* Note: The super-class of MethodHandle is Object. * Any other super-class visible in the Reference Implementation * will be removed before the Proposed Final Draft. * Also, the final version will not include any public or * protected constructors. *
* Method handles are strongly typed according to signature. * They are not distinguished by method name or enclosing class. * A method handle must be invoked under a signature which matches * the method handle's own {@linkplain MethodType method type}. *
* Every method handle reports its type via the {@link #type type} accessor. * The structure of this type is a series of classes, one of which is * the return type of the method (or {@code void.class} if none). *
* Every method handle appears as an object containing a method named * {@link #invokeExact invokeExact}, whose signature exactly matches * the method handle's type. * A Java method call expression, which compiles to an * {@code invokevirtual} instruction, * can invoke this method from Java source code. *
* Every call to a method handle specifies an intended method type, * which must exactly match the type of the method handle. * (The type is specified in the {@code invokevirtual} instruction, * via a {@code CONSTANT_NameAndType} constant pool entry.) * The call looks within the receiver object for a method * named {@code invokeExact} of the intended method type. * The call fails with a {@link WrongMethodTypeException} * if the method does not exist, even if there is an {@code invokeExact} * method of a closely similar signature. * As with other kinds * of methods in the JVM, signature matching during method linkage * is exact, and does not allow for language-level implicit conversions * such as {@code String} to {@code Object} or {@code short} to {@code int}. *
* Each individual method handle also contains a method named * {@link #invokeGeneric invokeGeneric}, whose type is the same * as {@code invokeExact}, and is therefore also reported by * the {@link #type type} accessor. * A call to {@code invokeGeneric} works the same as a call to * {@code invokeExact}, if the signature specified by the caller * exactly matches the method handle's own type. * If there is a type mismatch, {@code invokeGeneric} attempts * to adjust the type of the target method handle * (as if by a call to {@link #asType asType}) * to obtain an exactly invokable target. * This allows a more powerful negotiation of method type * between caller and callee. *
* A method handle is an unrestricted capability to call a method. * A method handle can be formed on a non-public method by a class * that has access to that method; the resulting handle can be used * in any place by any caller who receives a reference to it. Thus, access * checking is performed when the method handle is created, not * (as in reflection) every time it is called. Handles to non-public * methods, or in non-public classes, should generally be kept secret. * They should not be passed to untrusted code unless their use from * the untrusted code would be harmless. *
* Bytecode in the JVM can directly call a method handle's * {@code invokeExact} method from an {@code invokevirtual} instruction. * The receiver class type must be {@code MethodHandle} and the method name * must be {@code invokeExact}. The signature of the invocation * (after resolving symbolic type names) must exactly match the method type * of the target method. * Similarly, bytecode can directly call a method handle's {@code invokeGeneric} * method. The signature of the invocation (after resolving symbolic type names) * must either exactly match the method type or be a valid argument to * the target's {@link #asType asType} method. *
* Every {@code invokeExact} and {@code invokeGeneric} method always * throws {@link java.lang.Throwable Throwable}, * which is to say that there is no static restriction on what a method handle * can throw. Since the JVM does not distinguish between checked * and unchecked exceptions (other than by their class, of course), * there is no particular effect on bytecode shape from ascribing * checked exceptions to method handle invocations. But in Java source * code, methods which perform method handle calls must either explicitly * throw {@code java.lang.Throwable Throwable}, or else must catch all * throwables locally, rethrowing only those which are legal in the context, * and wrapping ones which are illegal. *
* Bytecode in the JVM can directly obtain a method handle * for any accessible method from a {@code ldc} instruction * which refers to a {@code CONSTANT_Methodref} or * {@code CONSTANT_InterfaceMethodref} constant pool entry. *
* Java code can also use a reflective API called * {@link java.dyn.MethodHandles.Lookup MethodHandles.Lookup} * for creating and calling method handles. * For example, a static method handle can be obtained * from {@link java.dyn.MethodHandles.Lookup#findStatic Lookup.findStatic}. * There are also bridge methods from Core Reflection API objects, * such as {@link java.dyn.MethodHandles.Lookup#unreflect Lookup.ureflect}. *
* A method reference may refer either to a static or non-static method. * In the non-static case, the method handle type includes an explicit * receiver argument, prepended before any other arguments. * In the method handle's type, the initial receiver argument is typed * according to the class under which the method was initially requested. * (E.g., if a non-static method handle is obtained via {@code ldc}, * the type of the receiver is the class named in the constant pool entry.) *
* When a method handle to a virtual method is invoked, the method is * always looked up in the receiver (that is, the first argument). *
* A non-virtual method handles to a specific virtual method implementation * can also be created. These do not perform virtual lookup based on * receiver type. Such a method handle simulates the effect of * an {@code invokespecial} instruction to the same method. *
* Here are some examples of usage: *
Object x, y; String s; int i;
MethodType mt; MethodHandle mh;
MethodHandles.Lookup lookup = MethodHandles.lookup();
// mt is {(char,char) => String}
mt = MethodType.methodType(String.class, char.class, char.class);
mh = lookup.findVirtual(String.class, "replace", mt);
// (Ljava/lang/String;CC)Ljava/lang/String;
s = (String) mh.invokeExact("daddy",'d','n');
assert(s.equals("nanny"));
// weakly typed invocation (using MHs.invoke)
s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
assert(s.equals("savvy"));
// mt is {Object[] => List}
mt = MethodType.methodType(java.util.List.class, Object[].class);
mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
// mt is {(Object,Object,Object) => Object}
mt = MethodType.genericMethodType(3);
mh = MethodHandles.collectArguments(mh, mt);
// mt is {(Object,Object,Object) => Object}
// (Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
x = mh.invokeExact((Object)1, (Object)2, (Object)3);
assert(x.equals(java.util.Arrays.asList(1,2,3)));
// mt is { => int}
mt = MethodType.methodType(int.class);
mh = lookup.findVirtual(java.util.List.class, "size", mt);
// (Ljava/util/List;)I
i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
assert(i == 3);
* * A note on generic typing: Method handles do not represent * their function types in terms of Java parameterized (generic) types, * because there are three mismatches between function types and parameterized * Java types. *
* Like classes and strings, method handles that correspond to accessible
* fields, methods, and constructors can be represented directly
* in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
* Loading such a constant causes the component classes of its type to be loaded as necessary.
*
* @see MethodType
* @see MethodHandles
* @author John Rose, JSR 292 EG
*/
public abstract class MethodHandle
// Note: This is an implementation inheritance hack, and will be removed
// with a JVM change which moves the required hidden state onto this class.
extends MethodHandleImpl
{
private static Access IMPL_TOKEN = Access.getToken();
// interface MethodHandle
* If the call site signature exactly matches this method handle's {@link #type type},
* the call proceeds as if by {@link #invokeExact invokeExact}.
*
* Otherwise, the call proceeds as if this method handle were first
* adjusted by calling {@link #asType asType} to adjust this method handle
* to the required type, and then the call proceeds as if by
* {@link #invokeExact invokeExact} on the adjusted method handle.
*
* There is no guarantee that the {@code asType} call is actually made.
* If the JVM can predict the results of making the call, it may perform
* adaptations directly on the caller's arguments,
* and call the target method handle according to its own exact type.
*
* If the method handle is equipped with a
* {@linkplain #withTypeHandler type handler}, the handler must produce
* an entry point of the call site's exact type.
* Otherwise, the signature at the call site of {@code invokeGeneric} must
* be a valid argument to the standard {@code asType} method.
* In particular, the caller must specify the same argument arity
* as the callee's type.
* @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's type signature
* @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
*/
public final native @PolymorphicSignature Object invokeGeneric(Object... args) throws Throwable;
/**
* Perform a varargs invocation, passing the arguments in the given array
* to the method handle, as if via {@link #invokeGeneric invokeGeneric} from a call site
* which mentions only the type {@code Object}, and whose arity is the length
* of the argument array.
*
* Specifically, execution proceeds as if by the following steps,
* although the methods are not guaranteed to be called if the JVM
* can predict their effects.
*
* Because of the action of the {@code asType} step, the following argument
* conversions are applied as necessary:
*
* The result returned by the call is boxed if it is a primitive,
* or forced to null if the return type is void.
*
* This call is equivalent to the following code:
*
* If the original type and new type are equal, returns {@code this}.
*
* This method provides the crucial behavioral difference between
* {@link #invokeExact invokeExact} and {@link #invokeGeneric invokeGeneric}. The two methods
* perform the same steps when the caller's type descriptor is identical
* with the callee's, but when the types differ, {@link #invokeGeneric invokeGeneric}
* also calls {@code asType} (or some internal equivalent) in order
* to match up the caller's and callee's types.
*
* This method is equivalent to {@link MethodHandles#convertArguments convertArguments},
* except for method handles produced by {@link #withTypeHandler withTypeHandler},
* in which case the specified type handler is used for calls to {@code asType}.
*
* Note that the default behavior of {@code asType} only performs
* pairwise argument conversion and return value conversion.
* Because of this, unless the method handle has a type handler,
* the original type and new type must have the same number of arguments.
*
* @param newType the expected type of the new method handle
* @return a method handle which delegates to {@code this} after performing
* any necessary argument conversions, and arranges for any
* necessary return value conversions
* @throws WrongMethodTypeException if the conversion cannot be made
* @see MethodHandles#convertArguments
*/
public MethodHandle asType(MethodType newType) {
return MethodHandles.convertArguments(this, newType);
}
/**
* Produce a method handle which adapts, as its target,
* the current method handle. The type of the adapter will be
* the same as the type of the target, except that the final
* {@code arrayLength} parameters of the target's type are replaced
* by a single array parameter of type {@code arrayType}.
*
* If the array element type differs from any of the corresponding
* argument types on original target,
* the original target is adapted to take the array elements directly,
* as if by a call to {@link #asType asType}.
*
* When called, the adapter replaces a trailing array argument
* by the array's elements, each as its own argument to the target.
* (The order of the arguments is preserved.)
* They are converted pairwise by casting and/or unboxing
* to the types of the trailing parameters of the target.
* Finally the target is called.
* What the target eventually returns is returned unchanged by the adapter.
*
* Before calling the target, the adapter verifies that the array
* contains exactly enough elements to provide a correct argument count
* to the target method handle.
* (The array may also be null when zero elements are required.)
* @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
* @param arrayLength the number of arguments to spread from an incoming array argument
* @return a new method handle which spreads its final array argument,
* before calling the original method handle
* @throws IllegalArgumentException if {@code arrayType} is not an array type
* @throws IllegalArgumentException if target does not have at least
* {@code arrayLength} parameter types
* @throws WrongMethodTypeException if the implied {@code asType} call fails
*/
public final MethodHandle asSpreader(Class arrayType, int arrayLength) {
Class arrayElement = arrayType.getComponentType();
if (arrayElement == null) throw newIllegalArgumentException("not an array type");
MethodType oldType = type();
int nargs = oldType.parameterCount();
if (nargs < arrayLength) throw newIllegalArgumentException("bad spread array length");
int keepPosArgs = nargs - arrayLength;
MethodType newType = oldType.dropParameterTypes(keepPosArgs, nargs);
newType = newType.insertParameterTypes(keepPosArgs, arrayElement);
return MethodHandles.spreadArguments(this, newType);
}
/**
* Produce a method handle which adapts, as its target,
* the current method handle. The type of the adapter will be
* the same as the type of the target, except that a single trailing
* parameter (usually of type {@code arrayType}) is replaced by
* {@code arrayLength} parameters whose type is element type of {@code arrayType}.
*
* If the array type differs from the final argument type on original target,
* the original target is adapted to take the array type directly,
* as if by a call to {@link #asType asType}.
*
* When called, the adapter replaces its trailing {@code arrayLength}
* arguments by a single new array of type {@code arrayType}, whose elements
* comprise (in order) the replaced arguments.
* Finally the target is called.
* What the target eventually returns is returned unchanged by the adapter.
*
* (The array may also be a shared constant when {@code arrayLength} is zero.)
* @param arrayType usually {@code Object[]}, the type of the array argument which will collect the arguments
* @param arrayLength the number of arguments to collect into a new array argument
* @return a new method handle which collects some trailing argument
* into an array, before calling the original method handle
* @throws IllegalArgumentException if {@code arrayType} is not an array type
or {@code arrayType} is not assignable to this method handle's trailing parameter type
* @throws IllegalArgumentException if {@code arrayLength} is not
* a legal array size
* @throws WrongMethodTypeException if the implied {@code asType} call fails
*/
public final MethodHandle asCollector(Class arrayType, int arrayLength) {
Class arrayElement = arrayType.getComponentType();
if (arrayElement == null) throw newIllegalArgumentException("not an array type");
MethodType oldType = type();
int nargs = oldType.parameterCount();
MethodType newType = oldType.dropParameterTypes(nargs-1, nargs);
newType = newType.insertParameterTypes(nargs-1,
java.util.Collections.
* When called, the bound handle inserts the given value {@code x}
* as a new leading argument to the target. The other arguments are
* also passed unchanged.
* What the target eventually returns is returned unchanged by the bound handle.
*
* The reference {@code x} must be convertible to the first parameter
* type of the target.
* @param x the value to bind to the first argument of the target
* @return a new method handle which collects some trailing argument
* into an array, before calling the original method handle
* @throws IllegalArgumentException if the target does not have a
* leading parameter type that is a reference type
* @throws ClassCastException if {@code x} cannot be converted
* to the leading parameter type of the target
* @see MethodHandles#insertArguments
*/
public final MethodHandle bindTo(Object x) {
return MethodHandles.insertArguments(this, 0, x);
}
/**
* PROVISIONAL API, WORK IN PROGRESS:
* Create a new method handle with the same type as this one,
* but whose {@code asType} method invokes the given
* {@code typeHandler} on this method handle,
* instead of the standard {@code MethodHandles.convertArguments}.
*
* The new method handle will have the same behavior as the
* old one when invoked by {@code invokeExact}.
* For {@code invokeGeneric} calls which exactly match
* the method type, the two method handles will also
* have the same behavior.
* For other {@code invokeGeneric} calls, the {@code typeHandler}
* will control the behavior of the new method handle.
*
* Thus, a method handle with an {@code asType} handler can
* be configured to accept more than one arity of {@code invokeGeneric}
* call, and potentially every possible arity.
* It can also be configured to supply default values for
* optional arguments, when the caller does not specify them.
*
* The given method handle must take two arguments and return
* one result. The result it returns must be a method handle
* of exactly the requested type. If the result returned by
* the target is null, a {@link NullPointerException} is thrown,
* else if the type of the target does not exactly match
* the requested type, a {@link WrongMethodTypeException} is thrown.
*
* Therefore, the type handler is invoked as if by this code:
*
* For example, here is a list-making variable-arity method handle:
*
*
*
*
*
* @param arguments the arguments to pass to the target
* @return the result returned by the target
* @throws WrongMethodTypeException if the target's type cannot be adjusted to take the arguments
* @throws Throwable anything thrown by the target method invocation
* @see MethodHandles#varargsInvoker
*/
public final Object invokeWithArguments(Object... arguments) throws Throwable {
int argc = arguments == null ? 0 : arguments.length;
MethodType type = type();
if (type.parameterCount() != argc) {
// simulate invokeGeneric
return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments);
}
if (argc <= 10) {
MethodHandle invoker = MethodHandles.invokers(type).genericInvoker();
switch (argc) {
case 0: return invoker.invokeExact(this);
case 1: return invoker.invokeExact(this,
arguments[0]);
case 2: return invoker.invokeExact(this,
arguments[0], arguments[1]);
case 3: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2]);
case 4: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3]);
case 5: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4]);
case 6: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4], arguments[5]);
case 7: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4], arguments[5],
arguments[6]);
case 8: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4], arguments[5],
arguments[6], arguments[7]);
case 9: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4], arguments[5],
arguments[6], arguments[7], arguments[8]);
case 10: return invoker.invokeExact(this,
arguments[0], arguments[1], arguments[2],
arguments[3], arguments[4], arguments[5],
arguments[6], arguments[7], arguments[8],
arguments[9]);
}
}
// more than ten arguments get boxed in a varargs list:
MethodHandle invoker = MethodHandles.invokers(type).varargsInvoker(0);
return invoker.invokeExact(this, arguments);
}
/** Equivalent to {@code invokeWithArguments(arguments.toArray())}. */
public final Object invokeWithArguments(java.util.List arguments) throws Throwable {
return invokeWithArguments(arguments.toArray());
}
@Deprecated
public final Object invokeVarargs(Object... arguments) throws Throwable {
return invokeWithArguments(arguments);
}
@Deprecated
public final Object invokeVarargs(java.util.List arguments) throws Throwable {
return invokeWithArguments(arguments.toArray());
}
/**
* Produce an adapter method handle which adapts the type of the
* current method handle to a new type
* The resulting method handle is guaranteed to report a type
* which is equal to the desired new type.
*
* MethodHandle invoker = MethodHandles.varargsInvoker(this.type(), 0);
* Object result = invoker.invokeExact(this, arguments);
*
*
* MethodHandle target = this; // original method handle
* MethodHandle adapter = ...; // adapted method handle
* MethodType requestedType = ...; // argument to asType()
* if (type().equals(requestedType))
* return adapter;
* MethodHandle result = (MethodHandle)
* typeHandler.invokeGeneric(target, requestedType);
* if (!result.type().equals(requestedType))
* throw new WrongMethodTypeException();
* return result;
*
MethodHandle makeEmptyList = MethodHandles.constant(List.class, Arrays.asList());
MethodHandle asList = lookup()
.findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
static MethodHandle collectingTypeHandler(MethodHandle base, MethodType newType) {
return asList.asCollector(Object[].class, newType.parameterCount()).asType(newType);
}
MethodHandle collectingTypeHandler = lookup()
.findStatic(lookup().lookupClass(), "collectingTypeHandler",
methodType(MethodHandle.class, MethodHandle.class, MethodType.class));
MethodHandle makeAnyList = makeEmptyList.withTypeHandler(collectingTypeHandler);
System.out.println(makeAnyList.invokeGeneric()); // prints []
System.out.println(makeAnyList.invokeGeneric(1)); // prints [1]
System.out.println(makeAnyList.invokeGeneric("two", "too")); // prints [two, too]
*
*/
public MethodHandle withTypeHandler(MethodHandle typeHandler) {
return MethodHandles.withTypeHandler(this, typeHandler);
}
}