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