1 /* 2 * Copyright (c) 2008, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 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 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length); 628 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments); 629 } 630 631 /** 632 * Performs a variable arity invocation, passing the arguments in the given array 633 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 634 * which mentions only the type {@code Object}, and whose arity is the length 635 * of the argument array. 636 * <p> 637 * This method is also equivalent to the following code: 638 * <blockquote><pre>{@code 639 * invokeWithArguments(arguments.toArray() 640 * }</pre></blockquote> 641 * 642 * @param arguments the arguments to pass to the target 643 * @return the result returned by the target 644 * @throws NullPointerException if {@code arguments} is a null reference 645 * @throws ClassCastException if an argument cannot be converted by reference casting 646 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 647 * @throws Throwable anything thrown by the target method invocation 648 */ 649 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { 650 return invokeWithArguments(arguments.toArray()); 651 } 652 653 /** 654 * Produces an adapter method handle which adapts the type of the 655 * current method handle to a new type. 656 * The resulting method handle is guaranteed to report a type 657 * which is equal to the desired new type. 658 * <p> 659 * If the original type and new type are equal, returns {@code this}. 660 * <p> 661 * The new method handle, when invoked, will perform the following 662 * steps: 663 * <ul> 664 * <li>Convert the incoming argument list to match the original 665 * method handle's argument list. 666 * <li>Invoke the original method handle on the converted argument list. 667 * <li>Convert any result returned by the original method handle 668 * to the return type of new method handle. 669 * </ul> 670 * <p> 671 * This method provides the crucial behavioral difference between 672 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. 673 * The two methods 674 * perform the same steps when the caller's type descriptor exactly matches 675 * the callee's, but when the types differ, plain {@link #invoke invoke} 676 * also calls {@code asType} (or some internal equivalent) in order 677 * to match up the caller's and callee's types. 678 * <p> 679 * If the current method is a variable arity method handle 680 * argument list conversion may involve the conversion and collection 681 * of several arguments into an array, as 682 * {@linkplain #asVarargsCollector described elsewhere}. 683 * In every other case, all conversions are applied <em>pairwise</em>, 684 * which means that each argument or return value is converted to 685 * exactly one argument or return value (or no return value). 686 * The applied conversions are defined by consulting the 687 * the corresponding component types of the old and new 688 * method handle types. 689 * <p> 690 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, 691 * or old and new return types. Specifically, for some valid index {@code i}, let 692 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. 693 * Or else, going the other way for return values, let 694 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. 695 * If the types are the same, the new method handle makes no change 696 * to the corresponding argument or return value (if any). 697 * Otherwise, one of the following conversions is applied 698 * if possible: 699 * <ul> 700 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. 701 * (The types do not need to be related in any particular way. 702 * This is because a dynamic value of null can convert to any reference type.) 703 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation 704 * conversion (JLS 5.3) is applied, if one exists. 705 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) 706 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, 707 * a Java casting conversion (JLS 5.5) is applied if one exists. 708 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, 709 * which is then widened as needed to <em>T1</em>.) 710 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing 711 * conversion will be applied at runtime, possibly followed 712 * by a Java method invocation conversion (JLS 5.3) 713 * on the primitive value. (These are the primitive widening conversions.) 714 * <em>T0</em> must be a wrapper class or a supertype of one. 715 * (In the case where <em>T0</em> is Object, these are the conversions 716 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) 717 * The unboxing conversion must have a possibility of success, which means that 718 * if <em>T0</em> is not itself a wrapper class, there must exist at least one 719 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed 720 * primitive value can be widened to <em>T1</em>. 721 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded 722 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. 723 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, 724 * a zero value is introduced. 725 * </ul> 726 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, 727 * because neither corresponds specifically to the <em>dynamic type</em> of any 728 * actual argument or return value.) 729 * <p> 730 * The method handle conversion cannot be made if any one of the required 731 * pairwise conversions cannot be made. 732 * <p> 733 * At runtime, the conversions applied to reference arguments 734 * or return values may require additional runtime checks which can fail. 735 * An unboxing operation may fail because the original reference is null, 736 * causing a {@link java.lang.NullPointerException NullPointerException}. 737 * An unboxing operation or a reference cast may also fail on a reference 738 * to an object of the wrong type, 739 * causing a {@link java.lang.ClassCastException ClassCastException}. 740 * Although an unboxing operation may accept several kinds of wrappers, 741 * if none are available, a {@code ClassCastException} will be thrown. 742 * 743 * @param newType the expected type of the new method handle 744 * @return a method handle which delegates to {@code this} after performing 745 * any necessary argument conversions, and arranges for any 746 * necessary return value conversions 747 * @throws NullPointerException if {@code newType} is a null reference 748 * @throws WrongMethodTypeException if the conversion cannot be made 749 * @see MethodHandles#explicitCastArguments 750 */ 751 public MethodHandle asType(MethodType newType) { 752 // Fast path alternative to a heavyweight {@code asType} call. 753 // Return 'this' if the conversion will be a no-op. 754 if (newType == type) { 755 return this; 756 } 757 // Return 'this.asTypeCache' if the conversion is already memoized. 758 MethodHandle atc = asTypeCached(newType); 759 if (atc != null) { 760 return atc; 761 } 762 return asTypeUncached(newType); 763 } 764 765 private MethodHandle asTypeCached(MethodType newType) { 766 MethodHandle atc = asTypeCache; 767 if (atc != null && newType == atc.type) { 768 return atc; 769 } 770 return null; 771 } 772 773 /** 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 = MethodHandleImpl.makePairwiseConvert(this, newType, 1); 778 } 779 780 /** 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}. 788 * <p> 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}. 793 * <p> 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. 801 * <p> 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.) 806 * <p> 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. 810 * <p> 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 826 } 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")); 845 } 846 } 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 867 * @see #asCollector 868 */ 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); 873 } 874 875 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))) { 885 sawProblem = true; 886 break; 887 } 888 } 889 if (sawProblem) { 890 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList()); 891 for (int i = nargs - arrayLength; i < nargs; i++) { 892 ptypes.set(i, arrayElement); 893 } 894 // elicit an error: 895 this.asType(MethodType.methodType(type().returnType(), ptypes)); 896 } 897 } 898 } 899 900 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); 911 } 912 } 913 914 /** 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}. 922 * <p> 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}. 926 * <p> 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. 932 * <p> 933 * (The array may also be a shared constant when {@code arrayLength} is zero.) 934 * <p> 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.) 939 * <p> 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. 942 * <p> 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 982 * @see #asSpreader 983 * @see #asVarargsCollector 984 */ 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 = MethodHandleImpl.makePairwiseConvert(this, type().changeParameterType(collectArgPos, arrayType), 1); 991 MethodHandle collector = MethodHandleImpl.varargsArray(arrayType, arrayLength); 992 return MethodHandles.collectArguments(target, collectArgPos, collector); 993 } 994 995 /** 996 * See if {@code asCollector} can be validly called with the given arguments. 997 * Return false if the last parameter is not an exact match to arrayType. 998 */ 999 /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) { 1000 spreadArrayChecks(arrayType, arrayLength); 1001 int nargs = type().parameterCount(); 1002 if (nargs != 0) { 1003 Class<?> lastParam = type().parameterType(nargs-1); 1004 if (lastParam == arrayType) return true; 1005 if (lastParam.isAssignableFrom(arrayType)) return false; 1006 } 1007 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType); 1008 } 1009 1010 /** 1011 * Makes a <em>variable arity</em> adapter which is able to accept 1012 * any number of trailing positional arguments and collect them 1013 * into an array argument. 1014 * <p> 1015 * The type and behavior of the adapter will be the same as 1016 * the type and behavior of the target, except that certain 1017 * {@code invoke} and {@code asType} requests can lead to 1018 * trailing positional arguments being collected into target's 1019 * trailing parameter. 1020 * Also, the last parameter type of the adapter will be 1021 * {@code arrayType}, even if the target has a different 1022 * last parameter type. 1023 * <p> 1024 * This transformation may return {@code this} if the method handle is 1025 * already of variable arity and its trailing parameter type 1026 * is identical to {@code arrayType}. 1027 * <p> 1028 * When called with {@link #invokeExact invokeExact}, the adapter invokes 1029 * the target with no argument changes. 1030 * (<em>Note:</em> This behavior is different from a 1031 * {@linkplain #asCollector fixed arity collector}, 1032 * since it accepts a whole array of indeterminate length, 1033 * rather than a fixed number of arguments.) 1034 * <p> 1035 * When called with plain, inexact {@link #invoke invoke}, if the caller 1036 * type is the same as the adapter, the adapter invokes the target as with 1037 * {@code invokeExact}. 1038 * (This is the normal behavior for {@code invoke} when types match.) 1039 * <p> 1040 * Otherwise, if the caller and adapter arity are the same, and the 1041 * trailing parameter type of the caller is a reference type identical to 1042 * or assignable to the trailing parameter type of the adapter, 1043 * the arguments and return values are converted pairwise, 1044 * as if by {@link #asType asType} on a fixed arity 1045 * method handle. 1046 * <p> 1047 * Otherwise, the arities differ, or the adapter's trailing parameter 1048 * type is not assignable from the corresponding caller type. 1049 * In this case, the adapter replaces all trailing arguments from 1050 * the original trailing argument position onward, by 1051 * a new array of type {@code arrayType}, whose elements 1052 * comprise (in order) the replaced arguments. 1053 * <p> 1054 * The caller type must provides as least enough arguments, 1055 * and of the correct type, to satisfy the target's requirement for 1056 * positional arguments before the trailing array argument. 1057 * Thus, the caller must supply, at a minimum, {@code N-1} arguments, 1058 * where {@code N} is the arity of the target. 1059 * Also, there must exist conversions from the incoming arguments 1060 * to the target's arguments. 1061 * As with other uses of plain {@code invoke}, if these basic 1062 * requirements are not fulfilled, a {@code WrongMethodTypeException} 1063 * may be thrown. 1064 * <p> 1065 * In all cases, what the target eventually returns is returned unchanged by the adapter. 1066 * <p> 1067 * In the final case, it is exactly as if the target method handle were 1068 * temporarily adapted with a {@linkplain #asCollector fixed arity collector} 1069 * to the arity required by the caller type. 1070 * (As with {@code asCollector}, if the array length is zero, 1071 * a shared constant may be used instead of a new array. 1072 * If the implied call to {@code asCollector} would throw 1073 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, 1074 * the call to the variable arity adapter must throw 1075 * {@code WrongMethodTypeException}.) 1076 * <p> 1077 * The behavior of {@link #asType asType} is also specialized for 1078 * variable arity adapters, to maintain the invariant that 1079 * plain, inexact {@code invoke} is always equivalent to an {@code asType} 1080 * call to adjust the target type, followed by {@code invokeExact}. 1081 * Therefore, a variable arity adapter responds 1082 * to an {@code asType} request by building a fixed arity collector, 1083 * if and only if the adapter and requested type differ either 1084 * in arity or trailing argument type. 1085 * The resulting fixed arity collector has its type further adjusted 1086 * (if necessary) to the requested type by pairwise conversion, 1087 * as if by another application of {@code asType}. 1088 * <p> 1089 * When a method handle is obtained by executing an {@code ldc} instruction 1090 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked 1091 * as a variable arity method (with the modifier bit {@code 0x0080}), 1092 * the method handle will accept multiple arities, as if the method handle 1093 * constant were created by means of a call to {@code asVarargsCollector}. 1094 * <p> 1095 * In order to create a collecting adapter which collects a predetermined 1096 * number of arguments, and whose type reflects this predetermined number, 1097 * use {@link #asCollector asCollector} instead. 1098 * <p> 1099 * No method handle transformations produce new method handles with 1100 * variable arity, unless they are documented as doing so. 1101 * Therefore, besides {@code asVarargsCollector}, 1102 * all methods in {@code MethodHandle} and {@code MethodHandles} 1103 * will return a method handle with fixed arity, 1104 * except in the cases where they are specified to return their original 1105 * operand (e.g., {@code asType} of the method handle's own type). 1106 * <p> 1107 * Calling {@code asVarargsCollector} on a method handle which is already 1108 * of variable arity will produce a method handle with the same type and behavior. 1109 * It may (or may not) return the original variable arity method handle. 1110 * <p> 1111 * Here is an example, of a list-making variable arity method handle: 1112 * <blockquote><pre>{@code 1113 MethodHandle deepToString = publicLookup() 1114 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1115 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); 1116 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); 1117 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); 1118 assertEquals("[won]", (String) ts1.invoke( "won" )); 1119 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); 1120 // findStatic of Arrays.asList(...) produces a variable arity method handle: 1121 MethodHandle asList = publicLookup() 1122 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); 1123 assertEquals(methodType(List.class, Object[].class), asList.type()); 1124 assert(asList.isVarargsCollector()); 1125 assertEquals("[]", asList.invoke().toString()); 1126 assertEquals("[1]", asList.invoke(1).toString()); 1127 assertEquals("[two, too]", asList.invoke("two", "too").toString()); 1128 String[] argv = { "three", "thee", "tee" }; 1129 assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); 1130 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); 1131 List ls = (List) asList.invoke((Object)argv); 1132 assertEquals(1, ls.size()); 1133 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); 1134 * }</pre></blockquote> 1135 * <p style="font-size:smaller;"> 1136 * <em>Discussion:</em> 1137 * These rules are designed as a dynamically-typed variation 1138 * of the Java rules for variable arity methods. 1139 * In both cases, callers to a variable arity method or method handle 1140 * can either pass zero or more positional arguments, or else pass 1141 * pre-collected arrays of any length. Users should be aware of the 1142 * special role of the final argument, and of the effect of a 1143 * type match on that final argument, which determines whether 1144 * or not a single trailing argument is interpreted as a whole 1145 * array or a single element of an array to be collected. 1146 * Note that the dynamic type of the trailing argument has no 1147 * effect on this decision, only a comparison between the symbolic 1148 * type descriptor of the call site and the type descriptor of the method handle.) 1149 * 1150 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1151 * @return a new method handle which can collect any number of trailing arguments 1152 * into an array, before calling the original method handle 1153 * @throws NullPointerException if {@code arrayType} is a null reference 1154 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1155 * or {@code arrayType} is not assignable to this method handle's trailing parameter type 1156 * @see #asCollector 1157 * @see #isVarargsCollector 1158 * @see #asFixedArity 1159 */ 1160 public MethodHandle asVarargsCollector(Class<?> arrayType) { 1161 arrayType.getClass(); // explicit NPE 1162 boolean lastMatch = asCollectorChecks(arrayType, 0); 1163 if (isVarargsCollector() && lastMatch) 1164 return this; 1165 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1166 } 1167 1168 /** 1169 * Determines if this method handle 1170 * supports {@linkplain #asVarargsCollector variable arity} calls. 1171 * Such method handles arise from the following sources: 1172 * <ul> 1173 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} 1174 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} 1175 * which resolves to a variable arity Java method or constructor 1176 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} 1177 * which resolves to a variable arity Java method or constructor 1178 * </ul> 1179 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls 1180 * @see #asVarargsCollector 1181 * @see #asFixedArity 1182 */ 1183 public boolean isVarargsCollector() { 1184 return false; 1185 } 1186 1187 /** 1188 * Makes a <em>fixed arity</em> method handle which is otherwise 1189 * equivalent to the current method handle. 1190 * <p> 1191 * If the current method handle is not of 1192 * {@linkplain #asVarargsCollector variable arity}, 1193 * the current method handle is returned. 1194 * This is true even if the current method handle 1195 * could not be a valid input to {@code asVarargsCollector}. 1196 * <p> 1197 * Otherwise, the resulting fixed-arity method handle has the same 1198 * type and behavior of the current method handle, 1199 * except that {@link #isVarargsCollector isVarargsCollector} 1200 * will be false. 1201 * The fixed-arity method handle may (or may not) be the 1202 * a previous argument to {@code asVarargsCollector}. 1203 * <p> 1204 * Here is an example, of a list-making variable arity method handle: 1205 * <blockquote><pre>{@code 1206 MethodHandle asListVar = publicLookup() 1207 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) 1208 .asVarargsCollector(Object[].class); 1209 MethodHandle asListFix = asListVar.asFixedArity(); 1210 assertEquals("[1]", asListVar.invoke(1).toString()); 1211 Exception caught = null; 1212 try { asListFix.invoke((Object)1); } 1213 catch (Exception ex) { caught = ex; } 1214 assert(caught instanceof ClassCastException); 1215 assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); 1216 try { asListFix.invoke("two", "too"); } 1217 catch (Exception ex) { caught = ex; } 1218 assert(caught instanceof WrongMethodTypeException); 1219 Object[] argv = { "three", "thee", "tee" }; 1220 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); 1221 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); 1222 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); 1223 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); 1224 * }</pre></blockquote> 1225 * 1226 * @return a new method handle which accepts only a fixed number of arguments 1227 * @see #asVarargsCollector 1228 * @see #isVarargsCollector 1229 */ 1230 public MethodHandle asFixedArity() { 1231 assert(!isVarargsCollector()); 1232 return this; 1233 } 1234 1235 /** 1236 * Binds a value {@code x} to the first argument of a method handle, without invoking it. 1237 * The new method handle adapts, as its <i>target</i>, 1238 * the current method handle by binding it to the given argument. 1239 * The type of the bound handle will be 1240 * the same as the type of the target, except that a single leading 1241 * reference parameter will be omitted. 1242 * <p> 1243 * When called, the bound handle inserts the given value {@code x} 1244 * as a new leading argument to the target. The other arguments are 1245 * also passed unchanged. 1246 * What the target eventually returns is returned unchanged by the bound handle. 1247 * <p> 1248 * The reference {@code x} must be convertible to the first parameter 1249 * type of the target. 1250 * <p> 1251 * (<em>Note:</em> Because method handles are immutable, the target method handle 1252 * retains its original type and behavior.) 1253 * @param x the value to bind to the first argument of the target 1254 * @return a new method handle which prepends the given value to the incoming 1255 * argument list, before calling the original method handle 1256 * @throws IllegalArgumentException if the target does not have a 1257 * leading parameter type that is a reference type 1258 * @throws ClassCastException if {@code x} cannot be converted 1259 * to the leading parameter type of the target 1260 * @see MethodHandles#insertArguments 1261 */ 1262 public MethodHandle bindTo(Object x) { 1263 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed 1264 return bindArgumentL(0, x); 1265 } 1266 1267 /** 1268 * Returns a string representation of the method handle, 1269 * starting with the string {@code "MethodHandle"} and 1270 * ending with the string representation of the method handle's type. 1271 * In other words, this method returns a string equal to the value of: 1272 * <blockquote><pre>{@code 1273 * "MethodHandle" + type().toString() 1274 * }</pre></blockquote> 1275 * <p> 1276 * (<em>Note:</em> Future releases of this API may add further information 1277 * to the string representation. 1278 * Therefore, the present syntax should not be parsed by applications.) 1279 * 1280 * @return a string representation of the method handle 1281 */ 1282 @Override 1283 public String toString() { 1284 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString(); 1285 return standardString(); 1286 } 1287 String standardString() { 1288 return "MethodHandle"+type; 1289 } 1290 /** Return a string with a several lines describing the method handle structure. 1291 * This string would be suitable for display in an IDE debugger. 1292 */ 1293 String debugString() { 1294 return type+" : "+internalForm()+internalProperties(); 1295 } 1296 1297 //// Implementation methods. 1298 //// Sub-classes can override these default implementations. 1299 //// All these methods assume arguments are already validated. 1300 1301 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch 1302 1303 BoundMethodHandle bindArgumentL(int pos, Object value) { 1304 return rebind().bindArgumentL(pos, value); 1305 } 1306 1307 /*non-public*/ 1308 MethodHandle setVarargs(MemberName member) throws IllegalAccessException { 1309 if (!member.isVarargs()) return this; 1310 Class<?> arrayType = type().lastParameterType(); 1311 if (arrayType.isArray()) { 1312 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1313 } 1314 throw member.makeAccessException("cannot make variable arity", null); 1315 } 1316 1317 /*non-public*/ 1318 MethodHandle viewAsType(MethodType newType) { 1319 // No actual conversions, just a new view of the same method. 1320 return MethodHandleImpl.makePairwiseConvert(this, newType, 0); 1321 } 1322 1323 // Decoding 1324 1325 /*non-public*/ 1326 LambdaForm internalForm() { 1327 return form; 1328 } 1329 1330 /*non-public*/ 1331 MemberName internalMemberName() { 1332 return null; // DMH returns DMH.member 1333 } 1334 1335 /*non-public*/ 1336 Class<?> internalCallerClass() { 1337 return null; // caller-bound MH for @CallerSensitive method returns caller 1338 } 1339 1340 /*non-public*/ 1341 MethodHandle withInternalMemberName(MemberName member) { 1342 if (member != null) { 1343 return MethodHandleImpl.makeWrappedMember(this, member); 1344 } else if (internalMemberName() == null) { 1345 // The required internaMemberName is null, and this MH (like most) doesn't have one. 1346 return this; 1347 } else { 1348 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. 1349 MethodHandle result = rebind(); 1350 assert (result.internalMemberName() == null); 1351 return result; 1352 } 1353 } 1354 1355 /*non-public*/ 1356 boolean isInvokeSpecial() { 1357 return false; // DMH.Special returns true 1358 } 1359 1360 /*non-public*/ 1361 Object internalValues() { 1362 return null; 1363 } 1364 1365 /*non-public*/ 1366 Object internalProperties() { 1367 // Override to something to follow this.form, like "\n& FOO=bar" 1368 return ""; 1369 } 1370 1371 //// Method handle implementation methods. 1372 //// Sub-classes can override these default implementations. 1373 //// All these methods assume arguments are already validated. 1374 1375 /*non-public*/ 1376 BoundMethodHandle rebind() { 1377 // Bind 'this' into a new invoker, of the known class BMH. 1378 MethodType type2 = type(); 1379 LambdaForm form2 = reinvokerForm(this); 1380 // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) } 1381 return BoundMethodHandle.bindSingle(type2, form2, this); 1382 } 1383 1384 /*non-public*/ 1385 MethodHandle reinvokerTarget() { 1386 throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this); 1387 } 1388 1389 /** Create a LF which simply reinvokes a target of the given basic type. 1390 * The target MH must override {@link #reinvokerTarget} to provide the target. 1391 */ 1392 static LambdaForm reinvokerForm(MethodHandle target) { 1393 MethodType mtype = target.type().basicType(); 1394 LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE); 1395 if (reinvoker != null) return reinvoker; 1396 if (mtype.parameterSlotCount() >= MethodType.MAX_MH_ARITY) 1397 return makeReinvokerForm(target.type(), target); // cannot cache this 1398 reinvoker = makeReinvokerForm(mtype, null); 1399 return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, reinvoker); 1400 } 1401 private static LambdaForm makeReinvokerForm(MethodType mtype, MethodHandle customTargetOrNull) { 1402 boolean customized = (customTargetOrNull != null); 1403 MethodHandle MH_invokeBasic = customized ? null : MethodHandles.basicInvoker(mtype); 1404 final int THIS_BMH = 0; 1405 final int ARG_BASE = 1; 1406 final int ARG_LIMIT = ARG_BASE + mtype.parameterCount(); 1407 int nameCursor = ARG_LIMIT; 1408 final int NEXT_MH = customized ? -1 : nameCursor++; 1409 final int REINVOKE = nameCursor++; 1410 LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType()); 1411 Object[] targetArgs; 1412 MethodHandle targetMH; 1413 if (customized) { 1414 targetArgs = Arrays.copyOfRange(names, ARG_BASE, ARG_LIMIT, Object[].class); 1415 targetMH = customTargetOrNull; 1416 } else { 1417 names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]); 1418 targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class); 1419 targetArgs[0] = names[NEXT_MH]; // overwrite this MH with next MH 1420 targetMH = MethodHandles.basicInvoker(mtype); 1421 } 1422 names[REINVOKE] = new LambdaForm.Name(targetMH, targetArgs); 1423 return new LambdaForm("BMH.reinvoke", ARG_LIMIT, names); 1424 } 1425 1426 private static final LambdaForm.NamedFunction NF_reinvokerTarget; 1427 static { 1428 try { 1429 NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class 1430 .getDeclaredMethod("reinvokerTarget")); 1431 } catch (ReflectiveOperationException ex) { 1432 throw newInternalError(ex); 1433 } 1434 } 1435 1436 /** 1437 * Replace the old lambda form of this method handle with a new one. 1438 * The new one must be functionally equivalent to the old one. 1439 * Threads may continue running the old form indefinitely, 1440 * but it is likely that the new one will be preferred for new executions. 1441 * Use with discretion. 1442 */ 1443 /*non-public*/ 1444 void updateForm(LambdaForm newForm) { 1445 if (form == newForm) return; 1446 assert(this instanceof DirectMethodHandle && this.internalMemberName().isStatic()); 1447 // ISSUE: Should we have a memory fence here? 1448 UNSAFE.putObject(this, FORM_OFFSET, newForm); 1449 this.form.prepare(); // as in MethodHandle.<init> 1450 } 1451 1452 private static final long FORM_OFFSET; 1453 static { 1454 try { 1455 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form")); 1456 } catch (ReflectiveOperationException ex) { 1457 throw newInternalError(ex); 1458 } 1459 } 1460 }