* 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. * *
* 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. * *
* 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.) * *
* 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. * *
* 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. * *
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
* * 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. * *
* 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 . * *
* 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. *
* 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}. * @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}. * @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. */ /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; /*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. */ /*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. */ /*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. */ /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; /** * 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. *
* 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: *
* 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 { int argc = arguments == null ? 0 : arguments.length; @SuppressWarnings("LocalVariableHidesMemberVariable") MethodType type = type(); if (type.parameterCount() != argc || isVarargsCollector()) { // simulate invoke return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments); } MethodHandle invoker = type.invokers().varargsInvoker(); return invoker.invokeExact(this, arguments); } /** * 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: *
* {@link #invokeWithArguments(Object...) invokeWithArguments}(arguments.toArray())
* * If the original type and new type are equal, returns {@code this}. *
* The new method handle, when invoked, will perform the following * steps: *
* 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: *
* 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) { if (!type.isConvertibleTo(newType)) { throw new WrongMethodTypeException("cannot convert "+this+" to "+newType); } return convertArguments(newType); } /** * 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.) *
* Here are some simple examples of array-spreading method handles: *
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" }));
// 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()));
* * 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: *
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));
* * 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: *
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) { Class arrayElement = arrayType.getComponentType(); boolean lastMatch = asCollectorChecks(arrayType, 0); if (isVarargsCollector() && lastMatch) return this; return MethodHandleImpl.makeVarargsCollector(this, arrayType); } /** * Determines if this method handle * supports {@linkplain #asVarargsCollector variable arity} calls. * Such method handles arise from the following sources: *
* 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: *
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());
* * 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) { Class ptype; @SuppressWarnings("LocalVariableHidesMemberVariable") MethodType type = type(); if (type.parameterCount() == 0 || (ptype = type.parameterType(0)).isPrimitive()) throw newIllegalArgumentException("no leading reference parameter", x); x = ptype.cast(x); // throw CCE if needed return bindReceiver(x); } /** * 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: *
* "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() {
if (DEBUG_METHOD_HANDLE_NAMES) return debugString();
return standardString();
}
String standardString() {
return "MethodHandle"+type;
}
String debugString() {
return standardString()+"="+internalForm()+internalValues();
}
//// Implementation methods.
//// Sub-classes can override these default implementations.
//// All these methods assume arguments are already validated.
// Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
/*non-public*/
MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
if (!member.isVarargs()) return this;
int argc = type().parameterCount();
if (argc != 0) {
Class arrayType = type().parameterType(argc-1);
if (arrayType.isArray()) {
return MethodHandleImpl.makeVarargsCollector(this, arrayType);
}
}
throw member.makeAccessException("cannot make variable arity", null);
}
/*non-public*/
MethodHandle viewAsType(MethodType newType) {
// No actual conversions, just a new view of the same method.
if (!type.isViewableAs(newType))
throw new InternalError();
return MethodHandleImpl.makePairwiseConvert(this, newType, 0);
}
// Decoding
/*non-public*/
LambdaForm internalForm() {
return form;
}
/*non-public*/
MemberName internalMemberName() {
return null; // DMH returns DMH.member
}
/*non-public*/
Object internalValues() {
return "";
}
//// Method handle implementation methods.
//// Sub-classes can override these default implementations.
//// All these methods assume arguments are already validated.
/*non-public*/ MethodHandle convertArguments(MethodType newType) {
// Override this if it can be improved.
return MethodHandleImpl.makePairwiseConvert(this, newType, 1);
}
/*non-public*/
MethodHandle bindArgument(int pos, char basicType, Object value) {
// Override this if it can be improved.
return rebind().bindArgument(pos, basicType, value);
}
/*non-public*/
MethodHandle bindReceiver(Object receiver) {
// Override this if it can be improved.
return bindArgument(0, 'L', receiver);
}
/*non-public*/
MethodHandle bindImmediate(int pos, char basicType, Object value) {
// Bind an immediate value to a position in the arguments.
// This means, elide the respective argument,
// and replace all references to it in NamedFunction args with the specified value.
// CURRENT RESTRICTIONS
// * only for pos 0 and UNSAFE (position is adjusted in MHImpl to make API usable for others)
assert pos == 0 && basicType == 'L' && value instanceof Unsafe;
MethodType type2 = type.dropParameterTypes(pos, pos + 1); // adjustment: ignore receiver!
LambdaForm form2 = form.bindImmediate(pos + 1, basicType, value); // adjust pos to form-relative pos
return copyWith(type2, form2);
}
/*non-public*/
MethodHandle copyWith(MethodType mt, LambdaForm lf) {
throw new InternalError("copyWith: " + this.getClass());
}
/*non-public*/
MethodHandle dropArguments(MethodType srcType, int pos, int drops) {
// Override this if it can be improved.
return rebind().dropArguments(srcType, pos, drops);
}
/*non-public*/
MethodHandle permuteArguments(MethodType newType, int[] reorder) {
// Override this if it can be improved.
return rebind().permuteArguments(newType, reorder);
}
/*non-public*/
MethodHandle rebind() {
// Bind 'this' into a new invoker, of the known class BMH.
MethodType type2 = type();
LambdaForm form2 = reinvokerForm(type2.basicType());
// form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) }
return BoundMethodHandle.bindSingle(type2, form2, this);
}
/*non-public*/
MethodHandle reinvokerTarget() {
throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this);
}
/** Create a LF which simply reinvokes a target of the given basic type.
* The target MH must override {@link #reinvokerTarget} to provide the target.
*/
static LambdaForm reinvokerForm(MethodType mtype) {
mtype = mtype.basicType();
LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE);
if (reinvoker != null) return reinvoker;
MethodHandle MH_invokeBasic = MethodHandles.basicInvoker(mtype);
final int THIS_BMH = 0;
final int ARG_BASE = 1;
final int ARG_LIMIT = ARG_BASE + mtype.parameterCount();
int nameCursor = ARG_LIMIT;
final int NEXT_MH = nameCursor++;
final int REINVOKE = nameCursor++;
LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType());
names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]);
Object[] targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class);
targetArgs[0] = names[NEXT_MH]; // overwrite this MH with next MH
names[REINVOKE] = new LambdaForm.Name(MH_invokeBasic, targetArgs);
return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, new LambdaForm("BMH.reinvoke", ARG_LIMIT, names));
}
private static final LambdaForm.NamedFunction NF_reinvokerTarget;
static {
try {
NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class
.getDeclaredMethod("reinvokerTarget"));
} catch (ReflectiveOperationException ex) {
throw new InternalError(ex);
}
}
/**
* Replace the old lambda form of this method handle with a new one.
* The new one must be functionally equivalent to the old one.
* Threads may continue running the old form indefinitely,
* but it is likely that the new one will be preferred for new executions.
* Use with discretion.
* @param newForm
*/
/*non-public*/
void updateForm(LambdaForm newForm) {
if (form == newForm) return;
// ISSUE: Should we have a memory fence here?
UNSAFE.putObject(this, FORM_OFFSET, newForm);
this.form.prepare(); // as in MethodHandle.