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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 jdk.internal.HotSpotIntrinsicCandidate;
31
32 import static java.lang.invoke.MethodHandleStatics.*;
33
34 /**
35 * A method handle is a typed, directly executable reference to an underlying method,
36 * constructor, field, or similar low-level operation, with optional
37 * transformations of arguments or return values.
38 * These transformations are quite general, and include such patterns as
39 * {@linkplain #asType conversion},
40 * {@linkplain #bindTo insertion},
41 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
42 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
43 *
44 * <h1>Method handle contents</h1>
45 * Method handles are dynamically and strongly typed according to their parameter and return types.
46 * They are not distinguished by the name or the defining class of their underlying methods.
47 * A method handle must be invoked using a symbolic type descriptor which matches
48 * the method handle's own {@linkplain #type type descriptor}.
49 * <p>
50 * Every method handle reports its type descriptor via the {@link #type type} accessor.
51 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
52 * whose structure is a series of classes, one of which is
53 * the return type of the method (or {@code void.class} if none).
54 * <p>
55 * A method handle's type controls the types of invocations it accepts,
56 * and the kinds of transformations that apply to it.
57 * <p>
58 * A method handle contains a pair of special invoker methods
59 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
60 * Both invoker methods provide direct access to the method handle's
61 * underlying method, constructor, field, or other operation,
62 * as modified by transformations of arguments and return values.
63 * Both invokers accept calls which exactly match the method handle's own type.
64 * The plain, inexact invoker also accepts a range of other call types.
65 * <p>
66 * Method handles are immutable and have no visible state.
67 * Of course, they can be bound to underlying methods or data which exhibit state.
68 * With respect to the Java Memory Model, any method handle will behave
69 * as if all of its (internal) fields are final variables. This means that any method
70 * handle made visible to the application will always be fully formed.
71 * This is true even if the method handle is published through a shared
72 * variable in a data race.
73 * <p>
74 * Method handles cannot be subclassed by the user.
75 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
76 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
77 * operation. The programmer should not draw conclusions about a method handle
78 * from its specific class, as the method handle class hierarchy (if any)
79 * may change from time to time or across implementations from different vendors.
80 *
81 * <h1>Method handle compilation</h1>
82 * A Java method call expression naming {@code invokeExact} or {@code invoke}
83 * can invoke a method handle from Java source code.
84 * From the viewpoint of source code, these methods can take any arguments
85 * and their result can be cast to any return type.
86 * Formally this is accomplished by giving the invoker methods
87 * {@code Object} return types and variable arity {@code Object} arguments,
88 * but they have an additional quality called <em>signature polymorphism</em>
89 * which connects this freedom of invocation directly to the JVM execution stack.
90 * <p>
91 * As is usual with virtual methods, source-level calls to {@code invokeExact}
92 * and {@code invoke} compile to an {@code invokevirtual} instruction.
93 * More unusually, the compiler must record the actual argument types,
94 * and may not perform method invocation conversions on the arguments.
95 * Instead, it must push them on the stack according to their own unconverted types.
96 * The method handle object itself is pushed on the stack before the arguments.
97 * The compiler then calls the method handle with a symbolic type descriptor which
98 * describes the argument and return types.
99 * <p>
100 * To issue a complete symbolic type descriptor, the compiler must also determine
101 * the return type. This is based on a cast on the method invocation expression,
102 * if there is one, or else {@code Object} if the invocation is an expression
103 * or else {@code void} if the invocation is a statement.
104 * The cast may be to a primitive type (but not {@code void}).
105 * <p>
106 * As a corner case, an uncasted {@code null} argument is given
107 * a symbolic type descriptor of {@code java.lang.Void}.
108 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
109 * {@code Void} except the null reference.
110 *
111 * <h1>Method handle invocation</h1>
112 * The first time a {@code invokevirtual} instruction is executed
113 * it is linked, by symbolically resolving the names in the instruction
114 * and verifying that the method call is statically legal.
115 * This is true of calls to {@code invokeExact} and {@code invoke}.
116 * In this case, the symbolic type descriptor emitted by the compiler is checked for
117 * correct syntax and names it contains are resolved.
118 * Thus, an {@code invokevirtual} instruction which invokes
119 * a method handle will always link, as long
120 * as the symbolic type descriptor is syntactically well-formed
121 * and the types exist.
122 * <p>
123 * When the {@code invokevirtual} is executed after linking,
124 * the receiving method handle's type is first checked by the JVM
125 * to ensure that it matches the symbolic type descriptor.
126 * If the type match fails, it means that the method which the
127 * caller is invoking is not present on the individual
128 * method handle being invoked.
129 * <p>
130 * In the case of {@code invokeExact}, the type descriptor of the invocation
131 * (after resolving symbolic type names) must exactly match the method type
132 * of the receiving method handle.
133 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
134 * must be a valid argument to the receiver's {@link #asType asType} method.
135 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
136 * <p>
137 * After type matching, a call to {@code invokeExact} directly
138 * and immediately invoke the method handle's underlying method
139 * (or other behavior, as the case may be).
140 * <p>
141 * A call to plain {@code invoke} works the same as a call to
142 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
143 * exactly matches the method handle's own type.
144 * If there is a type mismatch, {@code invoke} attempts
145 * to adjust the type of the receiving method handle,
146 * as if by a call to {@link #asType asType},
147 * to obtain an exactly invokable method handle {@code M2}.
148 * This allows a more powerful negotiation of method type
149 * between caller and callee.
150 * <p>
151 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
152 * and implementations are therefore not required to materialize it.)
153 *
154 * <h1>Invocation checking</h1>
155 * In typical programs, method handle type matching will usually succeed.
156 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
157 * either directly (in the case of {@code invokeExact}) or indirectly as if
158 * by a failed call to {@code asType} (in the case of {@code invoke}).
159 * <p>
160 * Thus, a method type mismatch which might show up as a linkage error
161 * in a statically typed program can show up as
162 * a dynamic {@code WrongMethodTypeException}
163 * in a program which uses method handles.
164 * <p>
165 * Because method types contain "live" {@code Class} objects,
166 * method type matching takes into account both types names and class loaders.
167 * Thus, even if a method handle {@code M} is created in one
168 * class loader {@code L1} and used in another {@code L2},
169 * method handle calls are type-safe, because the caller's symbolic type
170 * descriptor, as resolved in {@code L2},
171 * is matched against the original callee method's symbolic type descriptor,
172 * as resolved in {@code L1}.
173 * The resolution in {@code L1} happens when {@code M} is created
174 * and its type is assigned, while the resolution in {@code L2} happens
175 * when the {@code invokevirtual} instruction is linked.
176 * <p>
177 * Apart from the checking of type descriptors,
178 * a method handle's capability to call its underlying method is unrestricted.
179 * If a method handle is formed on a non-public method by a class
180 * that has access to that method, the resulting handle can be used
181 * in any place by any caller who receives a reference to it.
182 * <p>
183 * Unlike with the Core Reflection API, where access is checked every time
184 * a reflective method is invoked,
185 * method handle access checking is performed
186 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
187 * In the case of {@code ldc} (see below), access checking is performed as part of linking
188 * the constant pool entry underlying the constant method handle.
189 * <p>
190 * Thus, handles to non-public methods, or to methods in non-public classes,
191 * should generally be kept secret.
192 * They should not be passed to untrusted code unless their use from
193 * the untrusted code would be harmless.
194 *
195 * <h1>Method handle creation</h1>
196 * Java code can create a method handle that directly accesses
197 * any method, constructor, or field that is accessible to that code.
198 * This is done via a reflective, capability-based API called
199 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
200 * For example, a static method handle can be obtained
201 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
202 * There are also conversion methods from Core Reflection API objects,
203 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
204 * <p>
205 * Like classes and strings, method handles that correspond to accessible
206 * fields, methods, and constructors can also be represented directly
207 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
208 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
209 * refers directly to an associated {@code CONSTANT_Methodref},
210 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
211 * constant pool entry.
212 * (For full details on method handle constants,
213 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
214 * <p>
215 * Method handles produced by lookups or constant loads from methods or
216 * constructors with the variable arity modifier bit ({@code 0x0080})
217 * have a corresponding variable arity, as if they were defined with
218 * the help of {@link #asVarargsCollector asVarargsCollector}.
219 * <p>
220 * A method reference may refer either to a static or non-static method.
221 * In the non-static case, the method handle type includes an explicit
222 * receiver argument, prepended before any other arguments.
223 * In the method handle's type, the initial receiver argument is typed
224 * according to the class under which the method was initially requested.
225 * (E.g., if a non-static method handle is obtained via {@code ldc},
226 * the type of the receiver is the class named in the constant pool entry.)
227 * <p>
228 * Method handle constants are subject to the same link-time access checks
229 * their corresponding bytecode instructions, and the {@code ldc} instruction
230 * will throw corresponding linkage errors if the bytecode behaviors would
231 * throw such errors.
232 * <p>
233 * As a corollary of this, access to protected members is restricted
234 * to receivers only of the accessing class, or one of its subclasses,
235 * and the accessing class must in turn be a subclass (or package sibling)
236 * of the protected member's defining class.
237 * If a method reference refers to a protected non-static method or field
238 * of a class outside the current package, the receiver argument will
239 * be narrowed to the type of the accessing class.
240 * <p>
241 * When a method handle to a virtual method is invoked, the method is
242 * always looked up in the receiver (that is, the first argument).
243 * <p>
244 * A non-virtual method handle to a specific virtual method implementation
245 * can also be created. These do not perform virtual lookup based on
246 * receiver type. Such a method handle simulates the effect of
247 * an {@code invokespecial} instruction to the same method.
248 *
249 * <h1>Usage examples</h1>
250 * Here are some examples of usage:
251 * <blockquote><pre>{@code
252 Object x, y; String s; int i;
253 MethodType mt; MethodHandle mh;
254 MethodHandles.Lookup lookup = MethodHandles.lookup();
255 // mt is (char,char)String
256 mt = MethodType.methodType(String.class, char.class, char.class);
257 mh = lookup.findVirtual(String.class, "replace", mt);
258 s = (String) mh.invokeExact("daddy",'d','n');
259 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
260 assertEquals(s, "nanny");
261 // weakly typed invocation (using MHs.invoke)
262 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
263 assertEquals(s, "savvy");
264 // mt is (Object[])List
265 mt = MethodType.methodType(java.util.List.class, Object[].class);
266 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
267 assert(mh.isVarargsCollector());
268 x = mh.invoke("one", "two");
269 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
270 assertEquals(x, java.util.Arrays.asList("one","two"));
271 // mt is (Object,Object,Object)Object
272 mt = MethodType.genericMethodType(3);
273 mh = mh.asType(mt);
274 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
275 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
276 assertEquals(x, java.util.Arrays.asList(1,2,3));
277 // mt is ()int
278 mt = MethodType.methodType(int.class);
279 mh = lookup.findVirtual(java.util.List.class, "size", mt);
280 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
281 // invokeExact(Ljava/util/List;)I
282 assert(i == 3);
283 mt = MethodType.methodType(void.class, String.class);
284 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
285 mh.invokeExact(System.out, "Hello, world.");
286 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
287 * }</pre></blockquote>
288 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
289 * generates a single invokevirtual instruction with
290 * the symbolic type descriptor indicated in the following comment.
291 * In these examples, the helper method {@code assertEquals} is assumed to
292 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
293 * on its arguments, and asserts that the result is true.
294 *
295 * <h1>Exceptions</h1>
296 * The methods {@code invokeExact} and {@code invoke} are declared
297 * to throw {@link java.lang.Throwable Throwable},
298 * which is to say that there is no static restriction on what a method handle
299 * can throw. Since the JVM does not distinguish between checked
300 * and unchecked exceptions (other than by their class, of course),
301 * there is no particular effect on bytecode shape from ascribing
302 * checked exceptions to method handle invocations. But in Java source
303 * code, methods which perform method handle calls must either explicitly
304 * throw {@code Throwable}, or else must catch all
305 * throwables locally, rethrowing only those which are legal in the context,
306 * and wrapping ones which are illegal.
307 *
308 * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
309 * The unusual compilation and linkage behavior of
310 * {@code invokeExact} and plain {@code invoke}
311 * is referenced by the term <em>signature polymorphism</em>.
312 * As defined in the Java Language Specification,
313 * a signature polymorphic method is one which can operate with
314 * any of a wide range of call signatures and return types.
315 * <p>
316 * In source code, a call to a signature polymorphic method will
317 * compile, regardless of the requested symbolic type descriptor.
318 * As usual, the Java compiler emits an {@code invokevirtual}
319 * instruction with the given symbolic type descriptor against the named method.
320 * The unusual part is that the symbolic type descriptor is derived from
321 * the actual argument and return types, not from the method declaration.
322 * <p>
323 * When the JVM processes bytecode containing signature polymorphic calls,
324 * it will successfully link any such call, regardless of its symbolic type descriptor.
325 * (In order to retain type safety, the JVM will guard such calls with suitable
326 * dynamic type checks, as described elsewhere.)
327 * <p>
328 * Bytecode generators, including the compiler back end, are required to emit
329 * untransformed symbolic type descriptors for these methods.
330 * Tools which determine symbolic linkage are required to accept such
331 * untransformed descriptors, without reporting linkage errors.
332 *
333 * <h1>Interoperation between method handles and the Core Reflection API</h1>
334 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
335 * any class member represented by a Core Reflection API object
336 * can be converted to a behaviorally equivalent method handle.
337 * For example, a reflective {@link java.lang.reflect.Method Method} can
338 * be converted to a method handle using
339 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
340 * The resulting method handles generally provide more direct and efficient
341 * access to the underlying class members.
342 * <p>
343 * As a special case,
344 * when the Core Reflection API is used to view the signature polymorphic
345 * methods {@code invokeExact} or plain {@code invoke} in this class,
346 * they appear as ordinary non-polymorphic methods.
347 * Their reflective appearance, as viewed by
348 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
349 * is unaffected by their special status in this API.
350 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
351 * will report exactly those modifier bits required for any similarly
352 * declared method, including in this case {@code native} and {@code varargs} bits.
353 * <p>
354 * As with any reflected method, these methods (when reflected) may be
355 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
356 * However, such reflective calls do not result in method handle invocations.
357 * Such a call, if passed the required argument
358 * (a single one, of type {@code Object[]}), will ignore the argument and
359 * will throw an {@code UnsupportedOperationException}.
360 * <p>
361 * Since {@code invokevirtual} instructions can natively
362 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
363 * with the normal presentation of these methods via bytecodes.
364 * Thus, these two native methods, when reflectively viewed by
365 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
366 * <p>
367 * In order to obtain an invoker method for a particular type descriptor,
368 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
369 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
370 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
371 * API is also able to return a method handle
372 * to call {@code invokeExact} or plain {@code invoke},
373 * for any specified type descriptor .
374 *
375 * <h1>Interoperation between method handles and Java generics</h1>
376 * A method handle can be obtained on a method, constructor, or field
377 * which is declared with Java generic types.
378 * As with the Core Reflection API, the type of the method handle
379 * will constructed from the erasure of the source-level type.
380 * When a method handle is invoked, the types of its arguments
381 * or the return value cast type may be generic types or type instances.
382 * If this occurs, the compiler will replace those
383 * types by their erasures when it constructs the symbolic type descriptor
384 * for the {@code invokevirtual} instruction.
385 * <p>
386 * Method handles do not represent
387 * their function-like types in terms of Java parameterized (generic) types,
388 * because there are three mismatches between function-like types and parameterized
389 * Java types.
390 * <ul>
391 * <li>Method types range over all possible arities,
392 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
393 * Generics are not variadic, and so cannot represent this.</li>
394 * <li>Method types can specify arguments of primitive types,
395 * which Java generic types cannot range over.</li>
396 * <li>Higher order functions over method handles (combinators) are
397 * often generic across a wide range of function types, including
398 * those of multiple arities. It is impossible to represent such
399 * genericity with a Java type parameter.</li>
400 * </ul>
401 *
402 * <h1><a name="maxarity"></a>Arity limits</h1>
403 * The JVM imposes on all methods and constructors of any kind an absolute
404 * limit of 255 stacked arguments. This limit can appear more restrictive
405 * in certain cases:
406 * <ul>
407 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
408 * <li>A non-static method consumes an extra argument for the object on which the method is called.
409 * <li>A constructor consumes an extra argument for the object which is being constructed.
410 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
411 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
412 * </ul>
413 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
414 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
415 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
416 * In particular, a method handle’s type must not have an arity of the exact maximum 255.
417 *
418 * @see MethodType
419 * @see MethodHandles
420 * @author John Rose, JSR 292 EG
421 */
422 public abstract class MethodHandle {
423 static { MethodHandleImpl.initStatics(); }
424
425 /**
426 * Internal marker interface which distinguishes (to the Java compiler)
427 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
428 */
429 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
430 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
431 @interface PolymorphicSignature { }
432
433 private final MethodType type;
434 /*private*/ final LambdaForm form;
435 // form is not private so that invokers can easily fetch it
436 /*private*/ MethodHandle asTypeCache;
437 // asTypeCache is not private so that invokers can easily fetch it
438 /*non-public*/ byte customizationCount;
439 // customizationCount should be accessible from invokers
440
441 /**
442 * Reports the type of this method handle.
443 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
444 * @return the method handle type
445 */
446 public MethodType type() {
447 return type;
448 }
449
450 /**
451 * Package-private constructor for the method handle implementation hierarchy.
452 * Method handle inheritance will be contained completely within
453 * the {@code java.lang.invoke} package.
454 */
455 // @param type type (permanently assigned) of the new method handle
456 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
457 this.type = Objects.requireNonNull(type);
458 this.form = Objects.requireNonNull(form).uncustomize();
459
460 this.form.prepare(); // TO DO: Try to delay this step until just before invocation.
461 }
462
463 /**
464 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
465 * The symbolic type descriptor at the call site of {@code invokeExact} must
466 * exactly match this method handle's {@link #type type}.
467 * No conversions are allowed on arguments or return values.
468 * <p>
469 * When this method is observed via the Core Reflection API,
470 * it will appear as a single native method, taking an object array and returning an object.
471 * If this native method is invoked directly via
472 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
473 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
474 * it will throw an {@code UnsupportedOperationException}.
475 * @param args the signature-polymorphic parameter list, statically represented using varargs
476 * @return the signature-polymorphic result, statically represented using {@code Object}
477 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
478 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
479 */
480 @HotSpotIntrinsicCandidate
481 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
482
483 /**
484 * Invokes the method handle, allowing any caller type descriptor,
485 * and optionally performing conversions on arguments and return values.
486 * <p>
487 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
488 * the call proceeds as if by {@link #invokeExact invokeExact}.
489 * <p>
490 * Otherwise, the call proceeds as if this method handle were first
491 * adjusted by calling {@link #asType asType} to adjust this method handle
492 * to the required type, and then the call proceeds as if by
493 * {@link #invokeExact invokeExact} on the adjusted method handle.
494 * <p>
495 * There is no guarantee that the {@code asType} call is actually made.
496 * If the JVM can predict the results of making the call, it may perform
497 * adaptations directly on the caller's arguments,
498 * and call the target method handle according to its own exact type.
499 * <p>
500 * The resolved type descriptor at the call site of {@code invoke} must
501 * be a valid argument to the receivers {@code asType} method.
502 * In particular, the caller must specify the same argument arity
503 * as the callee's type,
504 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
505 * <p>
506 * When this method is observed via the Core Reflection API,
507 * it will appear as a single native method, taking an object array and returning an object.
508 * If this native method is invoked directly via
509 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
510 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
511 * it will throw an {@code UnsupportedOperationException}.
512 * @param args the signature-polymorphic parameter list, statically represented using varargs
513 * @return the signature-polymorphic result, statically represented using {@code Object}
514 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
515 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
516 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
517 */
518 @HotSpotIntrinsicCandidate
519 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
520
521 /**
522 * Private method for trusted invocation of a method handle respecting simplified signatures.
523 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
524 * <p>
525 * The caller signature is restricted to the following basic types:
526 * Object, int, long, float, double, and void return.
527 * <p>
528 * The caller is responsible for maintaining type correctness by ensuring
529 * that the each outgoing argument value is a member of the range of the corresponding
530 * callee argument type.
531 * (The caller should therefore issue appropriate casts and integer narrowing
532 * operations on outgoing argument values.)
533 * The caller can assume that the incoming result value is part of the range
534 * of the callee's return type.
535 * @param args the signature-polymorphic parameter list, statically represented using varargs
536 * @return the signature-polymorphic result, statically represented using {@code Object}
537 */
538 @HotSpotIntrinsicCandidate
539 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
540
541 /**
542 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
543 * The caller signature is restricted to basic types as with {@code invokeBasic}.
544 * The trailing (not leading) argument must be a MemberName.
545 * @param args the signature-polymorphic parameter list, statically represented using varargs
546 * @return the signature-polymorphic result, statically represented using {@code Object}
547 */
548 @HotSpotIntrinsicCandidate
549 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
550
551 /**
552 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
553 * The caller signature is restricted to basic types as with {@code invokeBasic}.
554 * The trailing (not leading) argument must be a MemberName.
555 * @param args the signature-polymorphic parameter list, statically represented using varargs
556 * @return the signature-polymorphic result, statically represented using {@code Object}
557 */
558 @HotSpotIntrinsicCandidate
559 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
560
561 /**
562 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
563 * The caller signature is restricted to basic types as with {@code invokeBasic}.
564 * The trailing (not leading) argument must be a MemberName.
565 * @param args the signature-polymorphic parameter list, statically represented using varargs
566 * @return the signature-polymorphic result, statically represented using {@code Object}
567 */
568 @HotSpotIntrinsicCandidate
569 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
570
571 /**
572 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
573 * The caller signature is restricted to basic types as with {@code invokeBasic}.
574 * The trailing (not leading) argument must be a MemberName.
575 * @param args the signature-polymorphic parameter list, statically represented using varargs
576 * @return the signature-polymorphic result, statically represented using {@code Object}
577 */
578 @HotSpotIntrinsicCandidate
579 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
580
581 /**
582 * Performs a variable arity invocation, passing the arguments in the given list
583 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
584 * which mentions only the type {@code Object}, and whose arity is the length
585 * of the argument list.
586 * <p>
587 * Specifically, execution proceeds as if by the following steps,
588 * although the methods are not guaranteed to be called if the JVM
589 * can predict their effects.
590 * <ul>
591 * <li>Determine the length of the argument array as {@code N}.
592 * For a null reference, {@code N=0}. </li>
593 * <li>Determine the general type {@code TN} of {@code N} arguments as
594 * as {@code TN=MethodType.genericMethodType(N)}.</li>
595 * <li>Force the original target method handle {@code MH0} to the
596 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
597 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
598 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
599 * MH1.invokeExact(A0, ...). </li>
600 * <li>Take the return value as an {@code Object} reference. </li>
601 * </ul>
602 * <p>
603 * Because of the action of the {@code asType} step, the following argument
604 * conversions are applied as necessary:
605 * <ul>
606 * <li>reference casting
607 * <li>unboxing
608 * <li>widening primitive conversions
609 * </ul>
610 * <p>
611 * The result returned by the call is boxed if it is a primitive,
612 * or forced to null if the return type is void.
613 * <p>
614 * This call is equivalent to the following code:
615 * <blockquote><pre>{@code
616 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
617 * Object result = invoker.invokeExact(this, arguments);
618 * }</pre></blockquote>
619 * <p>
620 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
621 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
622 * It can therefore be used as a bridge between native or reflective code and method handles.
623 *
624 * @param arguments the arguments to pass to the target
625 * @return the result returned by the target
626 * @throws ClassCastException if an argument cannot be converted by reference casting
627 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
628 * @throws Throwable anything thrown by the target method invocation
629 * @see MethodHandles#spreadInvoker
630 */
631 public Object invokeWithArguments(Object... arguments) throws Throwable {
632 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length);
633 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments);
634 }
635
636 /**
637 * Performs a variable arity invocation, passing the arguments in the given array
638 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
639 * which mentions only the type {@code Object}, and whose arity is the length
640 * of the argument array.
641 * <p>
642 * This method is also equivalent to the following code:
643 * <blockquote><pre>{@code
644 * invokeWithArguments(arguments.toArray()
645 * }</pre></blockquote>
646 *
647 * @param arguments the arguments to pass to the target
648 * @return the result returned by the target
649 * @throws NullPointerException if {@code arguments} is a null reference
650 * @throws ClassCastException if an argument cannot be converted by reference casting
651 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
652 * @throws Throwable anything thrown by the target method invocation
653 */
654 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
655 return invokeWithArguments(arguments.toArray());
656 }
657
658 /**
659 * Produces an adapter method handle which adapts the type of the
660 * current method handle to a new type.
661 * The resulting method handle is guaranteed to report a type
662 * which is equal to the desired new type.
663 * <p>
664 * If the original type and new type are equal, returns {@code this}.
665 * <p>
666 * The new method handle, when invoked, will perform the following
667 * steps:
668 * <ul>
669 * <li>Convert the incoming argument list to match the original
670 * method handle's argument list.
671 * <li>Invoke the original method handle on the converted argument list.
672 * <li>Convert any result returned by the original method handle
673 * to the return type of new method handle.
674 * </ul>
675 * <p>
676 * This method provides the crucial behavioral difference between
677 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
678 * The two methods
679 * perform the same steps when the caller's type descriptor exactly matches
680 * the callee's, but when the types differ, plain {@link #invoke invoke}
681 * also calls {@code asType} (or some internal equivalent) in order
682 * to match up the caller's and callee's types.
683 * <p>
684 * If the current method is a variable arity method handle
685 * argument list conversion may involve the conversion and collection
686 * of several arguments into an array, as
687 * {@linkplain #asVarargsCollector described elsewhere}.
688 * In every other case, all conversions are applied <em>pairwise</em>,
689 * which means that each argument or return value is converted to
690 * exactly one argument or return value (or no return value).
691 * The applied conversions are defined by consulting the
692 * the corresponding component types of the old and new
693 * method handle types.
694 * <p>
695 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
696 * or old and new return types. Specifically, for some valid index {@code i}, let
697 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
698 * Or else, going the other way for return values, let
699 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
700 * If the types are the same, the new method handle makes no change
701 * to the corresponding argument or return value (if any).
702 * Otherwise, one of the following conversions is applied
703 * if possible:
704 * <ul>
705 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
706 * (The types do not need to be related in any particular way.
707 * This is because a dynamic value of null can convert to any reference type.)
708 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
709 * conversion (JLS 5.3) is applied, if one exists.
710 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
711 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
712 * a Java casting conversion (JLS 5.5) is applied if one exists.
713 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
714 * which is then widened as needed to <em>T1</em>.)
715 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
716 * conversion will be applied at runtime, possibly followed
717 * by a Java method invocation conversion (JLS 5.3)
718 * on the primitive value. (These are the primitive widening conversions.)
719 * <em>T0</em> must be a wrapper class or a supertype of one.
720 * (In the case where <em>T0</em> is Object, these are the conversions
721 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
722 * The unboxing conversion must have a possibility of success, which means that
723 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
724 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
725 * primitive value can be widened to <em>T1</em>.
726 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
727 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
728 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
729 * a zero value is introduced.
730 * </ul>
731 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
732 * because neither corresponds specifically to the <em>dynamic type</em> of any
733 * actual argument or return value.)
734 * <p>
735 * The method handle conversion cannot be made if any one of the required
736 * pairwise conversions cannot be made.
737 * <p>
738 * At runtime, the conversions applied to reference arguments
739 * or return values may require additional runtime checks which can fail.
740 * An unboxing operation may fail because the original reference is null,
741 * causing a {@link java.lang.NullPointerException NullPointerException}.
742 * An unboxing operation or a reference cast may also fail on a reference
743 * to an object of the wrong type,
744 * causing a {@link java.lang.ClassCastException ClassCastException}.
745 * Although an unboxing operation may accept several kinds of wrappers,
746 * if none are available, a {@code ClassCastException} will be thrown.
747 *
748 * @param newType the expected type of the new method handle
749 * @return a method handle which delegates to {@code this} after performing
750 * any necessary argument conversions, and arranges for any
751 * necessary return value conversions
752 * @throws NullPointerException if {@code newType} is a null reference
753 * @throws WrongMethodTypeException if the conversion cannot be made
754 * @see MethodHandles#explicitCastArguments
755 */
756 public MethodHandle asType(MethodType newType) {
757 // Fast path alternative to a heavyweight {@code asType} call.
758 // Return 'this' if the conversion will be a no-op.
759 if (newType == type) {
760 return this;
761 }
762 // Return 'this.asTypeCache' if the conversion is already memoized.
763 MethodHandle atc = asTypeCached(newType);
764 if (atc != null) {
765 return atc;
766 }
767 return asTypeUncached(newType);
768 }
769
770 private MethodHandle asTypeCached(MethodType newType) {
771 MethodHandle atc = asTypeCache;
772 if (atc != null && newType == atc.type) {
773 return atc;
774 }
775 return null;
776 }
777
778 /** Override this to change asType behavior. */
779 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
780 if (!type.isConvertibleTo(newType))
781 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
782 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
783 }
784
785 /**
786 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
787 * and spreads its elements as positional arguments.
788 * The new method handle adapts, as its <i>target</i>,
789 * the current method handle. The type of the adapter will be
790 * the same as the type of the target, except that the final
791 * {@code arrayLength} parameters of the target's type are replaced
792 * by a single array parameter of type {@code arrayType}.
793 * <p>
794 * If the array element type differs from any of the corresponding
795 * argument types on the original target,
796 * the original target is adapted to take the array elements directly,
797 * as if by a call to {@link #asType asType}.
798 * <p>
799 * When called, the adapter replaces a trailing array argument
800 * by the array's elements, each as its own argument to the target.
801 * (The order of the arguments is preserved.)
802 * They are converted pairwise by casting and/or unboxing
803 * to the types of the trailing parameters of the target.
804 * Finally the target is called.
805 * What the target eventually returns is returned unchanged by the adapter.
806 * <p>
807 * Before calling the target, the adapter verifies that the array
808 * contains exactly enough elements to provide a correct argument count
809 * to the target method handle.
810 * (The array may also be null when zero elements are required.)
811 * <p>
812 * If, when the adapter is called, the supplied array argument does
813 * not have the correct number of elements, the adapter will throw
814 * an {@link IllegalArgumentException} instead of invoking the target.
815 * <p>
816 * Here are some simple examples of array-spreading method handles:
817 * <blockquote><pre>{@code
818 MethodHandle equals = publicLookup()
819 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
820 assert( (boolean) equals.invokeExact("me", (Object)"me"));
821 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
822 // spread both arguments from a 2-array:
823 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
824 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
825 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
826 // try to spread from anything but a 2-array:
827 for (int n = 0; n <= 10; n++) {
828 Object[] badArityArgs = (n == 2 ? null : new Object[n]);
829 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
830 catch (IllegalArgumentException ex) { } // OK
831 }
832 // spread both arguments from a String array:
833 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
834 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
835 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
836 // spread second arguments from a 1-array:
837 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
838 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
839 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
840 // spread no arguments from a 0-array or null:
841 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
842 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
843 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
844 // asSpreader and asCollector are approximate inverses:
845 for (int n = 0; n <= 2; n++) {
846 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
847 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
848 assert( (boolean) equals2.invokeWithArguments("me", "me"));
849 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
850 }
851 }
852 MethodHandle caToString = publicLookup()
853 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
854 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
855 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
856 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
857 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
858 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
859 * }</pre></blockquote>
860 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
861 * @param arrayLength the number of arguments to spread from an incoming array argument
862 * @return a new method handle which spreads its final array argument,
863 * before calling the original method handle
864 * @throws NullPointerException if {@code arrayType} is a null reference
865 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
866 * or if target does not have at least
867 * {@code arrayLength} parameter types,
868 * or if {@code arrayLength} is negative,
869 * or if the resulting method handle's type would have
870 * <a href="MethodHandle.html#maxarity">too many parameters</a>
871 * @throws WrongMethodTypeException if the implied {@code asType} call fails
872 * @see #asCollector
873 */
874 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
875 MethodType postSpreadType = asSpreaderChecks(arrayType, arrayLength);
876 int arity = type().parameterCount();
877 int spreadArgPos = arity - arrayLength;
878 MethodHandle afterSpread = this.asType(postSpreadType);
879 BoundMethodHandle mh = afterSpread.rebind();
880 LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
881 MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, arity, arrayType);
882 return mh.copyWith(preSpreadType, lform);
883 }
884
885 /**
886 * See if {@code asSpreader} can be validly called with the given arguments.
887 * Return the type of the method handle call after spreading but before conversions.
888 */
889 private MethodType asSpreaderChecks(Class<?> arrayType, int arrayLength) {
890 spreadArrayChecks(arrayType, arrayLength);
891 int nargs = type().parameterCount();
892 if (nargs < arrayLength || arrayLength < 0)
893 throw newIllegalArgumentException("bad spread array length");
894 Class<?> arrayElement = arrayType.getComponentType();
895 MethodType mtype = type();
896 boolean match = true, fail = false;
897 for (int i = nargs - arrayLength; i < nargs; i++) {
898 Class<?> ptype = mtype.parameterType(i);
899 if (ptype != arrayElement) {
900 match = false;
901 if (!MethodType.canConvert(arrayElement, ptype)) {
902 fail = true;
903 break;
904 }
905 }
906 }
907 if (match) return mtype;
908 MethodType needType = mtype.asSpreaderType(arrayType, arrayLength);
909 if (!fail) return needType;
910 // elicit an error:
911 this.asType(needType);
912 throw newInternalError("should not return", null);
913 }
914
915 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
916 Class<?> arrayElement = arrayType.getComponentType();
917 if (arrayElement == null)
918 throw newIllegalArgumentException("not an array type", arrayType);
919 if ((arrayLength & 0x7F) != arrayLength) {
920 if ((arrayLength & 0xFF) != arrayLength)
921 throw newIllegalArgumentException("array length is not legal", arrayLength);
922 assert(arrayLength >= 128);
923 if (arrayElement == long.class ||
924 arrayElement == double.class)
925 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
926 }
927 }
928
929 /**
930 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
931 * positional arguments and collects them into an array argument.
932 * The new method handle adapts, as its <i>target</i>,
933 * the current method handle. The type of the adapter will be
934 * the same as the type of the target, except that a single trailing
935 * parameter (usually of type {@code arrayType}) is replaced by
936 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
937 * <p>
938 * If the array type differs from the final argument type on the original target,
939 * the original target is adapted to take the array type directly,
940 * as if by a call to {@link #asType asType}.
941 * <p>
942 * When called, the adapter replaces its trailing {@code arrayLength}
943 * arguments by a single new array of type {@code arrayType}, whose elements
944 * comprise (in order) the replaced arguments.
945 * Finally the target is called.
946 * What the target eventually returns is returned unchanged by the adapter.
947 * <p>
948 * (The array may also be a shared constant when {@code arrayLength} is zero.)
949 * <p>
950 * (<em>Note:</em> The {@code arrayType} is often identical to the last
951 * parameter type of the original target.
952 * It is an explicit argument for symmetry with {@code asSpreader}, and also
953 * to allow the target to use a simple {@code Object} as its last parameter type.)
954 * <p>
955 * In order to create a collecting adapter which is not restricted to a particular
956 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
957 * <p>
958 * Here are some examples of array-collecting method handles:
959 * <blockquote><pre>{@code
960 MethodHandle deepToString = publicLookup()
961 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
962 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
963 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
964 assertEquals(methodType(String.class, Object.class), ts1.type());
965 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
966 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
967 // arrayType can be a subtype of Object[]
968 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
969 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
970 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
971 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
972 assertEquals("[]", (String) ts0.invokeExact());
973 // collectors can be nested, Lisp-style
974 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
975 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
976 // arrayType can be any primitive array type
977 MethodHandle bytesToString = publicLookup()
978 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
979 .asCollector(byte[].class, 3);
980 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
981 MethodHandle longsToString = publicLookup()
982 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
983 .asCollector(long[].class, 1);
984 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
985 * }</pre></blockquote>
986 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
987 * @param arrayLength the number of arguments to collect into a new array argument
988 * @return a new method handle which collects some trailing argument
989 * into an array, before calling the original method handle
990 * @throws NullPointerException if {@code arrayType} is a null reference
991 * @throws IllegalArgumentException if {@code arrayType} is not an array type
992 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
993 * or {@code arrayLength} is not a legal array size,
994 * or the resulting method handle's type would have
995 * <a href="MethodHandle.html#maxarity">too many parameters</a>
996 * @throws WrongMethodTypeException if the implied {@code asType} call fails
997 * @see #asSpreader
998 * @see #asVarargsCollector
999 */
1000 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1001 asCollectorChecks(arrayType, arrayLength);
1002 int collectArgPos = type().parameterCount() - 1;
1003 BoundMethodHandle mh = rebind();
1004 MethodType resultType = type().asCollectorType(arrayType, arrayLength);
1005 MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
1006 LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
1007 if (lform != null) {
1008 return mh.copyWith(resultType, lform);
1009 }
1010 lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
1011 return mh.copyWithExtendL(resultType, lform, newArray);
1012 }
1013
1014 /**
1015 * See if {@code asCollector} can be validly called with the given arguments.
1016 * Return false if the last parameter is not an exact match to arrayType.
1017 */
1018 /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
1019 spreadArrayChecks(arrayType, arrayLength);
1020 int nargs = type().parameterCount();
1021 if (nargs != 0) {
1022 Class<?> lastParam = type().parameterType(nargs-1);
1023 if (lastParam == arrayType) return true;
1024 if (lastParam.isAssignableFrom(arrayType)) return false;
1025 }
1026 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1027 }
1028
1029 /**
1030 * Makes a <em>variable arity</em> adapter which is able to accept
1031 * any number of trailing positional arguments and collect them
1032 * into an array argument.
1033 * <p>
1034 * The type and behavior of the adapter will be the same as
1035 * the type and behavior of the target, except that certain
1036 * {@code invoke} and {@code asType} requests can lead to
1037 * trailing positional arguments being collected into target's
1038 * trailing parameter.
1039 * Also, the last parameter type of the adapter will be
1040 * {@code arrayType}, even if the target has a different
1041 * last parameter type.
1042 * <p>
1043 * This transformation may return {@code this} if the method handle is
1044 * already of variable arity and its trailing parameter type
1045 * is identical to {@code arrayType}.
1046 * <p>
1047 * When called with {@link #invokeExact invokeExact}, the adapter invokes
1048 * the target with no argument changes.
1049 * (<em>Note:</em> This behavior is different from a
1050 * {@linkplain #asCollector fixed arity collector},
1051 * since it accepts a whole array of indeterminate length,
1052 * rather than a fixed number of arguments.)
1053 * <p>
1054 * When called with plain, inexact {@link #invoke invoke}, if the caller
1055 * type is the same as the adapter, the adapter invokes the target as with
1056 * {@code invokeExact}.
1057 * (This is the normal behavior for {@code invoke} when types match.)
1058 * <p>
1059 * Otherwise, if the caller and adapter arity are the same, and the
1060 * trailing parameter type of the caller is a reference type identical to
1061 * or assignable to the trailing parameter type of the adapter,
1062 * the arguments and return values are converted pairwise,
1063 * as if by {@link #asType asType} on a fixed arity
1064 * method handle.
1065 * <p>
1066 * Otherwise, the arities differ, or the adapter's trailing parameter
1067 * type is not assignable from the corresponding caller type.
1068 * In this case, the adapter replaces all trailing arguments from
1069 * the original trailing argument position onward, by
1070 * a new array of type {@code arrayType}, whose elements
1071 * comprise (in order) the replaced arguments.
1072 * <p>
1073 * The caller type must provides as least enough arguments,
1074 * and of the correct type, to satisfy the target's requirement for
1075 * positional arguments before the trailing array argument.
1076 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1077 * where {@code N} is the arity of the target.
1078 * Also, there must exist conversions from the incoming arguments
1079 * to the target's arguments.
1080 * As with other uses of plain {@code invoke}, if these basic
1081 * requirements are not fulfilled, a {@code WrongMethodTypeException}
1082 * may be thrown.
1083 * <p>
1084 * In all cases, what the target eventually returns is returned unchanged by the adapter.
1085 * <p>
1086 * In the final case, it is exactly as if the target method handle were
1087 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1088 * to the arity required by the caller type.
1089 * (As with {@code asCollector}, if the array length is zero,
1090 * a shared constant may be used instead of a new array.
1091 * If the implied call to {@code asCollector} would throw
1092 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1093 * the call to the variable arity adapter must throw
1094 * {@code WrongMethodTypeException}.)
1095 * <p>
1096 * The behavior of {@link #asType asType} is also specialized for
1097 * variable arity adapters, to maintain the invariant that
1098 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1099 * call to adjust the target type, followed by {@code invokeExact}.
1100 * Therefore, a variable arity adapter responds
1101 * to an {@code asType} request by building a fixed arity collector,
1102 * if and only if the adapter and requested type differ either
1103 * in arity or trailing argument type.
1104 * The resulting fixed arity collector has its type further adjusted
1105 * (if necessary) to the requested type by pairwise conversion,
1106 * as if by another application of {@code asType}.
1107 * <p>
1108 * When a method handle is obtained by executing an {@code ldc} instruction
1109 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1110 * as a variable arity method (with the modifier bit {@code 0x0080}),
1111 * the method handle will accept multiple arities, as if the method handle
1112 * constant were created by means of a call to {@code asVarargsCollector}.
1113 * <p>
1114 * In order to create a collecting adapter which collects a predetermined
1115 * number of arguments, and whose type reflects this predetermined number,
1116 * use {@link #asCollector asCollector} instead.
1117 * <p>
1118 * No method handle transformations produce new method handles with
1119 * variable arity, unless they are documented as doing so.
1120 * Therefore, besides {@code asVarargsCollector},
1121 * all methods in {@code MethodHandle} and {@code MethodHandles}
1122 * will return a method handle with fixed arity,
1123 * except in the cases where they are specified to return their original
1124 * operand (e.g., {@code asType} of the method handle's own type).
1125 * <p>
1126 * Calling {@code asVarargsCollector} on a method handle which is already
1127 * of variable arity will produce a method handle with the same type and behavior.
1128 * It may (or may not) return the original variable arity method handle.
1129 * <p>
1130 * Here is an example, of a list-making variable arity method handle:
1131 * <blockquote><pre>{@code
1132 MethodHandle deepToString = publicLookup()
1133 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1134 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1135 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1136 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1137 assertEquals("[won]", (String) ts1.invoke( "won" ));
1138 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1139 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1140 MethodHandle asList = publicLookup()
1141 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1142 assertEquals(methodType(List.class, Object[].class), asList.type());
1143 assert(asList.isVarargsCollector());
1144 assertEquals("[]", asList.invoke().toString());
1145 assertEquals("[1]", asList.invoke(1).toString());
1146 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1147 String[] argv = { "three", "thee", "tee" };
1148 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1149 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1150 List ls = (List) asList.invoke((Object)argv);
1151 assertEquals(1, ls.size());
1152 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1153 * }</pre></blockquote>
1154 * <p style="font-size:smaller;">
1155 * <em>Discussion:</em>
1156 * These rules are designed as a dynamically-typed variation
1157 * of the Java rules for variable arity methods.
1158 * In both cases, callers to a variable arity method or method handle
1159 * can either pass zero or more positional arguments, or else pass
1160 * pre-collected arrays of any length. Users should be aware of the
1161 * special role of the final argument, and of the effect of a
1162 * type match on that final argument, which determines whether
1163 * or not a single trailing argument is interpreted as a whole
1164 * array or a single element of an array to be collected.
1165 * Note that the dynamic type of the trailing argument has no
1166 * effect on this decision, only a comparison between the symbolic
1167 * type descriptor of the call site and the type descriptor of the method handle.)
1168 *
1169 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1170 * @return a new method handle which can collect any number of trailing arguments
1171 * into an array, before calling the original method handle
1172 * @throws NullPointerException if {@code arrayType} is a null reference
1173 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1174 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1175 * @see #asCollector
1176 * @see #isVarargsCollector
1177 * @see #asFixedArity
1178 */
1179 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1180 Objects.requireNonNull(arrayType);
1181 boolean lastMatch = asCollectorChecks(arrayType, 0);
1182 if (isVarargsCollector() && lastMatch)
1183 return this;
1184 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1185 }
1186
1187 /**
1188 * Determines if this method handle
1189 * supports {@linkplain #asVarargsCollector variable arity} calls.
1190 * Such method handles arise from the following sources:
1191 * <ul>
1192 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1193 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1194 * which resolves to a variable arity Java method or constructor
1195 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1196 * which resolves to a variable arity Java method or constructor
1197 * </ul>
1198 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1199 * @see #asVarargsCollector
1200 * @see #asFixedArity
1201 */
1202 public boolean isVarargsCollector() {
1203 return false;
1204 }
1205
1206 /**
1207 * Makes a <em>fixed arity</em> method handle which is otherwise
1208 * equivalent to the current method handle.
1209 * <p>
1210 * If the current method handle is not of
1211 * {@linkplain #asVarargsCollector variable arity},
1212 * the current method handle is returned.
1213 * This is true even if the current method handle
1214 * could not be a valid input to {@code asVarargsCollector}.
1215 * <p>
1216 * Otherwise, the resulting fixed-arity method handle has the same
1217 * type and behavior of the current method handle,
1218 * except that {@link #isVarargsCollector isVarargsCollector}
1219 * will be false.
1220 * The fixed-arity method handle may (or may not) be the
1221 * a previous argument to {@code asVarargsCollector}.
1222 * <p>
1223 * Here is an example, of a list-making variable arity method handle:
1224 * <blockquote><pre>{@code
1225 MethodHandle asListVar = publicLookup()
1226 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1227 .asVarargsCollector(Object[].class);
1228 MethodHandle asListFix = asListVar.asFixedArity();
1229 assertEquals("[1]", asListVar.invoke(1).toString());
1230 Exception caught = null;
1231 try { asListFix.invoke((Object)1); }
1232 catch (Exception ex) { caught = ex; }
1233 assert(caught instanceof ClassCastException);
1234 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1235 try { asListFix.invoke("two", "too"); }
1236 catch (Exception ex) { caught = ex; }
1237 assert(caught instanceof WrongMethodTypeException);
1238 Object[] argv = { "three", "thee", "tee" };
1239 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1240 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1241 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1242 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1243 * }</pre></blockquote>
1244 *
1245 * @return a new method handle which accepts only a fixed number of arguments
1246 * @see #asVarargsCollector
1247 * @see #isVarargsCollector
1248 */
1249 public MethodHandle asFixedArity() {
1250 assert(!isVarargsCollector());
1251 return this;
1252 }
1253
1254 /**
1255 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1256 * The new method handle adapts, as its <i>target</i>,
1257 * the current method handle by binding it to the given argument.
1258 * The type of the bound handle will be
1259 * the same as the type of the target, except that a single leading
1260 * reference parameter will be omitted.
1261 * <p>
1262 * When called, the bound handle inserts the given value {@code x}
1263 * as a new leading argument to the target. The other arguments are
1264 * also passed unchanged.
1265 * What the target eventually returns is returned unchanged by the bound handle.
1266 * <p>
1267 * The reference {@code x} must be convertible to the first parameter
1268 * type of the target.
1269 * <p>
1270 * (<em>Note:</em> Because method handles are immutable, the target method handle
1271 * retains its original type and behavior.)
1272 * @param x the value to bind to the first argument of the target
1273 * @return a new method handle which prepends the given value to the incoming
1274 * argument list, before calling the original method handle
1275 * @throws IllegalArgumentException if the target does not have a
1276 * leading parameter type that is a reference type
1277 * @throws ClassCastException if {@code x} cannot be converted
1278 * to the leading parameter type of the target
1279 * @see MethodHandles#insertArguments
1280 */
1281 public MethodHandle bindTo(Object x) {
1282 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed
1283 return bindArgumentL(0, x);
1284 }
1285
1286 /**
1287 * Returns a string representation of the method handle,
1288 * starting with the string {@code "MethodHandle"} and
1289 * ending with the string representation of the method handle's type.
1290 * In other words, this method returns a string equal to the value of:
1291 * <blockquote><pre>{@code
1292 * "MethodHandle" + type().toString()
1293 * }</pre></blockquote>
1294 * <p>
1295 * (<em>Note:</em> Future releases of this API may add further information
1296 * to the string representation.
1297 * Therefore, the present syntax should not be parsed by applications.)
1298 *
1299 * @return a string representation of the method handle
1300 */
1301 @Override
1302 public String toString() {
1303 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString();
1304 return standardString();
1305 }
1306 String standardString() {
1307 return "MethodHandle"+type;
1308 }
1309 /** Return a string with a several lines describing the method handle structure.
1310 * This string would be suitable for display in an IDE debugger.
1311 */
1312 String debugString() {
1313 return type+" : "+internalForm()+internalProperties();
1314 }
1315
1316 //// Implementation methods.
1317 //// Sub-classes can override these default implementations.
1318 //// All these methods assume arguments are already validated.
1319
1320 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
1321
1322 BoundMethodHandle bindArgumentL(int pos, Object value) {
1323 return rebind().bindArgumentL(pos, value);
1324 }
1325
1326 /*non-public*/
1327 MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1328 if (!member.isVarargs()) return this;
1329 Class<?> arrayType = type().lastParameterType();
1330 if (arrayType.isArray()) {
1331 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1332 }
1333 throw member.makeAccessException("cannot make variable arity", null);
1334 }
1335
1336 /*non-public*/
1337 MethodHandle viewAsType(MethodType newType, boolean strict) {
1338 // No actual conversions, just a new view of the same method.
1339 // Note that this operation must not produce a DirectMethodHandle,
1340 // because retyped DMHs, like any transformed MHs,
1341 // cannot be cracked into MethodHandleInfo.
1342 assert viewAsTypeChecks(newType, strict);
1343 BoundMethodHandle mh = rebind();
1344 assert(!((MethodHandle)mh instanceof DirectMethodHandle));
1345 return mh.copyWith(newType, mh.form);
1346 }
1347
1348 /*non-public*/
1349 boolean viewAsTypeChecks(MethodType newType, boolean strict) {
1350 if (strict) {
1351 assert(type().isViewableAs(newType, true))
1352 : Arrays.asList(this, newType);
1353 } else {
1354 assert(type().basicType().isViewableAs(newType.basicType(), true))
1355 : Arrays.asList(this, newType);
1356 }
1357 return true;
1358 }
1359
1360 // Decoding
1361
1362 /*non-public*/
1363 LambdaForm internalForm() {
1364 return form;
1365 }
1366
1367 /*non-public*/
1368 MemberName internalMemberName() {
1369 return null; // DMH returns DMH.member
1370 }
1371
1372 /*non-public*/
1373 Class<?> internalCallerClass() {
1374 return null; // caller-bound MH for @CallerSensitive method returns caller
1375 }
1376
1377 /*non-public*/
1378 MethodHandleImpl.Intrinsic intrinsicName() {
1379 // no special intrinsic meaning to most MHs
1380 return MethodHandleImpl.Intrinsic.NONE;
1381 }
1382
1383 /*non-public*/
1384 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
1385 if (member != null) {
1386 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
1387 } else if (internalMemberName() == null) {
1388 // The required internaMemberName is null, and this MH (like most) doesn't have one.
1389 return this;
1390 } else {
1391 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1392 MethodHandle result = rebind();
1393 assert (result.internalMemberName() == null);
1394 return result;
1395 }
1396 }
1397
1398 /*non-public*/
1399 boolean isInvokeSpecial() {
1400 return false; // DMH.Special returns true
1401 }
1402
1403 /*non-public*/
1404 Object internalValues() {
1405 return null;
1406 }
1407
1408 /*non-public*/
1409 Object internalProperties() {
1410 // Override to something to follow this.form, like "\n& FOO=bar"
1411 return "";
1412 }
1413
1414 //// Method handle implementation methods.
1415 //// Sub-classes can override these default implementations.
1416 //// All these methods assume arguments are already validated.
1417
1418 /*non-public*/
1419 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1420
1421 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
1422 * Many transforms are implemented only for BMHs.
1423 * @return a behaviorally equivalent BMH
1424 */
1425 abstract BoundMethodHandle rebind();
1426
1427 /**
1428 * Replace the old lambda form of this method handle with a new one.
1429 * The new one must be functionally equivalent to the old one.
1430 * Threads may continue running the old form indefinitely,
1431 * but it is likely that the new one will be preferred for new executions.
1432 * Use with discretion.
1433 */
1434 /*non-public*/
1435 void updateForm(LambdaForm newForm) {
1436 assert(newForm.customized == null || newForm.customized == this);
1437 if (form == newForm) return;
1438 newForm.prepare(); // as in MethodHandle.<init>
1439 UNSAFE.putObject(this, FORM_OFFSET, newForm);
1440 UNSAFE.fullFence();
1441 }
1442
1443 /** Craft a LambdaForm customized for this particular MethodHandle */
1444 /*non-public*/
1445 void customize() {
1446 if (form.customized == null) {
1447 LambdaForm newForm = form.customize(this);
1448 updateForm(newForm);
1449 } else {
1450 assert(form.customized == this);
1451 }
1452 }
1453
1454 private static final long FORM_OFFSET;
1455 static {
1456 try {
1457 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1458 } catch (ReflectiveOperationException ex) {
1459 throw newInternalError(ex);
1460 }
1461 }
1462 }