Batch of classes necessary to implement invoke dynamic interfaces. Taken from JDK8 build 132
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26 package java.lang.invoke;
30 import sun.invoke.util.*;
31 import sun.misc.Unsafe;
33 import static java.lang.invoke.MethodHandleStatics.*;
34 import java.util.logging.Level;
35 import java.util.logging.Logger;
38 * A method handle is a typed, directly executable reference to an underlying method,
39 * constructor, field, or similar low-level operation, with optional
40 * transformations of arguments or return values.
41 * These transformations are quite general, and include such patterns as
42 * {@linkplain #asType conversion},
43 * {@linkplain #bindTo insertion},
44 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
45 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
47 * <h1>Method handle contents</h1>
48 * Method handles are dynamically and strongly typed according to their parameter and return types.
49 * They are not distinguished by the name or the defining class of their underlying methods.
50 * A method handle must be invoked using a symbolic type descriptor which matches
51 * the method handle's own {@linkplain #type type descriptor}.
53 * Every method handle reports its type descriptor via the {@link #type type} accessor.
54 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
55 * whose structure is a series of classes, one of which is
56 * the return type of the method (or {@code void.class} if none).
58 * A method handle's type controls the types of invocations it accepts,
59 * and the kinds of transformations that apply to it.
61 * A method handle contains a pair of special invoker methods
62 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
63 * Both invoker methods provide direct access to the method handle's
64 * underlying method, constructor, field, or other operation,
65 * as modified by transformations of arguments and return values.
66 * Both invokers accept calls which exactly match the method handle's own type.
67 * The plain, inexact invoker also accepts a range of other call types.
69 * Method handles are immutable and have no visible state.
70 * Of course, they can be bound to underlying methods or data which exhibit state.
71 * With respect to the Java Memory Model, any method handle will behave
72 * as if all of its (internal) fields are final variables. This means that any method
73 * handle made visible to the application will always be fully formed.
74 * This is true even if the method handle is published through a shared
75 * variable in a data race.
77 * Method handles cannot be subclassed by the user.
78 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
79 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
80 * operation. The programmer should not draw conclusions about a method handle
81 * from its specific class, as the method handle class hierarchy (if any)
82 * may change from time to time or across implementations from different vendors.
84 * <h1>Method handle compilation</h1>
85 * A Java method call expression naming {@code invokeExact} or {@code invoke}
86 * can invoke a method handle from Java source code.
87 * From the viewpoint of source code, these methods can take any arguments
88 * and their result can be cast to any return type.
89 * Formally this is accomplished by giving the invoker methods
90 * {@code Object} return types and variable arity {@code Object} arguments,
91 * but they have an additional quality called <em>signature polymorphism</em>
92 * which connects this freedom of invocation directly to the JVM execution stack.
94 * As is usual with virtual methods, source-level calls to {@code invokeExact}
95 * and {@code invoke} compile to an {@code invokevirtual} instruction.
96 * More unusually, the compiler must record the actual argument types,
97 * and may not perform method invocation conversions on the arguments.
98 * Instead, it must push them on the stack according to their own unconverted types.
99 * The method handle object itself is pushed on the stack before the arguments.
100 * The compiler then calls the method handle with a symbolic type descriptor which
101 * describes the argument and return types.
103 * To issue a complete symbolic type descriptor, the compiler must also determine
104 * the return type. This is based on a cast on the method invocation expression,
105 * if there is one, or else {@code Object} if the invocation is an expression
106 * or else {@code void} if the invocation is a statement.
107 * The cast may be to a primitive type (but not {@code void}).
109 * As a corner case, an uncasted {@code null} argument is given
110 * a symbolic type descriptor of {@code java.lang.Void}.
111 * The ambiguity with the type {@code Void} is harmless, since there are no references of type
112 * {@code Void} except the null reference.
114 * <h1>Method handle invocation</h1>
115 * The first time a {@code invokevirtual} instruction is executed
116 * it is linked, by symbolically resolving the names in the instruction
117 * and verifying that the method call is statically legal.
118 * This is true of calls to {@code invokeExact} and {@code invoke}.
119 * In this case, the symbolic type descriptor emitted by the compiler is checked for
120 * correct syntax and names it contains are resolved.
121 * Thus, an {@code invokevirtual} instruction which invokes
122 * a method handle will always link, as long
123 * as the symbolic type descriptor is syntactically well-formed
124 * and the types exist.
126 * When the {@code invokevirtual} is executed after linking,
127 * the receiving method handle's type is first checked by the JVM
128 * to ensure that it matches the symbolic type descriptor.
129 * If the type match fails, it means that the method which the
130 * caller is invoking is not present on the individual
131 * method handle being invoked.
133 * In the case of {@code invokeExact}, the type descriptor of the invocation
134 * (after resolving symbolic type names) must exactly match the method type
135 * of the receiving method handle.
136 * In the case of plain, inexact {@code invoke}, the resolved type descriptor
137 * must be a valid argument to the receiver's {@link #asType asType} method.
138 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
140 * After type matching, a call to {@code invokeExact} directly
141 * and immediately invoke the method handle's underlying method
142 * (or other behavior, as the case may be).
144 * A call to plain {@code invoke} works the same as a call to
145 * {@code invokeExact}, if the symbolic type descriptor specified by the caller
146 * exactly matches the method handle's own type.
147 * If there is a type mismatch, {@code invoke} attempts
148 * to adjust the type of the receiving method handle,
149 * as if by a call to {@link #asType asType},
150 * to obtain an exactly invokable method handle {@code M2}.
151 * This allows a more powerful negotiation of method type
152 * between caller and callee.
154 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
155 * and implementations are therefore not required to materialize it.)
157 * <h1>Invocation checking</h1>
158 * In typical programs, method handle type matching will usually succeed.
159 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
160 * either directly (in the case of {@code invokeExact}) or indirectly as if
161 * by a failed call to {@code asType} (in the case of {@code invoke}).
163 * Thus, a method type mismatch which might show up as a linkage error
164 * in a statically typed program can show up as
165 * a dynamic {@code WrongMethodTypeException}
166 * in a program which uses method handles.
168 * Because method types contain "live" {@code Class} objects,
169 * method type matching takes into account both types names and class loaders.
170 * Thus, even if a method handle {@code M} is created in one
171 * class loader {@code L1} and used in another {@code L2},
172 * method handle calls are type-safe, because the caller's symbolic type
173 * descriptor, as resolved in {@code L2},
174 * is matched against the original callee method's symbolic type descriptor,
175 * as resolved in {@code L1}.
176 * The resolution in {@code L1} happens when {@code M} is created
177 * and its type is assigned, while the resolution in {@code L2} happens
178 * when the {@code invokevirtual} instruction is linked.
180 * Apart from the checking of type descriptors,
181 * a method handle's capability to call its underlying method is unrestricted.
182 * If a method handle is formed on a non-public method by a class
183 * that has access to that method, the resulting handle can be used
184 * in any place by any caller who receives a reference to it.
186 * Unlike with the Core Reflection API, where access is checked every time
187 * a reflective method is invoked,
188 * method handle access checking is performed
189 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
190 * In the case of {@code ldc} (see below), access checking is performed as part of linking
191 * the constant pool entry underlying the constant method handle.
193 * Thus, handles to non-public methods, or to methods in non-public classes,
194 * should generally be kept secret.
195 * They should not be passed to untrusted code unless their use from
196 * the untrusted code would be harmless.
198 * <h1>Method handle creation</h1>
199 * Java code can create a method handle that directly accesses
200 * any method, constructor, or field that is accessible to that code.
201 * This is done via a reflective, capability-based API called
202 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
203 * For example, a static method handle can be obtained
204 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
205 * There are also conversion methods from Core Reflection API objects,
206 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
208 * Like classes and strings, method handles that correspond to accessible
209 * fields, methods, and constructors can also be represented directly
210 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
211 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
212 * refers directly to an associated {@code CONSTANT_Methodref},
213 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
214 * constant pool entry.
215 * (For full details on method handle constants,
216 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
218 * Method handles produced by lookups or constant loads from methods or
219 * constructors with the variable arity modifier bit ({@code 0x0080})
220 * have a corresponding variable arity, as if they were defined with
221 * the help of {@link #asVarargsCollector asVarargsCollector}.
223 * A method reference may refer either to a static or non-static method.
224 * In the non-static case, the method handle type includes an explicit
225 * receiver argument, prepended before any other arguments.
226 * In the method handle's type, the initial receiver argument is typed
227 * according to the class under which the method was initially requested.
228 * (E.g., if a non-static method handle is obtained via {@code ldc},
229 * the type of the receiver is the class named in the constant pool entry.)
231 * Method handle constants are subject to the same link-time access checks
232 * their corresponding bytecode instructions, and the {@code ldc} instruction
233 * will throw corresponding linkage errors if the bytecode behaviors would
236 * As a corollary of this, access to protected members is restricted
237 * to receivers only of the accessing class, or one of its subclasses,
238 * and the accessing class must in turn be a subclass (or package sibling)
239 * of the protected member's defining class.
240 * If a method reference refers to a protected non-static method or field
241 * of a class outside the current package, the receiver argument will
242 * be narrowed to the type of the accessing class.
244 * When a method handle to a virtual method is invoked, the method is
245 * always looked up in the receiver (that is, the first argument).
247 * A non-virtual method handle to a specific virtual method implementation
248 * can also be created. These do not perform virtual lookup based on
249 * receiver type. Such a method handle simulates the effect of
250 * an {@code invokespecial} instruction to the same method.
252 * <h1>Usage examples</h1>
253 * Here are some examples of usage:
254 * <blockquote><pre>{@code
255 Object x, y; String s; int i;
256 MethodType mt; MethodHandle mh;
257 MethodHandles.Lookup lookup = MethodHandles.lookup();
258 // mt is (char,char)String
259 mt = MethodType.methodType(String.class, char.class, char.class);
260 mh = lookup.findVirtual(String.class, "replace", mt);
261 s = (String) mh.invokeExact("daddy",'d','n');
262 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
263 assertEquals(s, "nanny");
264 // weakly typed invocation (using MHs.invoke)
265 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
266 assertEquals(s, "savvy");
267 // mt is (Object[])List
268 mt = MethodType.methodType(java.util.List.class, Object[].class);
269 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
270 assert(mh.isVarargsCollector());
271 x = mh.invoke("one", "two");
272 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
273 assertEquals(x, java.util.Arrays.asList("one","two"));
274 // mt is (Object,Object,Object)Object
275 mt = MethodType.genericMethodType(3);
277 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
278 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
279 assertEquals(x, java.util.Arrays.asList(1,2,3));
281 mt = MethodType.methodType(int.class);
282 mh = lookup.findVirtual(java.util.List.class, "size", mt);
283 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
284 // invokeExact(Ljava/util/List;)I
286 mt = MethodType.methodType(void.class, String.class);
287 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
288 mh.invokeExact(System.out, "Hello, world.");
289 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
290 * }</pre></blockquote>
291 * Each of the above calls to {@code invokeExact} or plain {@code invoke}
292 * generates a single invokevirtual instruction with
293 * the symbolic type descriptor indicated in the following comment.
294 * In these examples, the helper method {@code assertEquals} is assumed to
295 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
296 * on its arguments, and asserts that the result is true.
298 * <h1>Exceptions</h1>
299 * The methods {@code invokeExact} and {@code invoke} are declared
300 * to throw {@link java.lang.Throwable Throwable},
301 * which is to say that there is no static restriction on what a method handle
302 * can throw. Since the JVM does not distinguish between checked
303 * and unchecked exceptions (other than by their class, of course),
304 * there is no particular effect on bytecode shape from ascribing
305 * checked exceptions to method handle invocations. But in Java source
306 * code, methods which perform method handle calls must either explicitly
307 * throw {@code Throwable}, or else must catch all
308 * throwables locally, rethrowing only those which are legal in the context,
309 * and wrapping ones which are illegal.
311 * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
312 * The unusual compilation and linkage behavior of
313 * {@code invokeExact} and plain {@code invoke}
314 * is referenced by the term <em>signature polymorphism</em>.
315 * As defined in the Java Language Specification,
316 * a signature polymorphic method is one which can operate with
317 * any of a wide range of call signatures and return types.
319 * In source code, a call to a signature polymorphic method will
320 * compile, regardless of the requested symbolic type descriptor.
321 * As usual, the Java compiler emits an {@code invokevirtual}
322 * instruction with the given symbolic type descriptor against the named method.
323 * The unusual part is that the symbolic type descriptor is derived from
324 * the actual argument and return types, not from the method declaration.
326 * When the JVM processes bytecode containing signature polymorphic calls,
327 * it will successfully link any such call, regardless of its symbolic type descriptor.
328 * (In order to retain type safety, the JVM will guard such calls with suitable
329 * dynamic type checks, as described elsewhere.)
331 * Bytecode generators, including the compiler back end, are required to emit
332 * untransformed symbolic type descriptors for these methods.
333 * Tools which determine symbolic linkage are required to accept such
334 * untransformed descriptors, without reporting linkage errors.
336 * <h1>Interoperation between method handles and the Core Reflection API</h1>
337 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
338 * any class member represented by a Core Reflection API object
339 * can be converted to a behaviorally equivalent method handle.
340 * For example, a reflective {@link java.lang.reflect.Method Method} can
341 * be converted to a method handle using
342 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
343 * The resulting method handles generally provide more direct and efficient
344 * access to the underlying class members.
347 * when the Core Reflection API is used to view the signature polymorphic
348 * methods {@code invokeExact} or plain {@code invoke} in this class,
349 * they appear as ordinary non-polymorphic methods.
350 * Their reflective appearance, as viewed by
351 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
352 * is unaffected by their special status in this API.
353 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
354 * will report exactly those modifier bits required for any similarly
355 * declared method, including in this case {@code native} and {@code varargs} bits.
357 * As with any reflected method, these methods (when reflected) may be
358 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
359 * However, such reflective calls do not result in method handle invocations.
360 * Such a call, if passed the required argument
361 * (a single one, of type {@code Object[]}), will ignore the argument and
362 * will throw an {@code UnsupportedOperationException}.
364 * Since {@code invokevirtual} instructions can natively
365 * invoke method handles under any symbolic type descriptor, this reflective view conflicts
366 * with the normal presentation of these methods via bytecodes.
367 * Thus, these two native methods, when reflectively viewed by
368 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
370 * In order to obtain an invoker method for a particular type descriptor,
371 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
372 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
373 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
374 * API is also able to return a method handle
375 * to call {@code invokeExact} or plain {@code invoke},
376 * for any specified type descriptor .
378 * <h1>Interoperation between method handles and Java generics</h1>
379 * A method handle can be obtained on a method, constructor, or field
380 * which is declared with Java generic types.
381 * As with the Core Reflection API, the type of the method handle
382 * will constructed from the erasure of the source-level type.
383 * When a method handle is invoked, the types of its arguments
384 * or the return value cast type may be generic types or type instances.
385 * If this occurs, the compiler will replace those
386 * types by their erasures when it constructs the symbolic type descriptor
387 * for the {@code invokevirtual} instruction.
389 * Method handles do not represent
390 * their function-like types in terms of Java parameterized (generic) types,
391 * because there are three mismatches between function-like types and parameterized
394 * <li>Method types range over all possible arities,
395 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
396 * Generics are not variadic, and so cannot represent this.</li>
397 * <li>Method types can specify arguments of primitive types,
398 * which Java generic types cannot range over.</li>
399 * <li>Higher order functions over method handles (combinators) are
400 * often generic across a wide range of function types, including
401 * those of multiple arities. It is impossible to represent such
402 * genericity with a Java type parameter.</li>
405 * <h1><a name="maxarity"></a>Arity limits</h1>
406 * The JVM imposes on all methods and constructors of any kind an absolute
407 * limit of 255 stacked arguments. This limit can appear more restrictive
410 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
411 * <li>A non-static method consumes an extra argument for the object on which the method is called.
412 * <li>A constructor consumes an extra argument for the object which is being constructed.
413 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
414 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
416 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
417 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
418 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
419 * In particular, a method handle’s type must not have an arity of the exact maximum 255.
423 * @author John Rose, JSR 292 EG
425 public abstract class MethodHandle {
426 static { MethodHandleImpl.initStatics(); }
429 * Internal marker interface which distinguishes (to the Java compiler)
430 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
432 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
433 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
434 @interface PolymorphicSignature { }
436 private final MethodType type;
437 /*private*/ final LambdaForm form;
438 // form is not private so that invokers can easily fetch it
439 /*private*/ MethodHandle asTypeCache;
440 // asTypeCache is not private so that invokers can easily fetch it
443 * Reports the type of this method handle.
444 * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
445 * @return the method handle type
447 public MethodType type() {
452 * Package-private constructor for the method handle implementation hierarchy.
453 * Method handle inheritance will be contained completely within
454 * the {@code java.lang.invoke} package.
456 // @param type type (permanently assigned) of the new method handle
457 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
458 type.getClass(); // explicit NPE
459 form.getClass(); // explicit NPE
463 form.prepare(); // TO DO: Try to delay this step until just before invocation.
467 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
468 * The symbolic type descriptor at the call site of {@code invokeExact} must
469 * exactly match this method handle's {@link #type type}.
470 * No conversions are allowed on arguments or return values.
472 * When this method is observed via the Core Reflection API,
473 * it will appear as a single native method, taking an object array and returning an object.
474 * If this native method is invoked directly via
475 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
476 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
477 * it will throw an {@code UnsupportedOperationException}.
478 * @param args the signature-polymorphic parameter list, statically represented using varargs
479 * @return the signature-polymorphic result, statically represented using {@code Object}
480 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
481 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
483 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
486 * Invokes the method handle, allowing any caller type descriptor,
487 * and optionally performing conversions on arguments and return values.
489 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
490 * the call proceeds as if by {@link #invokeExact invokeExact}.
492 * Otherwise, the call proceeds as if this method handle were first
493 * adjusted by calling {@link #asType asType} to adjust this method handle
494 * to the required type, and then the call proceeds as if by
495 * {@link #invokeExact invokeExact} on the adjusted method handle.
497 * There is no guarantee that the {@code asType} call is actually made.
498 * If the JVM can predict the results of making the call, it may perform
499 * adaptations directly on the caller's arguments,
500 * and call the target method handle according to its own exact type.
502 * The resolved type descriptor at the call site of {@code invoke} must
503 * be a valid argument to the receivers {@code asType} method.
504 * In particular, the caller must specify the same argument arity
505 * as the callee's type,
506 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
508 * When this method is observed via the Core Reflection API,
509 * it will appear as a single native method, taking an object array and returning an object.
510 * If this native method is invoked directly via
511 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
512 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
513 * it will throw an {@code UnsupportedOperationException}.
514 * @param args the signature-polymorphic parameter list, statically represented using varargs
515 * @return the signature-polymorphic result, statically represented using {@code Object}
516 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
517 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
518 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
520 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
523 * Private method for trusted invocation of a method handle respecting simplified signatures.
524 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
526 * The caller signature is restricted to the following basic types:
527 * Object, int, long, float, double, and void return.
529 * The caller is responsible for maintaining type correctness by ensuring
530 * that the each outgoing argument value is a member of the range of the corresponding
531 * callee argument type.
532 * (The caller should therefore issue appropriate casts and integer narrowing
533 * operations on outgoing argument values.)
534 * The caller can assume that the incoming result value is part of the range
535 * of the callee's return type.
536 * @param args the signature-polymorphic parameter list, statically represented using varargs
537 * @return the signature-polymorphic result, statically represented using {@code Object}
539 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
542 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
543 * The caller signature is restricted to basic types as with {@code invokeBasic}.
544 * The trailing (not leading) argument must be a MemberName.
545 * @param args the signature-polymorphic parameter list, statically represented using varargs
546 * @return the signature-polymorphic result, statically represented using {@code Object}
548 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
551 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
552 * The caller signature is restricted to basic types as with {@code invokeBasic}.
553 * The trailing (not leading) argument must be a MemberName.
554 * @param args the signature-polymorphic parameter list, statically represented using varargs
555 * @return the signature-polymorphic result, statically represented using {@code Object}
557 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
560 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
561 * The caller signature is restricted to basic types as with {@code invokeBasic}.
562 * The trailing (not leading) argument must be a MemberName.
563 * @param args the signature-polymorphic parameter list, statically represented using varargs
564 * @return the signature-polymorphic result, statically represented using {@code Object}
566 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
569 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
570 * The caller signature is restricted to basic types as with {@code invokeBasic}.
571 * The trailing (not leading) argument must be a MemberName.
572 * @param args the signature-polymorphic parameter list, statically represented using varargs
573 * @return the signature-polymorphic result, statically represented using {@code Object}
575 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
578 * Performs a variable arity invocation, passing the arguments in the given list
579 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
580 * which mentions only the type {@code Object}, and whose arity is the length
581 * of the argument list.
583 * Specifically, execution proceeds as if by the following steps,
584 * although the methods are not guaranteed to be called if the JVM
585 * can predict their effects.
587 * <li>Determine the length of the argument array as {@code N}.
588 * For a null reference, {@code N=0}. </li>
589 * <li>Determine the general type {@code TN} of {@code N} arguments as
590 * as {@code TN=MethodType.genericMethodType(N)}.</li>
591 * <li>Force the original target method handle {@code MH0} to the
592 * required type, as {@code MH1 = MH0.asType(TN)}. </li>
593 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
594 * <li>Invoke the type-adjusted method handle on the unpacked arguments:
595 * MH1.invokeExact(A0, ...). </li>
596 * <li>Take the return value as an {@code Object} reference. </li>
599 * Because of the action of the {@code asType} step, the following argument
600 * conversions are applied as necessary:
602 * <li>reference casting
604 * <li>widening primitive conversions
607 * The result returned by the call is boxed if it is a primitive,
608 * or forced to null if the return type is void.
610 * This call is equivalent to the following code:
611 * <blockquote><pre>{@code
612 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
613 * Object result = invoker.invokeExact(this, arguments);
614 * }</pre></blockquote>
616 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
617 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
618 * It can therefore be used as a bridge between native or reflective code and method handles.
620 * @param arguments the arguments to pass to the target
621 * @return the result returned by the target
622 * @throws ClassCastException if an argument cannot be converted by reference casting
623 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
624 * @throws Throwable anything thrown by the target method invocation
625 * @see MethodHandles#spreadInvoker
627 public Object invokeWithArguments(Object... arguments) throws Throwable {
628 int argc = arguments == null ? 0 : arguments.length;
629 @SuppressWarnings("LocalVariableHidesMemberVariable")
630 MethodType type = type();
631 if (type.parameterCount() != argc || isVarargsCollector()) {
633 return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments);
635 MethodHandle invoker = type.invokers().varargsInvoker();
636 return invoker.invokeExact(this, arguments);
640 * Performs a variable arity invocation, passing the arguments in the given array
641 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
642 * which mentions only the type {@code Object}, and whose arity is the length
643 * of the argument array.
645 * This method is also equivalent to the following code:
646 * <blockquote><pre>{@code
647 * invokeWithArguments(arguments.toArray()
648 * }</pre></blockquote>
650 * @param arguments the arguments to pass to the target
651 * @return the result returned by the target
652 * @throws NullPointerException if {@code arguments} is a null reference
653 * @throws ClassCastException if an argument cannot be converted by reference casting
654 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
655 * @throws Throwable anything thrown by the target method invocation
657 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
658 return invokeWithArguments(arguments.toArray());
662 * Produces an adapter method handle which adapts the type of the
663 * current method handle to a new type.
664 * The resulting method handle is guaranteed to report a type
665 * which is equal to the desired new type.
667 * If the original type and new type are equal, returns {@code this}.
669 * The new method handle, when invoked, will perform the following
672 * <li>Convert the incoming argument list to match the original
673 * method handle's argument list.
674 * <li>Invoke the original method handle on the converted argument list.
675 * <li>Convert any result returned by the original method handle
676 * to the return type of new method handle.
679 * This method provides the crucial behavioral difference between
680 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
682 * perform the same steps when the caller's type descriptor exactly m atches
683 * the callee's, but when the types differ, plain {@link #invoke invoke}
684 * also calls {@code asType} (or some internal equivalent) in order
685 * to match up the caller's and callee's types.
687 * If the current method is a variable arity method handle
688 * argument list conversion may involve the conversion and collection
689 * of several arguments into an array, as
690 * {@linkplain #asVarargsCollector described elsewhere}.
691 * In every other case, all conversions are applied <em>pairwise</em>,
692 * which means that each argument or return value is converted to
693 * exactly one argument or return value (or no return value).
694 * The applied conversions are defined by consulting the
695 * the corresponding component types of the old and new
696 * method handle types.
698 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
699 * or old and new return types. Specifically, for some valid index {@code i}, let
700 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
701 * Or else, going the other way for return values, let
702 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
703 * If the types are the same, the new method handle makes no change
704 * to the corresponding argument or return value (if any).
705 * Otherwise, one of the following conversions is applied
708 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
709 * (The types do not need to be related in any particular way.
710 * This is because a dynamic value of null can convert to any reference type.)
711 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
712 * conversion (JLS 5.3) is applied, if one exists.
713 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
714 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
715 * a Java casting conversion (JLS 5.5) is applied if one exists.
716 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
717 * which is then widened as needed to <em>T1</em>.)
718 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
719 * conversion will be applied at runtime, possibly followed
720 * by a Java method invocation conversion (JLS 5.3)
721 * on the primitive value. (These are the primitive widening conversions.)
722 * <em>T0</em> must be a wrapper class or a supertype of one.
723 * (In the case where <em>T0</em> is Object, these are the conversions
724 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
725 * The unboxing conversion must have a possibility of success, which means that
726 * if <em>T0</em> is not itself a wrapper class, there must exist at least one
727 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
728 * primitive value can be widened to <em>T1</em>.
729 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
730 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
731 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
732 * a zero value is introduced.
734 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
735 * because neither corresponds specifically to the <em>dynamic type</em> of any
736 * actual argument or return value.)
738 * The method handle conversion cannot be made if any one of the required
739 * pairwise conversions cannot be made.
741 * At runtime, the conversions applied to reference arguments
742 * or return values may require additional runtime checks which can fail.
743 * An unboxing operation may fail because the original reference is null,
744 * causing a {@link java.lang.NullPointerException NullPointerException}.
745 * An unboxing operation or a reference cast may also fail on a reference
746 * to an object of the wrong type,
747 * causing a {@link java.lang.ClassCastException ClassCastException}.
748 * Although an unboxing operation may accept several kinds of wrappers,
749 * if none are available, a {@code ClassCastException} will be thrown.
751 * @param newType the expected type of the new method handle
752 * @return a method handle which delegates to {@code this} after performing
753 * any necessary argument conversions, and arranges for any
754 * necessary return value conversions
755 * @throws NullPointerException if {@code newType} is a null reference
756 * @throws WrongMethodTypeException if the conversion cannot be made
757 * @see MethodHandles#explicitCastArguments
759 public MethodHandle asType(MethodType newType) {
760 // Fast path alternative to a heavyweight {@code asType} call.
761 // Return 'this' if the conversion will be a no-op.
762 if (newType == type) {
765 // Return 'this.asTypeCache' if the conversion is already memoized.
766 MethodHandle atc = asTypeCache;
767 if (atc != null && newType == atc.type) {
770 return asTypeUncached(newType);
773 /** Override this to change asType behavior. */
774 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
775 if (!type.isConvertibleTo(newType))
776 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
777 return asTypeCache = convertArguments(newType);
781 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
782 * and spreads its elements as positional arguments.
783 * The new method handle adapts, as its <i>target</i>,
784 * the current method handle. The type of the adapter will be
785 * the same as the type of the target, except that the final
786 * {@code arrayLength} parameters of the target's type are replaced
787 * by a single array parameter of type {@code arrayType}.
789 * If the array element type differs from any of the corresponding
790 * argument types on the original target,
791 * the original target is adapted to take the array elements directly,
792 * as if by a call to {@link #asType asType}.
794 * When called, the adapter replaces a trailing array argument
795 * by the array's elements, each as its own argument to the target.
796 * (The order of the arguments is preserved.)
797 * They are converted pairwise by casting and/or unboxing
798 * to the types of the trailing parameters of the target.
799 * Finally the target is called.
800 * What the target eventually returns is returned unchanged by the adapter.
802 * Before calling the target, the adapter verifies that the array
803 * contains exactly enough elements to provide a correct argument count
804 * to the target method handle.
805 * (The array may also be null when zero elements are required.)
807 * If, when the adapter is called, the supplied array argument does
808 * not have the correct number of elements, the adapter will throw
809 * an {@link IllegalArgumentException} instead of invoking the target.
811 * Here are some simple examples of array-spreading method handles:
812 * <blockquote><pre>{@code
813 MethodHandle equals = publicLookup()
814 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
815 assert( (boolean) equals.invokeExact("me", (Object)"me"));
816 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
817 // spread both arguments from a 2-array:
818 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
819 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
820 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
821 // try to spread from anything but a 2-array:
822 for (int n = 0; n <= 10; n++) {
823 Object[] badArityArgs = (n == 2 ? null : new Object[n]);
824 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
825 catch (IllegalArgumentException ex) { } // OK
827 // spread both arguments from a String array:
828 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
829 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
830 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
831 // spread second arguments from a 1-array:
832 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
833 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
834 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
835 // spread no arguments from a 0-array or null:
836 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
837 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
838 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
839 // asSpreader and asCollector are approximate inverses:
840 for (int n = 0; n <= 2; n++) {
841 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
842 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
843 assert( (boolean) equals2.invokeWithArguments("me", "me"));
844 assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
847 MethodHandle caToString = publicLookup()
848 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
849 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
850 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
851 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
852 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
853 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
854 * }</pre></blockquote>
855 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
856 * @param arrayLength the number of arguments to spread from an incoming array argument
857 * @return a new method handle which spreads its final array argument,
858 * before calling the original method handle
859 * @throws NullPointerException if {@code arrayType} is a null reference
860 * @throws IllegalArgumentException if {@code arrayType} is not an array type,
861 * or if target does not have at least
862 * {@code arrayLength} parameter types,
863 * or if {@code arrayLength} is negative,
864 * or if the resulting method handle's type would have
865 * <a href="MethodHandle.html#maxarity">too many parameters</a>
866 * @throws WrongMethodTypeException if the implied {@code asType} call fails
869 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
870 asSpreaderChecks(arrayType, arrayLength);
871 int spreadArgPos = type.parameterCount() - arrayLength;
872 return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength);
875 private void asSpreaderChecks(Class<?> arrayType, int arrayLength) {
876 spreadArrayChecks(arrayType, arrayLength);
877 int nargs = type().parameterCount();
878 if (nargs < arrayLength || arrayLength < 0)
879 throw newIllegalArgumentException("bad spread array length");
880 if (arrayType != Object[].class && arrayLength != 0) {
881 boolean sawProblem = false;
882 Class<?> arrayElement = arrayType.getComponentType();
883 for (int i = nargs - arrayLength; i < nargs; i++) {
884 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) {
890 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList());
891 for (int i = nargs - arrayLength; i < nargs; i++) {
892 ptypes.set(i, arrayElement);
895 this.asType(MethodType.methodType(type().returnType(), ptypes));
900 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
901 Class<?> arrayElement = arrayType.getComponentType();
902 if (arrayElement == null)
903 throw newIllegalArgumentException("not an array type", arrayType);
904 if ((arrayLength & 0x7F) != arrayLength) {
905 if ((arrayLength & 0xFF) != arrayLength)
906 throw newIllegalArgumentException("array length is not legal", arrayLength);
907 assert(arrayLength >= 128);
908 if (arrayElement == long.class ||
909 arrayElement == double.class)
910 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
915 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
916 * positional arguments and collects them into an array argument.
917 * The new method handle adapts, as its <i>target</i>,
918 * the current method handle. The type of the adapter will be
919 * the same as the type of the target, except that a single trailing
920 * parameter (usually of type {@code arrayType}) is replaced by
921 * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
923 * If the array type differs from the final argument type on the original target,
924 * the original target is adapted to take the array type directly,
925 * as if by a call to {@link #asType asType}.
927 * When called, the adapter replaces its trailing {@code arrayLength}
928 * arguments by a single new array of type {@code arrayType}, whose elements
929 * comprise (in order) the replaced arguments.
930 * Finally the target is called.
931 * What the target eventually returns is returned unchanged by the adapter.
933 * (The array may also be a shared constant when {@code arrayLength} is zero.)
935 * (<em>Note:</em> The {@code arrayType} is often identical to the last
936 * parameter type of the original target.
937 * It is an explicit argument for symmetry with {@code asSpreader}, and also
938 * to allow the target to use a simple {@code Object} as its last parameter type.)
940 * In order to create a collecting adapter which is not restricted to a particular
941 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
943 * Here are some examples of array-collecting method handles:
944 * <blockquote><pre>{@code
945 MethodHandle deepToString = publicLookup()
946 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
947 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
948 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
949 assertEquals(methodType(String.class, Object.class), ts1.type());
950 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
951 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
952 // arrayType can be a subtype of Object[]
953 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
954 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
955 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
956 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
957 assertEquals("[]", (String) ts0.invokeExact());
958 // collectors can be nested, Lisp-style
959 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
960 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
961 // arrayType can be any primitive array type
962 MethodHandle bytesToString = publicLookup()
963 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
964 .asCollector(byte[].class, 3);
965 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
966 MethodHandle longsToString = publicLookup()
967 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
968 .asCollector(long[].class, 1);
969 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
970 * }</pre></blockquote>
971 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
972 * @param arrayLength the number of arguments to collect into a new array argument
973 * @return a new method handle which collects some trailing argument
974 * into an array, before calling the original method handle
975 * @throws NullPointerException if {@code arrayType} is a null reference
976 * @throws IllegalArgumentException if {@code arrayType} is not an array type
977 * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
978 * or {@code arrayLength} is not a legal array size,
979 * or the resulting method handle's type would have
980 * <a href="MethodHandle.html#maxarity">too many parameters</a>
981 * @throws WrongMethodTypeException if the implied {@code asType} call fails
983 * @see #asVarargsCollector
985 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
986 asCollectorChecks(arrayType, arrayLength);
987 int collectArgPos = type().parameterCount()-1;
988 MethodHandle target = this;
989 if (arrayType != type().parameterType(collectArgPos))
990 target = convertArguments(type().changeParameterType(collectArgPos, arrayType));
991 MethodHandle collector = ValueConversions.varargsArray(arrayType, arrayLength);
992 return MethodHandles.collectArguments(target, collectArgPos, collector);
995 // private API: return true if last param exactly matches arrayType
996 private boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
997 spreadArrayChecks(arrayType, arrayLength);
998 int nargs = type().parameterCount();
1000 Class<?> lastParam = type().parameterType(nargs-1);
1001 if (lastParam == arrayType) return true;
1002 if (lastParam.isAssignableFrom(arrayType)) return false;
1004 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1008 * Makes a <em>variable arity</em> adapter which is able to accept
1009 * any number of trailing positional arguments and collect them
1010 * into an array argument.
1012 * The type and behavior of the adapter will be the same as
1013 * the type and behavior of the target, except that certain
1014 * {@code invoke} and {@code asType} requests can lead to
1015 * trailing positional arguments being collected into target's
1016 * trailing parameter.
1017 * Also, the last parameter type of the adapter will be
1018 * {@code arrayType}, even if the target has a different
1019 * last parameter type.
1021 * This transformation may return {@code this} if the method handle is
1022 * already of variable arity and its trailing parameter type
1023 * is identical to {@code arrayType}.
1025 * When called with {@link #invokeExact invokeExact}, the adapter invokes
1026 * the target with no argument changes.
1027 * (<em>Note:</em> This behavior is different from a
1028 * {@linkplain #asCollector fixed arity collector},
1029 * since it accepts a whole array of indeterminate length,
1030 * rather than a fixed number of arguments.)
1032 * When called with plain, inexact {@link #invoke invoke}, if the caller
1033 * type is the same as the adapter, the adapter invokes the target as with
1034 * {@code invokeExact}.
1035 * (This is the normal behavior for {@code invoke} when types match.)
1037 * Otherwise, if the caller and adapter arity are the same, and the
1038 * trailing parameter type of the caller is a reference type identical to
1039 * or assignable to the trailing parameter type of the adapter,
1040 * the arguments and return values are converted pairwise,
1041 * as if by {@link #asType asType} on a fixed arity
1044 * Otherwise, the arities differ, or the adapter's trailing parameter
1045 * type is not assignable from the corresponding caller type.
1046 * In this case, the adapter replaces all trailing arguments from
1047 * the original trailing argument position onward, by
1048 * a new array of type {@code arrayType}, whose elements
1049 * comprise (in order) the replaced arguments.
1051 * The caller type must provides as least enough arguments,
1052 * and of the correct type, to satisfy the target's requirement for
1053 * positional arguments before the trailing array argument.
1054 * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1055 * where {@code N} is the arity of the target.
1056 * Also, there must exist conversions from the incoming arguments
1057 * to the target's arguments.
1058 * As with other uses of plain {@code invoke}, if these basic
1059 * requirements are not fulfilled, a {@code WrongMethodTypeException}
1062 * In all cases, what the target eventually returns is returned unchanged by the adapter.
1064 * In the final case, it is exactly as if the target method handle were
1065 * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1066 * to the arity required by the caller type.
1067 * (As with {@code asCollector}, if the array length is zero,
1068 * a shared constant may be used instead of a new array.
1069 * If the implied call to {@code asCollector} would throw
1070 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1071 * the call to the variable arity adapter must throw
1072 * {@code WrongMethodTypeException}.)
1074 * The behavior of {@link #asType asType} is also specialized for
1075 * variable arity adapters, to maintain the invariant that
1076 * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1077 * call to adjust the target type, followed by {@code invokeExact}.
1078 * Therefore, a variable arity adapter responds
1079 * to an {@code asType} request by building a fixed arity collector,
1080 * if and only if the adapter and requested type differ either
1081 * in arity or trailing argument type.
1082 * The resulting fixed arity collector has its type further adjusted
1083 * (if necessary) to the requested type by pairwise conversion,
1084 * as if by another application of {@code asType}.
1086 * When a method handle is obtained by executing an {@code ldc} instruction
1087 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1088 * as a variable arity method (with the modifier bit {@code 0x0080}),
1089 * the method handle will accept multiple arities, as if the method handle
1090 * constant were created by means of a call to {@code asVarargsCollector}.
1092 * In order to create a collecting adapter which collects a predetermined
1093 * number of arguments, and whose type reflects this predetermined number,
1094 * use {@link #asCollector asCollector} instead.
1096 * No method handle transformations produce new method handles with
1097 * variable arity, unless they are documented as doing so.
1098 * Therefore, besides {@code asVarargsCollector},
1099 * all methods in {@code MethodHandle} and {@code MethodHandles}
1100 * will return a method handle with fixed arity,
1101 * except in the cases where they are specified to return their original
1102 * operand (e.g., {@code asType} of the method handle's own type).
1104 * Calling {@code asVarargsCollector} on a method handle which is already
1105 * of variable arity will produce a method handle with the same type and behavior.
1106 * It may (or may not) return the original variable arity method handle.
1108 * Here is an example, of a list-making variable arity method handle:
1109 * <blockquote><pre>{@code
1110 MethodHandle deepToString = publicLookup()
1111 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1112 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1113 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1114 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1115 assertEquals("[won]", (String) ts1.invoke( "won" ));
1116 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1117 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1118 MethodHandle asList = publicLookup()
1119 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1120 assertEquals(methodType(List.class, Object[].class), asList.type());
1121 assert(asList.isVarargsCollector());
1122 assertEquals("[]", asList.invoke().toString());
1123 assertEquals("[1]", asList.invoke(1).toString());
1124 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1125 String[] argv = { "three", "thee", "tee" };
1126 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1127 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1128 List ls = (List) asList.invoke((Object)argv);
1129 assertEquals(1, ls.size());
1130 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1131 * }</pre></blockquote>
1132 * <p style="font-size:smaller;">
1133 * <em>Discussion:</em>
1134 * These rules are designed as a dynamically-typed variation
1135 * of the Java rules for variable arity methods.
1136 * In both cases, callers to a variable arity method or method handle
1137 * can either pass zero or more positional arguments, or else pass
1138 * pre-collected arrays of any length. Users should be aware of the
1139 * special role of the final argument, and of the effect of a
1140 * type match on that final argument, which determines whether
1141 * or not a single trailing argument is interpreted as a whole
1142 * array or a single element of an array to be collected.
1143 * Note that the dynamic type of the trailing argument has no
1144 * effect on this decision, only a comparison between the symbolic
1145 * type descriptor of the call site and the type descriptor of the method handle.)
1147 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1148 * @return a new method handle which can collect any number of trailing arguments
1149 * into an array, before calling the original method handle
1150 * @throws NullPointerException if {@code arrayType} is a null reference
1151 * @throws IllegalArgumentException if {@code arrayType} is not an array type
1152 * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1154 * @see #isVarargsCollector
1155 * @see #asFixedArity
1157 public MethodHandle asVarargsCollector(Class<?> arrayType) {
1158 Class<?> arrayElement = arrayType.getComponentType();
1159 boolean lastMatch = asCollectorChecks(arrayType, 0);
1160 if (isVarargsCollector() && lastMatch)
1162 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1166 * Determines if this method handle
1167 * supports {@linkplain #asVarargsCollector variable arity} calls.
1168 * Such method handles arise from the following sources:
1170 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1171 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1172 * which resolves to a variable arity Java method or constructor
1173 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1174 * which resolves to a variable arity Java method or constructor
1176 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1177 * @see #asVarargsCollector
1178 * @see #asFixedArity
1180 public boolean isVarargsCollector() {
1185 * Makes a <em>fixed arity</em> method handle which is otherwise
1186 * equivalent to the current method handle.
1188 * If the current method handle is not of
1189 * {@linkplain #asVarargsCollector variable arity},
1190 * the current method handle is returned.
1191 * This is true even if the current method handle
1192 * could not be a valid input to {@code asVarargsCollector}.
1194 * Otherwise, the resulting fixed-arity method handle has the same
1195 * type and behavior of the current method handle,
1196 * except that {@link #isVarargsCollector isVarargsCollector}
1198 * The fixed-arity method handle may (or may not) be the
1199 * a previous argument to {@code asVarargsCollector}.
1201 * Here is an example, of a list-making variable arity method handle:
1202 * <blockquote><pre>{@code
1203 MethodHandle asListVar = publicLookup()
1204 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1205 .asVarargsCollector(Object[].class);
1206 MethodHandle asListFix = asListVar.asFixedArity();
1207 assertEquals("[1]", asListVar.invoke(1).toString());
1208 Exception caught = null;
1209 try { asListFix.invoke((Object)1); }
1210 catch (Exception ex) { caught = ex; }
1211 assert(caught instanceof ClassCastException);
1212 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1213 try { asListFix.invoke("two", "too"); }
1214 catch (Exception ex) { caught = ex; }
1215 assert(caught instanceof WrongMethodTypeException);
1216 Object[] argv = { "three", "thee", "tee" };
1217 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1218 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1219 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1220 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1221 * }</pre></blockquote>
1223 * @return a new method handle which accepts only a fixed number of arguments
1224 * @see #asVarargsCollector
1225 * @see #isVarargsCollector
1227 public MethodHandle asFixedArity() {
1228 assert(!isVarargsCollector());
1233 * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1234 * The new method handle adapts, as its <i>target</i>,
1235 * the current method handle by binding it to the given argument.
1236 * The type of the bound handle will be
1237 * the same as the type of the target, except that a single leading
1238 * reference parameter will be omitted.
1240 * When called, the bound handle inserts the given value {@code x}
1241 * as a new leading argument to the target. The other arguments are
1242 * also passed unchanged.
1243 * What the target eventually returns is returned unchanged by the bound handle.
1245 * The reference {@code x} must be convertible to the first parameter
1246 * type of the target.
1248 * (<em>Note:</em> Because method handles are immutable, the target method handle
1249 * retains its original type and behavior.)
1250 * @param x the value to bind to the first argument of the target
1251 * @return a new method handle which prepends the given value to the incoming
1252 * argument list, before calling the original method handle
1253 * @throws IllegalArgumentException if the target does not have a
1254 * leading parameter type that is a reference type
1255 * @throws ClassCastException if {@code x} cannot be converted
1256 * to the leading parameter type of the target
1257 * @see MethodHandles#insertArguments
1259 public MethodHandle bindTo(Object x) {
1261 @SuppressWarnings("LocalVariableHidesMemberVariable")
1262 MethodType type = type();
1263 if (type.parameterCount() == 0 ||
1264 (ptype = type.parameterType(0)).isPrimitive())
1265 throw newIllegalArgumentException("no leading reference parameter", x);
1266 x = ptype.cast(x); // throw CCE if needed
1267 return bindReceiver(x);
1271 * Returns a string representation of the method handle,
1272 * starting with the string {@code "MethodHandle"} and
1273 * ending with the string representation of the method handle's type.
1274 * In other words, this method returns a string equal to the value of:
1275 * <blockquote><pre>{@code
1276 * "MethodHandle" + type().toString()
1277 * }</pre></blockquote>
1279 * (<em>Note:</em> Future releases of this API may add further information
1280 * to the string representation.
1281 * Therefore, the present syntax should not be parsed by applications.)
1283 * @return a string representation of the method handle
1286 public String toString() {
1287 if (DEBUG_METHOD_HANDLE_NAMES) return debugString();
1288 return standardString();
1290 String standardString() {
1291 return "MethodHandle"+type;
1293 String debugString() {
1294 return standardString()+"/LF="+internalForm()+internalProperties();
1297 //// Implementation methods.
1298 //// Sub-classes can override these default implementations.
1299 //// All these methods assume arguments are already validated.
1301 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch
1304 MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1305 if (!member.isVarargs()) return this;
1306 int argc = type().parameterCount();
1308 Class<?> arrayType = type().parameterType(argc-1);
1309 if (arrayType.isArray()) {
1310 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1313 throw member.makeAccessException("cannot make variable arity", null);
1316 MethodHandle viewAsType(MethodType newType) {
1317 // No actual conversions, just a new view of the same method.
1318 return MethodHandleImpl.makePairwiseConvert(this, newType, 0);
1324 LambdaForm internalForm() {
1329 MemberName internalMemberName() {
1330 return null; // DMH returns DMH.member
1334 Class<?> internalCallerClass() {
1335 return null; // caller-bound MH for @CallerSensitive method returns caller
1339 MethodHandle withInternalMemberName(MemberName member) {
1340 if (member != null) {
1341 return MethodHandleImpl.makeWrappedMember(this, member);
1342 } else if (internalMemberName() == null) {
1343 // The required internaMemberName is null, and this MH (like most) doesn't have one.
1346 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1347 MethodHandle result = rebind();
1348 assert (result.internalMemberName() == null);
1354 boolean isInvokeSpecial() {
1355 return false; // DMH.Special returns true
1359 Object internalValues() {
1364 Object internalProperties() {
1365 // Override to something like "/FOO=bar"
1369 //// Method handle implementation methods.
1370 //// Sub-classes can override these default implementations.
1371 //// All these methods assume arguments are already validated.
1373 /*non-public*/ MethodHandle convertArguments(MethodType newType) {
1374 // Override this if it can be improved.
1375 return MethodHandleImpl.makePairwiseConvert(this, newType, 1);
1379 MethodHandle bindArgument(int pos, char basicType, Object value) {
1380 // Override this if it can be improved.
1381 return rebind().bindArgument(pos, basicType, value);
1385 MethodHandle bindReceiver(Object receiver) {
1386 // Override this if it can be improved.
1387 return bindArgument(0, 'L', receiver);
1391 MethodHandle bindImmediate(int pos, char basicType, Object value) {
1392 // Bind an immediate value to a position in the arguments.
1393 // This means, elide the respective argument,
1394 // and replace all references to it in NamedFunction args with the specified value.
1396 // CURRENT RESTRICTIONS
1397 // * only for pos 0 and UNSAFE (position is adjusted in MHImpl to make API usable for others)
1398 assert pos == 0 && basicType == 'L' && value instanceof Unsafe;
1399 MethodType type2 = type.dropParameterTypes(pos, pos + 1); // adjustment: ignore receiver!
1400 LambdaForm form2 = form.bindImmediate(pos + 1, basicType, value); // adjust pos to form-relative pos
1401 return copyWith(type2, form2);
1405 MethodHandle copyWith(MethodType mt, LambdaForm lf) {
1406 throw new InternalError("copyWith: " + this.getClass());
1410 MethodHandle dropArguments(MethodType srcType, int pos, int drops) {
1411 // Override this if it can be improved.
1412 return rebind().dropArguments(srcType, pos, drops);
1416 MethodHandle permuteArguments(MethodType newType, int[] reorder) {
1417 // Override this if it can be improved.
1418 return rebind().permuteArguments(newType, reorder);
1422 MethodHandle rebind() {
1423 // Bind 'this' into a new invoker, of the known class BMH.
1424 MethodType type2 = type();
1425 LambdaForm form2 = reinvokerForm(this);
1426 // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) }
1427 return BoundMethodHandle.bindSingle(type2, form2, this);
1431 MethodHandle reinvokerTarget() {
1432 throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this);
1435 /** Create a LF which simply reinvokes a target of the given basic type.
1436 * The target MH must override {@link #reinvokerTarget} to provide the target.
1438 static LambdaForm reinvokerForm(MethodHandle target) {
1439 MethodType mtype = target.type().basicType();
1440 LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE);
1441 if (reinvoker != null) return reinvoker;
1442 if (mtype.parameterSlotCount() >= MethodType.MAX_MH_ARITY)
1443 return makeReinvokerForm(target.type(), target); // cannot cache this
1444 reinvoker = makeReinvokerForm(mtype, null);
1445 return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, reinvoker);
1447 private static LambdaForm makeReinvokerForm(MethodType mtype, MethodHandle customTargetOrNull) {
1448 boolean customized = (customTargetOrNull != null);
1449 MethodHandle MH_invokeBasic = customized ? null : MethodHandles.basicInvoker(mtype);
1450 final int THIS_BMH = 0;
1451 final int ARG_BASE = 1;
1452 final int ARG_LIMIT = ARG_BASE + mtype.parameterCount();
1453 int nameCursor = ARG_LIMIT;
1454 final int NEXT_MH = customized ? -1 : nameCursor++;
1455 final int REINVOKE = nameCursor++;
1456 LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType());
1457 Object[] targetArgs;
1458 MethodHandle targetMH;
1460 targetArgs = Arrays.copyOfRange(names, ARG_BASE, ARG_LIMIT, Object[].class);
1461 targetMH = customTargetOrNull;
1463 names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]);
1464 targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class);
1465 targetArgs[0] = names[NEXT_MH]; // overwrite this MH with next MH
1466 targetMH = MethodHandles.basicInvoker(mtype);
1468 names[REINVOKE] = new LambdaForm.Name(targetMH, targetArgs);
1469 return new LambdaForm("BMH.reinvoke", ARG_LIMIT, names);
1472 private static final LambdaForm.NamedFunction NF_reinvokerTarget;
1475 NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class
1476 .getDeclaredMethod("reinvokerTarget"));
1477 } catch (ReflectiveOperationException ex) {
1478 throw newInternalError(ex);
1483 * Replace the old lambda form of this method handle with a new one.
1484 * The new one must be functionally equivalent to the old one.
1485 * Threads may continue running the old form indefinitely,
1486 * but it is likely that the new one will be preferred for new executions.
1487 * Use with discretion.
1490 void updateForm(LambdaForm newForm) {
1491 if (form == newForm) return;
1492 // ISSUE: Should we have a memory fence here?
1493 UNSAFE.putObject(this, FORM_OFFSET, newForm);
1494 this.form.prepare(); // as in MethodHandle.<init>
1497 private static final long FORM_OFFSET;
1500 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1501 } catch (ReflectiveOperationException ex) {
1502 throw newInternalError(ex);