java.lang.invoke.MethodHandle Maven / Gradle / Ivy
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package java.lang.invoke;
import java.util.*;
/**
* A method handle is a typed, directly executable reference to an underlying method,
* constructor, field, or similar low-level operation, with optional
* transformations of arguments or return values.
* These transformations are quite general, and include such patterns as
* {@linkplain #asType conversion},
* {@linkplain #bindTo insertion},
* {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
* and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
*
* Method handle contents
* Method handles are dynamically and strongly typed according to their parameter and return types.
* They are not distinguished by the name or the defining class of their underlying methods.
* A method handle must be invoked using a symbolic type descriptor which matches
* the method handle's own {@linkplain #type type descriptor}.
*
* Every method handle reports its type descriptor via the {@link #type type} accessor.
* This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
* whose structure is a series of classes, one of which is
* the return type of the method (or {@code void.class} if none).
*
* A method handle's type controls the types of invocations it accepts,
* and the kinds of transformations that apply to it.
*
* A method handle contains a pair of special invoker methods
* called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
* Both invoker methods provide direct access to the method handle's
* underlying method, constructor, field, or other operation,
* as modified by transformations of arguments and return values.
* Both invokers accept calls which exactly match the method handle's own type.
* The plain, inexact invoker also accepts a range of other call types.
*
* Method handles are immutable and have no visible state.
* Of course, they can be bound to underlying methods or data which exhibit state.
* With respect to the Java Memory Model, any method handle will behave
* as if all of its (internal) fields are final variables. This means that any method
* handle made visible to the application will always be fully formed.
* This is true even if the method handle is published through a shared
* variable in a data race.
*
* Method handles cannot be subclassed by the user.
* Implementations may (or may not) create internal subclasses of {@code MethodHandle}
* which may be visible via the {@link java.lang.Object#getClass Object.getClass}
* operation. The programmer should not draw conclusions about a method handle
* from its specific class, as the method handle class hierarchy (if any)
* may change from time to time or across implementations from different vendors.
*
*
Method handle compilation
* A Java method call expression naming {@code invokeExact} or {@code invoke}
* can invoke a method handle from Java source code.
* From the viewpoint of source code, these methods can take any arguments
* and their result can be cast to any return type.
* Formally this is accomplished by giving the invoker methods
* {@code Object} return types and variable arity {@code Object} arguments,
* but they have an additional quality called signature polymorphism
* which connects this freedom of invocation directly to the JVM execution stack.
*
* As is usual with virtual methods, source-level calls to {@code invokeExact}
* and {@code invoke} compile to an {@code invokevirtual} instruction.
* More unusually, the compiler must record the actual argument types,
* and may not perform method invocation conversions on the arguments.
* Instead, it must push them on the stack according to their own unconverted types.
* The method handle object itself is pushed on the stack before the arguments.
* The compiler then calls the method handle with a symbolic type descriptor which
* describes the argument and return types.
*
* To issue a complete symbolic type descriptor, the compiler must also determine
* the return type. This is based on a cast on the method invocation expression,
* if there is one, or else {@code Object} if the invocation is an expression
* or else {@code void} if the invocation is a statement.
* The cast may be to a primitive type (but not {@code void}).
*
* As a corner case, an uncasted {@code null} argument is given
* a symbolic type descriptor of {@code java.lang.Void}.
* The ambiguity with the type {@code Void} is harmless, since there are no references of type
* {@code Void} except the null reference.
*
*
Method handle invocation
* The first time a {@code invokevirtual} instruction is executed
* it is linked, by symbolically resolving the names in the instruction
* and verifying that the method call is statically legal.
* This is true of calls to {@code invokeExact} and {@code invoke}.
* In this case, the symbolic type descriptor emitted by the compiler is checked for
* correct syntax and names it contains are resolved.
* Thus, an {@code invokevirtual} instruction which invokes
* a method handle will always link, as long
* as the symbolic type descriptor is syntactically well-formed
* and the types exist.
*
* When the {@code invokevirtual} is executed after linking,
* the receiving method handle's type is first checked by the JVM
* to ensure that it matches the symbolic type descriptor.
* If the type match fails, it means that the method which the
* caller is invoking is not present on the individual
* method handle being invoked.
*
* In the case of {@code invokeExact}, the type descriptor of the invocation
* (after resolving symbolic type names) must exactly match the method type
* of the receiving method handle.
* In the case of plain, inexact {@code invoke}, the resolved type descriptor
* must be a valid argument to the receiver's {@link #asType asType} method.
* Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
*
* After type matching, a call to {@code invokeExact} directly
* and immediately invoke the method handle's underlying method
* (or other behavior, as the case may be).
*
* A call to plain {@code invoke} works the same as a call to
* {@code invokeExact}, if the symbolic type descriptor specified by the caller
* exactly matches the method handle's own type.
* If there is a type mismatch, {@code invoke} attempts
* to adjust the type of the receiving method handle,
* as if by a call to {@link #asType asType},
* to obtain an exactly invokable method handle {@code M2}.
* This allows a more powerful negotiation of method type
* between caller and callee.
*
* (Note: The adjusted method handle {@code M2} is not directly observable,
* and implementations are therefore not required to materialize it.)
*
*
Invocation checking
* In typical programs, method handle type matching will usually succeed.
* But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
* either directly (in the case of {@code invokeExact}) or indirectly as if
* by a failed call to {@code asType} (in the case of {@code invoke}).
*
* Thus, a method type mismatch which might show up as a linkage error
* in a statically typed program can show up as
* a dynamic {@code WrongMethodTypeException}
* in a program which uses method handles.
*
* Because method types contain "live" {@code Class} objects,
* method type matching takes into account both types names and class loaders.
* Thus, even if a method handle {@code M} is created in one
* class loader {@code L1} and used in another {@code L2},
* method handle calls are type-safe, because the caller's symbolic type
* descriptor, as resolved in {@code L2},
* is matched against the original callee method's symbolic type descriptor,
* as resolved in {@code L1}.
* The resolution in {@code L1} happens when {@code M} is created
* and its type is assigned, while the resolution in {@code L2} happens
* when the {@code invokevirtual} instruction is linked.
*
* Apart from the checking of type descriptors,
* a method handle's capability to call its underlying method is unrestricted.
* If a method handle is 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.
*
* Unlike with the Core Reflection API, where access is checked every time
* a reflective method is invoked,
* method handle access checking is performed
* when the method handle is created.
* In the case of {@code ldc} (see below), access checking is performed as part of linking
* the constant pool entry underlying the constant method handle.
*
* Thus, handles to non-public methods, or to methods 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.
*
*
Method handle creation
* Java code can create a method handle that directly accesses
* any method, constructor, or field that is accessible to that code.
* This is done via a reflective, capability-based API called
* {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
* For example, a static method handle can be obtained
* from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
* There are also conversion methods from Core Reflection API objects,
* such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
*
* Like classes and strings, method handles that correspond to accessible
* fields, methods, and constructors can also be represented directly
* in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
* A new type of constant pool entry, {@code CONSTANT_MethodHandle},
* refers directly to an associated {@code CONSTANT_Methodref},
* {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
* constant pool entry.
* (For full details on method handle constants,
* see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
*
* Method handles produced by lookups or constant loads from methods or
* constructors with the variable arity modifier bit ({@code 0x0080})
* have a corresponding variable arity, as if they were defined with
* the help of {@link #asVarargsCollector asVarargsCollector}.
*
* 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.)
*
* Method handle constants are subject to the same link-time access checks
* their corresponding bytecode instructions, and the {@code ldc} instruction
* will throw corresponding linkage errors if the bytecode behaviors would
* throw such errors.
*
* As a corollary of this, access to protected members is restricted
* to receivers only of the accessing class, or one of its subclasses,
* and the accessing class must in turn be a subclass (or package sibling)
* of the protected member's defining class.
* If a method reference refers to a protected non-static method or field
* of a class outside the current package, the receiver argument will
* be narrowed to the type of the accessing class.
*
* 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 handle 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.
*
*
Usage examples
* Here are some examples of usage:
* {@code
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);
s = (String) mh.invokeExact("daddy",'d','n');
// invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
assertEquals(s, "nanny");
// weakly typed invocation (using MHs.invoke)
s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
assertEquals(s, "savvy");
// mt is (Object[])List
mt = MethodType.methodType(java.util.List.class, Object[].class);
mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
assert(mh.isVarargsCollector());
x = mh.invoke("one", "two");
// invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
assertEquals(x, java.util.Arrays.asList("one","two"));
// mt is (Object,Object,Object)Object
mt = MethodType.genericMethodType(3);
mh = mh.asType(mt);
x = mh.invokeExact((Object)1, (Object)2, (Object)3);
// invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
assertEquals(x, java.util.Arrays.asList(1,2,3));
// mt is ()int
mt = MethodType.methodType(int.class);
mh = lookup.findVirtual(java.util.List.class, "size", mt);
i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
// invokeExact(Ljava/util/List;)I
assert(i == 3);
mt = MethodType.methodType(void.class, String.class);
mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
mh.invokeExact(System.out, "Hello, world.");
// invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
* }
* Each of the above calls to {@code invokeExact} or plain {@code invoke}
* generates a single invokevirtual instruction with
* the symbolic type descriptor indicated in the following comment.
* In these examples, the helper method {@code assertEquals} is assumed to
* be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
* on its arguments, and asserts that the result is true.
*
* Exceptions
* The methods {@code invokeExact} and {@code invoke} are declared
* to throw {@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 Throwable}, or else must catch all
* throwables locally, rethrowing only those which are legal in the context,
* and wrapping ones which are illegal.
*
* Signature polymorphism
* The unusual compilation and linkage behavior of
* {@code invokeExact} and plain {@code invoke}
* is referenced by the term signature polymorphism.
* As defined in the Java Language Specification,
* a signature polymorphic method is one which can operate with
* any of a wide range of call signatures and return types.
*
* In source code, a call to a signature polymorphic method will
* compile, regardless of the requested symbolic type descriptor.
* As usual, the Java compiler emits an {@code invokevirtual}
* instruction with the given symbolic type descriptor against the named method.
* The unusual part is that the symbolic type descriptor is derived from
* the actual argument and return types, not from the method declaration.
*
* When the JVM processes bytecode containing signature polymorphic calls,
* it will successfully link any such call, regardless of its symbolic type descriptor.
* (In order to retain type safety, the JVM will guard such calls with suitable
* dynamic type checks, as described elsewhere.)
*
* Bytecode generators, including the compiler back end, are required to emit
* untransformed symbolic type descriptors for these methods.
* Tools which determine symbolic linkage are required to accept such
* untransformed descriptors, without reporting linkage errors.
*
*
Interoperation between method handles and the Core Reflection API
* Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
* any class member represented by a Core Reflection API object
* can be converted to a behaviorally equivalent method handle.
* For example, a reflective {@link java.lang.reflect.Method Method} can
* be converted to a method handle using
* {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
* The resulting method handles generally provide more direct and efficient
* access to the underlying class members.
*
* As a special case,
* when the Core Reflection API is used to view the signature polymorphic
* methods {@code invokeExact} or plain {@code invoke} in this class,
* they appear as ordinary non-polymorphic methods.
* Their reflective appearance, as viewed by
* {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
* is unaffected by their special status in this API.
* For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
* will report exactly those modifier bits required for any similarly
* declared method, including in this case {@code native} and {@code varargs} bits.
*
* As with any reflected method, these methods (when reflected) may be
* invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
* However, such reflective calls do not result in method handle invocations.
* Such a call, if passed the required argument
* (a single one, of type {@code Object[]}), will ignore the argument and
* will throw an {@code UnsupportedOperationException}.
*
* Since {@code invokevirtual} instructions can natively
* invoke method handles under any symbolic type descriptor, this reflective view conflicts
* with the normal presentation of these methods via bytecodes.
* Thus, these two native methods, when reflectively viewed by
* {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
*
* In order to obtain an invoker method for a particular type descriptor,
* use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
* or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
* The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
* API is also able to return a method handle
* to call {@code invokeExact} or plain {@code invoke},
* for any specified type descriptor .
*
*
Interoperation between method handles and Java generics
* A method handle can be obtained on a method, constructor, or field
* which is declared with Java generic types.
* As with the Core Reflection API, the type of the method handle
* will constructed from the erasure of the source-level type.
* When a method handle is invoked, the types of its arguments
* or the return value cast type may be generic types or type instances.
* If this occurs, the compiler will replace those
* types by their erasures when it constructs the symbolic type descriptor
* for the {@code invokevirtual} instruction.
*
* Method handles do not represent
* their function-like types in terms of Java parameterized (generic) types,
* because there are three mismatches between function-like types and parameterized
* Java types.
*
* - Method types range over all possible arities,
* from no arguments to up to the maximum number of allowed arguments.
* Generics are not variadic, and so cannot represent this.
* - Method types can specify arguments of primitive types,
* which Java generic types cannot range over.
* - Higher order functions over method handles (combinators) are
* often generic across a wide range of function types, including
* those of multiple arities. It is impossible to represent such
* genericity with a Java type parameter.
*
*
* Arity limits
* The JVM imposes on all methods and constructors of any kind an absolute
* limit of 255 stacked arguments. This limit can appear more restrictive
* in certain cases:
*
* - A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
*
- A non-static method consumes an extra argument for the object on which the method is called.
*
- A constructor consumes an extra argument for the object which is being constructed.
*
- Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
* it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
*
* These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
* For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
* Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
* In particular, a method handle’s type must not have an arity of the exact maximum 255.
*
* @see MethodType
* @see MethodHandles
* @author John Rose, JSR 292 EG
*/
public abstract class MethodHandle {
/**
* Internal marker interface which distinguishes (to the Java compiler)
* those methods which are signature polymorphic.
*/
@java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
@java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
@interface PolymorphicSignature { }
/**
* Reports the type of this method handle.
* Every invocation of this method handle via {@code invokeExact} must exactly match this type.
* @return the method handle type
*/
public MethodType type() {
throw new IllegalStateException();
}
/**
* Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
* The symbolic type descriptor at the call site of {@code invokeExact} must
* exactly match this method handle's {@link #type type}.
* No conversions are allowed on arguments or return values.
*
* When this method is observed via the Core Reflection API,
* it will appear as a single native method, taking an object array and returning an object.
* If this native method is invoked directly via
* {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
* or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
* it will throw an {@code UnsupportedOperationException}.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
* @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
* @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
*/
public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
/**
* Invokes the method handle, allowing any caller type descriptor,
* and optionally performing conversions on arguments and return values.
*
* If the call site's symbolic type descriptor 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.
*
* The resolved type descriptor at the call site of {@code invoke} must
* be a valid argument to the receivers {@code asType} method.
* In particular, the caller must specify the same argument arity
* as the callee's type,
* if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
*
* When this method is observed via the Core Reflection API,
* it will appear as a single native method, taking an object array and returning an object.
* If this native method is invoked directly via
* {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
* or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
* it will throw an {@code UnsupportedOperationException}.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
* @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
* @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
* @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
*/
public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
/**
* Private method for trusted invocation of a method handle respecting simplified signatures.
* Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
*
* The caller signature is restricted to the following basic types:
* Object, int, long, float, double, and void return.
*
* The caller is responsible for maintaining type correctness by ensuring
* that the each outgoing argument value is a member of the range of the corresponding
* callee argument type.
* (The caller should therefore issue appropriate casts and integer narrowing
* operations on outgoing argument values.)
* The caller can assume that the incoming result value is part of the range
* of the callee's return type.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
*/
/*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
/**
* Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
* The caller signature is restricted to basic types as with {@code invokeBasic}.
* The trailing (not leading) argument must be a MemberName.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
*/
/*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
/**
* Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
* The caller signature is restricted to basic types as with {@code invokeBasic}.
* The trailing (not leading) argument must be a MemberName.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
*/
/*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
/**
* Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
* The caller signature is restricted to basic types as with {@code invokeBasic}.
* The trailing (not leading) argument must be a MemberName.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
*/
/*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
/**
* Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
* The caller signature is restricted to basic types as with {@code invokeBasic}.
* The trailing (not leading) argument must be a MemberName.
* @param args the signature-polymorphic parameter list, statically represented using varargs
* @return the signature-polymorphic result, statically represented using {@code Object}
*/
/*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
/**
* Performs a variable arity invocation, passing the arguments in the given list
* to the method handle, as if via an inexact {@link #invoke invoke} from a call site
* which mentions only the type {@code Object}, and whose arity is the length
* of the argument list.
*
* 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.
*
* - Determine the length of the argument array as {@code N}.
* For a null reference, {@code N=0}.
* - Determine the general type {@code TN} of {@code N} arguments as
* as {@code TN=MethodType.genericMethodType(N)}.
* - Force the original target method handle {@code MH0} to the
* required type, as {@code MH1 = MH0.asType(TN)}.
* - Spread the array into {@code N} separate arguments {@code A0, ...}.
* - Invoke the type-adjusted method handle on the unpacked arguments:
* MH1.invokeExact(A0, ...).
* - Take the return value as an {@code Object} reference.
*
*
* Because of the action of the {@code asType} step, the following argument
* conversions are applied as necessary:
*
* - reference casting
*
- unboxing
*
- widening primitive conversions
*
*
* 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:
*
{@code
* MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
* Object result = invoker.invokeExact(this, arguments);
* }
*
* Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
* {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
* It can therefore be used as a bridge between native or reflective code and method handles.
*
* @param arguments the arguments to pass to the target
* @return the result returned by the target
* @throws ClassCastException if an argument cannot be converted by reference casting
* @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
* @throws Throwable anything thrown by the target method invocation
* @see MethodHandles#spreadInvoker
*/
public Object invokeWithArguments(Object... arguments) throws Throwable {
throw new IllegalStateException();
}
/**
* Performs a variable arity invocation, passing the arguments in the given array
* to the method handle, as if via an inexact {@link #invoke invoke} from a call site
* which mentions only the type {@code Object}, and whose arity is the length
* of the argument array.
*
* This method is also equivalent to the following code:
*
{@code
* invokeWithArguments(arguments.toArray()
* }
*
* @param arguments the arguments to pass to the target
* @return the result returned by the target
* @throws NullPointerException if {@code arguments} is a null reference
* @throws ClassCastException if an argument cannot be converted by reference casting
* @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
* @throws Throwable anything thrown by the target method invocation
*/
public Object invokeWithArguments(java.util.List arguments) throws Throwable {
return invokeWithArguments(arguments.toArray());
}
/**
* Produces 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.
*
* If the original type and new type are equal, returns {@code this}.
*
* The new method handle, when invoked, will perform the following
* steps:
*
* - Convert the incoming argument list to match the original
* method handle's argument list.
*
- Invoke the original method handle on the converted argument list.
*
- Convert any result returned by the original method handle
* to the return type of new method handle.
*
*
* This method provides the crucial behavioral difference between
* {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
* The two methods
* perform the same steps when the caller's type descriptor exactly m atches
* the callee's, but when the types differ, plain {@link #invoke invoke}
* also calls {@code asType} (or some internal equivalent) in order
* to match up the caller's and callee's types.
*
* If the current method is a variable arity method handle
* argument list conversion may involve the conversion and collection
* of several arguments into an array, as
* {@linkplain #asVarargsCollector described elsewhere}.
* In every other case, all conversions are applied pairwise,
* which means that each argument or return value is converted to
* exactly one argument or return value (or no return value).
* The applied conversions are defined by consulting the
* the corresponding component types of the old and new
* method handle types.
*
* Let T0 and T1 be corresponding new and old parameter types,
* or old and new return types. Specifically, for some valid index {@code i}, let
* T0{@code =newType.parameterType(i)} and T1{@code =this.type().parameterType(i)}.
* Or else, going the other way for return values, let
* T0{@code =this.type().returnType()} and T1{@code =newType.returnType()}.
* If the types are the same, the new method handle makes no change
* to the corresponding argument or return value (if any).
* Otherwise, one of the following conversions is applied
* if possible:
*
* - If T0 and T1 are references, then a cast to T1 is applied.
* (The types do not need to be related in any particular way.
* This is because a dynamic value of null can convert to any reference type.)
*
- If T0 and T1 are primitives, then a Java method invocation
* conversion (JLS 5.3) is applied, if one exists.
* (Specifically, T0 must convert to T1 by a widening primitive conversion.)
*
- If T0 is a primitive and T1 a reference,
* a Java casting conversion (JLS 5.5) is applied if one exists.
* (Specifically, the value is boxed from T0 to its wrapper class,
* which is then widened as needed to T1.)
*
- If T0 is a reference and T1 a primitive, an unboxing
* conversion will be applied at runtime, possibly followed
* by a Java method invocation conversion (JLS 5.3)
* on the primitive value. (These are the primitive widening conversions.)
* T0 must be a wrapper class or a supertype of one.
* (In the case where T0 is Object, these are the conversions
* allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
* The unboxing conversion must have a possibility of success, which means that
* if T0 is not itself a wrapper class, there must exist at least one
* wrapper class TW which is a subtype of T0 and whose unboxed
* primitive value can be widened to T1.
*
- If the return type T1 is marked as void, any returned value is discarded
*
- If the return type T0 is void and T1 a reference, a null value is introduced.
*
- If the return type T0 is void and T1 a primitive,
* a zero value is introduced.
*
* (Note: Both T0 and T1 may be regarded as static types,
* because neither corresponds specifically to the dynamic type of any
* actual argument or return value.)
*
* The method handle conversion cannot be made if any one of the required
* pairwise conversions cannot be made.
*
* At runtime, the conversions applied to reference arguments
* or return values may require additional runtime checks which can fail.
* An unboxing operation may fail because the original reference is null,
* causing a {@link java.lang.NullPointerException NullPointerException}.
* An unboxing operation or a reference cast may also fail on a reference
* to an object of the wrong type,
* causing a {@link java.lang.ClassCastException ClassCastException}.
* Although an unboxing operation may accept several kinds of wrappers,
* if none are available, a {@code ClassCastException} will be thrown.
*
* @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 NullPointerException if {@code newType} is a null reference
* @throws WrongMethodTypeException if the conversion cannot be made
* @see MethodHandles#explicitCastArguments
*/
public MethodHandle asType(MethodType newType) {
throw new IllegalStateException();
}
/**
* Makes an array-spreading method handle, which accepts a trailing array argument
* and spreads its elements as positional arguments.
* The new method handle 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 the 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.)
*
* If, when the adapter is called, the supplied array argument does
* not have the correct number of elements, the adapter will throw
* an {@link IllegalArgumentException} instead of invoking the target.
*
* Here are some simple examples of array-spreading method handles:
*
{@code
MethodHandle equals = publicLookup()
.findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
assert( (boolean) equals.invokeExact("me", (Object)"me"));
assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
// spread both arguments from a 2-array:
MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
// try to spread from anything but a 2-array:
for (int n = 0; n <= 10; n++) {
Object[] badArityArgs = (n == 2 ? null : new Object[n]);
try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
catch (IllegalArgumentException ex) { } // OK
}
// spread both arguments from a String array:
MethodHandle eq2s = equals.asSpreader(String[].class, 2);
assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
// spread second arguments from a 1-array:
MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
// spread no arguments from a 0-array or null:
MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
// asSpreader and asCollector are approximate inverses:
for (int n = 0; n <= 2; n++) {
for (Class a : new Class[]{Object[].class, String[].class, CharSequence[].class}) {
MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
assert( (boolean) equals2.invokeWithArguments("me", "me"));
assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
}
}
MethodHandle caToString = publicLookup()
.findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
MethodHandle caString3 = caToString.asCollector(char[].class, 3);
assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
* }
* @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 NullPointerException if {@code arrayType} is a null reference
* @throws IllegalArgumentException if {@code arrayType} is not an array type,
* or if target does not have at least
* {@code arrayLength} parameter types,
* or if {@code arrayLength} is negative,
* or if the resulting method handle's type would have
* too many parameters
* @throws WrongMethodTypeException if the implied {@code asType} call fails
* @see #asCollector
*/
public MethodHandle asSpreader(Class arrayType, int arrayLength) {
throw new IllegalStateException();
}
/**
* Makes an array-collecting method handle, which accepts a given number of trailing
* positional arguments and collects them into an array argument.
* The new method handle 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 the 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.)
*
* (Note: The {@code arrayType} is often identical to the last
* parameter type of the original target.
* It is an explicit argument for symmetry with {@code asSpreader}, and also
* to allow the target to use a simple {@code Object} as its last parameter type.)
*
* In order to create a collecting adapter which is not restricted to a particular
* number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
*
* Here are some examples of array-collecting method handles:
*
{@code
MethodHandle deepToString = publicLookup()
.findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
assertEquals(methodType(String.class, Object.class), ts1.type());
//assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
// arrayType can be a subtype of Object[]
MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
assertEquals(methodType(String.class, String.class, String.class), ts2.type());
assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
assertEquals("[]", (String) ts0.invokeExact());
// collectors can be nested, Lisp-style
MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
// arrayType can be any primitive array type
MethodHandle bytesToString = publicLookup()
.findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
.asCollector(byte[].class, 3);
assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
MethodHandle longsToString = publicLookup()
.findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
.asCollector(long[].class, 1);
assertEquals("[123]", (String) longsToString.invokeExact((long)123));
* }
* @param arrayType often {@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 NullPointerException if {@code arrayType} is a null reference
* @throws IllegalArgumentException if {@code arrayType} is not an array type
* or {@code arrayType} is not assignable to this method handle's trailing parameter type,
* or {@code arrayLength} is not a legal array size,
* or the resulting method handle's type would have
* too many parameters
* @throws WrongMethodTypeException if the implied {@code asType} call fails
* @see #asSpreader
* @see #asVarargsCollector
*/
public MethodHandle asCollector(Class arrayType, int arrayLength) {
throw new IllegalStateException();
}
/**
* Makes a variable arity adapter which is able to accept
* any number of trailing positional arguments and collect them
* into an array argument.
*
* The type and behavior of the adapter will be the same as
* the type and behavior of the target, except that certain
* {@code invoke} and {@code asType} requests can lead to
* trailing positional arguments being collected into target's
* trailing parameter.
* Also, the last parameter type of the adapter will be
* {@code arrayType}, even if the target has a different
* last parameter type.
*
* This transformation may return {@code this} if the method handle is
* already of variable arity and its trailing parameter type
* is identical to {@code arrayType}.
*
* When called with {@link #invokeExact invokeExact}, the adapter invokes
* the target with no argument changes.
* (Note: This behavior is different from a
* {@linkplain #asCollector fixed arity collector},
* since it accepts a whole array of indeterminate length,
* rather than a fixed number of arguments.)
*
* When called with plain, inexact {@link #invoke invoke}, if the caller
* type is the same as the adapter, the adapter invokes the target as with
* {@code invokeExact}.
* (This is the normal behavior for {@code invoke} when types match.)
*
* Otherwise, if the caller and adapter arity are the same, and the
* trailing parameter type of the caller is a reference type identical to
* or assignable to the trailing parameter type of the adapter,
* the arguments and return values are converted pairwise,
* as if by {@link #asType asType} on a fixed arity
* method handle.
*
* Otherwise, the arities differ, or the adapter's trailing parameter
* type is not assignable from the corresponding caller type.
* In this case, the adapter replaces all trailing arguments from
* the original trailing argument position onward, by
* a new array of type {@code arrayType}, whose elements
* comprise (in order) the replaced arguments.
*
* The caller type must provides as least enough arguments,
* and of the correct type, to satisfy the target's requirement for
* positional arguments before the trailing array argument.
* Thus, the caller must supply, at a minimum, {@code N-1} arguments,
* where {@code N} is the arity of the target.
* Also, there must exist conversions from the incoming arguments
* to the target's arguments.
* As with other uses of plain {@code invoke}, if these basic
* requirements are not fulfilled, a {@code WrongMethodTypeException}
* may be thrown.
*
* In all cases, what the target eventually returns is returned unchanged by the adapter.
*
* In the final case, it is exactly as if the target method handle were
* temporarily adapted with a {@linkplain #asCollector fixed arity collector}
* to the arity required by the caller type.
* (As with {@code asCollector}, if the array length is zero,
* a shared constant may be used instead of a new array.
* If the implied call to {@code asCollector} would throw
* an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
* the call to the variable arity adapter must throw
* {@code WrongMethodTypeException}.)
*
* The behavior of {@link #asType asType} is also specialized for
* variable arity adapters, to maintain the invariant that
* plain, inexact {@code invoke} is always equivalent to an {@code asType}
* call to adjust the target type, followed by {@code invokeExact}.
* Therefore, a variable arity adapter responds
* to an {@code asType} request by building a fixed arity collector,
* if and only if the adapter and requested type differ either
* in arity or trailing argument type.
* The resulting fixed arity collector has its type further adjusted
* (if necessary) to the requested type by pairwise conversion,
* as if by another application of {@code asType}.
*
* When a method handle is obtained by executing an {@code ldc} instruction
* of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
* as a variable arity method (with the modifier bit {@code 0x0080}),
* the method handle will accept multiple arities, as if the method handle
* constant were created by means of a call to {@code asVarargsCollector}.
*
* In order to create a collecting adapter which collects a predetermined
* number of arguments, and whose type reflects this predetermined number,
* use {@link #asCollector asCollector} instead.
*
* No method handle transformations produce new method handles with
* variable arity, unless they are documented as doing so.
* Therefore, besides {@code asVarargsCollector},
* all methods in {@code MethodHandle} and {@code MethodHandles}
* will return a method handle with fixed arity,
* except in the cases where they are specified to return their original
* operand (e.g., {@code asType} of the method handle's own type).
*
* Calling {@code asVarargsCollector} on a method handle which is already
* of variable arity will produce a method handle with the same type and behavior.
* It may (or may not) return the original variable arity method handle.
*
* Here is an example, of a list-making variable arity method handle:
*
{@code
MethodHandle deepToString = publicLookup()
.findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
assertEquals("[won]", (String) ts1.invoke( "won" ));
assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
// findStatic of Arrays.asList(...) produces a variable arity method handle:
MethodHandle asList = publicLookup()
.findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
assertEquals(methodType(List.class, Object[].class), asList.type());
assert(asList.isVarargsCollector());
assertEquals("[]", asList.invoke().toString());
assertEquals("[1]", asList.invoke(1).toString());
assertEquals("[two, too]", asList.invoke("two", "too").toString());
String[] argv = { "three", "thee", "tee" };
assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
List ls = (List) asList.invoke((Object)argv);
assertEquals(1, ls.size());
assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
* }
*
* Discussion:
* These rules are designed as a dynamically-typed variation
* of the Java rules for variable arity methods.
* In both cases, callers to a variable arity method or method handle
* can either pass zero or more positional arguments, or else pass
* pre-collected arrays of any length. Users should be aware of the
* special role of the final argument, and of the effect of a
* type match on that final argument, which determines whether
* or not a single trailing argument is interpreted as a whole
* array or a single element of an array to be collected.
* Note that the dynamic type of the trailing argument has no
* effect on this decision, only a comparison between the symbolic
* type descriptor of the call site and the type descriptor of the method handle.)
*
* @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
* @return a new method handle which can collect any number of trailing arguments
* into an array, before calling the original method handle
* @throws NullPointerException if {@code arrayType} is a null reference
* @throws IllegalArgumentException if {@code arrayType} is not an array type
* or {@code arrayType} is not assignable to this method handle's trailing parameter type
* @see #asCollector
* @see #isVarargsCollector
* @see #asFixedArity
*/
public MethodHandle asVarargsCollector(Class arrayType) {
throw new IllegalStateException();
}
/**
* Determines if this method handle
* supports {@linkplain #asVarargsCollector variable arity} calls.
* Such method handles arise from the following sources:
*
* - a call to {@linkplain #asVarargsCollector asVarargsCollector}
*
- a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
* which resolves to a variable arity Java method or constructor
*
- an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
* which resolves to a variable arity Java method or constructor
*
* @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
* @see #asVarargsCollector
* @see #asFixedArity
*/
public boolean isVarargsCollector() {
return false;
}
/**
* Makes a fixed arity method handle which is otherwise
* equivalent to the current method handle.
*
* If the current method handle is not of
* {@linkplain #asVarargsCollector variable arity},
* the current method handle is returned.
* This is true even if the current method handle
* could not be a valid input to {@code asVarargsCollector}.
*
* Otherwise, the resulting fixed-arity method handle has the same
* type and behavior of the current method handle,
* except that {@link #isVarargsCollector isVarargsCollector}
* will be false.
* The fixed-arity method handle may (or may not) be the
* a previous argument to {@code asVarargsCollector}.
*
* Here is an example, of a list-making variable arity method handle:
*
{@code
MethodHandle asListVar = publicLookup()
.findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
.asVarargsCollector(Object[].class);
MethodHandle asListFix = asListVar.asFixedArity();
assertEquals("[1]", asListVar.invoke(1).toString());
Exception caught = null;
try { asListFix.invoke((Object)1); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof ClassCastException);
assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
try { asListFix.invoke("two", "too"); }
catch (Exception ex) { caught = ex; }
assert(caught instanceof WrongMethodTypeException);
Object[] argv = { "three", "thee", "tee" };
assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
* }
*
* @return a new method handle which accepts only a fixed number of arguments
* @see #asVarargsCollector
* @see #isVarargsCollector
*/
public MethodHandle asFixedArity() {
assert(!isVarargsCollector());
return this;
}
/**
* Binds a value {@code x} to the first argument of a method handle, without invoking it.
* The new method handle adapts, as its target,
* the current method handle by binding it to the given argument.
* The type of the bound handle will be
* the same as the type of the target, except that a single leading
* reference parameter will be omitted.
*
* 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.
*
* (Note: Because method handles are immutable, the target method handle
* retains its original type and behavior.)
* @param x the value to bind to the first argument of the target
* @return a new method handle which prepends the given value to the incoming
* argument list, 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 MethodHandle bindTo(Object x) {
throw new IllegalStateException();
}
/**
* Returns a string representation of the method handle,
* starting with the string {@code "MethodHandle"} and
* ending with the string representation of the method handle's type.
* In other words, this method returns a string equal to the value of:
*
{@code
* "MethodHandle" + type().toString()
* }
*
* (Note: Future releases of this API may add further information
* to the string representation.
* Therefore, the present syntax should not be parsed by applications.)
*
* @return a string representation of the method handle
*/
@Override
public String toString() {
return standardString();
}
String standardString() {
throw new IllegalStateException();
}
}