable types followed by the external parameter types is called the "internal parameter list".
Step 1C: Determine loop return type.
- Examine fini function return types, disregarding omitted fini functions.
- If there are no fini functions, the loop return type is
void
. - Otherwise, the common return type
R
of the fini functions (their return types must be identical) defines the loop return type.
Step 1D: Check other types.
- There must be at least one non-omitted pred function.
- Every non-omitted pred function must have a
boolean
return type.
Step 2: Determine parameter lists.
- The parameter list for the resulting loop handle will be the external parameter list
(A...)
. - The parameter list for init functions will be adjusted to the external parameter list. (Note that their parameter lists are already effectively identical to this list.)
- The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
effectively identical to the internal parameter list
(V... A...)
.
Step 3: Fill in omitted functions.
- If an init function is omitted, use a default value for the clause's iteration variable type.
- If a step function is omitted, use an identity function of the clause's iteration
variable type; insert dropped argument parameters before the identity function parameter for the non-
void
iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.) - If a pred function is omitted, use a constant
true
function. (This will keep the loop going, as far as this clause is concerned. Note that in such cases the corresponding fini function is unreachable.) - If a fini function is omitted, use a default value for the loop return type.
Step 4: Fill in missing parameter types.
- At this point, every init function parameter list is effectively identical to the external parameter list
(A...)
, but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list. - At this point, every non-init function parameter list is effectively identical to the internal parameter
list
(V... A...)
, but some lists may be shorter. For every non-init function with a short parameter list, pad out the end of the list. - Argument lists are padded out by dropping unused trailing arguments.
Final observations.
- After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
- All init functions have a common parameter type list
(A...)
, which the final loop handle will also have. - All fini functions have a common return type
R
, which the final loop handle will also have. - All non-init functions have a common parameter type list
(V... A...)
, of (non-void
) iteration variablesV
followed by loop parameters. - Each pair of init and step functions agrees in their return type
V
. - Each non-init function will be able to observe the current values
(v...)
of all iteration variables. - Every function will be able to observe the incoming values
(a...)
of all loop parameters.
Example. As a consequence of step 1A above, the
loop
combinator has the following property:- Given
N
clausesCn = {null, Sn, Pn}
withn = 1..N
. - Suppose predicate handles
Pn
are eithernull
or have no parameters. (Only onePn
has to be non-null
.) - Suppose step handles
Sn
have signatures(B1..BX)Rn
, for some constantX>=N
. - Suppose
Q
is the count of non-void typesRn
, and(V1...VQ)
is the sequence of those types. - It must be that
Vn == Bn
forn = 1..min(X,Q)
. - The parameter types
Vn
will be interpreted as loop-local state elements(V...)
. - Any remaining types
BQ+1..BX
(ifQ<X
) will determine the resulting loop handle's parameter types(A...)
.
(A...)
were derived from the step functions, which is natural if most of the loop computation happens in the steps. For some loops, the burden of computation might be heaviest in the pred functions, and so the pred functions might need to accept the loop parameter values. For loops with complex exit logic, the fini functions might need to accept loop parameters, and likewise for loops with complex entry logic, where the init functions will need the extra parameters. For such reasons, the rules for determining these parameters are as symmetric as possible, across all clause parts. In general, the loop parameters function as common invariant values across the whole loop, while the iteration variables function as common variant values, or (if there is no step function) as internal loop invariant temporaries.Loop execution.
- When the loop is called, the loop input values are saved in locals, to be passed to every clause function. These locals are loop invariant.
- Each init function is executed in clause order (passing the external arguments
(a...)
) and the non-void
values are saved (as the iteration variables(v...)
) into locals. These locals will be loop varying (unless their steps behave as identity functions, as noted above). - All function executions (except init functions) will be passed the internal parameter list, consisting of
the non-
void
iteration values(v...)
(in clause order) and then the loop inputs(a...)
(in argument order). - The step and pred functions are then executed, in clause order (step before pred), until a pred function
returns
false
. - The non-
void
result from a step function call is used to update the corresponding value in the sequence(v...)
of loop variables. The updated value is immediately visible to all subsequent function calls. - If a pred function returns
false
, the corresponding fini function is called, and the resulting value (of typeR
) is returned from the loop as a whole. - If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit except by throwing an exception.
Usage tips.
- Although each step function will receive the current values of all the loop variables,
sometimes a step function only needs to observe the current value of its own variable.
In that case, the step function may need to explicitly drop all preceding loop variables.
This will require mentioning their types, in an expression like
dropArguments(step, 0, V0.class, ...)
. - Loop variables are not required to vary; they can be loop invariant. A clause can create a loop invariant by a suitable init function with no step, pred, or fini function. This may be useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
- If some of the clause functions are virtual methods on an instance, the instance
itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
like
new MethodHandle[]{identity(ObjType.class)}
. In that case, the instance reference will be the first iteration variable value, and it will be easy to use virtual methods as clause parts, since all of them will take a leading instance reference matching that value.
Here is pseudocode for the resulting loop handle. As above,
V
andv
represent the types and values of loop variables;A
anda
represent arguments passed to the whole loop; andR
is the common result type of all finalizers as well as of the resulting loop.
Note that the parameter type listsV... init...(A...); boolean pred...(V..., A...); V... step...(V..., A...); R fini...(V..., A...); R loop(A... a) { V... v... = init...(a...); for (;;) { for ((v, p, s, f) in (v..., pred..., step..., fini...)) { v = s(v..., a...); if (!p(v..., a...)) { return f(v..., a...); } } } }
(V...)
and(A...)
have been expanded to their full length, even though individual clause functions may neglect to take them all. As noted above, missing parameters are filled in as if bydropArgumentsToMatch(MethodHandle, int, List, int)
.- API Note:
- Example:
The same example, dropping arguments and using combinators:// iterative implementation of the factorial function as a loop handle static int one(int k) { return 1; } static int inc(int i, int acc, int k) { return i + 1; } static int mult(int i, int acc, int k) { return i * acc; } static boolean pred(int i, int acc, int k) { return i < k; } static int fin(int i, int acc, int k) { return acc; } // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods // null initializer for counter, should initialize to 0 MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause); assertEquals(120, loop.invoke(5));
A similar example, using a helper object to hold a loop parameter:// simplified implementation of the factorial function as a loop handle static int inc(int i) { return i + 1; } // drop acc, k static int mult(int i, int acc) { return i * acc; } //drop k static boolean cmp(int i, int k) { return i < k; } // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods // null initializer for counter, should initialize to 0 MethodHandle MH_one = MethodHandles.constant(int.class, 1); MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause); assertEquals(720, loop.invoke(6));
// instance-based implementation of the factorial function as a loop handle static class FacLoop { final int k; FacLoop(int k) { this.k = k; } int inc(int i) { return i + 1; } int mult(int i, int acc) { return i * acc; } boolean pred(int i) { return i < k; } int fin(int i, int acc) { return acc; } } // assume MH_FacLoop is a handle to the constructor // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods // null initializer for counter, should initialize to 0 MethodHandle MH_one = MethodHandles.constant(int.class, 1); MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop}; MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause); assertEquals(5040, loop.invoke(7));
- Parameters:
clauses
- an array of arrays (4-tuples) ofMethodHandle
s adhering to the rules described above.- Returns:
- a method handle embodying the looping behavior as defined by the arguments.
- Throws:
IllegalArgumentException
- in case any of the constraints described above is violated.- Since:
- 9
- See Also:
whileLoop(MethodHandle, MethodHandle, MethodHandle)
,doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
,countedLoop(MethodHandle, MethodHandle, MethodHandle)
,iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
- init: Before the loop executes, the initialization of an iteration variable
-
whileLoop
public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body)
Constructs awhile
loop from an initializer, a body, and a predicate. This is a convenience wrapper for the generic loop combinator.The
pred
handle describes the loop condition; andbody
, its body. The loop resulting from this method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate evaluates totrue
). The loop will terminate once the predicate evaluates tofalse
(the body will not be executed in this case).The
init
handle describes the initial value of an additional optional loop-local variable. In each iteration, this loop-local variable, if present, will be passed to thebody
and updated with the value returned from its invocation. The result of loop execution will be the final value of the additional loop-local variable (if present).The following rules hold for these argument handles:
- The
body
handle must not benull
; its type must be of the form(V A...)V
, whereV
is non-void
, or else(A...)void
. (In thevoid
case, we assign the typevoid
to the nameV
, and we will write(V A...)V
with the understanding that avoid
typeV
is quietly dropped from the parameter list, leaving(A...)V
.) - The parameter list
(V A...)
of the body is called the internal parameter list. It will constrain the parameter lists of the other loop parts. - If the iteration variable type
V
is dropped from the internal parameter list, the resulting shorter list(A...)
is called the external parameter list. - The body return type
V
, if non-void
, determines the type of an additional state variable of the loop. The body must both accept and return a value of this typeV
. - If
init
is non-null
, it must have return typeV
. Its parameter list (of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
init
isnull
, the loop variable will be initialized to its default value. - The
pred
handle must not benull
. It must haveboolean
as its return type. Its parameter list (either empty or of the form(V A*)
) must be effectively identical to the internal parameter list.
The resulting loop handle's result type and parameter signature are determined as follows:
- The loop handle's result type is the result type
V
of the body. - The loop handle's parameter types are the types
(A...)
, from the external parameter list.
Here is pseudocode for the resulting loop handle. In the code,
V
/v
represent the type / value of the sole loop variable as well as the result type of the loop; andA
/a
, that of the argument passed to the loop.V init(A...); boolean pred(V, A...); V body(V, A...); V whileLoop(A... a...) { V v = init(a...); while (pred(v, a...)) { v = body(v, a...); } return v; }
- API Note:
- Example:
, The implementation of this method can be expressed as follows:// implement the zip function for lists as a loop handle static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); } static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); } static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) { zip.add(a.next()); zip.add(b.next()); return zip; } // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep); List<String> a = Arrays.asList("a", "b", "c", "d"); List<String> b = Arrays.asList("e", "f", "g", "h"); List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h"); assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) { MethodHandle fini = (body.type().returnType() == void.class ? null : identity(body.type().returnType())); MethodHandle[] checkExit = { null, null, pred, fini }, varBody = { init, body }; return loop(checkExit, varBody); }
- Parameters:
init
- optional initializer, providing the initial value of the loop variable. May benull
, implying a default initial value. See above for other constraints.pred
- condition for the loop, which may not benull
. Its result type must beboolean
. See above for other constraints.body
- body of the loop, which may not benull
. It controls the loop parameters and result type. See above for other constraints.- Returns:
- a method handle implementing the
while
loop as described by the arguments. - Throws:
IllegalArgumentException
- if the rules for the arguments are violated.NullPointerException
- ifpred
orbody
arenull
.- Since:
- 9
- See Also:
loop(MethodHandle[][])
,doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
- The
-
doWhileLoop
public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred)
Constructs ado-while
loop from an initializer, a body, and a predicate. This is a convenience wrapper for the generic loop combinator.The
pred
handle describes the loop condition; andbody
, its body. The loop resulting from this method will, in each iteration, first execute its body and then evaluate the predicate. The loop will terminate once the predicate evaluates tofalse
after an execution of the body.The
init
handle describes the initial value of an additional optional loop-local variable. In each iteration, this loop-local variable, if present, will be passed to thebody
and updated with the value returned from its invocation. The result of loop execution will be the final value of the additional loop-local variable (if present).The following rules hold for these argument handles:
- The
body
handle must not benull
; its type must be of the form(V A...)V
, whereV
is non-void
, or else(A...)void
. (In thevoid
case, we assign the typevoid
to the nameV
, and we will write(V A...)V
with the understanding that avoid
typeV
is quietly dropped from the parameter list, leaving(A...)V
.) - The parameter list
(V A...)
of the body is called the internal parameter list. It will constrain the parameter lists of the other loop parts. - If the iteration variable type
V
is dropped from the internal parameter list, the resulting shorter list(A...)
is called the external parameter list. - The body return type
V
, if non-void
, determines the type of an additional state variable of the loop. The body must both accept and return a value of this typeV
. - If
init
is non-null
, it must have return typeV
. Its parameter list (of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
init
isnull
, the loop variable will be initialized to its default value. - The
pred
handle must not benull
. It must haveboolean
as its return type. Its parameter list (either empty or of the form(V A*)
) must be effectively identical to the internal parameter list.
The resulting loop handle's result type and parameter signature are determined as follows:
- The loop handle's result type is the result type
V
of the body. - The loop handle's parameter types are the types
(A...)
, from the external parameter list.
Here is pseudocode for the resulting loop handle. In the code,
V
/v
represent the type / value of the sole loop variable as well as the result type of the loop; andA
/a
, that of the argument passed to the loop.V init(A...); boolean pred(V, A...); V body(V, A...); V doWhileLoop(A... a...) { V v = init(a...); do { v = body(v, a...); } while (pred(v, a...)); return v; }
- API Note:
- Example:
, The implementation of this method can be expressed as follows:// int i = 0; while (i < limit) { ++i; } return i; => limit static int zero(int limit) { return 0; } static int step(int i, int limit) { return i + 1; } static boolean pred(int i, int limit) { return i < limit; } // assume MH_zero, MH_step, and MH_pred are handles to the above methods MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred); assertEquals(23, loop.invoke(23));
MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) { MethodHandle fini = (body.type().returnType() == void.class ? null : identity(body.type().returnType())); MethodHandle[] clause = { init, body, pred, fini }; return loop(clause); }
- Parameters:
init
- optional initializer, providing the initial value of the loop variable. May benull
, implying a default initial value. See above for other constraints.body
- body of the loop, which may not benull
. It controls the loop parameters and result type. See above for other constraints.pred
- condition for the loop, which may not benull
. Its result type must beboolean
. See above for other constraints.- Returns:
- a method handle implementing the
while
loop as described by the arguments. - Throws:
IllegalArgumentException
- if the rules for the arguments are violated.NullPointerException
- ifpred
orbody
arenull
.- Since:
- 9
- See Also:
loop(MethodHandle[][])
,whileLoop(MethodHandle, MethodHandle, MethodHandle)
- The
-
countedLoop
public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body)
Constructs a loop that runs a given number of iterations. This is a convenience wrapper for the generic loop combinator.The number of iterations is determined by the
iterations
handle evaluation result. The loop counteri
is an extra loop iteration variable of typeint
. It will be initialized to 0 and incremented by 1 in each iteration.If the
body
handle returns a non-void
typeV
, a leading loop iteration variable of that type is also present. This variable is initialized using the optionalinit
handle, or to the default value of typeV
if that handle isnull
.In each iteration, the iteration variables are passed to an invocation of the
body
handle. A non-void
value returned from the body (of typeV
) updates the leading iteration variable. The result of the loop handle execution will be the finalV
value of that variable (orvoid
if there is noV
variable).The following rules hold for the argument handles:
- The
iterations
handle must not benull
, and must return the typeint
, referred to here asI
in parameter type lists. - The
body
handle must not benull
; its type must be of the form(V I A...)V
, whereV
is non-void
, or else(I A...)void
. (In thevoid
case, we assign the typevoid
to the nameV
, and we will write(V I A...)V
with the understanding that avoid
typeV
is quietly dropped from the parameter list, leaving(I A...)V
.) - The parameter list
(V I A...)
of the body contributes to a list of types called the internal parameter list. It will constrain the parameter lists of the other loop parts. - As a special case, if the body contributes only
V
andI
types, with no additionalA
types, then the internal parameter list is extended by the argument typesA...
of theiterations
handle. - If the iteration variable types
(V I)
are dropped from the internal parameter list, the resulting shorter list(A...)
is called the external parameter list. - The body return type
V
, if non-void
, determines the type of an additional state variable of the loop. The body must both accept a leading parameter and return a value of this typeV
. - If
init
is non-null
, it must have return typeV
. Its parameter list (of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
init
isnull
, the loop variable will be initialized to its default value. - The parameter list of
iterations
(of some form(A*)
) must be effectively identical to the external parameter list(A...)
.
The resulting loop handle's result type and parameter signature are determined as follows:
- The loop handle's result type is the result type
V
of the body. - The loop handle's parameter types are the types
(A...)
, from the external parameter list.
Here is pseudocode for the resulting loop handle. In the code,
V
/v
represent the type / value of the second loop variable as well as the result type of the loop; andA...
/a...
represent arguments passed to the loop.int iterations(A...); V init(A...); V body(V, int, A...); V countedLoop(A... a...) { int end = iterations(a...); V v = init(a...); for (int i = 0; i < end; ++i) { v = body(v, i, a...); } return v; }
- API Note:
- Example with a fully conformant body method:
, Example with the simplest possible body method type, and passing the number of iterations to the loop invocation:// String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s; // => a variation on a well known theme static String step(String v, int counter, String init) { return "na " + v; } // assume MH_step is a handle to the method above MethodHandle fit13 = MethodHandles.constant(int.class, 13); MethodHandle start = MethodHandles.identity(String.class); MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step); assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
, Example that treats the number of iterations, string to append to, and string to append as loop parameters:// String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s; // => a variation on a well known theme static String step(String v, int counter ) { return "na " + v; } // assume MH_step is a handle to the method above MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class); MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class); MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step); // (v, i) -> "na " + v assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
, Example that illustrates the usage of// String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s; // => a variation on a well known theme static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; } // assume MH_step is a handle to the method above MethodHandle count = MethodHandles.identity(int.class); MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class); MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step); // (v, i, _, pre, _) -> pre + " " + v assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
dropArgumentsToMatch(MethodHandle, int, List, int)
to enforce a loop type:
, The implementation of this method can be expressed as follows:// String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s; // => a variation on a well known theme static String step(String v, int counter, String pre) { return pre + " " + v; } // assume MH_step is a handle to the method above MethodType loopType = methodType(String.class, String.class, int.class, String.class); MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class), 0, loopType.parameterList(), 1); MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2); MethodHandle body = MethodHandles.dropArgumentsToMatch(MH_step, 2, loopType.parameterList(), 0); MethodHandle loop = MethodHandles.countedLoop(count, start, body); // (v, i, pre, _, _) -> pre + " " + v assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) { return countedLoop(empty(iterations.type()), iterations, init, body); }
- Parameters:
iterations
- a non-null
handle to return the number of iterations this loop should run. The handle's result type must beint
. See above for other constraints.init
- optional initializer, providing the initial value of the loop variable. May benull
, implying a default initial value. See above for other constraints.body
- body of the loop, which may not benull
. It controls the loop parameters and result type in the standard case (see above for details). It must accept its own return type (if non-void) plus anint
parameter (for the counter), and may accept any number of additional types. See above for other constraints.- Returns:
- a method handle representing the loop.
- Throws:
NullPointerException
- if either of theiterations
orbody
handles isnull
.IllegalArgumentException
- if any argument violates the rules formulated above.- Since:
- 9
- See Also:
countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
- The
-
countedLoop
public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body)
Constructs a loop that counts over a range of numbers. This is a convenience wrapper for the generic loop combinator.The loop counter
i
is a loop iteration variable of typeint
. Thestart
andend
handles determine the start (inclusive) and end (exclusive) values of the loop counter. The loop counter will be initialized to theint
value returned from the evaluation of thestart
handle and run to the value returned fromend
(exclusively) with a step width of 1.If the
body
handle returns a non-void
typeV
, a leading loop iteration variable of that type is also present. This variable is initialized using the optionalinit
handle, or to the default value of typeV
if that handle isnull
.In each iteration, the iteration variables are passed to an invocation of the
body
handle. A non-void
value returned from the body (of typeV
) updates the leading iteration variable. The result of the loop handle execution will be the finalV
value of that variable (orvoid
if there is noV
variable).The following rules hold for the argument handles:
- The
start
andend
handles must not benull
, and must both return the common typeint
, referred to here asI
in parameter type lists. - The
body
handle must not benull
; its type must be of the form(V I A...)V
, whereV
is non-void
, or else(I A...)void
. (In thevoid
case, we assign the typevoid
to the nameV
, and we will write(V I A...)V
with the understanding that avoid
typeV
is quietly dropped from the parameter list, leaving(I A...)V
.) - The parameter list
(V I A...)
of the body contributes to a list of types called the internal parameter list. It will constrain the parameter lists of the other loop parts. - As a special case, if the body contributes only
V
andI
types, with no additionalA
types, then the internal parameter list is extended by the argument typesA...
of theend
handle. - If the iteration variable types
(V I)
are dropped from the internal parameter list, the resulting shorter list(A...)
is called the external parameter list. - The body return type
V
, if non-void
, determines the type of an additional state variable of the loop. The body must both accept a leading parameter and return a value of this typeV
. - If
init
is non-null
, it must have return typeV
. Its parameter list (of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
init
isnull
, the loop variable will be initialized to its default value. - The parameter list of
start
(of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - Likewise, the parameter list of
end
must be effectively identical to the external parameter list.
The resulting loop handle's result type and parameter signature are determined as follows:
- The loop handle's result type is the result type
V
of the body. - The loop handle's parameter types are the types
(A...)
, from the external parameter list.
Here is pseudocode for the resulting loop handle. In the code,
V
/v
represent the type / value of the second loop variable as well as the result type of the loop; andA...
/a...
represent arguments passed to the loop.int start(A...); int end(A...); V init(A...); V body(V, int, A...); V countedLoop(A... a...) { int e = end(a...); int s = start(a...); V v = init(a...); for (int i = s; i < e; ++i) { v = body(v, i, a...); } return v; }
- API Note:
- The implementation of this method can be expressed as follows:
MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) { MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class); // assume MH_increment and MH_predicate are handles to implementation-internal methods with // the following semantics: // MH_increment: (int limit, int counter) -> counter + 1 // MH_predicate: (int limit, int counter) -> counter < limit Class<?> counterType = start.type().returnType(); // int Class<?> returnType = body.type().returnType(); MethodHandle incr = MH_increment, pred = MH_predicate, retv = null; if (returnType != void.class) { // ignore the V variable incr = dropArguments(incr, 1, returnType); // (limit, v, i) => (limit, i) pred = dropArguments(pred, 1, returnType); // ditto retv = dropArguments(identity(returnType), 0, counterType); // ignore limit } body = dropArguments(body, 0, counterType); // ignore the limit variable MethodHandle[] loopLimit = { end, null, pred, retv }, // limit = end(); i < limit || return v bodyClause = { init, body }, // v = init(); v = body(v, i) indexVar = { start, incr }; // i = start(); i = i + 1 return loop(loopLimit, bodyClause, indexVar); }
- Parameters:
start
- a non-null
handle to return the start value of the loop counter, which must beint
. See above for other constraints.end
- a non-null
handle to return the end value of the loop counter (the loop will run toend-1
). The result type must beint
. See above for other constraints.init
- optional initializer, providing the initial value of the loop variable. May benull
, implying a default initial value. See above for other constraints.body
- body of the loop, which may not benull
. It controls the loop parameters and result type in the standard case (see above for details). It must accept its own return type (if non-void) plus anint
parameter (for the counter), and may accept any number of additional types. See above for other constraints.- Returns:
- a method handle representing the loop.
- Throws:
NullPointerException
- if any of thestart
,end
, orbody
handles isnull
.IllegalArgumentException
- if any argument violates the rules formulated above.- Since:
- 9
- See Also:
countedLoop(MethodHandle, MethodHandle, MethodHandle)
- The
-
iteratedLoop
public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body)
Constructs a loop that ranges over the values produced by anIterator<T>
. This is a convenience wrapper for the generic loop combinator.The iterator itself will be determined by the evaluation of the
iterator
handle. Each value it produces will be stored in a loop iteration variable of typeT
.If the
body
handle returns a non-void
typeV
, a leading loop iteration variable of that type is also present. This variable is initialized using the optionalinit
handle, or to the default value of typeV
if that handle isnull
.In each iteration, the iteration variables are passed to an invocation of the
body
handle. A non-void
value returned from the body (of typeV
) updates the leading iteration variable. The result of the loop handle execution will be the finalV
value of that variable (orvoid
if there is noV
variable).The following rules hold for the argument handles:
- The
body
handle must not benull
; its type must be of the form(V T A...)V
, whereV
is non-void
, or else(T A...)void
. (In thevoid
case, we assign the typevoid
to the nameV
, and we will write(V T A...)V
with the understanding that avoid
typeV
is quietly dropped from the parameter list, leaving(T A...)V
.) - The parameter list
(V T A...)
of the body contributes to a list of types called the internal parameter list. It will constrain the parameter lists of the other loop parts. - As a special case, if the body contributes only
V
andT
types, with no additionalA
types, then the internal parameter list is extended by the argument typesA...
of theiterator
handle; if it isnull
the single typeIterable
is added and constitutes theA...
list. - If the iteration variable types
(V T)
are dropped from the internal parameter list, the resulting shorter list(A...)
is called the external parameter list. - The body return type
V
, if non-void
, determines the type of an additional state variable of the loop. The body must both accept a leading parameter and return a value of this typeV
. - If
init
is non-null
, it must have return typeV
. Its parameter list (of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
init
isnull
, the loop variable will be initialized to its default value. - If the
iterator
handle is non-null
, it must have the return typejava.util.Iterator
or a subtype thereof. The iterator it produces when the loop is executed will be assumed to yield values which can be converted to typeT
. - The parameter list of an
iterator
that is non-null
(of some form(A*)
) must be effectively identical to the external parameter list(A...)
. - If
iterator
isnull
it defaults to a method handle which behaves likeIterable.iterator()
. In that case, the internal parameter list(V T A...)
must have at least oneA
type, and the default iterator handle parameter is adjusted to accept the leadingA
type, as if by theasType
conversion method. The leadingA
type must beIterable
or a subtype thereof. This conversion step, done at loop construction time, must not throw aWrongMethodTypeException
.
The type
T
may be either a primitive or reference. Since typeIterator<T>
is erased in the method handle representation to the raw typeIterator
, theiteratedLoop
combinator adjusts the leading argument type forbody
toObject
as if by theasType
conversion method. Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur as the result of dynamic conversions performed byMethodHandle.asType(MethodType)
.The resulting loop handle's result type and parameter signature are determined as follows:
- The loop handle's result type is the result type
V
of the body. - The loop handle's parameter types are the types
(A...)
, from the external parameter list.
Here is pseudocode for the resulting loop handle. In the code,
V
/v
represent the type / value of the loop variable as well as the result type of the loop;T
/t
, that of the elements of the structure the loop iterates over, andA...
/a...
represent arguments passed to the loop.Iterator<T> iterator(A...); // defaults to Iterable::iterator V init(A...); V body(V,T,A...); V iteratedLoop(A... a...) { Iterator<T> it = iterator(a...); V v = init(a...); while (it.hasNext()) { T t = it.next(); v = body(v, t, a...); } return v; }
- API Note:
- Example:
, The implementation of this method can be expressed approximately as follows:// get an iterator from a list static List<String> reverseStep(List<String> r, String e) { r.add(0, e); return r; } static List<String> newArrayList() { return new ArrayList<>(); } // assume MH_reverseStep and MH_newArrayList are handles to the above methods MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep); List<String> list = Arrays.asList("a", "b", "c", "d", "e"); List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a"); assertEquals(reversedList, (List<String>) loop.invoke(list));
MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) { // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable Class<?> returnType = body.type().returnType(); Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1); MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype)); MethodHandle retv = null, step = body, startIter = iterator; if (returnType != void.class) { // the simple thing first: in (I V A...), drop the I to get V retv = dropArguments(identity(returnType), 0, Iterator.class); // body type signature (V T A...), internal loop types (I V A...) step = swapArguments(body, 0, 1); // swap V <-> T } if (startIter == null) startIter = MH_getIter; MethodHandle[] iterVar = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext()) bodyClause = { init, filterArguments(step, 0, nextVal) }; // v = body(v, t, a) return loop(iterVar, bodyClause); }
- Parameters:
iterator
- an optional handle to return the iterator to start the loop. If non-null
, the handle must returnIterator
or a subtype. See above for other constraints.init
- optional initializer, providing the initial value of the loop variable. May benull
, implying a default initial value. See above for other constraints.body
- body of the loop, which may not benull
. It controls the loop parameters and result type in the standard case (see above for details). It must accept its own return type (if non-void) plus aT
parameter (for the iterated values), and may accept any number of additional types. See above for other constraints.- Returns:
- a method handle embodying the iteration loop functionality.
- Throws:
NullPointerException
- if thebody
handle isnull
.IllegalArgumentException
- if any argument violates the above requirements.- Since:
- 9
- The
-
tryFinally
public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup)
Makes a method handle that adapts atarget
method handle by wrapping it in atry-finally
block. Another method handle,cleanup
, represents the functionality of thefinally
block. Any exception thrown during the execution of thetarget
handle will be passed to thecleanup
handle. The exception will be rethrown, unlesscleanup
handle throws an exception first. The value returned from thecleanup
handle's execution will be the result of the execution of thetry-finally
handle.The
cleanup
handle will be passed one or two additional leading arguments. The first is the exception thrown during the execution of thetarget
handle, ornull
if no exception was thrown. The second is the result of the execution of thetarget
handle, or, if it throws an exception, anull
, zero, orfalse
value of the required type is supplied as a placeholder. The second argument is not present if thetarget
handle has avoid
return type. (Note that, except for argument type conversions, combinators representvoid
values in parameter lists by omitting the corresponding paradoxical arguments, not by insertingnull
or zero values.)The
target
andcleanup
handles must have the same corresponding argument and return types, except that thecleanup
handle may omit trailing arguments. Also, thecleanup
handle must have one or two extra leading parameters:- a
Throwable
, which will carry the exception thrown by thetarget
handle (if any); and - a parameter of the same type as the return type of both
target
andcleanup
, which will carry the result from the execution of thetarget
handle. This parameter is not present if thetarget
returnsvoid
.
The pseudocode for the resulting adapter looks as follows. In the code,
V
represents the result type of thetry/finally
construct;A
/a
, the types and values of arguments to the resulting handle consumed by the cleanup; andB
/b
, those of arguments to the resulting handle discarded by the cleanup.V target(A..., B...); V cleanup(Throwable, V, A...); V adapter(A... a, B... b) { V result = (zero value for V); Throwable throwable = null; try { result = target(a..., b...); } catch (Throwable t) { throwable = t; throw t; } finally { result = cleanup(throwable, result, a...); } return result; }
Note that the saved arguments (
a...
in the pseudocode) cannot be modified by execution of the target, and so are passed unchanged from the caller to the cleanup, if it is invoked.The target and cleanup must return the same type, even if the cleanup always throws. To create such a throwing cleanup, compose the cleanup logic with
throwException
, in order to create a method handle of the correct return type.Note that
tryFinally
never converts exceptions into normal returns. In rare cases where exceptions must be converted in that way, first wrap the target withcatchException(MethodHandle, Class, MethodHandle)
to capture an outgoing exception, and then wrap withtryFinally
.It is recommended that the first parameter type of
cleanup
be declaredThrowable
rather than a narrower subtype. This ensurescleanup
will always be invoked with whatever exception thattarget
throws. Declaring a narrower type may result in aClassCastException
being thrown by thetry-finally
handle if the type of the exception thrown bytarget
is not assignable to the first parameter type ofcleanup
. Note that various exception types ofVirtualMachineError
,LinkageError
, andRuntimeException
can in principle be thrown by almost any kind of Java code, and a finally clause that catches (say) onlyIOException
would mask any of the others behind aClassCastException
.- Parameters:
target
- the handle whose execution is to be wrapped in atry
block.cleanup
- the handle that is invoked in the finally block.- Returns:
- a method handle embodying the
try-finally
block composed of the two arguments. - Throws:
NullPointerException
- if any argument is nullIllegalArgumentException
- ifcleanup
does not accept the required leading arguments, or if the method handle types do not match in their return types and their corresponding trailing parameters- Since:
- 9
- See Also:
catchException(MethodHandle, Class, MethodHandle)
- a
-
-