1 /*
2 * Copyright (c) 2008, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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25
26 package java.lang.invoke;
27
28
29 import java.util.*;
30 import sun.invoke.util.*;
31 import sun.misc.Unsafe;
32
33 import static java.lang.invoke.MethodHandleStatics.*;
34 import java.util.logging.Level;
35 import java.util.logging.Logger;
36
37 /**
38 * A method handle is a typed, directly executable reference to an underlying method,
39 * constructor, field, or similar low-level operation, with optional
40 * transformations of arguments or return values.
41 * These transformations are quite general, and include such patterns as
42 * {@linkplain #asType conversion},
43 * {@linkplain #bindTo insertion},
44 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
45 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
46 *
47 * <h3>Method handle contents</h3>
48 * Method handles are dynamically and strongly typed according to their parameter and return types.
49 * They are not distinguished by the name or the defining class of their underlying methods.
50 * A method handle must be invoked using a symbolic type descriptor which matches
51 * the method handle's own {@linkplain #type type descriptor}.
52 * <p>
53 * Every method handle reports its type descriptor via the {@link #type type} accessor.
54 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
55 * whose structure is a series of classes, one of which is
56 * the return type of the method (or {@code void.class} if none).
57 * <p>
58 * A method handle's type controls the types of invocations it accepts,
59 * and the kinds of transformations that apply to it.
60 * <p>
61 * A method handle contains a pair of special invoker methods
62 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
63 * Both invoker methods provide direct access to the method handle's
64 * underlying method, constructor, field, or other operation,
65 * as modified by transformations of arguments and return values.
66 * Both invokers accept calls which exactly match the method handle's own type.
67 * The plain, inexact invoker also accepts a range of other call types.
68 * <p>
69 * Method handles are immutable and have no visible state.
70 * Of course, they can be bound to underlying methods or data which exhibit state.
71 * With respect to the Java Memory Model, any method handle will behave
72 * as if all of its (internal) fields are final variables. This means that any method
73 * handle made visible to the application will always be fully formed.
74 * This is true even if the method handle is published through a shared
75 * variable in a data race.
76 * <p>
77 * Method handles cannot be subclassed by the user.
78 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
79 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
80 * operation. The programmer should not draw conclusions about a method handle
81 * from its specific class, as the method handle class hierarchy (if any)
82 * may change from time to time or across implementations from different vendors.
83 *
84 * <h3>Method handle compilation</h3>
85 * A Java method call expression naming {@code invokeExact} or {@code invoke}
86 * can invoke a method handle from Java source code.
87 * From the viewpoint of source code, these methods can take any arguments
88 * and their result can be cast to any return type.
89 * Formally this is accomplished by giving the invoker methods
90 * {@code Object} return types and variable arity {@code Object} arguments,
91 * but they have an additional quality called <em>signature polymorphism</em>
92 * which connects this freedom of invocation directly to the JVM execution stack.
93 * <p>
94 * As is usual with virtual methods, source-level calls to {@code invokeExact}
95 * and {@code invoke} compile to an {@code invokevirtual} instruction.
96 * More unusually, the compiler must record the actual argument types,
97 * and may not perform method invocation conversions on the arguments.
98 * Instead, it must push them on the stack according to their own unconverted types.
99 * The method handle object itself is pushed on the stack before the arguments.
100 * The compiler then calls the method handle with a symbolic type descriptor which
101 * describes the argument and return types.
102 * <p>
103 * To issue a complete symbolic type descriptor, the compiler must also determine
104 * the return type. This is based on a cast on the method invocation expression,
105 * if there is one, or else {@code Object} if the invocation is an expression
106 * or else {@code void} if the invocation is a statement.
107 * The cast may be to a primitive type (but not {@code void}).
108 * <p>
109 * As a corner case, an uncasted {@code null} argument is given
110 * a symbolic type descriptor of {@code java.lang.Void}.
111 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
112 * {@code Void} except the null reference.
113 *
114 * <h3>Method handle invocation</h3>
115 * The first time a {@code invokevirtual} instruction is executed
116 * it is linked, by symbolically resolving the names in the instruction
117 * and verifying that the method call is statically legal.
118 * This is true of calls to {@code invokeExact} and {@code invoke}.
119 * In this case, the symbolic type descriptor emitted by the compiler is checked for
120 * correct syntax and names it contains are resolved.
121 * Thus, an {@code invokevirtual} instruction which invokes
122 * a method handle will always link, as long
123 * as the symbolic type descriptor is syntactically well-formed
124 * and the types exist.
125 * <p>
126 * When the {@code invokevirtual} is executed after linking,
127 * the receiving method handle's type is first checked by the JVM
128 * to ensure that it matches the symbolic type descriptor.
129 * If the type match fails, it means that the method which the
130 * caller is invoking is not present on the individual
131 * method handle being invoked.
132 * <p>
133 * In the case of {@code invokeExact}, the type descriptor of the invocation
134 * (after resolving symbolic type names) must exactly match the method type
135 * of the receiving method handle.
136 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
137 * must be a valid argument to the receiver's {@link #asType asType} method.
138 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
139 * <p>
140 * After type matching, a call to {@code invokeExact} directly
141 * and immediately invoke the method handle's underlying method
142 * (or other behavior, as the case may be).
143 * <p>
144 * A call to plain {@code invoke} works the same as a call to
145 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
146 * exactly matches the method handle's own type.
147 * If there is a type mismatch, {@code invoke} attempts
148 * to adjust the type of the receiving method handle,
149 * as if by a call to {@link #asType asType},
150 * to obtain an exactly invokable method handle {@code M2}.
151 * This allows a more powerful negotiation of method type
152 * between caller and callee.
153 * <p>
154 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
155 * and implementations are therefore not required to materialize it.)
156 *
157 * <h3>Invocation checking</h3>
158 * In typical programs, method handle type matching will usually succeed.
159 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
160 * either directly (in the case of {@code invokeExact}) or indirectly as if
161 * by a failed call to {@code asType} (in the case of {@code invoke}).
162 * <p>
163 * Thus, a method type mismatch which might show up as a linkage error
164 * in a statically typed program can show up as
165 * a dynamic {@code WrongMethodTypeException}
166 * in a program which uses method handles.
167 * <p>
168 * Because method types contain "live" {@code Class} objects,
169 * method type matching takes into account both types names and class loaders.
170 * Thus, even if a method handle {@code M} is created in one
171 * class loader {@code L1} and used in another {@code L2},
172 * method handle calls are type-safe, because the caller's symbolic type
173 * descriptor, as resolved in {@code L2},
174 * is matched against the original callee method's symbolic type descriptor,
175 * as resolved in {@code L1}.
176 * The resolution in {@code L1} happens when {@code M} is created
177 * and its type is assigned, while the resolution in {@code L2} happens
178 * when the {@code invokevirtual} instruction is linked.
179 * <p>
180 * Apart from the checking of type descriptors,
181 * a method handle's capability to call its underlying method is unrestricted.
182 * If a method handle is formed on a non-public method by a class
183 * that has access to that method, the resulting handle can be used
184 * in any place by any caller who receives a reference to it.
185 * <p>
186 * Unlike with the Core Reflection API, where access is checked every time
187 * a reflective method is invoked,
188 * method handle access checking is performed
189 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
190 * In the case of {@code ldc} (see below), access checking is performed as part of linking
191 * the constant pool entry underlying the constant method handle.
192 * <p>
193 * Thus, handles to non-public methods, or to methods in non-public classes,
194 * should generally be kept secret.
195 * They should not be passed to untrusted code unless their use from
196 * the untrusted code would be harmless.
197 *
198 * <h3>Method handle creation</h3>
199 * Java code can create a method handle that directly accesses
200 * any method, constructor, or field that is accessible to that code.
201 * This is done via a reflective, capability-based API called
202 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
203 * For example, a static method handle can be obtained
204 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
205 * There are also conversion methods from Core Reflection API objects,
206 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
207 * <p>
208 * Like classes and strings, method handles that correspond to accessible
209 * fields, methods, and constructors can also be represented directly
210 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
211 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
212 * refers directly to an associated {@code CONSTANT_Methodref},
213 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
214 * constant pool entry.
215 * (For full details on method handle constants,
216 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
217 * <p>
218 * Method handles produced by lookups or constant loads from methods or
219 * constructors with the variable arity modifier bit ({@code 0x0080})
220 * have a corresponding variable arity, as if they were defined with
221 * the help of {@link #asVarargsCollector asVarargsCollector}.
222 * <p>
223 * A method reference may refer either to a static or non-static method.
224 * In the non-static case, the method handle type includes an explicit
225 * receiver argument, prepended before any other arguments.
226 * In the method handle's type, the initial receiver argument is typed
227 * according to the class under which the method was initially requested.
228 * (E.g., if a non-static method handle is obtained via {@code ldc},
229 * the type of the receiver is the class named in the constant pool entry.)
230 * <p>
231 * Method handle constants are subject to the same link-time access checks
232 * their corresponding bytecode instructions, and the {@code ldc} instruction
233 * will throw corresponding linkage errors if the bytecode behaviors would
234 * throw such errors.
235 * <p>
236 * As a corollary of this, access to protected members is restricted
237 * to receivers only of the accessing class, or one of its subclasses,
238 * and the accessing class must in turn be a subclass (or package sibling)
239 * of the protected member's defining class.
240 * If a method reference refers to a protected non-static method or field
241 * of a class outside the current package, the receiver argument will
242 * be narrowed to the type of the accessing class.
243 * <p>
244 * When a method handle to a virtual method is invoked, the method is
245 * always looked up in the receiver (that is, the first argument).
246 * <p>
247 * A non-virtual method handle to a specific virtual method implementation
248 * can also be created. These do not perform virtual lookup based on
249 * receiver type. Such a method handle simulates the effect of
250 * an {@code invokespecial} instruction to the same method.
251 *
252 * <h3>Usage examples</h3>
253 * Here are some examples of usage:
254 * <p><blockquote><pre>{@code
255 Object x, y; String s; int i;
256 MethodType mt; MethodHandle mh;
257 MethodHandles.Lookup lookup = MethodHandles.lookup();
258 // mt is (char,char)String
259 mt = MethodType.methodType(String.class, char.class, char.class);
260 mh = lookup.findVirtual(String.class, "replace", mt);
261 s = (String) mh.invokeExact("daddy",'d','n');
262 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
263 assertEquals(s, "nanny");
264 // weakly typed invocation (using MHs.invoke)
265 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
266 assertEquals(s, "savvy");
267 // mt is (Object[])List
268 mt = MethodType.methodType(java.util.List.class, Object[].class);
269 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
270 assert(mh.isVarargsCollector());
271 x = mh.invoke("one", "two");
272 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
273 assertEquals(x, java.util.Arrays.asList("one","two"));
274 // mt is (Object,Object,Object)Object
275 mt = MethodType.genericMethodType(3);
276 mh = mh.asType(mt);
277 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
278 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
279 assertEquals(x, java.util.Arrays.asList(1,2,3));
280 // mt is ()int
281 mt = MethodType.methodType(int.class);
282 mh = lookup.findVirtual(java.util.List.class, "size", mt);
283 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
284 // invokeExact(Ljava/util/List;)I
285 assert(i == 3);
286 mt = MethodType.methodType(void.class, String.class);
287 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
288 mh.invokeExact(System.out, "Hello, world.");
289 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
290 * }</pre></blockquote>
291 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
292 * generates a single invokevirtual instruction with
293 * the symbolic type descriptor indicated in the following comment.
294 * In these examples, the helper method {@code assertEquals} is assumed to
295 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals }
296 * on its arguments, and asserts that the result is true.
297 *
298 * <h3>Exceptions</h3>
299 * The methods {@code invokeExact} and {@code invoke} are declared
300 * to throw {@link java.lang.Throwable Throwable},
301 * which is to say that there is no static restriction on what a method handle
302 * can throw. Since the JVM does not distinguish between checked
303 * and unchecked exceptions (other than by their class, of course),
304 * there is no particular effect on bytecode shape from ascribing
305 * checked exceptions to method handle invocations. But in Java source
306 * code, methods which perform method handle calls must either explicitly
307 * throw {@code Throwable}, or else must catch all
308 * throwables locally, rethrowing only those which are legal in the context,
309 * and wrapping ones which are illegal.
310 *
311 * <h3><a name="sigpoly"></a>Signature polymorphism</h3>
312 * The unusual compilation and linkage behavior of
313 * {@code invokeExact} and plain {@code invoke}
314 * is referenced by the term <em>signature polymorphism</em>.
315 * As defined in the Java Language Specification,
316 * a signature polymorphic method is one which can operate with
317 * any of a wide range of call signatures and return types.
318 * <p>
319 * In source code, a call to a signature polymorphic method will
320 * compile, regardless of the requested symbolic type descriptor.
321 * As usual, the Java compiler emits an {@code invokevirtual}
322 * instruction with the given symbolic type descriptor against the named method.
323 * The unusual part is that the symbolic type descriptor is derived from
324 * the actual argument and return types, not from the method declaration.
325 * <p>
326 * When the JVM processes bytecode containing signature polymorphic calls,
327 * it will successfully link any such call, regardless of its symbolic type descriptor.
328 * (In order to retain type safety, the JVM will guard such calls with suitable
329 * dynamic type checks, as described elsewhere.)
330 * <p>
331 * Bytecode generators, including the compiler back end, are required to emit
332 * untransformed symbolic type descriptors for these methods.
333 * Tools which determine symbolic linkage are required to accept such
334 * untransformed descriptors, without reporting linkage errors.
335 *
336 * <h3>Interoperation between method handles and the Core Reflection API</h3>
337 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
338 * any class member represented by a Core Reflection API object
339 * can be converted to a behaviorally equivalent method handle.
340 * For example, a reflective {@link java.lang.reflect.Method Method} can
341 * be converted to a method handle using
342 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
343 * The resulting method handles generally provide more direct and efficient
344 * access to the underlying class members.
345 * <p>
346 * As a special case,
347 * when the Core Reflection API is used to view the signature polymorphic
348 * methods {@code invokeExact} or plain {@code invoke} in this class,
349 * they appear as ordinary non-polymorphic methods.
350 * Their reflective appearance, as viewed by
351 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
352 * is unaffected by their special status in this API.
353 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
354 * will report exactly those modifier bits required for any similarly
355 * declared method, including in this case {@code native} and {@code varargs} bits.
356 * <p>
357 * As with any reflected method, these methods (when reflected) may be
358 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
359 * However, such reflective calls do not result in method handle invocations.
360 * Such a call, if passed the required argument
361 * (a single one, of type {@code Object[]}), will ignore the argument and
362 * will throw an {@code UnsupportedOperationException}.
363 * <p>
364 * Since {@code invokevirtual} instructions can natively
365 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
366 * with the normal presentation of these methods via bytecodes.
367 * Thus, these two native methods, when reflectively viewed by
368 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
369 * <p>
370 * In order to obtain an invoker method for a particular type descriptor,
371 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
372 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
373 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
374 * API is also able to return a method handle
375 * to call {@code invokeExact} or plain {@code invoke},
376 * for any specified type descriptor .
377 *
378 * <h3>Interoperation between method handles and Java generics</h3>
379 * A method handle can be obtained on a method, constructor, or field
380 * which is declared with Java generic types.
381 * As with the Core Reflection API, the type of the method handle
382 * will constructed from the erasure of the source-level type.
383 * When a method handle is invoked, the types of its arguments
384 * or the return value cast type may be generic types or type instances.
385 * If this occurs, the compiler will replace those
386 * types by their erasures when it constructs the symbolic type descriptor
387 * for the {@code invokevirtual} instruction.
388 * <p>
389 * Method handles do not represent
390 * their function-like types in terms of Java parameterized (generic) types,
391 * because there are three mismatches between function-like types and parameterized
392 * Java types.
393 * <ul>
394 * <li>Method types range over all possible arities,
395 * from no arguments to up to 255 of arguments (a limit imposed by the JVM).
396 * Generics are not variadic, and so cannot represent this.</li>
397 * <li>Method types can specify arguments of primitive types,
398 * which Java generic types cannot range over.</li>
399 * <li>Higher order functions over method handles (combinators) are
400 * often generic across a wide range of function types, including
401 * those of multiple arities. It is impossible to represent such
402 * genericity with a Java type parameter.</li>
403 * </ul>
404 *
405 * @see MethodType
406 * @see MethodHandles
407 * @author John Rose, JSR 292 EG
408 */
409 public abstract class MethodHandle {
410 static { MethodHandleImpl.initStatics(); }
411
412 /**
413 * Internal marker interface which distinguishes (to the Java compiler)
414 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
415 */
416 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
417 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
418 @interface PolymorphicSignature { }
419
420 private final MethodType type;
421 /*private*/ final LambdaForm form;
422 // form is not private so that invokers can easily fetch it
423
424 /**
425 * Reports the type of this method handle.
426 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
427 * @return the method handle type
428 */
429 public MethodType type() {
430 return type;
431 }
432
433 /**
434 * Package-private constructor for the method handle implementation hierarchy.
435 * Method handle inheritance will be contained completely within
436 * the {@code java.lang.invoke} package.
437 */
438 // @param type type (permanently assigned) of the new method handle
439 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
440 type.getClass(); // explicit NPE
441 form.getClass(); // explicit NPE
442 this.type = type;
443 this.form = form;
444
445 form.prepare(); // TO DO: Try to delay this step until just before invocation.
446 }
447
448 /**
449 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
450 * The symbolic type descriptor at the call site of {@code invokeExact} must
451 * exactly match this method handle's {@link #type type}.
452 * No conversions are allowed on arguments or return values.
453 * <p>
454 * When this method is observed via the Core Reflection API,
455 * it will appear as a single native method, taking an object array and returning an object.
456 * If this native method is invoked directly via
457 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
458 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
459 * it will throw an {@code UnsupportedOperationException}.
460 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
461 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
462 */
463 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
464
465 /**
466 * Invokes the method handle, allowing any caller type descriptor,
467 * and optionally performing conversions on arguments and return values.
468 * <p>
469 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
470 * the call proceeds as if by {@link #invokeExact invokeExact}.
471 * <p>
472 * Otherwise, the call proceeds as if this method handle were first
473 * adjusted by calling {@link #asType asType} to adjust this method handle
474 * to the required type, and then the call proceeds as if by
475 * {@link #invokeExact invokeExact} on the adjusted method handle.
476 * <p>
477 * There is no guarantee that the {@code asType} call is actually made.
478 * If the JVM can predict the results of making the call, it may perform
479 * adaptations directly on the caller's arguments,
480 * and call the target method handle according to its own exact type.
481 * <p>
482 * The resolved type descriptor at the call site of {@code invoke} must
483 * be a valid argument to the receivers {@code asType} method.
484 * In particular, the caller must specify the same argument arity
485 * as the callee's type,
486 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
487 * <p>
488 * When this method is observed via the Core Reflection API,
489 * it will appear as a single native method, taking an object array and returning an object.
490 * If this native method is invoked directly via
491 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
492 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
493 * it will throw an {@code UnsupportedOperationException}.
494 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
495 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
496 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
497 */
498 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
499
500 /**
501 * Private method for trusted invocation of a method handle respecting simplified signatures.
502 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
503 * <p>
504 * The caller signature is restricted to the following basic types:
505 * Object, int, long, float, double, and void return.
506 * <p>
507 * The caller is responsible for maintaining type correctness by ensuring
508 * that the each outgoing argument value is a member of the range of the corresponding
509 * callee argument type.
510 * (The caller should therefore issue appropriate casts and integer narrowing
511 * operations on outgoing argument values.)
512 * The caller can assume that the incoming result value is part of the range
513 * of the callee's return type.
514 */
515 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
516
517 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
518
519 /**
520 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
521 * The caller signature is restricted to basic types as with {@code invokeBasic}.
522 * The trailing (not leading) argument must be a MemberName.
523 */
524 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
525
526 /**
527 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
528 * The caller signature is restricted to basic types as with {@code invokeBasic}.
529 * The trailing (not leading) argument must be a MemberName.
530 */
531 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
532
533 /**
534 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
535 * The caller signature is restricted to basic types as with {@code invokeBasic}.
536 * The trailing (not leading) argument must be a MemberName.
537 */
538 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
539
540 /**
541 * Performs a variable arity invocation, passing the arguments in the given array
542 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
543 * which mentions only the type {@code Object}, and whose arity is the length
544 * of the argument array.
545 * <p>
546 * Specifically, execution proceeds as if by the following steps,
547 * although the methods are not guaranteed to be called if the JVM
548 * can predict their effects.
549 * <ul>
550 * <li>Determine the length of the argument array as {@code N}.
551 * For a null reference, {@code N=0}. </li>
552 * <li>Determine the general type {@code TN} of {@code N} arguments as
553 * as {@code TN=MethodType.genericMethodType(N)}.</li>
554 * <li>Force the original target method handle {@code MH0} to the
555 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
556 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
557 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
558 * MH1.invokeExact(A0, ...). </li>
559 * <li>Take the return value as an {@code Object} reference. </li>
560 * </ul>
561 * <p>
562 * Because of the action of the {@code asType} step, the following argument
563 * conversions are applied as necessary:
564 * <ul>
565 * <li>reference casting
566 * <li>unboxing
567 * <li>widening primitive conversions
568 * </ul>
569 * <p>
570 * The result returned by the call is boxed if it is a primitive,
571 * or forced to null if the return type is void.
572 * <p>
573 * This call is equivalent to the following code:
574 * <p><blockquote><pre>
575 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
576 * Object result = invoker.invokeExact(this, arguments);
577 * </pre></blockquote>
578 * <p>
579 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
580 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
581 * It can therefore be used as a bridge between native or reflective code and method handles.
582 *
583 * @param arguments the arguments to pass to the target
584 * @return the result returned by the target
585 * @throws ClassCastException if an argument cannot be converted by reference casting
586 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
587 * @throws Throwable anything thrown by the target method invocation
588 * @see MethodHandles#spreadInvoker
589 */
590 public Object invokeWithArguments(Object... arguments) throws Throwable {
591 int argc = arguments == null ? 0 : arguments.length;
592 @SuppressWarnings("LocalVariableHidesMemberVariable")
593 MethodType type = type();
594 if (type.parameterCount() != argc || isVarargsCollector()) {
595 // simulate invoke
596 return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments);
597 }
598 MethodHandle invoker = type.invokers().varargsInvoker();
599 return invoker.invokeExact(this, arguments);
600 }
601
602 /**
603 * Performs a variable arity invocation, passing the arguments in the given array
604 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
605 * which mentions only the type {@code Object}, and whose arity is the length
606 * of the argument array.
607 * <p>
608 * This method is also equivalent to the following code:
609 * <p><blockquote><pre>
610 * {@link #invokeWithArguments(Object...) invokeWithArguments}(arguments.toArray())
611 * </pre></blockquote>
612 *
613 * @param arguments the arguments to pass to the target
614 * @return the result returned by the target
615 * @throws NullPointerException if {@code arguments} is a null reference
616 * @throws ClassCastException if an argument cannot be converted by reference casting
617 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
618 * @throws Throwable anything thrown by the target method invocation
619 */
620 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
621 return invokeWithArguments(arguments.toArray());
622 }
623
624 /**
625 * Produces an adapter method handle which adapts the type of the
626 * current method handle to a new type.
627 * The resulting method handle is guaranteed to report a type
628 * which is equal to the desired new type.
629 * <p>
630 * If the original type and new type are equal, returns {@code this}.
631 * <p>
632 * The new method handle, when invoked, will perform the following
633 * steps:
634 * <ul>
635 * <li>Convert the incoming argument list to match the original
636 * method handle's argument list.
637 * <li>Invoke the original method handle on the converted argument list.
638 * <li>Convert any result returned by the original method handle
639 * to the return type of new method handle.
640 * </ul>
641 * <p>
642 * This method provides the crucial behavioral difference between
643 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
644 * The two methods
645 * perform the same steps when the caller's type descriptor exactly m atches
646 * the callee's, but when the types differ, plain {@link #invoke invoke}
647 * also calls {@code asType} (or some internal equivalent) in order
648 * to match up the caller's and callee's types.
649 * <p>
650 * If the current method is a variable arity method handle
651 * argument list conversion may involve the conversion and collection
652 * of several arguments into an array, as
653 * {@linkplain #asVarargsCollector described elsewhere}.
654 * In every other case, all conversions are applied <em>pairwise</em>,
655 * which means that each argument or return value is converted to
656 * exactly one argument or return value (or no return value).
657 * The applied conversions are defined by consulting the
658 * the corresponding component types of the old and new
659 * method handle types.
660 * <p>
661 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
662 * or old and new return types. Specifically, for some valid index {@code i}, let
663 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
664 * Or else, going the other way for return values, let
665 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
666 * If the types are the same, the new method handle makes no change
667 * to the corresponding argument or return value (if any).
668 * Otherwise, one of the following conversions is applied
669 * if possible:
670 * <ul>
671 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
672 * (The types do not need to be related in any particular way.
673 * This is because a dynamic value of null can convert to any reference type.)
674 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
675 * conversion (JLS 5.3) is applied, if one exists.
676 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
677 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
678 * a Java casting conversion (JLS 5.5) is applied if one exists.
679 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
680 * which is then widened as needed to <em>T1</em>.)
681 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
682 * conversion will be applied at runtime, possibly followed
683 * by a Java method invocation conversion (JLS 5.3)
684 * on the primitive value. (These are the primitive widening conversions.)
685 * <em>T0</em> must be a wrapper class or a supertype of one.
686 * (In the case where <em>T0</em> is Object, these are the conversions
687 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
688 * The unboxing conversion must have a possibility of success, which means that
689 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
690 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
691 * primitive value can be widened to <em>T1</em>.
692 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
693 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
694 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
695 * a zero value is introduced.
696 * </ul>
697 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
698 * because neither corresponds specifically to the <em>dynamic type</em> of any
699 * actual argument or return value.)
700 * <p>
701 * The method handle conversion cannot be made if any one of the required
702 * pairwise conversions cannot be made.
703 * <p>
704 * At runtime, the conversions applied to reference arguments
705 * or return values may require additional runtime checks which can fail.
706 * An unboxing operation may fail because the original reference is null,
707 * causing a {@link java.lang.NullPointerException NullPointerException}.
708 * An unboxing operation or a reference cast may also fail on a reference
709 * to an object of the wrong type,
710 * causing a {@link java.lang.ClassCastException ClassCastException}.
711 * Although an unboxing operation may accept several kinds of wrappers,
712 * if none are available, a {@code ClassCastException} will be thrown.
713 *
714 * @param newType the expected type of the new method handle
715 * @return a method handle which delegates to {@code this} after performing
716 * any necessary argument conversions, and arranges for any
717 * necessary return value conversions
718 * @throws NullPointerException if {@code newType} is a null reference
719 * @throws WrongMethodTypeException if the conversion cannot be made
720 * @see MethodHandles#explicitCastArguments
721 */
722 public MethodHandle asType(MethodType newType) {
723 if (!type.isConvertibleTo(newType)) {
724 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
725 }
726 return convertArguments(newType);
727 }
728
729 /**
730 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
731 * and spreads its elements as positional arguments.
732 * The new method handle adapts, as its <i>target</i>,
733 * the current method handle. The type of the adapter will be
734 * the same as the type of the target, except that the final
735 * {@code arrayLength} parameters of the target's type are replaced
736 * by a single array parameter of type {@code arrayType}.
737 * <p>
738 * If the array element type differs from any of the corresponding
739 * argument types on the original target,
740 * the original target is adapted to take the array elements directly,
741 * as if by a call to {@link #asType asType}.
742 * <p>
743 * When called, the adapter replaces a trailing array argument
744 * by the array's elements, each as its own argument to the target.
745 * (The order of the arguments is preserved.)
746 * They are converted pairwise by casting and/or unboxing
747 * to the types of the trailing parameters of the target.
748 * Finally the target is called.
749 * What the target eventually returns is returned unchanged by the adapter.
750 * <p>
751 * Before calling the target, the adapter verifies that the array
752 * contains exactly enough elements to provide a correct argument count
753 * to the target method handle.
754 * (The array may also be null when zero elements are required.)
755 * <p>
756 * Here are some simple examples of array-spreading method handles:
757 * <blockquote><pre>{@code
758 MethodHandle equals = publicLookup()
759 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
760 assert( (boolean) equals.invokeExact("me", (Object)"me"));
761 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
762 // spread both arguments from a 2-array:
763 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
764 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
765 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
766 // spread both arguments from a String array:
767 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
768 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
769 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
770 // spread second arguments from a 1-array:
771 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
772 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
773 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
774 // spread no arguments from a 0-array or null:
775 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
776 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
777 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
778 // asSpreader and asCollector are approximate inverses:
779 for (int n = 0; n <= 2; n++) {
780 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
781 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
782 assert( (boolean) equals2.invokeWithArguments("me", "me"));
783 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
784 }
785 }
786 MethodHandle caToString = publicLookup()
787 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
788 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
789 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
790 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
791 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
792 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
793 * }</pre></blockquote>
794 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
795 * @param arrayLength the number of arguments to spread from an incoming array argument
796 * @return a new method handle which spreads its final array argument,
797 * before calling the original method handle
798 * @throws NullPointerException if {@code arrayType} is a null reference
799 * @throws IllegalArgumentException if {@code arrayType} is not an array type
800 * @throws IllegalArgumentException if target does not have at least
801 * {@code arrayLength} parameter types,
802 * or if {@code arrayLength} is negative
803 * @throws WrongMethodTypeException if the implied {@code asType} call fails
804 * @see #asCollector
805 */
806 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
807 asSpreaderChecks(arrayType, arrayLength);
808 int spreadArgPos = type.parameterCount() - arrayLength;
809 return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength);
810 }
811
812 private void asSpreaderChecks(Class<?> arrayType, int arrayLength) {
813 spreadArrayChecks(arrayType, arrayLength);
814 int nargs = type().parameterCount();
815 if (nargs < arrayLength || arrayLength < 0)
816 throw newIllegalArgumentException("bad spread array length");
817 if (arrayType != Object[].class && arrayLength != 0) {
818 boolean sawProblem = false;
819 Class<?> arrayElement = arrayType.getComponentType();
820 for (int i = nargs - arrayLength; i < nargs; i++) {
821 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) {
822 sawProblem = true;
823 break;
824 }
825 }
826 if (sawProblem) {
827 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList());
828 for (int i = nargs - arrayLength; i < nargs; i++) {
829 ptypes.set(i, arrayElement);
830 }
831 // elicit an error:
832 this.asType(MethodType.methodType(type().returnType(), ptypes));
833 }
834 }
835 }
836
837 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
838 Class<?> arrayElement = arrayType.getComponentType();
839 if (arrayElement == null)
840 throw newIllegalArgumentException("not an array type", arrayType);
841 if ((arrayLength & 0x7F) != arrayLength) {
842 if ((arrayLength & 0xFF) != arrayLength)
843 throw newIllegalArgumentException("array length is not legal", arrayLength);
844 assert(arrayLength >= 128);
845 if (arrayElement == long.class ||
846 arrayElement == double.class)
847 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
848 }
849 }
850
851 /**
852 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
853 * positional arguments and collects them into an array argument.
854 * The new method handle adapts, as its <i>target</i>,
855 * the current method handle. The type of the adapter will be
856 * the same as the type of the target, except that a single trailing
857 * parameter (usually of type {@code arrayType}) is replaced by
858 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
859 * <p>
860 * If the array type differs from the final argument type on the original target,
861 * the original target is adapted to take the array type directly,
862 * as if by a call to {@link #asType asType}.
863 * <p>
864 * When called, the adapter replaces its trailing {@code arrayLength}
865 * arguments by a single new array of type {@code arrayType}, whose elements
866 * comprise (in order) the replaced arguments.
867 * Finally the target is called.
868 * What the target eventually returns is returned unchanged by the adapter.
869 * <p>
870 * (The array may also be a shared constant when {@code arrayLength} is zero.)
871 * <p>
872 * (<em>Note:</em> The {@code arrayType} is often identical to the last
873 * parameter type of the original target.
874 * It is an explicit argument for symmetry with {@code asSpreader}, and also
875 * to allow the target to use a simple {@code Object} as its last parameter type.)
876 * <p>
877 * In order to create a collecting adapter which is not restricted to a particular
878 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
879 * <p>
880 * Here are some examples of array-collecting method handles:
881 * <blockquote><pre>{@code
882 MethodHandle deepToString = publicLookup()
883 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
884 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
885 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
886 assertEquals(methodType(String.class, Object.class), ts1.type());
887 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
888 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
889 // arrayType can be a subtype of Object[]
890 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
891 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
892 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
893 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
894 assertEquals("[]", (String) ts0.invokeExact());
895 // collectors can be nested, Lisp-style
896 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
897 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
898 // arrayType can be any primitive array type
899 MethodHandle bytesToString = publicLookup()
900 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
901 .asCollector(byte[].class, 3);
902 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
903 MethodHandle longsToString = publicLookup()
904 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
905 .asCollector(long[].class, 1);
906 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
907 * }</pre></blockquote>
908 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
909 * @param arrayLength the number of arguments to collect into a new array argument
910 * @return a new method handle which collects some trailing argument
911 * into an array, before calling the original method handle
912 * @throws NullPointerException if {@code arrayType} is a null reference
913 * @throws IllegalArgumentException if {@code arrayType} is not an array type
914 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
915 * or {@code arrayLength} is not a legal array size
916 * @throws WrongMethodTypeException if the implied {@code asType} call fails
917 * @see #asSpreader
918 * @see #asVarargsCollector
919 */
920 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
921 asCollectorChecks(arrayType, arrayLength);
922 int collectArgPos = type().parameterCount()-1;
923 MethodHandle target = this;
924 if (arrayType != type().parameterType(collectArgPos))
925 target = convertArguments(type().changeParameterType(collectArgPos, arrayType));
926 MethodHandle collector = ValueConversions.varargsArray(arrayType, arrayLength);
927 return MethodHandles.collectArguments(target, collectArgPos, collector);
928 }
929
930 // private API: return true if last param exactly matches arrayType
931 private boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
932 spreadArrayChecks(arrayType, arrayLength);
933 int nargs = type().parameterCount();
934 if (nargs != 0) {
935 Class<?> lastParam = type().parameterType(nargs-1);
936 if (lastParam == arrayType) return true;
937 if (lastParam.isAssignableFrom(arrayType)) return false;
938 }
939 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
940 }
941
942 /**
943 * Makes a <em>variable arity</em> adapter which is able to accept
944 * any number of trailing positional arguments and collect them
945 * into an array argument.
946 * <p>
947 * The type and behavior of the adapter will be the same as
948 * the type and behavior of the target, except that certain
949 * {@code invoke} and {@code asType} requests can lead to
950 * trailing positional arguments being collected into target's
951 * trailing parameter.
952 * Also, the last parameter type of the adapter will be
953 * {@code arrayType}, even if the target has a different
954 * last parameter type.
955 * <p>
956 * This transformation may return {@code this} if the method handle is
957 * already of variable arity and its trailing parameter type
958 * is identical to {@code arrayType}.
959 * <p>
960 * When called with {@link #invokeExact invokeExact}, the adapter invokes
961 * the target with no argument changes.
962 * (<em>Note:</em> This behavior is different from a
963 * {@linkplain #asCollector fixed arity collector},
964 * since it accepts a whole array of indeterminate length,
965 * rather than a fixed number of arguments.)
966 * <p>
967 * When called with plain, inexact {@link #invoke invoke}, if the caller
968 * type is the same as the adapter, the adapter invokes the target as with
969 * {@code invokeExact}.
970 * (This is the normal behavior for {@code invoke} when types match.)
971 * <p>
972 * Otherwise, if the caller and adapter arity are the same, and the
973 * trailing parameter type of the caller is a reference type identical to
974 * or assignable to the trailing parameter type of the adapter,
975 * the arguments and return values are converted pairwise,
976 * as if by {@link #asType asType} on a fixed arity
977 * method handle.
978 * <p>
979 * Otherwise, the arities differ, or the adapter's trailing parameter
980 * type is not assignable from the corresponding caller type.
981 * In this case, the adapter replaces all trailing arguments from
982 * the original trailing argument position onward, by
983 * a new array of type {@code arrayType}, whose elements
984 * comprise (in order) the replaced arguments.
985 * <p>
986 * The caller type must provides as least enough arguments,
987 * and of the correct type, to satisfy the target's requirement for
988 * positional arguments before the trailing array argument.
989 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
990 * where {@code N} is the arity of the target.
991 * Also, there must exist conversions from the incoming arguments
992 * to the target's arguments.
993 * As with other uses of plain {@code invoke}, if these basic
994 * requirements are not fulfilled, a {@code WrongMethodTypeException}
995 * may be thrown.
996 * <p>
997 * In all cases, what the target eventually returns is returned unchanged by the adapter.
998 * <p>
999 * In the final case, it is exactly as if the target method handle were
1000 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1001 * to the arity required by the caller type.
1002 * (As with {@code asCollector}, if the array length is zero,
1003 * a shared constant may be used instead of a new array.
1004 * If the implied call to {@code asCollector} would throw
1005 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1006 * the call to the variable arity adapter must throw
1007 * {@code WrongMethodTypeException}.)
1008 * <p>
1009 * The behavior of {@link #asType asType} is also specialized for
1010 * variable arity adapters, to maintain the invariant that
1011 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1012 * call to adjust the target type, followed by {@code invokeExact}.
1013 * Therefore, a variable arity adapter responds
1014 * to an {@code asType} request by building a fixed arity collector,
1015 * if and only if the adapter and requested type differ either
1016 * in arity or trailing argument type.
1017 * The resulting fixed arity collector has its type further adjusted
1018 * (if necessary) to the requested type by pairwise conversion,
1019 * as if by another application of {@code asType}.
1020 * <p>
1021 * When a method handle is obtained by executing an {@code ldc} instruction
1022 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1023 * as a variable arity method (with the modifier bit {@code 0x0080}),
1024 * the method handle will accept multiple arities, as if the method handle
1025 * constant were created by means of a call to {@code asVarargsCollector}.
1026 * <p>
1027 * In order to create a collecting adapter which collects a predetermined
1028 * number of arguments, and whose type reflects this predetermined number,
1029 * use {@link #asCollector asCollector} instead.
1030 * <p>
1031 * No method handle transformations produce new method handles with
1032 * variable arity, unless they are documented as doing so.
1033 * Therefore, besides {@code asVarargsCollector},
1034 * all methods in {@code MethodHandle} and {@code MethodHandles}
1035 * will return a method handle with fixed arity,
1036 * except in the cases where they are specified to return their original
1037 * operand (e.g., {@code asType} of the method handle's own type).
1038 * <p>
1039 * Calling {@code asVarargsCollector} on a method handle which is already
1040 * of variable arity will produce a method handle with the same type and behavior.
1041 * It may (or may not) return the original variable arity method handle.
1042 * <p>
1043 * Here is an example, of a list-making variable arity method handle:
1044 * <blockquote><pre>{@code
1045 MethodHandle deepToString = publicLookup()
1046 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1047 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1048 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1049 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1050 assertEquals("[won]", (String) ts1.invoke( "won" ));
1051 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1052 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1053 MethodHandle asList = publicLookup()
1054 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1055 assertEquals(methodType(List.class, Object[].class), asList.type());
1056 assert(asList.isVarargsCollector());
1057 assertEquals("[]", asList.invoke().toString());
1058 assertEquals("[1]", asList.invoke(1).toString());
1059 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1060 String[] argv = { "three", "thee", "tee" };
1061 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1062 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1063 List ls = (List) asList.invoke((Object)argv);
1064 assertEquals(1, ls.size());
1065 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1066 * }</pre></blockquote>
1067 * <p style="font-size:smaller;">
1068 * <em>Discussion:</em>
1069 * These rules are designed as a dynamically-typed variation
1070 * of the Java rules for variable arity methods.
1071 * In both cases, callers to a variable arity method or method handle
1072 * can either pass zero or more positional arguments, or else pass
1073 * pre-collected arrays of any length. Users should be aware of the
1074 * special role of the final argument, and of the effect of a
1075 * type match on that final argument, which determines whether
1076 * or not a single trailing argument is interpreted as a whole
1077 * array or a single element of an array to be collected.
1078 * Note that the dynamic type of the trailing argument has no
1079 * effect on this decision, only a comparison between the symbolic
1080 * type descriptor of the call site and the type descriptor of the method handle.)
1081 *
1082 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1083 * @return a new method handle which can collect any number of trailing arguments
1084 * into an array, before calling the original method handle
1085 * @throws NullPointerException if {@code arrayType} is a null reference
1086 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1087 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1088 * @see #asCollector
1089 * @see #isVarargsCollector
1090 * @see #asFixedArity
1091 */
1092 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1093 Class<?> arrayElement = arrayType.getComponentType();
1094 boolean lastMatch = asCollectorChecks(arrayType, 0);
1095 if (isVarargsCollector() && lastMatch)
1096 return this;
1097 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1098 }
1099
1100 /**
1101 * Determines if this method handle
1102 * supports {@linkplain #asVarargsCollector variable arity} calls.
1103 * Such method handles arise from the following sources:
1104 * <ul>
1105 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1106 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1107 * which resolves to a variable arity Java method or constructor
1108 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1109 * which resolves to a variable arity Java method or constructor
1110 * </ul>
1111 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1112 * @see #asVarargsCollector
1113 * @see #asFixedArity
1114 */
1115 public boolean isVarargsCollector() {
1116 return false;
1117 }
1118
1119 /**
1120 * Makes a <em>fixed arity</em> method handle which is otherwise
1121 * equivalent to the the current method handle.
1122 * <p>
1123 * If the current method handle is not of
1124 * {@linkplain #asVarargsCollector variable arity},
1125 * the current method handle is returned.
1126 * This is true even if the current method handle
1127 * could not be a valid input to {@code asVarargsCollector}.
1128 * <p>
1129 * Otherwise, the resulting fixed-arity method handle has the same
1130 * type and behavior of the current method handle,
1131 * except that {@link #isVarargsCollector isVarargsCollector}
1132 * will be false.
1133 * The fixed-arity method handle may (or may not) be the
1134 * a previous argument to {@code asVarargsCollector}.
1135 * <p>
1136 * Here is an example, of a list-making variable arity method handle:
1137 * <blockquote><pre>{@code
1138 MethodHandle asListVar = publicLookup()
1139 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1140 .asVarargsCollector(Object[].class);
1141 MethodHandle asListFix = asListVar.asFixedArity();
1142 assertEquals("[1]", asListVar.invoke(1).toString());
1143 Exception caught = null;
1144 try { asListFix.invoke((Object)1); }
1145 catch (Exception ex) { caught = ex; }
1146 assert(caught instanceof ClassCastException);
1147 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1148 try { asListFix.invoke("two", "too"); }
1149 catch (Exception ex) { caught = ex; }
1150 assert(caught instanceof WrongMethodTypeException);
1151 Object[] argv = { "three", "thee", "tee" };
1152 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1153 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1154 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1155 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1156 * }</pre></blockquote>
1157 *
1158 * @return a new method handle which accepts only a fixed number of arguments
1159 * @see #asVarargsCollector
1160 * @see #isVarargsCollector
1161 */
1162 public MethodHandle asFixedArity() {
1163 assert(!isVarargsCollector());
1164 return this;
1165 }
1166
1167 /**
1168 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1169 * The new method handle adapts, as its <i>target</i>,
1170 * the current method handle by binding it to the given argument.
1171 * The type of the bound handle will be
1172 * the same as the type of the target, except that a single leading
1173 * reference parameter will be omitted.
1174 * <p>
1175 * When called, the bound handle inserts the given value {@code x}
1176 * as a new leading argument to the target. The other arguments are
1177 * also passed unchanged.
1178 * What the target eventually returns is returned unchanged by the bound handle.
1179 * <p>
1180 * The reference {@code x} must be convertible to the first parameter
1181 * type of the target.
1182 * <p>
1183 * (<em>Note:</em> Because method handles are immutable, the target method handle
1184 * retains its original type and behavior.)
1185 * @param x the value to bind to the first argument of the target
1186 * @return a new method handle which prepends the given value to the incoming
1187 * argument list, before calling the original method handle
1188 * @throws IllegalArgumentException if the target does not have a
1189 * leading parameter type that is a reference type
1190 * @throws ClassCastException if {@code x} cannot be converted
1191 * to the leading parameter type of the target
1192 * @see MethodHandles#insertArguments
1193 */
1194 public MethodHandle bindTo(Object x) {
1195 Class<?> ptype;
1196 @SuppressWarnings("LocalVariableHidesMemberVariable")
1197 MethodType type = type();
1198 if (type.parameterCount() == 0 ||
1199 (ptype = type.parameterType(0)).isPrimitive())
1200 throw newIllegalArgumentException("no leading reference parameter", x);
1201 x = ptype.cast(x); // throw CCE if needed
1202 return bindReceiver(x);
1203 }
1204
1205 /**
1206 * Returns a string representation of the method handle,
1207 * starting with the string {@code "MethodHandle"} and
1208 * ending with the string representation of the method handle's type.
1209 * In other words, this method returns a string equal to the value of:
1210 * <blockquote><pre>
1211 * "MethodHandle" + type().toString()
1212 * </pre></blockquote>
1213 * <p>
1214 * (<em>Note:</em> Future releases of this API may add further information
1215 * to the string representation.
1216 * Therefore, the present syntax should not be parsed by applications.)
1217 *
1218 * @return a string representation of the method handle
1219 */
1220 @Override
1221 public String toString() {
1222 if (DEBUG_METHOD_HANDLE_NAMES) return debugString();
1223 return standardString();
1224 }
1225 String standardString() {
1226 return "MethodHandle"+type;
1227 }
1228 String debugString() {
1229 return standardString()+"/LF="+internalForm()+internalProperties();
1230 }
1231
1232 //// Implementation methods.
1233 //// Sub-classes can override these default implementations.
1234 //// All these methods assume arguments are already validated.
1235
1236 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
1237
1238 /*non-public*/
1239 MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1240 if (!member.isVarargs()) return this;
1241 int argc = type().parameterCount();
1242 if (argc != 0) {
1243 Class<?> arrayType = type().parameterType(argc-1);
1244 if (arrayType.isArray()) {
1245 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1246 }
1247 }
1248 throw member.makeAccessException("cannot make variable arity", null);
1249 }
1250 /*non-public*/
1251 MethodHandle viewAsType(MethodType newType) {
1252 // No actual conversions, just a new view of the same method.
1253 return MethodHandleImpl.makePairwiseConvert(this, newType, 0);
1254 }
1255
1256 // Decoding
1257
1258 /*non-public*/
1259 LambdaForm internalForm() {
1260 return form;
1261 }
1262
1263 /*non-public*/
1264 MemberName internalMemberName() {
1265 return null; // DMH returns DMH.member
1266 }
1267
1268 /*non-public*/
1269 boolean isInvokeSpecial() {
1270 return false; // DMH.Special returns true
1271 }
1272
1273 /*non-public*/
1274 Object internalValues() {
1275 return null;
1276 }
1277
1278 /*non-public*/
1279 Object internalProperties() {
1280 // Override to something like "/FOO=bar"
1281 return "";
1282 }
1283
1284 //// Method handle implementation methods.
1285 //// Sub-classes can override these default implementations.
1286 //// All these methods assume arguments are already validated.
1287
1288 /*non-public*/ MethodHandle convertArguments(MethodType newType) {
1289 // Override this if it can be improved.
1290 return MethodHandleImpl.makePairwiseConvert(this, newType, 1);
1291 }
1292
1293 /*non-public*/
1294 MethodHandle bindArgument(int pos, char basicType, Object value) {
1295 // Override this if it can be improved.
1296 return rebind().bindArgument(pos, basicType, value);
1297 }
1298
1299 /*non-public*/
1300 MethodHandle bindReceiver(Object receiver) {
1301 // Override this if it can be improved.
1302 return bindArgument(0, 'L', receiver);
1303 }
1304
1305 /*non-public*/
1306 MethodHandle bindImmediate(int pos, char basicType, Object value) {
1307 // Bind an immediate value to a position in the arguments.
1308 // This means, elide the respective argument,
1309 // and replace all references to it in NamedFunction args with the specified value.
1310
1311 // CURRENT RESTRICTIONS
1312 // * only for pos 0 and UNSAFE (position is adjusted in MHImpl to make API usable for others)
1313 assert pos == 0 && basicType == 'L' && value instanceof Unsafe;
1314 MethodType type2 = type.dropParameterTypes(pos, pos + 1); // adjustment: ignore receiver!
1315 LambdaForm form2 = form.bindImmediate(pos + 1, basicType, value); // adjust pos to form-relative pos
1316 return copyWith(type2, form2);
1317 }
1318
1319 /*non-public*/
1320 MethodHandle copyWith(MethodType mt, LambdaForm lf) {
1321 throw new InternalError("copyWith: " + this.getClass());
1322 }
1323
1324 /*non-public*/
1325 MethodHandle dropArguments(MethodType srcType, int pos, int drops) {
1326 // Override this if it can be improved.
1327 return rebind().dropArguments(srcType, pos, drops);
1328 }
1329
1330 /*non-public*/
1331 MethodHandle permuteArguments(MethodType newType, int[] reorder) {
1332 // Override this if it can be improved.
1333 return rebind().permuteArguments(newType, reorder);
1334 }
1335
1336 /*non-public*/
1337 MethodHandle rebind() {
1338 // Bind 'this' into a new invoker, of the known class BMH.
1339 MethodType type2 = type();
1340 LambdaForm form2 = reinvokerForm(type2.basicType());
1341 // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) }
1342 return BoundMethodHandle.bindSingle(type2, form2, this);
1343 }
1344
1345 /*non-public*/
1346 MethodHandle reinvokerTarget() {
1347 throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this);
1348 }
1349
1350 /** Create a LF which simply reinvokes a target of the given basic type.
1351 * The target MH must override {@link #reinvokerTarget} to provide the target.
1352 */
1353 static LambdaForm reinvokerForm(MethodType mtype) {
1354 mtype = mtype.basicType();
1355 LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE);
1356 if (reinvoker != null) return reinvoker;
1357 MethodHandle MH_invokeBasic = MethodHandles.basicInvoker(mtype);
1358 final int THIS_BMH = 0;
1359 final int ARG_BASE = 1;
1360 final int ARG_LIMIT = ARG_BASE + mtype.parameterCount();
1361 int nameCursor = ARG_LIMIT;
1362 final int NEXT_MH = nameCursor++;
1363 final int REINVOKE = nameCursor++;
1364 LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType());
1365 names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]);
1366 Object[] targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class);
1367 targetArgs[0] = names[NEXT_MH]; // overwrite this MH with next MH
1368 names[REINVOKE] = new LambdaForm.Name(MH_invokeBasic, targetArgs);
1369 return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, new LambdaForm("BMH.reinvoke", ARG_LIMIT, names));
1370 }
1371
1372 private static final LambdaForm.NamedFunction NF_reinvokerTarget;
1373 static {
1374 try {
1375 NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class
1376 .getDeclaredMethod("reinvokerTarget"));
1377 } catch (ReflectiveOperationException ex) {
1378 throw newInternalError(ex);
1379 }
1380 }
1381
1382 /**
1383 * Replace the old lambda form of this method handle with a new one.
1384 * The new one must be functionally equivalent to the old one.
1385 * Threads may continue running the old form indefinitely,
1386 * but it is likely that the new one will be preferred for new executions.
1387 * Use with discretion.
1388 * @param newForm
1389 */
1390 /*non-public*/
1391 void updateForm(LambdaForm newForm) {
1392 if (form == newForm) return;
1393 // ISSUE: Should we have a memory fence here?
1394 UNSAFE.putObject(this, FORM_OFFSET, newForm);
1395 this.form.prepare(); // as in MethodHandle.<init>
1396 }
1397
1398 private static final long FORM_OFFSET;
1399 static {
1400 try {
1401 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1402 } catch (ReflectiveOperationException ex) {
1403 throw newInternalError(ex);
1404 }
1405 }
1406 }