+ * 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. + * + *
{@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
+ * }+ * 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}. + * @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. + *
+ * 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: + *
{@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()
+ * }+ * 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) { + 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()));
+ * }+ * 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));
+ * }+ * 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: + *
+ * 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());
+ * }+ * 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(); + } +}