1 /*
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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 jdk.internal.HotSpotIntrinsicCandidate;
30
31 import java.lang.constant.ClassDesc;
32 import java.lang.constant.Constable;
33 import java.lang.constant.ConstantDesc;
34 import java.lang.constant.DirectMethodHandleDesc;
35 import java.lang.constant.MethodHandleDesc;
36 import java.lang.constant.MethodTypeDesc;
37 import java.util.Arrays;
38 import java.util.Objects;
39 import java.util.Optional;
40
41 import static java.lang.invoke.MethodHandleInfo.*;
42 import static java.lang.invoke.MethodHandleStatics.*;
43 import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
44
45 /**
46 * A method handle is a typed, directly executable reference to an underlying method,
47 * constructor, field, or similar low-level operation, with optional
48 * transformations of arguments or return values.
49 * These transformations are quite general, and include such patterns as
50 * {@linkplain #asType conversion},
51 * {@linkplain #bindTo insertion},
52 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
53 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
54 *
55 * <h2>Method handle contents</h2>
56 * Method handles are dynamically and strongly typed according to their parameter and return types.
57 * They are not distinguished by the name or the defining class of their underlying methods.
58 * A method handle must be invoked using a symbolic type descriptor which matches
59 * the method handle's own {@linkplain #type() type descriptor}.
60 * <p>
61 * Every method handle reports its type descriptor via the {@link #type() type} accessor.
62 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
63 * whose structure is a series of classes, one of which is
64 * the return type of the method (or {@code void.class} if none).
65 * <p>
66 * A method handle's type controls the types of invocations it accepts,
67 * and the kinds of transformations that apply to it.
68 * <p>
69 * A method handle contains a pair of special invoker methods
70 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
71 * Both invoker methods provide direct access to the method handle's
72 * underlying method, constructor, field, or other operation,
73 * as modified by transformations of arguments and return values.
74 * Both invokers accept calls which exactly match the method handle's own type.
75 * The plain, inexact invoker also accepts a range of other call types.
76 * <p>
77 * Method handles are immutable and have no visible state.
78 * Of course, they can be bound to underlying methods or data which exhibit state.
79 * With respect to the Java Memory Model, any method handle will behave
80 * as if all of its (internal) fields are final variables. This means that any method
81 * handle made visible to the application will always be fully formed.
82 * This is true even if the method handle is published through a shared
83 * variable in a data race.
84 * <p>
85 * Method handles cannot be subclassed by the user.
86 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
87 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
88 * operation. The programmer should not draw conclusions about a method handle
89 * from its specific class, as the method handle class hierarchy (if any)
90 * may change from time to time or across implementations from different vendors.
91 *
92 * <h2>Method handle compilation</h2>
93 * A Java method call expression naming {@code invokeExact} or {@code invoke}
94 * can invoke a method handle from Java source code.
95 * From the viewpoint of source code, these methods can take any arguments
96 * and their result can be cast to any return type.
97 * Formally this is accomplished by giving the invoker methods
98 * {@code Object} return types and variable arity {@code Object} arguments,
99 * but they have an additional quality called <em>signature polymorphism</em>
100 * which connects this freedom of invocation directly to the JVM execution stack.
101 * <p>
102 * As is usual with virtual methods, source-level calls to {@code invokeExact}
103 * and {@code invoke} compile to an {@code invokevirtual} instruction.
104 * More unusually, the compiler must record the actual argument types,
105 * and may not perform method invocation conversions on the arguments.
106 * Instead, it must generate instructions that push them on the stack according
107 * to their own unconverted types. The method handle object itself is pushed on
108 * the stack before the arguments.
109 * The compiler then generates an {@code invokevirtual} instruction that invokes
110 * the method handle with a symbolic type descriptor which describes the argument
111 * and return types.
112 * <p>
113 * To issue a complete symbolic type descriptor, the compiler must also determine
114 * the return type. This is based on a cast on the method invocation expression,
115 * if there is one, or else {@code Object} if the invocation is an expression,
116 * or else {@code void} if the invocation is a statement.
117 * The cast may be to a primitive type (but not {@code void}).
118 * <p>
119 * As a corner case, an uncasted {@code null} argument is given
120 * a symbolic type descriptor of {@code java.lang.Void}.
121 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
122 * {@code Void} except the null reference.
123 *
124 * <h2>Method handle invocation</h2>
125 * The first time an {@code invokevirtual} instruction is executed
126 * it is linked by symbolically resolving the names in the instruction
127 * and verifying that the method call is statically legal.
128 * This also holds for calls to {@code invokeExact} and {@code invoke}.
129 * In this case, the symbolic type descriptor emitted by the compiler is checked for
130 * correct syntax, and names it contains are resolved.
131 * Thus, an {@code invokevirtual} instruction which invokes
132 * a method handle will always link, as long
133 * as the symbolic type descriptor is syntactically well-formed
134 * and the types exist.
135 * <p>
136 * When the {@code invokevirtual} is executed after linking,
137 * the receiving method handle's type is first checked by the JVM
138 * to ensure that it matches the symbolic type descriptor.
139 * If the type match fails, it means that the method which the
140 * caller is invoking is not present on the individual
141 * method handle being invoked.
142 * <p>
143 * In the case of {@code invokeExact}, the type descriptor of the invocation
144 * (after resolving symbolic type names) must exactly match the method type
145 * of the receiving method handle.
146 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
147 * must be a valid argument to the receiver's {@link #asType asType} method.
148 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
149 * <p>
150 * After type matching, a call to {@code invokeExact} directly
151 * and immediately invoke the method handle's underlying method
152 * (or other behavior, as the case may be).
153 * <p>
154 * A call to plain {@code invoke} works the same as a call to
155 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
156 * exactly matches the method handle's own type.
157 * If there is a type mismatch, {@code invoke} attempts
158 * to adjust the type of the receiving method handle,
159 * as if by a call to {@link #asType asType},
160 * to obtain an exactly invokable method handle {@code M2}.
161 * This allows a more powerful negotiation of method type
162 * between caller and callee.
163 * <p>
164 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
165 * and implementations are therefore not required to materialize it.)
166 *
167 * <h2>Invocation checking</h2>
168 * In typical programs, method handle type matching will usually succeed.
169 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
170 * either directly (in the case of {@code invokeExact}) or indirectly as if
171 * by a failed call to {@code asType} (in the case of {@code invoke}).
172 * <p>
173 * Thus, a method type mismatch which might show up as a linkage error
174 * in a statically typed program can show up as
175 * a dynamic {@code WrongMethodTypeException}
176 * in a program which uses method handles.
177 * <p>
178 * Because method types contain "live" {@code Class} objects,
179 * method type matching takes into account both type names and class loaders.
180 * Thus, even if a method handle {@code M} is created in one
181 * class loader {@code L1} and used in another {@code L2},
182 * method handle calls are type-safe, because the caller's symbolic type
183 * descriptor, as resolved in {@code L2},
184 * is matched against the original callee method's symbolic type descriptor,
185 * as resolved in {@code L1}.
186 * The resolution in {@code L1} happens when {@code M} is created
187 * and its type is assigned, while the resolution in {@code L2} happens
188 * when the {@code invokevirtual} instruction is linked.
189 * <p>
190 * Apart from type descriptor checks,
191 * a method handle's capability to call its underlying method is unrestricted.
192 * If a method handle is formed on a non-public method by a class
193 * that has access to that method, the resulting handle can be used
194 * in any place by any caller who receives a reference to it.
195 * <p>
196 * Unlike with the Core Reflection API, where access is checked every time
197 * a reflective method is invoked,
198 * method handle access checking is performed
199 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
200 * In the case of {@code ldc} (see below), access checking is performed as part of linking
201 * the constant pool entry underlying the constant method handle.
202 * <p>
203 * Thus, handles to non-public methods, or to methods in non-public classes,
204 * should generally be kept secret.
205 * They should not be passed to untrusted code unless their use from
206 * the untrusted code would be harmless.
207 *
208 * <h2>Method handle creation</h2>
209 * Java code can create a method handle that directly accesses
210 * any method, constructor, or field that is accessible to that code.
211 * This is done via a reflective, capability-based API called
212 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}.
213 * For example, a static method handle can be obtained
214 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
215 * There are also conversion methods from Core Reflection API objects,
216 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
217 * <p>
218 * Like classes and strings, method handles that correspond to accessible
219 * fields, methods, and constructors can also be represented directly
220 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
221 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
222 * refers directly to an associated {@code CONSTANT_Methodref},
223 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
224 * constant pool entry.
225 * (For full details on method handle constants,
226 * see sections {@jvms 4.4.8} and {@jvms 5.4.3.5} of the Java Virtual Machine Specification.)
227 * <p>
228 * Method handles produced by lookups or constant loads from methods or
229 * constructors with the variable arity modifier bit ({@code 0x0080})
230 * have a corresponding variable arity, as if they were defined with
231 * the help of {@link #asVarargsCollector asVarargsCollector}
232 * or {@link #withVarargs withVarargs}.
233 * <p>
234 * A method reference may refer either to a static or non-static method.
235 * In the non-static case, the method handle type includes an explicit
236 * receiver argument, prepended before any other arguments.
237 * In the method handle's type, the initial receiver argument is typed
238 * according to the class under which the method was initially requested.
239 * (E.g., if a non-static method handle is obtained via {@code ldc},
240 * the type of the receiver is the class named in the constant pool entry.)
241 * <p>
242 * Method handle constants are subject to the same link-time access checks
243 * their corresponding bytecode instructions, and the {@code ldc} instruction
244 * will throw corresponding linkage errors if the bytecode behaviors would
245 * throw such errors.
246 * <p>
247 * As a corollary of this, access to protected members is restricted
248 * to receivers only of the accessing class, or one of its subclasses,
249 * and the accessing class must in turn be a subclass (or package sibling)
250 * of the protected member's defining class.
251 * If a method reference refers to a protected non-static method or field
252 * of a class outside the current package, the receiver argument will
253 * be narrowed to the type of the accessing class.
254 * <p>
255 * When a method handle to a virtual method is invoked, the method is
256 * always looked up in the receiver (that is, the first argument).
257 * <p>
258 * A non-virtual method handle to a specific virtual method implementation
259 * can also be created. These do not perform virtual lookup based on
260 * receiver type. Such a method handle simulates the effect of
261 * an {@code invokespecial} instruction to the same method.
262 * A non-virtual method handle can also be created to simulate the effect
263 * of an {@code invokevirtual} or {@code invokeinterface} instruction on
264 * a private method (as applicable).
265 *
266 * <h2>Usage examples</h2>
267 * Here are some examples of usage:
268 * <blockquote><pre>{@code
269 Object x, y; String s; int i;
270 MethodType mt; MethodHandle mh;
271 MethodHandles.Lookup lookup = MethodHandles.lookup();
272 // mt is (char,char)String
273 mt = MethodType.methodType(String.class, char.class, char.class);
274 mh = lookup.findVirtual(String.class, "replace", mt);
275 s = (String) mh.invokeExact("daddy",'d','n');
276 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
277 assertEquals(s, "nanny");
278 // weakly typed invocation (using MHs.invoke)
279 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
280 assertEquals(s, "savvy");
281 // mt is (Object[])List
282 mt = MethodType.methodType(java.util.List.class, Object[].class);
283 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
284 assert(mh.isVarargsCollector());
285 x = mh.invoke("one", "two");
286 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
287 assertEquals(x, java.util.Arrays.asList("one","two"));
288 // mt is (Object,Object,Object)Object
289 mt = MethodType.genericMethodType(3);
290 mh = mh.asType(mt);
291 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
292 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
293 assertEquals(x, java.util.Arrays.asList(1,2,3));
294 // mt is ()int
295 mt = MethodType.methodType(int.class);
296 mh = lookup.findVirtual(java.util.List.class, "size", mt);
297 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
298 // invokeExact(Ljava/util/List;)I
299 assert(i == 3);
300 mt = MethodType.methodType(void.class, String.class);
301 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
302 mh.invokeExact(System.out, "Hello, world.");
303 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
304 * }</pre></blockquote>
305 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
306 * generates a single invokevirtual instruction with
307 * the symbolic type descriptor indicated in the following comment.
308 * In these examples, the helper method {@code assertEquals} is assumed to
309 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
310 * on its arguments, and asserts that the result is true.
311 *
312 * <h2>Exceptions</h2>
313 * The methods {@code invokeExact} and {@code invoke} are declared
314 * to throw {@link java.lang.Throwable Throwable},
315 * which is to say that there is no static restriction on what a method handle
316 * can throw. Since the JVM does not distinguish between checked
317 * and unchecked exceptions (other than by their class, of course),
318 * there is no particular effect on bytecode shape from ascribing
319 * checked exceptions to method handle invocations. But in Java source
320 * code, methods which perform method handle calls must either explicitly
321 * throw {@code Throwable}, or else must catch all
322 * throwables locally, rethrowing only those which are legal in the context,
323 * and wrapping ones which are illegal.
324 *
325 * <h2><a id="sigpoly"></a>Signature polymorphism</h2>
326 * The unusual compilation and linkage behavior of
327 * {@code invokeExact} and plain {@code invoke}
328 * is referenced by the term <em>signature polymorphism</em>.
329 * As defined in the Java Language Specification,
330 * a signature polymorphic method is one which can operate with
331 * any of a wide range of call signatures and return types.
332 * <p>
333 * In source code, a call to a signature polymorphic method will
334 * compile, regardless of the requested symbolic type descriptor.
335 * As usual, the Java compiler emits an {@code invokevirtual}
336 * instruction with the given symbolic type descriptor against the named method.
337 * The unusual part is that the symbolic type descriptor is derived from
338 * the actual argument and return types, not from the method declaration.
339 * <p>
340 * When the JVM processes bytecode containing signature polymorphic calls,
341 * it will successfully link any such call, regardless of its symbolic type descriptor.
342 * (In order to retain type safety, the JVM will guard such calls with suitable
343 * dynamic type checks, as described elsewhere.)
344 * <p>
345 * Bytecode generators, including the compiler back end, are required to emit
346 * untransformed symbolic type descriptors for these methods.
347 * Tools which determine symbolic linkage are required to accept such
348 * untransformed descriptors, without reporting linkage errors.
349 *
350 * <h2>Interoperation between method handles and the Core Reflection API</h2>
351 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
352 * any class member represented by a Core Reflection API object
353 * can be converted to a behaviorally equivalent method handle.
354 * For example, a reflective {@link java.lang.reflect.Method Method} can
355 * be converted to a method handle using
356 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
357 * The resulting method handles generally provide more direct and efficient
358 * access to the underlying class members.
359 * <p>
360 * As a special case,
361 * when the Core Reflection API is used to view the signature polymorphic
362 * methods {@code invokeExact} or plain {@code invoke} in this class,
363 * they appear as ordinary non-polymorphic methods.
364 * Their reflective appearance, as viewed by
365 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
366 * is unaffected by their special status in this API.
367 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
368 * will report exactly those modifier bits required for any similarly
369 * declared method, including in this case {@code native} and {@code varargs} bits.
370 * <p>
371 * As with any reflected method, these methods (when reflected) may be
372 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
373 * However, such reflective calls do not result in method handle invocations.
374 * Such a call, if passed the required argument
375 * (a single one, of type {@code Object[]}), will ignore the argument and
376 * will throw an {@code UnsupportedOperationException}.
377 * <p>
378 * Since {@code invokevirtual} instructions can natively
379 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
380 * with the normal presentation of these methods via bytecodes.
381 * Thus, these two native methods, when reflectively viewed by
382 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
383 * <p>
384 * In order to obtain an invoker method for a particular type descriptor,
385 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
386 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
387 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
388 * API is also able to return a method handle
389 * to call {@code invokeExact} or plain {@code invoke},
390 * for any specified type descriptor .
391 *
392 * <h2>Interoperation between method handles and Java generics</h2>
393 * A method handle can be obtained on a method, constructor, or field
394 * which is declared with Java generic types.
395 * As with the Core Reflection API, the type of the method handle
396 * will be constructed from the erasure of the source-level type.
397 * When a method handle is invoked, the types of its arguments
398 * or the return value cast type may be generic types or type instances.
399 * If this occurs, the compiler will replace those
400 * types by their erasures when it constructs the symbolic type descriptor
401 * for the {@code invokevirtual} instruction.
402 * <p>
403 * Method handles do not represent
404 * their function-like types in terms of Java parameterized (generic) types,
405 * because there are three mismatches between function-like types and parameterized
406 * Java types.
407 * <ul>
408 * <li>Method types range over all possible arities,
409 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
410 * Generics are not variadic, and so cannot represent this.</li>
411 * <li>Method types can specify arguments of primitive types,
412 * which Java generic types cannot range over.</li>
413 * <li>Higher order functions over method handles (combinators) are
414 * often generic across a wide range of function types, including
415 * those of multiple arities. It is impossible to represent such
416 * genericity with a Java type parameter.</li>
417 * </ul>
418 *
419 * <h2><a id="maxarity"></a>Arity limits</h2>
420 * The JVM imposes on all methods and constructors of any kind an absolute
421 * limit of 255 stacked arguments. This limit can appear more restrictive
422 * in certain cases:
423 * <ul>
424 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
425 * <li>A non-static method consumes an extra argument for the object on which the method is called.
426 * <li>A constructor consumes an extra argument for the object which is being constructed.
427 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
428 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
429 * </ul>
430 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
431 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
432 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
433 * In particular, a method handle’s type must not have an arity of the exact maximum 255.
434 *
435 * @see MethodType
436 * @see MethodHandles
437 * @author John Rose, JSR 292 EG
438 * @since 1.7
439 */
440 public abstract class MethodHandle implements Constable {
441
442 /**
443 * Internal marker interface which distinguishes (to the Java compiler)
444 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
445 */
446 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
447 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
448 @interface PolymorphicSignature { }
449
450 private final MethodType type;
451 /*private*/
452 final LambdaForm form;
453 // form is not private so that invokers can easily fetch it
454 /*private*/
455 MethodHandle asTypeCache;
456 // asTypeCache is not private so that invokers can easily fetch it
457 /*non-public*/
458 byte customizationCount;
459 // customizationCount should be accessible from invokers
460
461 /**
462 * Reports the type of this method handle.
463 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
464 * @return the method handle type
465 */
466 public MethodType type() {
467 return type;
468 }
469
470 /**
471 * Package-private constructor for the method handle implementation hierarchy.
472 * Method handle inheritance will be contained completely within
473 * the {@code java.lang.invoke} package.
474 */
475 // @param type type (permanently assigned) of the new method handle
476 /*non-public*/
477 MethodHandle(MethodType type, LambdaForm form) {
478 this.type = Objects.requireNonNull(type);
479 this.form = Objects.requireNonNull(form).uncustomize();
480
481 this.form.prepare(); // TO DO: Try to delay this step until just before invocation.
482 }
483
484 /**
485 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
486 * The symbolic type descriptor at the call site of {@code invokeExact} must
487 * exactly match this method handle's {@link #type() type}.
488 * No conversions are allowed on arguments or return values.
489 * <p>
490 * When this method is observed via the Core Reflection API,
491 * it will appear as a single native method, taking an object array and returning an object.
492 * If this native method is invoked directly via
493 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
494 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
495 * it will throw an {@code UnsupportedOperationException}.
496 * @param args the signature-polymorphic parameter list, statically represented using varargs
497 * @return the signature-polymorphic result, statically represented using {@code Object}
498 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
499 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
500 */
501 @HotSpotIntrinsicCandidate
502 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
503
504 /**
505 * Invokes the method handle, allowing any caller type descriptor,
506 * and optionally performing conversions on arguments and return values.
507 * <p>
508 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type() type},
509 * the call proceeds as if by {@link #invokeExact invokeExact}.
510 * <p>
511 * Otherwise, the call proceeds as if this method handle were first
512 * adjusted by calling {@link #asType asType} to adjust this method handle
513 * to the required type, and then the call proceeds as if by
514 * {@link #invokeExact invokeExact} on the adjusted method handle.
515 * <p>
516 * There is no guarantee that the {@code asType} call is actually made.
517 * If the JVM can predict the results of making the call, it may perform
518 * adaptations directly on the caller's arguments,
519 * and call the target method handle according to its own exact type.
520 * <p>
521 * The resolved type descriptor at the call site of {@code invoke} must
522 * be a valid argument to the receivers {@code asType} method.
523 * In particular, the caller must specify the same argument arity
524 * as the callee's type,
525 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
526 * <p>
527 * When this method is observed via the Core Reflection API,
528 * it will appear as a single native method, taking an object array and returning an object.
529 * If this native method is invoked directly via
530 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
531 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
532 * it will throw an {@code UnsupportedOperationException}.
533 * @param args the signature-polymorphic parameter list, statically represented using varargs
534 * @return the signature-polymorphic result, statically represented using {@code Object}
535 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
536 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
537 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
538 */
539 @HotSpotIntrinsicCandidate
540 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
541
542 /**
543 * Private method for trusted invocation of a method handle respecting simplified signatures.
544 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
545 * <p>
546 * The caller signature is restricted to the following basic types:
547 * Object, int, long, float, double, and void return.
548 * <p>
549 * The caller is responsible for maintaining type correctness by ensuring
550 * that the each outgoing argument value is a member of the range of the corresponding
551 * callee argument type.
552 * (The caller should therefore issue appropriate casts and integer narrowing
553 * operations on outgoing argument values.)
554 * The caller can assume that the incoming result value is part of the range
555 * of the callee's return type.
556 * @param args the signature-polymorphic parameter list, statically represented using varargs
557 * @return the signature-polymorphic result, statically represented using {@code Object}
558 */
559 @HotSpotIntrinsicCandidate
560 /*non-public*/
561 final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
562
563 /**
564 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
565 * The caller signature is restricted to basic types as with {@code invokeBasic}.
566 * The trailing (not leading) argument must be a MemberName.
567 * @param args the signature-polymorphic parameter list, statically represented using varargs
568 * @return the signature-polymorphic result, statically represented using {@code Object}
569 */
570 @HotSpotIntrinsicCandidate
571 /*non-public*/
572 static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
573
574 /**
575 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
576 * The caller signature is restricted to basic types as with {@code invokeBasic}.
577 * The trailing (not leading) argument must be a MemberName.
578 * @param args the signature-polymorphic parameter list, statically represented using varargs
579 * @return the signature-polymorphic result, statically represented using {@code Object}
580 */
581 @HotSpotIntrinsicCandidate
582 /*non-public*/
583 static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
584
585 /**
586 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
587 * The caller signature is restricted to basic types as with {@code invokeBasic}.
588 * The trailing (not leading) argument must be a MemberName.
589 * @param args the signature-polymorphic parameter list, statically represented using varargs
590 * @return the signature-polymorphic result, statically represented using {@code Object}
591 */
592 @HotSpotIntrinsicCandidate
593 /*non-public*/
594 static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
595
596 /**
597 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
598 * The caller signature is restricted to basic types as with {@code invokeBasic}.
599 * The trailing (not leading) argument must be a MemberName.
600 * @param args the signature-polymorphic parameter list, statically represented using varargs
601 * @return the signature-polymorphic result, statically represented using {@code Object}
602 */
603 @HotSpotIntrinsicCandidate
604 /*non-public*/
605 static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
606
607 /**
608 * Performs a variable arity invocation, passing the arguments in the given array
609 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
610 * which mentions only the type {@code Object}, and whose actual argument count is the length
611 * of the argument array.
612 * <p>
613 * Specifically, execution proceeds as if by the following steps,
614 * although the methods are not guaranteed to be called if the JVM
615 * can predict their effects.
616 * <ul>
617 * <li>Determine the length of the argument array as {@code N}.
618 * For a null reference, {@code N=0}. </li>
619 * <li>Collect the {@code N} elements of the array as a logical
620 * argument list, each argument statically typed as an {@code Object}. </li>
621 * <li>Determine, as {@code M}, the parameter count of the type of this
622 * method handle. </li>
623 * <li>Determine the general type {@code TN} of {@code N} arguments or
624 * {@code M} arguments, if smaller than {@code N}, as
625 * {@code TN=MethodType.genericMethodType(Math.min(N, M))}.</li>
626 * <li>If {@code N} is greater than {@code M}, perform the following
627 * checks and actions to shorten the logical argument list: <ul>
628 * <li>Check that this method handle has variable arity with a
629 * {@linkplain MethodType#lastParameterType trailing parameter}
630 * of some array type {@code A[]}. If not, fail with a
631 * {@code WrongMethodTypeException}. </li>
632 * <li>Collect the trailing elements (there are {@code N-M+1} of them)
633 * from the logical argument list into a single array of
634 * type {@code A[]}, using {@code asType} conversions to
635 * convert each trailing argument to type {@code A}. </li>
636 * <li>If any of these conversions proves impossible, fail with either
637 * a {@code ClassCastException} if any trailing element cannot be
638 * cast to {@code A} or a {@code NullPointerException} if any
639 * trailing element is {@code null} and {@code A} is not a reference
640 * type. </li>
641 * <li>Replace the logical arguments gathered into the array of
642 * type {@code A[]} with the array itself, thus shortening
643 * the argument list to length {@code M}. This final argument
644 * retains the static type {@code A[]}.</li>
645 * <li>Adjust the type {@code TN} by changing the {@code N}th
646 * parameter type from {@code Object} to {@code A[]}.
647 * </ul>
648 * <li>Force the original target method handle {@code MH0} to the
649 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
650 * <li>Spread the argument list into {@code N} separate arguments {@code A0, ...}. </li>
651 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
652 * MH1.invokeExact(A0, ...). </li>
653 * <li>Take the return value as an {@code Object} reference. </li>
654 * </ul>
655 * <p>
656 * If the target method handle has variable arity, and the argument list is longer
657 * than that arity, the excess arguments, starting at the position of the trailing
658 * array argument, will be gathered (if possible, as if by {@code asType} conversions)
659 * into an array of the appropriate type, and invocation will proceed on the
660 * shortened argument list.
661 * In this way, <em>jumbo argument lists</em> which would spread into more
662 * than 254 slots can still be processed uniformly.
663 * <p>
664 * Unlike the {@link #invoke(Object...) generic} invocation mode, which can
665 * "recycle" an array argument, passing it directly to the target method,
666 * this invocation mode <em>always</em> creates a new array parameter, even
667 * if the original array passed to {@code invokeWithArguments} would have
668 * been acceptable as a direct argument to the target method.
669 * Even if the number {@code M} of actual arguments is the arity {@code N},
670 * and the last argument is dynamically a suitable array of type {@code A[]},
671 * it will still be boxed into a new one-element array, since the call
672 * site statically types the argument as {@code Object}, not an array type.
673 * This is not a special rule for this method, but rather a regular effect
674 * of the {@linkplain #asVarargsCollector rules for variable-arity invocation}.
675 * <p>
676 * Because of the action of the {@code asType} step, the following argument
677 * conversions are applied as necessary:
678 * <ul>
679 * <li>reference casting
680 * <li>unboxing
681 * <li>widening primitive conversions
682 * <li>variable arity conversion
683 * </ul>
684 * <p>
685 * The result returned by the call is boxed if it is a primitive,
686 * or forced to null if the return type is void.
687 * <p>
688 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
689 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
690 * It can therefore be used as a bridge between native or reflective code and method handles.
691 * @apiNote
692 * This call is approximately equivalent to the following code:
693 * <blockquote><pre>{@code
694 * // for jumbo argument lists, adapt varargs explicitly:
695 * int N = (arguments == null? 0: arguments.length);
696 * int M = this.type.parameterCount();
697 * int MAX_SAFE = 127; // 127 longs require 254 slots, which is OK
698 * if (N > MAX_SAFE && N > M && this.isVarargsCollector()) {
699 * Class<?> arrayType = this.type().lastParameterType();
700 * Class<?> elemType = arrayType.getComponentType();
701 * if (elemType != null) {
702 * Object args2 = Array.newInstance(elemType, M);
703 * MethodHandle arraySetter = MethodHandles.arrayElementSetter(arrayType);
704 * for (int i = 0; i < M; i++) {
705 * arraySetter.invoke(args2, i, arguments[M-1 + i]);
706 * }
707 * arguments = Arrays.copyOf(arguments, M);
708 * arguments[M-1] = args2;
709 * return this.asFixedArity().invokeWithArguments(arguments);
710 * }
711 * } // done with explicit varargs processing
712 *
713 * // Handle fixed arity and non-jumbo variable arity invocation.
714 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
715 * Object result = invoker.invokeExact(this, arguments);
716 * }</pre></blockquote>
717 *
718 * @param arguments the arguments to pass to the target
719 * @return the result returned by the target
720 * @throws ClassCastException if an argument cannot be converted by reference casting
721 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
722 * @throws Throwable anything thrown by the target method invocation
723 * @see MethodHandles#spreadInvoker
724 */
725 public Object invokeWithArguments(Object... arguments) throws Throwable {
726 // Note: Jumbo argument lists are handled in the variable-arity subclass.
727 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length);
728 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments);
729 }
730
731 /**
732 * Performs a variable arity invocation, passing the arguments in the given list
733 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
734 * which mentions only the type {@code Object}, and whose actual argument count is the length
735 * of the argument list.
736 * <p>
737 * This method is also equivalent to the following code:
738 * <blockquote><pre>{@code
739 * invokeWithArguments(arguments.toArray())
740 * }</pre></blockquote>
741 * <p>
742 * Jumbo-sized lists are acceptable if this method handle has variable arity.
743 * See {@link #invokeWithArguments(Object[])} for details.
744 *
745 * @param arguments the arguments to pass to the target
746 * @return the result returned by the target
747 * @throws NullPointerException if {@code arguments} is a null reference
748 * @throws ClassCastException if an argument cannot be converted by reference casting
749 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
750 * @throws Throwable anything thrown by the target method invocation
751 */
752 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
753 return invokeWithArguments(arguments.toArray());
754 }
755
756 /**
757 * Produces an adapter method handle which adapts the type of the
758 * current method handle to a new type.
759 * The resulting method handle is guaranteed to report a type
760 * which is equal to the desired new type.
761 * <p>
762 * If the original type and new type are equal, returns {@code this}.
763 * <p>
764 * The new method handle, when invoked, will perform the following
765 * steps:
766 * <ul>
767 * <li>Convert the incoming argument list to match the original
768 * method handle's argument list.
769 * <li>Invoke the original method handle on the converted argument list.
770 * <li>Convert any result returned by the original method handle
771 * to the return type of new method handle.
772 * </ul>
773 * <p>
774 * This method provides the crucial behavioral difference between
775 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
776 * The two methods
777 * perform the same steps when the caller's type descriptor exactly matches
778 * the callee's, but when the types differ, plain {@link #invoke invoke}
779 * also calls {@code asType} (or some internal equivalent) in order
780 * to match up the caller's and callee's types.
781 * <p>
782 * If the current method is a variable arity method handle
783 * argument list conversion may involve the conversion and collection
784 * of several arguments into an array, as
785 * {@linkplain #asVarargsCollector described elsewhere}.
786 * In every other case, all conversions are applied <em>pairwise</em>,
787 * which means that each argument or return value is converted to
788 * exactly one argument or return value (or no return value).
789 * The applied conversions are defined by consulting
790 * the corresponding component types of the old and new
791 * method handle types.
792 * <p>
793 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
794 * or old and new return types. Specifically, for some valid index {@code i}, let
795 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
796 * Or else, going the other way for return values, let
797 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
798 * If the types are the same, the new method handle makes no change
799 * to the corresponding argument or return value (if any).
800 * Otherwise, one of the following conversions is applied
801 * if possible:
802 * <ul>
803 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
804 * (The types do not need to be related in any particular way.
805 * This is because a dynamic value of null can convert to any reference type.)
806 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
807 * conversion (JLS 5.3) is applied, if one exists.
808 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
809 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
810 * a Java casting conversion (JLS 5.5) is applied if one exists.
811 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
812 * which is then widened as needed to <em>T1</em>.)
813 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
814 * conversion will be applied at runtime, possibly followed
815 * by a Java method invocation conversion (JLS 5.3)
816 * on the primitive value. (These are the primitive widening conversions.)
817 * <em>T0</em> must be a wrapper class or a supertype of one.
818 * (In the case where <em>T0</em> is Object, these are the conversions
819 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
820 * The unboxing conversion must have a possibility of success, which means that
821 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
822 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
823 * primitive value can be widened to <em>T1</em>.
824 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
825 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
826 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
827 * a zero value is introduced.
828 * </ul>
829 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
830 * because neither corresponds specifically to the <em>dynamic type</em> of any
831 * actual argument or return value.)
832 * <p>
833 * The method handle conversion cannot be made if any one of the required
834 * pairwise conversions cannot be made.
835 * <p>
836 * At runtime, the conversions applied to reference arguments
837 * or return values may require additional runtime checks which can fail.
838 * An unboxing operation may fail because the original reference is null,
839 * causing a {@link java.lang.NullPointerException NullPointerException}.
840 * An unboxing operation or a reference cast may also fail on a reference
841 * to an object of the wrong type,
842 * causing a {@link java.lang.ClassCastException ClassCastException}.
843 * Although an unboxing operation may accept several kinds of wrappers,
844 * if none are available, a {@code ClassCastException} will be thrown.
845 *
846 * @param newType the expected type of the new method handle
847 * @return a method handle which delegates to {@code this} after performing
848 * any necessary argument conversions, and arranges for any
849 * necessary return value conversions
850 * @throws NullPointerException if {@code newType} is a null reference
851 * @throws WrongMethodTypeException if the conversion cannot be made
852 * @see MethodHandles#explicitCastArguments
853 */
854 public MethodHandle asType(MethodType newType) {
855 // Fast path alternative to a heavyweight {@code asType} call.
856 // Return 'this' if the conversion will be a no-op.
857 if (newType == type) {
858 return this;
859 }
860 // Return 'this.asTypeCache' if the conversion is already memoized.
861 MethodHandle atc = asTypeCached(newType);
862 if (atc != null) {
863 return atc;
864 }
865 return asTypeUncached(newType);
866 }
867
868 private MethodHandle asTypeCached(MethodType newType) {
869 MethodHandle atc = asTypeCache;
870 if (atc != null && newType == atc.type) {
871 return atc;
872 }
873 return null;
874 }
875
876 /** Override this to change asType behavior. */
877 /*non-public*/
878 MethodHandle asTypeUncached(MethodType newType) {
879 if (!type.isConvertibleTo(newType))
880 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
881 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
882 }
883
884 /**
885 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
886 * and spreads its elements as positional arguments.
887 * The new method handle adapts, as its <i>target</i>,
888 * the current method handle. The type of the adapter will be
889 * the same as the type of the target, except that the final
890 * {@code arrayLength} parameters of the target's type are replaced
891 * by a single array parameter of type {@code arrayType}.
892 * <p>
893 * If the array element type differs from any of the corresponding
894 * argument types on the original target,
895 * the original target is adapted to take the array elements directly,
896 * as if by a call to {@link #asType asType}.
897 * <p>
898 * When called, the adapter replaces a trailing array argument
899 * by the array's elements, each as its own argument to the target.
900 * (The order of the arguments is preserved.)
901 * They are converted pairwise by casting and/or unboxing
902 * to the types of the trailing parameters of the target.
903 * Finally the target is called.
904 * What the target eventually returns is returned unchanged by the adapter.
905 * <p>
906 * Before calling the target, the adapter verifies that the array
907 * contains exactly enough elements to provide a correct argument count
908 * to the target method handle.
909 * (The array may also be null when zero elements are required.)
910 * <p>
911 * When the adapter is called, the length of the supplied {@code array}
912 * argument is queried as if by {@code array.length} or {@code arraylength}
913 * bytecode. If the adapter accepts a zero-length trailing array argument,
914 * the supplied {@code array} argument can either be a zero-length array or
915 * {@code null}; otherwise, the adapter will throw a {@code NullPointerException}
916 * if the array is {@code null} and throw an {@link IllegalArgumentException}
917 * if the array does not have the correct number of elements.
918 * <p>
919 * Here are some simple examples of array-spreading method handles:
920 * <blockquote><pre>{@code
921 MethodHandle equals = publicLookup()
922 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
923 assert( (boolean) equals.invokeExact("me", (Object)"me"));
924 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
925 // spread both arguments from a 2-array:
926 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
927 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
928 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
929 // try to spread from anything but a 2-array:
930 for (int n = 0; n <= 10; n++) {
931 Object[] badArityArgs = (n == 2 ? new Object[0] : new Object[n]);
932 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
933 catch (IllegalArgumentException ex) { } // OK
934 }
935 // spread both arguments from a String array:
936 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
937 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
938 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
939 // spread second arguments from a 1-array:
940 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
941 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
942 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
943 // spread no arguments from a 0-array or null:
944 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
945 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
946 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
947 // asSpreader and asCollector are approximate inverses:
948 for (int n = 0; n <= 2; n++) {
949 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
950 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
951 assert( (boolean) equals2.invokeWithArguments("me", "me"));
952 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
953 }
954 }
955 MethodHandle caToString = publicLookup()
956 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
957 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
958 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
959 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
960 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
961 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
962 * }</pre></blockquote>
963 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
964 * @param arrayLength the number of arguments to spread from an incoming array argument
965 * @return a new method handle which spreads its final array argument,
966 * before calling the original method handle
967 * @throws NullPointerException if {@code arrayType} is a null reference
968 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
969 * or if target does not have at least
970 * {@code arrayLength} parameter types,
971 * or if {@code arrayLength} is negative,
972 * or if the resulting method handle's type would have
973 * <a href="MethodHandle.html#maxarity">too many parameters</a>
974 * @throws WrongMethodTypeException if the implied {@code asType} call fails
975 * @see #asCollector
976 */
977 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
978 return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength);
979 }
980
981 /**
982 * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads
983 * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>,
984 * the current method handle. The type of the adapter will be the same as the type of the target, except that the
985 * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos},
986 * are replaced by a single array parameter of type {@code arrayType}.
987 * <p>
988 * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos}
989 * argument to indicate at which position in the parameter list the spreading should take place.
990 *
991 * @apiNote Example:
992 * <blockquote><pre>{@code
993 MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class));
994 MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2);
995 Object[] ints = new Object[]{3, 9, 7, 7};
996 Comparator<Integer> cmp = (a, b) -> a - b;
997 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0);
998 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0);
999 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0);
1000 * }</pre></blockquote>
1001 * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start.
1002 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
1003 * @param arrayLength the number of arguments to spread from an incoming array argument
1004 * @return a new method handle which spreads an array argument at a given position,
1005 * before calling the original method handle
1006 * @throws NullPointerException if {@code arrayType} is a null reference
1007 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
1008 * or if target does not have at least
1009 * {@code arrayLength} parameter types,
1010 * or if {@code arrayLength} is negative,
1011 * or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the
1012 * number of arguments),
1013 * or if the resulting method handle's type would have
1014 * <a href="MethodHandle.html#maxarity">too many parameters</a>
1015 * @throws WrongMethodTypeException if the implied {@code asType} call fails
1016 *
1017 * @see #asSpreader(Class, int)
1018 * @since 9
1019 */
1020 public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) {
1021 MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength);
1022 MethodHandle afterSpread = this.asType(postSpreadType);
1023 BoundMethodHandle mh = afterSpread.rebind();
1024 LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
1025 MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType);
1026 return mh.copyWith(preSpreadType, lform);
1027 }
1028
1029 /**
1030 * See if {@code asSpreader} can be validly called with the given arguments.
1031 * Return the type of the method handle call after spreading but before conversions.
1032 */
1033 private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) {
1034 spreadArrayChecks(arrayType, arrayLength);
1035 int nargs = type().parameterCount();
1036 if (nargs < arrayLength || arrayLength < 0)
1037 throw newIllegalArgumentException("bad spread array length");
1038 if (pos < 0 || pos + arrayLength > nargs) {
1039 throw newIllegalArgumentException("bad spread position");
1040 }
1041 Class<?> arrayElement = arrayType.getComponentType();
1042 MethodType mtype = type();
1043 boolean match = true, fail = false;
1044 for (int i = pos; i < pos + arrayLength; i++) {
1045 Class<?> ptype = mtype.parameterType(i);
1046 if (ptype != arrayElement) {
1047 match = false;
1048 if (!MethodType.canConvert(arrayElement, ptype)) {
1049 fail = true;
1050 break;
1051 }
1052 }
1053 }
1054 if (match) return mtype;
1055 MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength);
1056 if (!fail) return needType;
1057 // elicit an error:
1058 this.asType(needType);
1059 throw newInternalError("should not return");
1060 }
1061
1062 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
1063 Class<?> arrayElement = arrayType.getComponentType();
1064 if (arrayElement == null)
1065 throw newIllegalArgumentException("not an array type", arrayType);
1066 if ((arrayLength & 0x7F) != arrayLength) {
1067 if ((arrayLength & 0xFF) != arrayLength)
1068 throw newIllegalArgumentException("array length is not legal", arrayLength);
1069 assert(arrayLength >= 128);
1070 if (arrayElement == long.class ||
1071 arrayElement == double.class)
1072 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
1073 }
1074 }
1075 /**
1076 * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity}
1077 * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}.
1078 * If the method handle is already of the proper arity mode, it is returned
1079 * unchanged.
1080 * @apiNote
1081 * <p>This method is sometimes useful when adapting a method handle that
1082 * may be variable arity, to ensure that the resulting adapter is also
1083 * variable arity if and only if the original handle was. For example,
1084 * this code changes the first argument of a handle {@code mh} to {@code int} without
1085 * disturbing its variable arity property:
1086 * {@code mh.asType(mh.type().changeParameterType(0,int.class))
1087 * .withVarargs(mh.isVarargsCollector())}
1088 * <p>
1089 * This call is approximately equivalent to the following code:
1090 * <blockquote><pre>{@code
1091 * if (makeVarargs == isVarargsCollector())
1092 * return this;
1093 * else if (makeVarargs)
1094 * return asVarargsCollector(type().lastParameterType());
1095 * else
1096 * return asFixedArity();
1097 * }</pre></blockquote>
1098 * @param makeVarargs true if the return method handle should have variable arity behavior
1099 * @return a method handle of the same type, with possibly adjusted variable arity behavior
1100 * @throws IllegalArgumentException if {@code makeVarargs} is true and
1101 * this method handle does not have a trailing array parameter
1102 * @since 9
1103 * @see #asVarargsCollector
1104 * @see #asFixedArity
1105 */
1106 public MethodHandle withVarargs(boolean makeVarargs) {
1107 assert(!isVarargsCollector()); // subclass responsibility
1108 if (makeVarargs) {
1109 return asVarargsCollector(type().lastParameterType());
1110 } else {
1111 return this;
1112 }
1113 }
1114
1115 /**
1116 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
1117 * positional arguments and collects them into an array argument.
1118 * The new method handle adapts, as its <i>target</i>,
1119 * the current method handle. The type of the adapter will be
1120 * the same as the type of the target, except that a single trailing
1121 * parameter (usually of type {@code arrayType}) is replaced by
1122 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1123 * <p>
1124 * If the array type differs from the final argument type on the original target,
1125 * the original target is adapted to take the array type directly,
1126 * as if by a call to {@link #asType asType}.
1127 * <p>
1128 * When called, the adapter replaces its trailing {@code arrayLength}
1129 * arguments by a single new array of type {@code arrayType}, whose elements
1130 * comprise (in order) the replaced arguments.
1131 * Finally the target is called.
1132 * What the target eventually returns is returned unchanged by the adapter.
1133 * <p>
1134 * (The array may also be a shared constant when {@code arrayLength} is zero.)
1135 * <p>
1136 * (<em>Note:</em> The {@code arrayType} is often identical to the
1137 * {@linkplain MethodType#lastParameterType last parameter type}
1138 * of the original target.
1139 * It is an explicit argument for symmetry with {@code asSpreader}, and also
1140 * to allow the target to use a simple {@code Object} as its last parameter type.)
1141 * <p>
1142 * In order to create a collecting adapter which is not restricted to a particular
1143 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector}
1144 * or {@link #withVarargs withVarargs} instead.
1145 * <p>
1146 * Here are some examples of array-collecting method handles:
1147 * <blockquote><pre>{@code
1148 MethodHandle deepToString = publicLookup()
1149 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1150 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
1151 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
1152 assertEquals(methodType(String.class, Object.class), ts1.type());
1153 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
1154 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
1155 // arrayType can be a subtype of Object[]
1156 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
1157 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
1158 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
1159 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
1160 assertEquals("[]", (String) ts0.invokeExact());
1161 // collectors can be nested, Lisp-style
1162 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
1163 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
1164 // arrayType can be any primitive array type
1165 MethodHandle bytesToString = publicLookup()
1166 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
1167 .asCollector(byte[].class, 3);
1168 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
1169 MethodHandle longsToString = publicLookup()
1170 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
1171 .asCollector(long[].class, 1);
1172 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
1173 * }</pre></blockquote>
1174 * <p>
1175 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1176 * variable-arity method handle}, even if the original target method handle was.
1177 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1178 * @param arrayLength the number of arguments to collect into a new array argument
1179 * @return a new method handle which collects some trailing argument
1180 * into an array, before calling the original method handle
1181 * @throws NullPointerException if {@code arrayType} is a null reference
1182 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1183 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
1184 * or {@code arrayLength} is not a legal array size,
1185 * or the resulting method handle's type would have
1186 * <a href="MethodHandle.html#maxarity">too many parameters</a>
1187 * @throws WrongMethodTypeException if the implied {@code asType} call fails
1188 * @see #asSpreader
1189 * @see #asVarargsCollector
1190 */
1191 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1192 return asCollector(type().parameterCount() - 1, arrayType, arrayLength);
1193 }
1194
1195 /**
1196 * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting
1197 * at a given position, and collects them into an array argument. The new method handle adapts, as its
1198 * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target,
1199 * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType})
1200 * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1201 * <p>
1202 * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code
1203 * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This
1204 * index is zero-based.
1205 *
1206 * @apiNote Examples:
1207 * <blockquote><pre>{@code
1208 StringWriter swr = new StringWriter();
1209 MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr);
1210 MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4);
1211 swWrite4.invoke('A', 'B', 'C', 'D', 1, 2);
1212 assertEquals("BC", swr.toString());
1213 swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4);
1214 assertEquals("BCPQRS", swr.toString());
1215 swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1);
1216 assertEquals("BCPQRSZ", swr.toString());
1217 * }</pre></blockquote>
1218 * <p>
1219 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1220 * variable-arity method handle}, even if the original target method handle was.
1221 * @param collectArgPos the zero-based position in the parameter list at which to start collecting.
1222 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1223 * @param arrayLength the number of arguments to collect into a new array argument
1224 * @return a new method handle which collects some arguments
1225 * into an array, before calling the original method handle
1226 * @throws NullPointerException if {@code arrayType} is a null reference
1227 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1228 * or {@code arrayType} is not assignable to this method handle's array parameter type,
1229 * or {@code arrayLength} is not a legal array size,
1230 * or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments),
1231 * or the resulting method handle's type would have
1232 * <a href="MethodHandle.html#maxarity">too many parameters</a>
1233 * @throws WrongMethodTypeException if the implied {@code asType} call fails
1234 *
1235 * @see #asCollector(Class, int)
1236 * @since 9
1237 */
1238 public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) {
1239 asCollectorChecks(arrayType, collectArgPos, arrayLength);
1240 BoundMethodHandle mh = rebind();
1241 MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength);
1242 MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
1243 LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
1244 if (lform != null) {
1245 return mh.copyWith(resultType, lform);
1246 }
1247 lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
1248 return mh.copyWithExtendL(resultType, lform, newArray);
1249 }
1250
1251 /**
1252 * See if {@code asCollector} can be validly called with the given arguments.
1253 * Return false if the last parameter is not an exact match to arrayType.
1254 */
1255 /*non-public*/
1256 boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) {
1257 spreadArrayChecks(arrayType, arrayLength);
1258 int nargs = type().parameterCount();
1259 if (pos < 0 || pos >= nargs) {
1260 throw newIllegalArgumentException("bad collect position");
1261 }
1262 if (nargs != 0) {
1263 Class<?> param = type().parameterType(pos);
1264 if (param == arrayType) return true;
1265 if (param.isAssignableFrom(arrayType)) return false;
1266 }
1267 throw newIllegalArgumentException("array type not assignable to argument", this, arrayType);
1268 }
1269
1270 /**
1271 * Makes a <em>variable arity</em> adapter which is able to accept
1272 * any number of trailing positional arguments and collect them
1273 * into an array argument.
1274 * <p>
1275 * The type and behavior of the adapter will be the same as
1276 * the type and behavior of the target, except that certain
1277 * {@code invoke} and {@code asType} requests can lead to
1278 * trailing positional arguments being collected into target's
1279 * trailing parameter.
1280 * Also, the
1281 * {@linkplain MethodType#lastParameterType last parameter type}
1282 * of the adapter will be
1283 * {@code arrayType}, even if the target has a different
1284 * last parameter type.
1285 * <p>
1286 * This transformation may return {@code this} if the method handle is
1287 * already of variable arity and its trailing parameter type
1288 * is identical to {@code arrayType}.
1289 * <p>
1290 * When called with {@link #invokeExact invokeExact}, the adapter invokes
1291 * the target with no argument changes.
1292 * (<em>Note:</em> This behavior is different from a
1293 * {@linkplain #asCollector fixed arity collector},
1294 * since it accepts a whole array of indeterminate length,
1295 * rather than a fixed number of arguments.)
1296 * <p>
1297 * When called with plain, inexact {@link #invoke invoke}, if the caller
1298 * type is the same as the adapter, the adapter invokes the target as with
1299 * {@code invokeExact}.
1300 * (This is the normal behavior for {@code invoke} when types match.)
1301 * <p>
1302 * Otherwise, if the caller and adapter arity are the same, and the
1303 * trailing parameter type of the caller is a reference type identical to
1304 * or assignable to the trailing parameter type of the adapter,
1305 * the arguments and return values are converted pairwise,
1306 * as if by {@link #asType asType} on a fixed arity
1307 * method handle.
1308 * <p>
1309 * Otherwise, the arities differ, or the adapter's trailing parameter
1310 * type is not assignable from the corresponding caller type.
1311 * In this case, the adapter replaces all trailing arguments from
1312 * the original trailing argument position onward, by
1313 * a new array of type {@code arrayType}, whose elements
1314 * comprise (in order) the replaced arguments.
1315 * <p>
1316 * The caller type must provides as least enough arguments,
1317 * and of the correct type, to satisfy the target's requirement for
1318 * positional arguments before the trailing array argument.
1319 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1320 * where {@code N} is the arity of the target.
1321 * Also, there must exist conversions from the incoming arguments
1322 * to the target's arguments.
1323 * As with other uses of plain {@code invoke}, if these basic
1324 * requirements are not fulfilled, a {@code WrongMethodTypeException}
1325 * may be thrown.
1326 * <p>
1327 * In all cases, what the target eventually returns is returned unchanged by the adapter.
1328 * <p>
1329 * In the final case, it is exactly as if the target method handle were
1330 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1331 * to the arity required by the caller type.
1332 * (As with {@code asCollector}, if the array length is zero,
1333 * a shared constant may be used instead of a new array.
1334 * If the implied call to {@code asCollector} would throw
1335 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1336 * the call to the variable arity adapter must throw
1337 * {@code WrongMethodTypeException}.)
1338 * <p>
1339 * The behavior of {@link #asType asType} is also specialized for
1340 * variable arity adapters, to maintain the invariant that
1341 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1342 * call to adjust the target type, followed by {@code invokeExact}.
1343 * Therefore, a variable arity adapter responds
1344 * to an {@code asType} request by building a fixed arity collector,
1345 * if and only if the adapter and requested type differ either
1346 * in arity or trailing argument type.
1347 * The resulting fixed arity collector has its type further adjusted
1348 * (if necessary) to the requested type by pairwise conversion,
1349 * as if by another application of {@code asType}.
1350 * <p>
1351 * When a method handle is obtained by executing an {@code ldc} instruction
1352 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1353 * as a variable arity method (with the modifier bit {@code 0x0080}),
1354 * the method handle will accept multiple arities, as if the method handle
1355 * constant were created by means of a call to {@code asVarargsCollector}.
1356 * <p>
1357 * In order to create a collecting adapter which collects a predetermined
1358 * number of arguments, and whose type reflects this predetermined number,
1359 * use {@link #asCollector asCollector} instead.
1360 * <p>
1361 * No method handle transformations produce new method handles with
1362 * variable arity, unless they are documented as doing so.
1363 * Therefore, besides {@code asVarargsCollector} and {@code withVarargs},
1364 * all methods in {@code MethodHandle} and {@code MethodHandles}
1365 * will return a method handle with fixed arity,
1366 * except in the cases where they are specified to return their original
1367 * operand (e.g., {@code asType} of the method handle's own type).
1368 * <p>
1369 * Calling {@code asVarargsCollector} on a method handle which is already
1370 * of variable arity will produce a method handle with the same type and behavior.
1371 * It may (or may not) return the original variable arity method handle.
1372 * <p>
1373 * Here is an example, of a list-making variable arity method handle:
1374 * <blockquote><pre>{@code
1375 MethodHandle deepToString = publicLookup()
1376 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1377 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1378 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1379 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1380 assertEquals("[won]", (String) ts1.invoke( "won" ));
1381 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1382 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1383 MethodHandle asList = publicLookup()
1384 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1385 assertEquals(methodType(List.class, Object[].class), asList.type());
1386 assert(asList.isVarargsCollector());
1387 assertEquals("[]", asList.invoke().toString());
1388 assertEquals("[1]", asList.invoke(1).toString());
1389 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1390 String[] argv = { "three", "thee", "tee" };
1391 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1392 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1393 List ls = (List) asList.invoke((Object)argv);
1394 assertEquals(1, ls.size());
1395 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1396 * }</pre></blockquote>
1397 * <p style="font-size:smaller;">
1398 * <em>Discussion:</em>
1399 * These rules are designed as a dynamically-typed variation
1400 * of the Java rules for variable arity methods.
1401 * In both cases, callers to a variable arity method or method handle
1402 * can either pass zero or more positional arguments, or else pass
1403 * pre-collected arrays of any length. Users should be aware of the
1404 * special role of the final argument, and of the effect of a
1405 * type match on that final argument, which determines whether
1406 * or not a single trailing argument is interpreted as a whole
1407 * array or a single element of an array to be collected.
1408 * Note that the dynamic type of the trailing argument has no
1409 * effect on this decision, only a comparison between the symbolic
1410 * type descriptor of the call site and the type descriptor of the method handle.)
1411 *
1412 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1413 * @return a new method handle which can collect any number of trailing arguments
1414 * into an array, before calling the original method handle
1415 * @throws NullPointerException if {@code arrayType} is a null reference
1416 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1417 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1418 * @see #asCollector
1419 * @see #isVarargsCollector
1420 * @see #withVarargs
1421 * @see #asFixedArity
1422 */
1423 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1424 Objects.requireNonNull(arrayType);
1425 boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0);
1426 if (isVarargsCollector() && lastMatch)
1427 return this;
1428 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1429 }
1430
1431 /**
1432 * Determines if this method handle
1433 * supports {@linkplain #asVarargsCollector variable arity} calls.
1434 * Such method handles arise from the following sources:
1435 * <ul>
1436 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1437 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1438 * which resolves to a variable arity Java method or constructor
1439 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1440 * which resolves to a variable arity Java method or constructor
1441 * </ul>
1442 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1443 * @see #asVarargsCollector
1444 * @see #asFixedArity
1445 */
1446 public boolean isVarargsCollector() {
1447 return false;
1448 }
1449
1450 /**
1451 * Makes a <em>fixed arity</em> method handle which is otherwise
1452 * equivalent to the current method handle.
1453 * <p>
1454 * If the current method handle is not of
1455 * {@linkplain #asVarargsCollector variable arity},
1456 * the current method handle is returned.
1457 * This is true even if the current method handle
1458 * could not be a valid input to {@code asVarargsCollector}.
1459 * <p>
1460 * Otherwise, the resulting fixed-arity method handle has the same
1461 * type and behavior of the current method handle,
1462 * except that {@link #isVarargsCollector isVarargsCollector}
1463 * will be false.
1464 * The fixed-arity method handle may (or may not) be the
1465 * a previous argument to {@code asVarargsCollector}.
1466 * <p>
1467 * Here is an example, of a list-making variable arity method handle:
1468 * <blockquote><pre>{@code
1469 MethodHandle asListVar = publicLookup()
1470 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1471 .asVarargsCollector(Object[].class);
1472 MethodHandle asListFix = asListVar.asFixedArity();
1473 assertEquals("[1]", asListVar.invoke(1).toString());
1474 Exception caught = null;
1475 try { asListFix.invoke((Object)1); }
1476 catch (Exception ex) { caught = ex; }
1477 assert(caught instanceof ClassCastException);
1478 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1479 try { asListFix.invoke("two", "too"); }
1480 catch (Exception ex) { caught = ex; }
1481 assert(caught instanceof WrongMethodTypeException);
1482 Object[] argv = { "three", "thee", "tee" };
1483 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1484 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1485 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1486 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1487 * }</pre></blockquote>
1488 *
1489 * @return a new method handle which accepts only a fixed number of arguments
1490 * @see #asVarargsCollector
1491 * @see #isVarargsCollector
1492 * @see #withVarargs
1493 */
1494 public MethodHandle asFixedArity() {
1495 assert(!isVarargsCollector());
1496 return this;
1497 }
1498
1499 /**
1500 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1501 * The new method handle adapts, as its <i>target</i>,
1502 * the current method handle by binding it to the given argument.
1503 * The type of the bound handle will be
1504 * the same as the type of the target, except that a single leading
1505 * reference parameter will be omitted.
1506 * <p>
1507 * When called, the bound handle inserts the given value {@code x}
1508 * as a new leading argument to the target. The other arguments are
1509 * also passed unchanged.
1510 * What the target eventually returns is returned unchanged by the bound handle.
1511 * <p>
1512 * The reference {@code x} must be convertible to the first parameter
1513 * type of the target.
1514 * <p>
1515 * <em>Note:</em> Because method handles are immutable, the target method handle
1516 * retains its original type and behavior.
1517 * <p>
1518 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1519 * variable-arity method handle}, even if the original target method handle was.
1520 * @param x the value to bind to the first argument of the target
1521 * @return a new method handle which prepends the given value to the incoming
1522 * argument list, before calling the original method handle
1523 * @throws IllegalArgumentException if the target does not have a
1524 * leading parameter type that is a reference type
1525 * @throws ClassCastException if {@code x} cannot be converted
1526 * to the leading parameter type of the target
1527 * @see MethodHandles#insertArguments
1528 */
1529 public MethodHandle bindTo(Object x) {
1530 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed
1531 return bindArgumentL(0, x);
1532 }
1533
1534 /**
1535 * Return a nominal descriptor for this instance, if one can be
1536 * constructed, or an empty {@link Optional} if one cannot be.
1537 *
1538 * @return An {@link Optional} containing the resulting nominal descriptor,
1539 * or an empty {@link Optional} if one cannot be constructed.
1540 * @since 12
1541 */
1542 @Override
1543 public Optional<MethodHandleDesc> describeConstable() {
1544 MethodHandleInfo info;
1545 ClassDesc owner;
1546 String name;
1547 MethodTypeDesc type;
1548 boolean isInterface;
1549 try {
1550 info = IMPL_LOOKUP.revealDirect(this);
1551 isInterface = info.getDeclaringClass().isInterface();
1552 owner = info.getDeclaringClass().describeConstable().orElseThrow();
1553 type = info.getMethodType().describeConstable().orElseThrow();
1554 name = info.getName();
1555 }
1556 catch (Exception e) {
1557 return Optional.empty();
1558 }
1559
1560 switch (info.getReferenceKind()) {
1561 case REF_getField:
1562 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.GETTER, owner, name, type.returnType()));
1563 case REF_putField:
1564 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.SETTER, owner, name, type.parameterType(0)));
1565 case REF_getStatic:
1566 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_GETTER, owner, name, type.returnType()));
1567 case REF_putStatic:
1568 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_SETTER, owner, name, type.parameterType(0)));
1569 case REF_invokeVirtual:
1570 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.VIRTUAL, owner, name, type));
1571 case REF_invokeStatic:
1572 return isInterface ?
1573 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_STATIC, owner, name, type)) :
1574 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.STATIC, owner, name, type));
1575 case REF_invokeSpecial:
1576 return isInterface ?
1577 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_SPECIAL, owner, name, type)) :
1578 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.SPECIAL, owner, name, type));
1579 case REF_invokeInterface:
1580 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_VIRTUAL, owner, name, type));
1581 case REF_newInvokeSpecial:
1582 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.CONSTRUCTOR, owner, name, type));
1583 default:
1584 return Optional.empty();
1585 }
1586 }
1587
1588 /**
1589 * Returns a string representation of the method handle,
1590 * starting with the string {@code "MethodHandle"} and
1591 * ending with the string representation of the method handle's type.
1592 * In other words, this method returns a string equal to the value of:
1593 * <blockquote><pre>{@code
1594 * "MethodHandle" + type().toString()
1595 * }</pre></blockquote>
1596 * <p>
1597 * (<em>Note:</em> Future releases of this API may add further information
1598 * to the string representation.
1599 * Therefore, the present syntax should not be parsed by applications.)
1600 *
1601 * @return a string representation of the method handle
1602 */
1603 @Override
1604 public String toString() {
1605 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString();
1606 return standardString();
1607 }
1608 String standardString() {
1609 return "MethodHandle"+type;
1610 }
1611 /** Return a string with a several lines describing the method handle structure.
1612 * This string would be suitable for display in an IDE debugger.
1613 */
1614 String debugString() {
1615 return type+" : "+internalForm()+internalProperties();
1616 }
1617
1618 //// Implementation methods.
1619 //// Sub-classes can override these default implementations.
1620 //// All these methods assume arguments are already validated.
1621
1622 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
1623
1624 BoundMethodHandle bindArgumentL(int pos, Object value) {
1625 return rebind().bindArgumentL(pos, value);
1626 }
1627
1628 /*non-public*/
1629 MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1630 if (!member.isVarargs()) return this;
1631 try {
1632 return this.withVarargs(true);
1633 } catch (IllegalArgumentException ex) {
1634 throw member.makeAccessException("cannot make variable arity", null);
1635 }
1636 }
1637
1638 /*non-public*/
1639 MethodHandle viewAsType(MethodType newType, boolean strict) {
1640 // No actual conversions, just a new view of the same method.
1641 // Note that this operation must not produce a DirectMethodHandle,
1642 // because retyped DMHs, like any transformed MHs,
1643 // cannot be cracked into MethodHandleInfo.
1644 assert viewAsTypeChecks(newType, strict);
1645 BoundMethodHandle mh = rebind();
1646 return mh.copyWith(newType, mh.form);
1647 }
1648
1649 /*non-public*/
1650 boolean viewAsTypeChecks(MethodType newType, boolean strict) {
1651 if (strict) {
1652 assert(type().isViewableAs(newType, true))
1653 : Arrays.asList(this, newType);
1654 } else {
1655 assert(type().basicType().isViewableAs(newType.basicType(), true))
1656 : Arrays.asList(this, newType);
1657 }
1658 return true;
1659 }
1660
1661 // Decoding
1662
1663 /*non-public*/
1664 LambdaForm internalForm() {
1665 return form;
1666 }
1667
1668 /*non-public*/
1669 MemberName internalMemberName() {
1670 return null; // DMH returns DMH.member
1671 }
1672
1673 /*non-public*/
1674 Class<?> internalCallerClass() {
1675 return null; // caller-bound MH for @CallerSensitive method returns caller
1676 }
1677
1678 /*non-public*/
1679 MethodHandleImpl.Intrinsic intrinsicName() {
1680 // no special intrinsic meaning to most MHs
1681 return MethodHandleImpl.Intrinsic.NONE;
1682 }
1683
1684 /*non-public*/
1685 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
1686 if (member != null) {
1687 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
1688 } else if (internalMemberName() == null) {
1689 // The required internaMemberName is null, and this MH (like most) doesn't have one.
1690 return this;
1691 } else {
1692 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1693 MethodHandle result = rebind();
1694 assert (result.internalMemberName() == null);
1695 return result;
1696 }
1697 }
1698
1699 /*non-public*/
1700 boolean isInvokeSpecial() {
1701 return false; // DMH.Special returns true
1702 }
1703
1704 /*non-public*/
1705 Object internalValues() {
1706 return null;
1707 }
1708
1709 /*non-public*/
1710 Object internalProperties() {
1711 // Override to something to follow this.form, like "\n& FOO=bar"
1712 return "";
1713 }
1714
1715 //// Method handle implementation methods.
1716 //// Sub-classes can override these default implementations.
1717 //// All these methods assume arguments are already validated.
1718
1719 /*non-public*/
1720 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1721
1722 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
1723 * Many transforms are implemented only for BMHs.
1724 * @return a behaviorally equivalent BMH
1725 */
1726 abstract BoundMethodHandle rebind();
1727
1728 /**
1729 * Replace the old lambda form of this method handle with a new one.
1730 * The new one must be functionally equivalent to the old one.
1731 * Threads may continue running the old form indefinitely,
1732 * but it is likely that the new one will be preferred for new executions.
1733 * Use with discretion.
1734 */
1735 /*non-public*/
1736 void updateForm(LambdaForm newForm) {
1737 assert(newForm.customized == null || newForm.customized == this);
1738 if (form == newForm) return;
1739 newForm.prepare(); // as in MethodHandle.<init>
1740 UNSAFE.putReference(this, FORM_OFFSET, newForm);
1741 UNSAFE.fullFence();
1742 }
1743
1744 /** Craft a LambdaForm customized for this particular MethodHandle */
1745 /*non-public*/
1746 void customize() {
1747 final LambdaForm form = this.form;
1748 if (form.customized == null) {
1749 LambdaForm newForm = form.customize(this);
1750 updateForm(newForm);
1751 } else {
1752 assert(form.customized == this);
1753 }
1754 }
1755
1756 private static final long FORM_OFFSET
1757 = UNSAFE.objectFieldOffset(MethodHandle.class, "form");
1758 }