1 /* 2 * Copyright (c) 2008, 2018, 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 jdk.internal.HotSpotIntrinsicCandidate; 30 31 import java.util.Arrays; 32 import java.util.Objects; 33 34 import static java.lang.invoke.MethodHandleStatics.*; 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 generate instructions that push them on the stack according 98 * to their own unconverted types. The method handle object itself is pushed on 99 * the stack before the arguments. 100 * The compiler then generates an {@code invokevirtual} instruction that invokes 101 * the method handle with a symbolic type descriptor which describes the argument 102 * and return types. 103 * <p> 104 * To issue a complete symbolic type descriptor, the compiler must also determine 105 * the return type. This is based on a cast on the method invocation expression, 106 * if there is one, or else {@code Object} if the invocation is an expression, 107 * or else {@code void} if the invocation is a statement. 108 * The cast may be to a primitive type (but not {@code void}). 109 * <p> 110 * As a corner case, an uncasted {@code null} argument is given 111 * a symbolic type descriptor of {@code java.lang.Void}. 112 * The ambiguity with the type {@code Void} is harmless, since there are no references of type 113 * {@code Void} except the null reference. 114 * 115 * <h1>Method handle invocation</h1> 116 * The first time an {@code invokevirtual} instruction is executed 117 * it is linked by symbolically resolving the names in the instruction 118 * and verifying that the method call is statically legal. 119 * This also holds for calls to {@code invokeExact} and {@code invoke}. 120 * In this case, the symbolic type descriptor emitted by the compiler is checked for 121 * correct syntax, and names it contains are resolved. 122 * Thus, an {@code invokevirtual} instruction which invokes 123 * a method handle will always link, as long 124 * as the symbolic type descriptor is syntactically well-formed 125 * and the types exist. 126 * <p> 127 * When the {@code invokevirtual} is executed after linking, 128 * the receiving method handle's type is first checked by the JVM 129 * to ensure that it matches the symbolic type descriptor. 130 * If the type match fails, it means that the method which the 131 * caller is invoking is not present on the individual 132 * method handle being invoked. 133 * <p> 134 * In the case of {@code invokeExact}, the type descriptor of the invocation 135 * (after resolving symbolic type names) must exactly match the method type 136 * of the receiving method handle. 137 * In the case of plain, inexact {@code invoke}, the resolved type descriptor 138 * must be a valid argument to the receiver's {@link #asType asType} method. 139 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}. 140 * <p> 141 * After type matching, a call to {@code invokeExact} directly 142 * and immediately invoke the method handle's underlying method 143 * (or other behavior, as the case may be). 144 * <p> 145 * A call to plain {@code invoke} works the same as a call to 146 * {@code invokeExact}, if the symbolic type descriptor specified by the caller 147 * exactly matches the method handle's own type. 148 * If there is a type mismatch, {@code invoke} attempts 149 * to adjust the type of the receiving method handle, 150 * as if by a call to {@link #asType asType}, 151 * to obtain an exactly invokable method handle {@code M2}. 152 * This allows a more powerful negotiation of method type 153 * between caller and callee. 154 * <p> 155 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable, 156 * and implementations are therefore not required to materialize it.) 157 * 158 * <h1>Invocation checking</h1> 159 * In typical programs, method handle type matching will usually succeed. 160 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, 161 * either directly (in the case of {@code invokeExact}) or indirectly as if 162 * by a failed call to {@code asType} (in the case of {@code invoke}). 163 * <p> 164 * Thus, a method type mismatch which might show up as a linkage error 165 * in a statically typed program can show up as 166 * a dynamic {@code WrongMethodTypeException} 167 * in a program which uses method handles. 168 * <p> 169 * Because method types contain "live" {@code Class} objects, 170 * method type matching takes into account both type names and class loaders. 171 * Thus, even if a method handle {@code M} is created in one 172 * class loader {@code L1} and used in another {@code L2}, 173 * method handle calls are type-safe, because the caller's symbolic type 174 * descriptor, as resolved in {@code L2}, 175 * is matched against the original callee method's symbolic type descriptor, 176 * as resolved in {@code L1}. 177 * The resolution in {@code L1} happens when {@code M} is created 178 * and its type is assigned, while the resolution in {@code L2} happens 179 * when the {@code invokevirtual} instruction is linked. 180 * <p> 181 * Apart from type descriptor checks, 182 * a method handle's capability to call its underlying method is unrestricted. 183 * If a method handle is formed on a non-public method by a class 184 * that has access to that method, the resulting handle can be used 185 * in any place by any caller who receives a reference to it. 186 * <p> 187 * Unlike with the Core Reflection API, where access is checked every time 188 * a reflective method is invoked, 189 * method handle access checking is performed 190 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>. 191 * In the case of {@code ldc} (see below), access checking is performed as part of linking 192 * the constant pool entry underlying the constant method handle. 193 * <p> 194 * Thus, handles to non-public methods, or to methods in non-public classes, 195 * should generally be kept secret. 196 * They should not be passed to untrusted code unless their use from 197 * the untrusted code would be harmless. 198 * 199 * <h1>Method handle creation</h1> 200 * Java code can create a method handle that directly accesses 201 * any method, constructor, or field that is accessible to that code. 202 * This is done via a reflective, capability-based API called 203 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}. 204 * For example, a static method handle can be obtained 205 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}. 206 * There are also conversion methods from Core Reflection API objects, 207 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 208 * <p> 209 * Like classes and strings, method handles that correspond to accessible 210 * fields, methods, and constructors can also be represented directly 211 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes. 212 * A new type of constant pool entry, {@code CONSTANT_MethodHandle}, 213 * refers directly to an associated {@code CONSTANT_Methodref}, 214 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref} 215 * constant pool entry. 216 * (For full details on method handle constants, 217 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.) 218 * <p> 219 * Method handles produced by lookups or constant loads from methods or 220 * constructors with the variable arity modifier bit ({@code 0x0080}) 221 * have a corresponding variable arity, as if they were defined with 222 * the help of {@link #asVarargsCollector asVarargsCollector} 223 * or {@link #withVarargs withVarargs}. 224 * <p> 225 * A method reference may refer either to a static or non-static method. 226 * In the non-static case, the method handle type includes an explicit 227 * receiver argument, prepended before any other arguments. 228 * In the method handle's type, the initial receiver argument is typed 229 * according to the class under which the method was initially requested. 230 * (E.g., if a non-static method handle is obtained via {@code ldc}, 231 * the type of the receiver is the class named in the constant pool entry.) 232 * <p> 233 * Method handle constants are subject to the same link-time access checks 234 * their corresponding bytecode instructions, and the {@code ldc} instruction 235 * will throw corresponding linkage errors if the bytecode behaviors would 236 * throw such errors. 237 * <p> 238 * As a corollary of this, access to protected members is restricted 239 * to receivers only of the accessing class, or one of its subclasses, 240 * and the accessing class must in turn be a subclass (or package sibling) 241 * of the protected member's defining class. 242 * If a method reference refers to a protected non-static method or field 243 * of a class outside the current package, the receiver argument will 244 * be narrowed to the type of the accessing class. 245 * <p> 246 * When a method handle to a virtual method is invoked, the method is 247 * always looked up in the receiver (that is, the first argument). 248 * <p> 249 * A non-virtual method handle to a specific virtual method implementation 250 * can also be created. These do not perform virtual lookup based on 251 * receiver type. Such a method handle simulates the effect of 252 * an {@code invokespecial} instruction to the same method. 253 * A non-virtual method handle can also be created to simulate the effect 254 * of an {@code invokevirtual} or {@code invokeinterface} instruction on 255 * a private method (as applicable). 256 * 257 * <h1>Usage examples</h1> 258 * Here are some examples of usage: 259 * <blockquote><pre>{@code 260 Object x, y; String s; int i; 261 MethodType mt; MethodHandle mh; 262 MethodHandles.Lookup lookup = MethodHandles.lookup(); 263 // mt is (char,char)String 264 mt = MethodType.methodType(String.class, char.class, char.class); 265 mh = lookup.findVirtual(String.class, "replace", mt); 266 s = (String) mh.invokeExact("daddy",'d','n'); 267 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String; 268 assertEquals(s, "nanny"); 269 // weakly typed invocation (using MHs.invoke) 270 s = (String) mh.invokeWithArguments("sappy", 'p', 'v'); 271 assertEquals(s, "savvy"); 272 // mt is (Object[])List 273 mt = MethodType.methodType(java.util.List.class, Object[].class); 274 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt); 275 assert(mh.isVarargsCollector()); 276 x = mh.invoke("one", "two"); 277 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object; 278 assertEquals(x, java.util.Arrays.asList("one","two")); 279 // mt is (Object,Object,Object)Object 280 mt = MethodType.genericMethodType(3); 281 mh = mh.asType(mt); 282 x = mh.invokeExact((Object)1, (Object)2, (Object)3); 283 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object; 284 assertEquals(x, java.util.Arrays.asList(1,2,3)); 285 // mt is ()int 286 mt = MethodType.methodType(int.class); 287 mh = lookup.findVirtual(java.util.List.class, "size", mt); 288 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3)); 289 // invokeExact(Ljava/util/List;)I 290 assert(i == 3); 291 mt = MethodType.methodType(void.class, String.class); 292 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt); 293 mh.invokeExact(System.out, "Hello, world."); 294 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V 295 * }</pre></blockquote> 296 * Each of the above calls to {@code invokeExact} or plain {@code invoke} 297 * generates a single invokevirtual instruction with 298 * the symbolic type descriptor indicated in the following comment. 299 * In these examples, the helper method {@code assertEquals} is assumed to 300 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals} 301 * on its arguments, and asserts that the result is true. 302 * 303 * <h1>Exceptions</h1> 304 * The methods {@code invokeExact} and {@code invoke} are declared 305 * to throw {@link java.lang.Throwable Throwable}, 306 * which is to say that there is no static restriction on what a method handle 307 * can throw. Since the JVM does not distinguish between checked 308 * and unchecked exceptions (other than by their class, of course), 309 * there is no particular effect on bytecode shape from ascribing 310 * checked exceptions to method handle invocations. But in Java source 311 * code, methods which perform method handle calls must either explicitly 312 * throw {@code Throwable}, or else must catch all 313 * throwables locally, rethrowing only those which are legal in the context, 314 * and wrapping ones which are illegal. 315 * 316 * <h1><a id="sigpoly"></a>Signature polymorphism</h1> 317 * The unusual compilation and linkage behavior of 318 * {@code invokeExact} and plain {@code invoke} 319 * is referenced by the term <em>signature polymorphism</em>. 320 * As defined in the Java Language Specification, 321 * a signature polymorphic method is one which can operate with 322 * any of a wide range of call signatures and return types. 323 * <p> 324 * In source code, a call to a signature polymorphic method will 325 * compile, regardless of the requested symbolic type descriptor. 326 * As usual, the Java compiler emits an {@code invokevirtual} 327 * instruction with the given symbolic type descriptor against the named method. 328 * The unusual part is that the symbolic type descriptor is derived from 329 * the actual argument and return types, not from the method declaration. 330 * <p> 331 * When the JVM processes bytecode containing signature polymorphic calls, 332 * it will successfully link any such call, regardless of its symbolic type descriptor. 333 * (In order to retain type safety, the JVM will guard such calls with suitable 334 * dynamic type checks, as described elsewhere.) 335 * <p> 336 * Bytecode generators, including the compiler back end, are required to emit 337 * untransformed symbolic type descriptors for these methods. 338 * Tools which determine symbolic linkage are required to accept such 339 * untransformed descriptors, without reporting linkage errors. 340 * 341 * <h1>Interoperation between method handles and the Core Reflection API</h1> 342 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, 343 * any class member represented by a Core Reflection API object 344 * can be converted to a behaviorally equivalent method handle. 345 * For example, a reflective {@link java.lang.reflect.Method Method} can 346 * be converted to a method handle using 347 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 348 * The resulting method handles generally provide more direct and efficient 349 * access to the underlying class members. 350 * <p> 351 * As a special case, 352 * when the Core Reflection API is used to view the signature polymorphic 353 * methods {@code invokeExact} or plain {@code invoke} in this class, 354 * they appear as ordinary non-polymorphic methods. 355 * Their reflective appearance, as viewed by 356 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, 357 * is unaffected by their special status in this API. 358 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} 359 * will report exactly those modifier bits required for any similarly 360 * declared method, including in this case {@code native} and {@code varargs} bits. 361 * <p> 362 * As with any reflected method, these methods (when reflected) may be 363 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}. 364 * However, such reflective calls do not result in method handle invocations. 365 * Such a call, if passed the required argument 366 * (a single one, of type {@code Object[]}), will ignore the argument and 367 * will throw an {@code UnsupportedOperationException}. 368 * <p> 369 * Since {@code invokevirtual} instructions can natively 370 * invoke method handles under any symbolic type descriptor, this reflective view conflicts 371 * with the normal presentation of these methods via bytecodes. 372 * Thus, these two native methods, when reflectively viewed by 373 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only. 374 * <p> 375 * In order to obtain an invoker method for a particular type descriptor, 376 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker}, 377 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}. 378 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual} 379 * API is also able to return a method handle 380 * to call {@code invokeExact} or plain {@code invoke}, 381 * for any specified type descriptor . 382 * 383 * <h1>Interoperation between method handles and Java generics</h1> 384 * A method handle can be obtained on a method, constructor, or field 385 * which is declared with Java generic types. 386 * As with the Core Reflection API, the type of the method handle 387 * will constructed from the erasure of the source-level type. 388 * When a method handle is invoked, the types of its arguments 389 * or the return value cast type may be generic types or type instances. 390 * If this occurs, the compiler will replace those 391 * types by their erasures when it constructs the symbolic type descriptor 392 * for the {@code invokevirtual} instruction. 393 * <p> 394 * Method handles do not represent 395 * their function-like types in terms of Java parameterized (generic) types, 396 * because there are three mismatches between function-like types and parameterized 397 * Java types. 398 * <ul> 399 * <li>Method types range over all possible arities, 400 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments. 401 * Generics are not variadic, and so cannot represent this.</li> 402 * <li>Method types can specify arguments of primitive types, 403 * which Java generic types cannot range over.</li> 404 * <li>Higher order functions over method handles (combinators) are 405 * often generic across a wide range of function types, including 406 * those of multiple arities. It is impossible to represent such 407 * genericity with a Java type parameter.</li> 408 * </ul> 409 * 410 * <h1><a id="maxarity"></a>Arity limits</h1> 411 * The JVM imposes on all methods and constructors of any kind an absolute 412 * limit of 255 stacked arguments. This limit can appear more restrictive 413 * in certain cases: 414 * <ul> 415 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots. 416 * <li>A non-static method consumes an extra argument for the object on which the method is called. 417 * <li>A constructor consumes an extra argument for the object which is being constructed. 418 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual, 419 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object. 420 * </ul> 421 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments. 422 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it. 423 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}. 424 * In particular, a method handle’s type must not have an arity of the exact maximum 255. 425 * 426 * @see MethodType 427 * @see MethodHandles 428 * @author John Rose, JSR 292 EG 429 * @since 1.7 430 */ 431 public abstract class MethodHandle { 432 433 /** 434 * Internal marker interface which distinguishes (to the Java compiler) 435 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>. 436 */ 437 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD}) 438 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME) 439 @interface PolymorphicSignature { } 440 441 private final MethodType type; 442 /*private*/ final LambdaForm form; 443 // form is not private so that invokers can easily fetch it 444 /*private*/ MethodHandle asTypeCache; 445 // asTypeCache is not private so that invokers can easily fetch it 446 /*non-public*/ byte customizationCount; 447 // customizationCount should be accessible from invokers 448 449 /** 450 * Reports the type of this method handle. 451 * Every invocation of this method handle via {@code invokeExact} must exactly match this type. 452 * @return the method handle type 453 */ 454 public MethodType type() { 455 return type; 456 } 457 458 /** 459 * Package-private constructor for the method handle implementation hierarchy. 460 * Method handle inheritance will be contained completely within 461 * the {@code java.lang.invoke} package. 462 */ 463 // @param type type (permanently assigned) of the new method handle 464 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) { 465 this.type = Objects.requireNonNull(type); 466 this.form = Objects.requireNonNull(form).uncustomize(); 467 468 this.form.prepare(); // TO DO: Try to delay this step until just before invocation. 469 } 470 471 /** 472 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match. 473 * The symbolic type descriptor at the call site of {@code invokeExact} must 474 * exactly match this method handle's {@link #type() type}. 475 * No conversions are allowed on arguments or return values. 476 * <p> 477 * When this method is observed via the Core Reflection API, 478 * it will appear as a single native method, taking an object array and returning an object. 479 * If this native method is invoked directly via 480 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 481 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 482 * it will throw an {@code UnsupportedOperationException}. 483 * @param args the signature-polymorphic parameter list, statically represented using varargs 484 * @return the signature-polymorphic result, statically represented using {@code Object} 485 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor 486 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 487 */ 488 @HotSpotIntrinsicCandidate 489 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable; 490 491 /** 492 * Invokes the method handle, allowing any caller type descriptor, 493 * and optionally performing conversions on arguments and return values. 494 * <p> 495 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type() type}, 496 * the call proceeds as if by {@link #invokeExact invokeExact}. 497 * <p> 498 * Otherwise, the call proceeds as if this method handle were first 499 * adjusted by calling {@link #asType asType} to adjust this method handle 500 * to the required type, and then the call proceeds as if by 501 * {@link #invokeExact invokeExact} on the adjusted method handle. 502 * <p> 503 * There is no guarantee that the {@code asType} call is actually made. 504 * If the JVM can predict the results of making the call, it may perform 505 * adaptations directly on the caller's arguments, 506 * and call the target method handle according to its own exact type. 507 * <p> 508 * The resolved type descriptor at the call site of {@code invoke} must 509 * be a valid argument to the receivers {@code asType} method. 510 * In particular, the caller must specify the same argument arity 511 * as the callee's type, 512 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}. 513 * <p> 514 * When this method is observed via the Core Reflection API, 515 * it will appear as a single native method, taking an object array and returning an object. 516 * If this native method is invoked directly via 517 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 518 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 519 * it will throw an {@code UnsupportedOperationException}. 520 * @param args the signature-polymorphic parameter list, statically represented using varargs 521 * @return the signature-polymorphic result, statically represented using {@code Object} 522 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor 523 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails 524 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 525 */ 526 @HotSpotIntrinsicCandidate 527 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable; 528 529 /** 530 * Private method for trusted invocation of a method handle respecting simplified signatures. 531 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM. 532 * <p> 533 * The caller signature is restricted to the following basic types: 534 * Object, int, long, float, double, and void return. 535 * <p> 536 * The caller is responsible for maintaining type correctness by ensuring 537 * that the each outgoing argument value is a member of the range of the corresponding 538 * callee argument type. 539 * (The caller should therefore issue appropriate casts and integer narrowing 540 * operations on outgoing argument values.) 541 * The caller can assume that the incoming result value is part of the range 542 * of the callee's return type. 543 * @param args the signature-polymorphic parameter list, statically represented using varargs 544 * @return the signature-polymorphic result, statically represented using {@code Object} 545 */ 546 @HotSpotIntrinsicCandidate 547 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; 548 549 /** 550 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}. 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 @HotSpotIntrinsicCandidate 557 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable; 558 559 /** 560 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}. 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} 565 */ 566 @HotSpotIntrinsicCandidate 567 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable; 568 569 /** 570 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}. 571 * The caller signature is restricted to basic types as with {@code invokeBasic}. 572 * The trailing (not leading) argument must be a MemberName. 573 * @param args the signature-polymorphic parameter list, statically represented using varargs 574 * @return the signature-polymorphic result, statically represented using {@code Object} 575 */ 576 @HotSpotIntrinsicCandidate 577 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable; 578 579 /** 580 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}. 581 * The caller signature is restricted to basic types as with {@code invokeBasic}. 582 * The trailing (not leading) argument must be a MemberName. 583 * @param args the signature-polymorphic parameter list, statically represented using varargs 584 * @return the signature-polymorphic result, statically represented using {@code Object} 585 */ 586 @HotSpotIntrinsicCandidate 587 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; 588 589 /** 590 * Performs a variable arity invocation, passing the arguments in the given array 591 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 592 * which mentions only the type {@code Object}, and whose actual argument count is the length 593 * of the argument array. 594 * <p> 595 * Specifically, execution proceeds as if by the following steps, 596 * although the methods are not guaranteed to be called if the JVM 597 * can predict their effects. 598 * <ul> 599 * <li>Determine the length of the argument array as {@code N}. 600 * For a null reference, {@code N=0}. </li> 601 * <li>Collect the {@code N} elements of the array as a logical 602 * argument list, each argument statically typed as an {@code Object}. </li> 603 * <li>Determine, as {@code M}, the parameter count of the type of this 604 * method handle. </li> 605 * <li>Determine the general type {@code TN} of {@code N} arguments or 606 * {@code M} arguments, if smaller than {@code N}, as 607 * {@code TN=MethodType.genericMethodType(Math.min(N, M))}.</li> 608 * <li>If {@code N} is greater than {@code M}, perform the following 609 * checks and actions to shorten the logical argument list: <ul> 610 * <li>Check that this method handle has variable arity with a 611 * {@linkplain MethodType#lastParameterType trailing parameter} 612 * of some array type {@code A[]}. If not, fail with a 613 * {@code WrongMethodTypeException}. </li> 614 * <li>Collect the trailing elements (there are {@code N-M+1} of them) 615 * from the logical argument list into a single array of 616 * type {@code A[]}, using {@code asType} conversions to 617 * convert each trailing argument to type {@code A}. </li> 618 * <li>If any of these conversions proves impossible, fail with either 619 * a {@code ClassCastException} if any trailing element cannot be 620 * cast to {@code A} or a {@code NullPointerException} if any 621 * trailing element is {@code null} and {@code A} is not a reference 622 * type. </li> 623 * <li>Replace the logical arguments gathered into the array of 624 * type {@code A[]} with the array itself, thus shortening 625 * the argument list to length {@code M}. This final argument 626 * retains the static type {@code A[]}.</li> 627 * <li>Adjust the type {@code TN} by changing the {@code N}th 628 * parameter type from {@code Object} to {@code A[]}. 629 * </ul> 630 * <li>Force the original target method handle {@code MH0} to the 631 * required type, as {@code MH1 = MH0.asType(TN)}. </li> 632 * <li>Spread the argument list into {@code N} separate arguments {@code A0, ...}. </li> 633 * <li>Invoke the type-adjusted method handle on the unpacked arguments: 634 * MH1.invokeExact(A0, ...). </li> 635 * <li>Take the return value as an {@code Object} reference. </li> 636 * </ul> 637 * <p> 638 * If the target method handle has variable arity, and the argument list is longer 639 * than that arity, the excess arguments, starting at the position of the trailing 640 * array argument, will be gathered (if possible, as if by {@code asType} conversions) 641 * into an array of the appropriate type, and invocation will proceed on the 642 * shortened argument list. 643 * In this way, <em>jumbo argument lists</em> which would spread into more 644 * than 254 slots can still be processed uniformly. 645 * <p> 646 * Unlike the {@link #invoke(Object...) generic} invocation mode, which can 647 * "recycle" an array argument, passing it directly to the target method, 648 * this invocation mode <em>always</em> creates a new array parameter, even 649 * if the original array passed to {@code invokeWithArguments} would have 650 * been acceptable as a direct argument to the target method. 651 * Even if the number {@code M} of actual arguments is the arity {@code N}, 652 * and the last argument is dynamically a suitable array of type {@code A[]}, 653 * it will still be boxed into a new one-element array, since the call 654 * site statically types the argument as {@code Object}, not an array type. 655 * This is not a special rule for this method, but rather a regular effect 656 * of the {@linkplain #asVarargsCollector rules for variable-arity invocation}. 657 * <p> 658 * Because of the action of the {@code asType} step, the following argument 659 * conversions are applied as necessary: 660 * <ul> 661 * <li>reference casting 662 * <li>unboxing 663 * <li>widening primitive conversions 664 * <li>variable arity conversion 665 * </ul> 666 * <p> 667 * The result returned by the call is boxed if it is a primitive, 668 * or forced to null if the return type is void. 669 * <p> 670 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke}, 671 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI. 672 * It can therefore be used as a bridge between native or reflective code and method handles. 673 * @apiNote 674 * This call is approximately equivalent to the following code: 675 * <blockquote><pre>{@code 676 * // for jumbo argument lists, adapt varargs explicitly: 677 * int N = (arguments == null? 0: arguments.length); 678 * int M = this.type.parameterCount(); 679 * int MAX_SAFE = 127; // 127 longs require 254 slots, which is OK 680 * if (N > MAX_SAFE && N > M && this.isVarargsCollector()) { 681 * Class<?> arrayType = this.type().lastParameterType(); 682 * Class<?> elemType = arrayType.getComponentType(); 683 * if (elemType != null) { 684 * Object args2 = Array.newInstance(elemType, M); 685 * MethodHandle arraySetter = MethodHandles.arrayElementSetter(arrayType); 686 * for (int i = 0; i < M; i++) { 687 * arraySetter.invoke(args2, i, arguments[M-1 + i]); 688 * } 689 * arguments = Arrays.copyOf(arguments, M); 690 * arguments[M-1] = args2; 691 * return this.asFixedArity().invokeWithArguments(arguments); 692 * } 693 * } // done with explicit varargs processing 694 * 695 * // Handle fixed arity and non-jumbo variable arity invocation. 696 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0); 697 * Object result = invoker.invokeExact(this, arguments); 698 * }</pre></blockquote> 699 * 700 * @param arguments the arguments to pass to the target 701 * @return the result returned by the target 702 * @throws ClassCastException if an argument cannot be converted by reference casting 703 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 704 * @throws Throwable anything thrown by the target method invocation 705 * @see MethodHandles#spreadInvoker 706 */ 707 public Object invokeWithArguments(Object... arguments) throws Throwable { 708 // Note: Jumbo argument lists are handled in the variable-arity subclass. 709 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length); 710 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments); 711 } 712 713 /** 714 * Performs a variable arity invocation, passing the arguments in the given list 715 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 716 * which mentions only the type {@code Object}, and whose actual argument count is the length 717 * of the argument list. 718 * <p> 719 * This method is also equivalent to the following code: 720 * <blockquote><pre>{@code 721 * invokeWithArguments(arguments.toArray()) 722 * }</pre></blockquote> 723 * <p> 724 * Jumbo-sized lists are acceptable if this method handle has variable arity. 725 * See {@link #invokeWithArguments(Object[])} for details. 726 * 727 * @param arguments the arguments to pass to the target 728 * @return the result returned by the target 729 * @throws NullPointerException if {@code arguments} is a null reference 730 * @throws ClassCastException if an argument cannot be converted by reference casting 731 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 732 * @throws Throwable anything thrown by the target method invocation 733 */ 734 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { 735 return invokeWithArguments(arguments.toArray()); 736 } 737 738 /** 739 * Produces an adapter method handle which adapts the type of the 740 * current method handle to a new type. 741 * The resulting method handle is guaranteed to report a type 742 * which is equal to the desired new type. 743 * <p> 744 * If the original type and new type are equal, returns {@code this}. 745 * <p> 746 * The new method handle, when invoked, will perform the following 747 * steps: 748 * <ul> 749 * <li>Convert the incoming argument list to match the original 750 * method handle's argument list. 751 * <li>Invoke the original method handle on the converted argument list. 752 * <li>Convert any result returned by the original method handle 753 * to the return type of new method handle. 754 * </ul> 755 * <p> 756 * This method provides the crucial behavioral difference between 757 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. 758 * The two methods 759 * perform the same steps when the caller's type descriptor exactly matches 760 * the callee's, but when the types differ, plain {@link #invoke invoke} 761 * also calls {@code asType} (or some internal equivalent) in order 762 * to match up the caller's and callee's types. 763 * <p> 764 * If the current method is a variable arity method handle 765 * argument list conversion may involve the conversion and collection 766 * of several arguments into an array, as 767 * {@linkplain #asVarargsCollector described elsewhere}. 768 * In every other case, all conversions are applied <em>pairwise</em>, 769 * which means that each argument or return value is converted to 770 * exactly one argument or return value (or no return value). 771 * The applied conversions are defined by consulting 772 * the corresponding component types of the old and new 773 * method handle types. 774 * <p> 775 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, 776 * or old and new return types. Specifically, for some valid index {@code i}, let 777 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. 778 * Or else, going the other way for return values, let 779 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. 780 * If the types are the same, the new method handle makes no change 781 * to the corresponding argument or return value (if any). 782 * Otherwise, one of the following conversions is applied 783 * if possible: 784 * <ul> 785 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. 786 * (The types do not need to be related in any particular way. 787 * This is because a dynamic value of null can convert to any reference type.) 788 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation 789 * conversion (JLS 5.3) is applied, if one exists. 790 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) 791 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, 792 * a Java casting conversion (JLS 5.5) is applied if one exists. 793 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, 794 * which is then widened as needed to <em>T1</em>.) 795 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing 796 * conversion will be applied at runtime, possibly followed 797 * by a Java method invocation conversion (JLS 5.3) 798 * on the primitive value. (These are the primitive widening conversions.) 799 * <em>T0</em> must be a wrapper class or a supertype of one. 800 * (In the case where <em>T0</em> is Object, these are the conversions 801 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) 802 * The unboxing conversion must have a possibility of success, which means that 803 * if <em>T0</em> is not itself a wrapper class, there must exist at least one 804 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed 805 * primitive value can be widened to <em>T1</em>. 806 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded 807 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. 808 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, 809 * a zero value is introduced. 810 * </ul> 811 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, 812 * because neither corresponds specifically to the <em>dynamic type</em> of any 813 * actual argument or return value.) 814 * <p> 815 * The method handle conversion cannot be made if any one of the required 816 * pairwise conversions cannot be made. 817 * <p> 818 * At runtime, the conversions applied to reference arguments 819 * or return values may require additional runtime checks which can fail. 820 * An unboxing operation may fail because the original reference is null, 821 * causing a {@link java.lang.NullPointerException NullPointerException}. 822 * An unboxing operation or a reference cast may also fail on a reference 823 * to an object of the wrong type, 824 * causing a {@link java.lang.ClassCastException ClassCastException}. 825 * Although an unboxing operation may accept several kinds of wrappers, 826 * if none are available, a {@code ClassCastException} will be thrown. 827 * 828 * @param newType the expected type of the new method handle 829 * @return a method handle which delegates to {@code this} after performing 830 * any necessary argument conversions, and arranges for any 831 * necessary return value conversions 832 * @throws NullPointerException if {@code newType} is a null reference 833 * @throws WrongMethodTypeException if the conversion cannot be made 834 * @see MethodHandles#explicitCastArguments 835 */ 836 public MethodHandle asType(MethodType newType) { 837 // Fast path alternative to a heavyweight {@code asType} call. 838 // Return 'this' if the conversion will be a no-op. 839 if (newType == type) { 840 return this; 841 } 842 // Return 'this.asTypeCache' if the conversion is already memoized. 843 MethodHandle atc = asTypeCached(newType); 844 if (atc != null) { 845 return atc; 846 } 847 return asTypeUncached(newType); 848 } 849 850 private MethodHandle asTypeCached(MethodType newType) { 851 MethodHandle atc = asTypeCache; 852 if (atc != null && newType == atc.type) { 853 return atc; 854 } 855 return null; 856 } 857 858 /** Override this to change asType behavior. */ 859 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) { 860 if (!type.isConvertibleTo(newType)) 861 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType); 862 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true); 863 } 864 865 /** 866 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument 867 * and spreads its elements as positional arguments. 868 * The new method handle adapts, as its <i>target</i>, 869 * the current method handle. The type of the adapter will be 870 * the same as the type of the target, except that the final 871 * {@code arrayLength} parameters of the target's type are replaced 872 * by a single array parameter of type {@code arrayType}. 873 * <p> 874 * If the array element type differs from any of the corresponding 875 * argument types on the original target, 876 * the original target is adapted to take the array elements directly, 877 * as if by a call to {@link #asType asType}. 878 * <p> 879 * When called, the adapter replaces a trailing array argument 880 * by the array's elements, each as its own argument to the target. 881 * (The order of the arguments is preserved.) 882 * They are converted pairwise by casting and/or unboxing 883 * to the types of the trailing parameters of the target. 884 * Finally the target is called. 885 * What the target eventually returns is returned unchanged by the adapter. 886 * <p> 887 * Before calling the target, the adapter verifies that the array 888 * contains exactly enough elements to provide a correct argument count 889 * to the target method handle. 890 * (The array may also be null when zero elements are required.) 891 * <p> 892 * When the adapter is called, the length of the supplied {@code array} 893 * argument is queried as if by {@code array.length} or {@code arraylength} 894 * bytecode. If the adapter accepts a zero-length trailing array argument, 895 * the supplied {@code array} argument can either be a zero-length array or 896 * {@code null}; otherwise, the adapter will throw a {@code NullPointerException} 897 * if the array is {@code null} and throw an {@link IllegalArgumentException} 898 * if the array does not have the correct number of elements. 899 * <p> 900 * Here are some simple examples of array-spreading method handles: 901 * <blockquote><pre>{@code 902 MethodHandle equals = publicLookup() 903 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class)); 904 assert( (boolean) equals.invokeExact("me", (Object)"me")); 905 assert(!(boolean) equals.invokeExact("me", (Object)"thee")); 906 // spread both arguments from a 2-array: 907 MethodHandle eq2 = equals.asSpreader(Object[].class, 2); 908 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" })); 909 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" })); 910 // try to spread from anything but a 2-array: 911 for (int n = 0; n <= 10; n++) { 912 Object[] badArityArgs = (n == 2 ? new Object[0] : new Object[n]); 913 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); } 914 catch (IllegalArgumentException ex) { } // OK 915 } 916 // spread both arguments from a String array: 917 MethodHandle eq2s = equals.asSpreader(String[].class, 2); 918 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" })); 919 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" })); 920 // spread second arguments from a 1-array: 921 MethodHandle eq1 = equals.asSpreader(Object[].class, 1); 922 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" })); 923 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" })); 924 // spread no arguments from a 0-array or null: 925 MethodHandle eq0 = equals.asSpreader(Object[].class, 0); 926 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0])); 927 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null)); 928 // asSpreader and asCollector are approximate inverses: 929 for (int n = 0; n <= 2; n++) { 930 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) { 931 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n); 932 assert( (boolean) equals2.invokeWithArguments("me", "me")); 933 assert(!(boolean) equals2.invokeWithArguments("me", "thee")); 934 } 935 } 936 MethodHandle caToString = publicLookup() 937 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class)); 938 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray())); 939 MethodHandle caString3 = caToString.asCollector(char[].class, 3); 940 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C')); 941 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2); 942 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray())); 943 * }</pre></blockquote> 944 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 945 * @param arrayLength the number of arguments to spread from an incoming array argument 946 * @return a new method handle which spreads its final array argument, 947 * before calling the original method handle 948 * @throws NullPointerException if {@code arrayType} is a null reference 949 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 950 * or if target does not have at least 951 * {@code arrayLength} parameter types, 952 * or if {@code arrayLength} is negative, 953 * or if the resulting method handle's type would have 954 * <a href="MethodHandle.html#maxarity">too many parameters</a> 955 * @throws WrongMethodTypeException if the implied {@code asType} call fails 956 * @see #asCollector 957 */ 958 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) { 959 return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength); 960 } 961 962 /** 963 * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads 964 * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>, 965 * the current method handle. The type of the adapter will be the same as the type of the target, except that the 966 * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos}, 967 * are replaced by a single array parameter of type {@code arrayType}. 968 * <p> 969 * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos} 970 * argument to indicate at which position in the parameter list the spreading should take place. 971 * 972 * @apiNote Example: 973 * <blockquote><pre>{@code 974 MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class)); 975 MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2); 976 Object[] ints = new Object[]{3, 9, 7, 7}; 977 Comparator<Integer> cmp = (a, b) -> a - b; 978 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0); 979 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0); 980 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0); 981 * }</pre></blockquote> 982 * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start. 983 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 984 * @param arrayLength the number of arguments to spread from an incoming array argument 985 * @return a new method handle which spreads an array argument at a given position, 986 * before calling the original method handle 987 * @throws NullPointerException if {@code arrayType} is a null reference 988 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 989 * or if target does not have at least 990 * {@code arrayLength} parameter types, 991 * or if {@code arrayLength} is negative, 992 * or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the 993 * number of arguments), 994 * or if the resulting method handle's type would have 995 * <a href="MethodHandle.html#maxarity">too many parameters</a> 996 * @throws WrongMethodTypeException if the implied {@code asType} call fails 997 * 998 * @see #asSpreader(Class, int) 999 * @since 9 1000 */ 1001 public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) { 1002 MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength); 1003 MethodHandle afterSpread = this.asType(postSpreadType); 1004 BoundMethodHandle mh = afterSpread.rebind(); 1005 LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength); 1006 MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType); 1007 return mh.copyWith(preSpreadType, lform); 1008 } 1009 1010 /** 1011 * See if {@code asSpreader} can be validly called with the given arguments. 1012 * Return the type of the method handle call after spreading but before conversions. 1013 */ 1014 private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) { 1015 spreadArrayChecks(arrayType, arrayLength); 1016 int nargs = type().parameterCount(); 1017 if (nargs < arrayLength || arrayLength < 0) 1018 throw newIllegalArgumentException("bad spread array length"); 1019 if (pos < 0 || pos + arrayLength > nargs) { 1020 throw newIllegalArgumentException("bad spread position"); 1021 } 1022 Class<?> arrayElement = arrayType.getComponentType(); 1023 MethodType mtype = type(); 1024 boolean match = true, fail = false; 1025 for (int i = pos; i < pos + arrayLength; i++) { 1026 Class<?> ptype = mtype.parameterType(i); 1027 if (ptype != arrayElement) { 1028 match = false; 1029 if (!MethodType.canConvert(arrayElement, ptype)) { 1030 fail = true; 1031 break; 1032 } 1033 } 1034 } 1035 if (match) return mtype; 1036 MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength); 1037 if (!fail) return needType; 1038 // elicit an error: 1039 this.asType(needType); 1040 throw newInternalError("should not return"); 1041 } 1042 1043 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) { 1044 Class<?> arrayElement = arrayType.getComponentType(); 1045 if (arrayElement == null) 1046 throw newIllegalArgumentException("not an array type", arrayType); 1047 if ((arrayLength & 0x7F) != arrayLength) { 1048 if ((arrayLength & 0xFF) != arrayLength) 1049 throw newIllegalArgumentException("array length is not legal", arrayLength); 1050 assert(arrayLength >= 128); 1051 if (arrayElement == long.class || 1052 arrayElement == double.class) 1053 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength); 1054 } 1055 } 1056 /** 1057 * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity} 1058 * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}. 1059 * If the method handle is already of the proper arity mode, it is returned 1060 * unchanged. 1061 * @apiNote 1062 * <p>This method is sometimes useful when adapting a method handle that 1063 * may be variable arity, to ensure that the resulting adapter is also 1064 * variable arity if and only if the original handle was. For example, 1065 * this code changes the first argument of a handle {@code mh} to {@code int} without 1066 * disturbing its variable arity property: 1067 * {@code mh.asType(mh.type().changeParameterType(0,int.class)) 1068 * .withVarargs(mh.isVarargsCollector())} 1069 * <p> 1070 * This call is approximately equivalent to the following code: 1071 * <blockquote><pre>{@code 1072 * if (makeVarargs == isVarargsCollector()) 1073 * return this; 1074 * else if (makeVarargs) 1075 * return asVarargsCollector(type().lastParameterType()); 1076 * else 1077 * return return asFixedArity(); 1078 * }</pre></blockquote> 1079 * @param makeVarargs true if the return method handle should have variable arity behavior 1080 * @return a method handle of the same type, with possibly adjusted variable arity behavior 1081 * @throws IllegalArgumentException if {@code makeVarargs} is true and 1082 * this method handle does not have a trailing array parameter 1083 * @since 9 1084 * @see #asVarargsCollector 1085 * @see #asFixedArity 1086 */ 1087 public MethodHandle withVarargs(boolean makeVarargs) { 1088 assert(!isVarargsCollector()); // subclass responsibility 1089 if (makeVarargs) { 1090 return asVarargsCollector(type().lastParameterType()); 1091 } else { 1092 return this; 1093 } 1094 } 1095 1096 /** 1097 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing 1098 * positional arguments and collects them into an array argument. 1099 * The new method handle adapts, as its <i>target</i>, 1100 * the current method handle. The type of the adapter will be 1101 * the same as the type of the target, except that a single trailing 1102 * parameter (usually of type {@code arrayType}) is replaced by 1103 * {@code arrayLength} parameters whose type is element type of {@code arrayType}. 1104 * <p> 1105 * If the array type differs from the final argument type on the original target, 1106 * the original target is adapted to take the array type directly, 1107 * as if by a call to {@link #asType asType}. 1108 * <p> 1109 * When called, the adapter replaces its trailing {@code arrayLength} 1110 * arguments by a single new array of type {@code arrayType}, whose elements 1111 * comprise (in order) the replaced arguments. 1112 * Finally the target is called. 1113 * What the target eventually returns is returned unchanged by the adapter. 1114 * <p> 1115 * (The array may also be a shared constant when {@code arrayLength} is zero.) 1116 * <p> 1117 * (<em>Note:</em> The {@code arrayType} is often identical to the 1118 * {@linkplain MethodType#lastParameterType last parameter type} 1119 * of the original target. 1120 * It is an explicit argument for symmetry with {@code asSpreader}, and also 1121 * to allow the target to use a simple {@code Object} as its last parameter type.) 1122 * <p> 1123 * In order to create a collecting adapter which is not restricted to a particular 1124 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} 1125 * or {@link #withVarargs withVarargs} instead. 1126 * <p> 1127 * Here are some examples of array-collecting method handles: 1128 * <blockquote><pre>{@code 1129 MethodHandle deepToString = publicLookup() 1130 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1131 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"})); 1132 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1); 1133 assertEquals(methodType(String.class, Object.class), ts1.type()); 1134 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL 1135 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"})); 1136 // arrayType can be a subtype of Object[] 1137 MethodHandle ts2 = deepToString.asCollector(String[].class, 2); 1138 assertEquals(methodType(String.class, String.class, String.class), ts2.type()); 1139 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too")); 1140 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0); 1141 assertEquals("[]", (String) ts0.invokeExact()); 1142 // collectors can be nested, Lisp-style 1143 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2); 1144 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D"))); 1145 // arrayType can be any primitive array type 1146 MethodHandle bytesToString = publicLookup() 1147 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class)) 1148 .asCollector(byte[].class, 3); 1149 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3)); 1150 MethodHandle longsToString = publicLookup() 1151 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class)) 1152 .asCollector(long[].class, 1); 1153 assertEquals("[123]", (String) longsToString.invokeExact((long)123)); 1154 * }</pre></blockquote> 1155 * <p> 1156 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1157 * variable-arity method handle}, even if the original target method handle was. 1158 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1159 * @param arrayLength the number of arguments to collect into a new array argument 1160 * @return a new method handle which collects some trailing argument 1161 * into an array, before calling the original method handle 1162 * @throws NullPointerException if {@code arrayType} is a null reference 1163 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1164 * or {@code arrayType} is not assignable to this method handle's trailing parameter type, 1165 * or {@code arrayLength} is not a legal array size, 1166 * or the resulting method handle's type would have 1167 * <a href="MethodHandle.html#maxarity">too many parameters</a> 1168 * @throws WrongMethodTypeException if the implied {@code asType} call fails 1169 * @see #asSpreader 1170 * @see #asVarargsCollector 1171 */ 1172 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) { 1173 return asCollector(type().parameterCount() - 1, arrayType, arrayLength); 1174 } 1175 1176 /** 1177 * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting 1178 * at a given position, and collects them into an array argument. The new method handle adapts, as its 1179 * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target, 1180 * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType}) 1181 * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}. 1182 * <p> 1183 * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code 1184 * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This 1185 * index is zero-based. 1186 * 1187 * @apiNote Examples: 1188 * <blockquote><pre>{@code 1189 StringWriter swr = new StringWriter(); 1190 MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr); 1191 MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4); 1192 swWrite4.invoke('A', 'B', 'C', 'D', 1, 2); 1193 assertEquals("BC", swr.toString()); 1194 swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4); 1195 assertEquals("BCPQRS", swr.toString()); 1196 swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1); 1197 assertEquals("BCPQRSZ", swr.toString()); 1198 * }</pre></blockquote> 1199 * <p> 1200 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1201 * variable-arity method handle}, even if the original target method handle was. 1202 * @param collectArgPos the zero-based position in the parameter list at which to start collecting. 1203 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1204 * @param arrayLength the number of arguments to collect into a new array argument 1205 * @return a new method handle which collects some arguments 1206 * into an array, before calling the original method handle 1207 * @throws NullPointerException if {@code arrayType} is a null reference 1208 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1209 * or {@code arrayType} is not assignable to this method handle's array parameter type, 1210 * or {@code arrayLength} is not a legal array size, 1211 * or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments), 1212 * or the resulting method handle's type would have 1213 * <a href="MethodHandle.html#maxarity">too many parameters</a> 1214 * @throws WrongMethodTypeException if the implied {@code asType} call fails 1215 * 1216 * @see #asCollector(Class, int) 1217 * @since 9 1218 */ 1219 public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) { 1220 asCollectorChecks(arrayType, collectArgPos, arrayLength); 1221 BoundMethodHandle mh = rebind(); 1222 MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength); 1223 MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength); 1224 LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray); 1225 if (lform != null) { 1226 return mh.copyWith(resultType, lform); 1227 } 1228 lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType()); 1229 return mh.copyWithExtendL(resultType, lform, newArray); 1230 } 1231 1232 /** 1233 * See if {@code asCollector} can be validly called with the given arguments. 1234 * Return false if the last parameter is not an exact match to arrayType. 1235 */ 1236 /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) { 1237 spreadArrayChecks(arrayType, arrayLength); 1238 int nargs = type().parameterCount(); 1239 if (pos < 0 || pos >= nargs) { 1240 throw newIllegalArgumentException("bad collect position"); 1241 } 1242 if (nargs != 0) { 1243 Class<?> param = type().parameterType(pos); 1244 if (param == arrayType) return true; 1245 if (param.isAssignableFrom(arrayType)) return false; 1246 } 1247 throw newIllegalArgumentException("array type not assignable to argument", this, arrayType); 1248 } 1249 1250 /** 1251 * Makes a <em>variable arity</em> adapter which is able to accept 1252 * any number of trailing positional arguments and collect them 1253 * into an array argument. 1254 * <p> 1255 * The type and behavior of the adapter will be the same as 1256 * the type and behavior of the target, except that certain 1257 * {@code invoke} and {@code asType} requests can lead to 1258 * trailing positional arguments being collected into target's 1259 * trailing parameter. 1260 * Also, the 1261 * {@linkplain MethodType#lastParameterType last parameter type} 1262 * of the adapter will be 1263 * {@code arrayType}, even if the target has a different 1264 * last parameter type. 1265 * <p> 1266 * This transformation may return {@code this} if the method handle is 1267 * already of variable arity and its trailing parameter type 1268 * is identical to {@code arrayType}. 1269 * <p> 1270 * When called with {@link #invokeExact invokeExact}, the adapter invokes 1271 * the target with no argument changes. 1272 * (<em>Note:</em> This behavior is different from a 1273 * {@linkplain #asCollector fixed arity collector}, 1274 * since it accepts a whole array of indeterminate length, 1275 * rather than a fixed number of arguments.) 1276 * <p> 1277 * When called with plain, inexact {@link #invoke invoke}, if the caller 1278 * type is the same as the adapter, the adapter invokes the target as with 1279 * {@code invokeExact}. 1280 * (This is the normal behavior for {@code invoke} when types match.) 1281 * <p> 1282 * Otherwise, if the caller and adapter arity are the same, and the 1283 * trailing parameter type of the caller is a reference type identical to 1284 * or assignable to the trailing parameter type of the adapter, 1285 * the arguments and return values are converted pairwise, 1286 * as if by {@link #asType asType} on a fixed arity 1287 * method handle. 1288 * <p> 1289 * Otherwise, the arities differ, or the adapter's trailing parameter 1290 * type is not assignable from the corresponding caller type. 1291 * In this case, the adapter replaces all trailing arguments from 1292 * the original trailing argument position onward, by 1293 * a new array of type {@code arrayType}, whose elements 1294 * comprise (in order) the replaced arguments. 1295 * <p> 1296 * The caller type must provides as least enough arguments, 1297 * and of the correct type, to satisfy the target's requirement for 1298 * positional arguments before the trailing array argument. 1299 * Thus, the caller must supply, at a minimum, {@code N-1} arguments, 1300 * where {@code N} is the arity of the target. 1301 * Also, there must exist conversions from the incoming arguments 1302 * to the target's arguments. 1303 * As with other uses of plain {@code invoke}, if these basic 1304 * requirements are not fulfilled, a {@code WrongMethodTypeException} 1305 * may be thrown. 1306 * <p> 1307 * In all cases, what the target eventually returns is returned unchanged by the adapter. 1308 * <p> 1309 * In the final case, it is exactly as if the target method handle were 1310 * temporarily adapted with a {@linkplain #asCollector fixed arity collector} 1311 * to the arity required by the caller type. 1312 * (As with {@code asCollector}, if the array length is zero, 1313 * a shared constant may be used instead of a new array. 1314 * If the implied call to {@code asCollector} would throw 1315 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, 1316 * the call to the variable arity adapter must throw 1317 * {@code WrongMethodTypeException}.) 1318 * <p> 1319 * The behavior of {@link #asType asType} is also specialized for 1320 * variable arity adapters, to maintain the invariant that 1321 * plain, inexact {@code invoke} is always equivalent to an {@code asType} 1322 * call to adjust the target type, followed by {@code invokeExact}. 1323 * Therefore, a variable arity adapter responds 1324 * to an {@code asType} request by building a fixed arity collector, 1325 * if and only if the adapter and requested type differ either 1326 * in arity or trailing argument type. 1327 * The resulting fixed arity collector has its type further adjusted 1328 * (if necessary) to the requested type by pairwise conversion, 1329 * as if by another application of {@code asType}. 1330 * <p> 1331 * When a method handle is obtained by executing an {@code ldc} instruction 1332 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked 1333 * as a variable arity method (with the modifier bit {@code 0x0080}), 1334 * the method handle will accept multiple arities, as if the method handle 1335 * constant were created by means of a call to {@code asVarargsCollector}. 1336 * <p> 1337 * In order to create a collecting adapter which collects a predetermined 1338 * number of arguments, and whose type reflects this predetermined number, 1339 * use {@link #asCollector asCollector} instead. 1340 * <p> 1341 * No method handle transformations produce new method handles with 1342 * variable arity, unless they are documented as doing so. 1343 * Therefore, besides {@code asVarargsCollector} and {@code withVarargs}, 1344 * all methods in {@code MethodHandle} and {@code MethodHandles} 1345 * will return a method handle with fixed arity, 1346 * except in the cases where they are specified to return their original 1347 * operand (e.g., {@code asType} of the method handle's own type). 1348 * <p> 1349 * Calling {@code asVarargsCollector} on a method handle which is already 1350 * of variable arity will produce a method handle with the same type and behavior. 1351 * It may (or may not) return the original variable arity method handle. 1352 * <p> 1353 * Here is an example, of a list-making variable arity method handle: 1354 * <blockquote><pre>{@code 1355 MethodHandle deepToString = publicLookup() 1356 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1357 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); 1358 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); 1359 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); 1360 assertEquals("[won]", (String) ts1.invoke( "won" )); 1361 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); 1362 // findStatic of Arrays.asList(...) produces a variable arity method handle: 1363 MethodHandle asList = publicLookup() 1364 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); 1365 assertEquals(methodType(List.class, Object[].class), asList.type()); 1366 assert(asList.isVarargsCollector()); 1367 assertEquals("[]", asList.invoke().toString()); 1368 assertEquals("[1]", asList.invoke(1).toString()); 1369 assertEquals("[two, too]", asList.invoke("two", "too").toString()); 1370 String[] argv = { "three", "thee", "tee" }; 1371 assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); 1372 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); 1373 List ls = (List) asList.invoke((Object)argv); 1374 assertEquals(1, ls.size()); 1375 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); 1376 * }</pre></blockquote> 1377 * <p style="font-size:smaller;"> 1378 * <em>Discussion:</em> 1379 * These rules are designed as a dynamically-typed variation 1380 * of the Java rules for variable arity methods. 1381 * In both cases, callers to a variable arity method or method handle 1382 * can either pass zero or more positional arguments, or else pass 1383 * pre-collected arrays of any length. Users should be aware of the 1384 * special role of the final argument, and of the effect of a 1385 * type match on that final argument, which determines whether 1386 * or not a single trailing argument is interpreted as a whole 1387 * array or a single element of an array to be collected. 1388 * Note that the dynamic type of the trailing argument has no 1389 * effect on this decision, only a comparison between the symbolic 1390 * type descriptor of the call site and the type descriptor of the method handle.) 1391 * 1392 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1393 * @return a new method handle which can collect any number of trailing arguments 1394 * into an array, before calling the original method handle 1395 * @throws NullPointerException if {@code arrayType} is a null reference 1396 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1397 * or {@code arrayType} is not assignable to this method handle's trailing parameter type 1398 * @see #asCollector 1399 * @see #isVarargsCollector 1400 * @see #withVarargs 1401 * @see #asFixedArity 1402 */ 1403 public MethodHandle asVarargsCollector(Class<?> arrayType) { 1404 Objects.requireNonNull(arrayType); 1405 boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0); 1406 if (isVarargsCollector() && lastMatch) 1407 return this; 1408 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1409 } 1410 1411 /** 1412 * Determines if this method handle 1413 * supports {@linkplain #asVarargsCollector variable arity} calls. 1414 * Such method handles arise from the following sources: 1415 * <ul> 1416 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} 1417 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} 1418 * which resolves to a variable arity Java method or constructor 1419 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} 1420 * which resolves to a variable arity Java method or constructor 1421 * </ul> 1422 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls 1423 * @see #asVarargsCollector 1424 * @see #asFixedArity 1425 */ 1426 public boolean isVarargsCollector() { 1427 return false; 1428 } 1429 1430 /** 1431 * Makes a <em>fixed arity</em> method handle which is otherwise 1432 * equivalent to the current method handle. 1433 * <p> 1434 * If the current method handle is not of 1435 * {@linkplain #asVarargsCollector variable arity}, 1436 * the current method handle is returned. 1437 * This is true even if the current method handle 1438 * could not be a valid input to {@code asVarargsCollector}. 1439 * <p> 1440 * Otherwise, the resulting fixed-arity method handle has the same 1441 * type and behavior of the current method handle, 1442 * except that {@link #isVarargsCollector isVarargsCollector} 1443 * will be false. 1444 * The fixed-arity method handle may (or may not) be the 1445 * a previous argument to {@code asVarargsCollector}. 1446 * <p> 1447 * Here is an example, of a list-making variable arity method handle: 1448 * <blockquote><pre>{@code 1449 MethodHandle asListVar = publicLookup() 1450 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) 1451 .asVarargsCollector(Object[].class); 1452 MethodHandle asListFix = asListVar.asFixedArity(); 1453 assertEquals("[1]", asListVar.invoke(1).toString()); 1454 Exception caught = null; 1455 try { asListFix.invoke((Object)1); } 1456 catch (Exception ex) { caught = ex; } 1457 assert(caught instanceof ClassCastException); 1458 assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); 1459 try { asListFix.invoke("two", "too"); } 1460 catch (Exception ex) { caught = ex; } 1461 assert(caught instanceof WrongMethodTypeException); 1462 Object[] argv = { "three", "thee", "tee" }; 1463 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); 1464 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); 1465 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); 1466 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); 1467 * }</pre></blockquote> 1468 * 1469 * @return a new method handle which accepts only a fixed number of arguments 1470 * @see #asVarargsCollector 1471 * @see #isVarargsCollector 1472 * @see #withVarargs 1473 */ 1474 public MethodHandle asFixedArity() { 1475 assert(!isVarargsCollector()); 1476 return this; 1477 } 1478 1479 /** 1480 * Binds a value {@code x} to the first argument of a method handle, without invoking it. 1481 * The new method handle adapts, as its <i>target</i>, 1482 * the current method handle by binding it to the given argument. 1483 * The type of the bound handle will be 1484 * the same as the type of the target, except that a single leading 1485 * reference parameter will be omitted. 1486 * <p> 1487 * When called, the bound handle inserts the given value {@code x} 1488 * as a new leading argument to the target. The other arguments are 1489 * also passed unchanged. 1490 * What the target eventually returns is returned unchanged by the bound handle. 1491 * <p> 1492 * The reference {@code x} must be convertible to the first parameter 1493 * type of the target. 1494 * <p> 1495 * <em>Note:</em> Because method handles are immutable, the target method handle 1496 * retains its original type and behavior. 1497 * <p> 1498 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1499 * variable-arity method handle}, even if the original target method handle was. 1500 * @param x the value to bind to the first argument of the target 1501 * @return a new method handle which prepends the given value to the incoming 1502 * argument list, before calling the original method handle 1503 * @throws IllegalArgumentException if the target does not have a 1504 * leading parameter type that is a reference type 1505 * @throws ClassCastException if {@code x} cannot be converted 1506 * to the leading parameter type of the target 1507 * @see MethodHandles#insertArguments 1508 */ 1509 public MethodHandle bindTo(Object x) { 1510 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed 1511 return bindArgumentL(0, x); 1512 } 1513 1514 /** 1515 * Returns a string representation of the method handle, 1516 * starting with the string {@code "MethodHandle"} and 1517 * ending with the string representation of the method handle's type. 1518 * In other words, this method returns a string equal to the value of: 1519 * <blockquote><pre>{@code 1520 * "MethodHandle" + type().toString() 1521 * }</pre></blockquote> 1522 * <p> 1523 * (<em>Note:</em> Future releases of this API may add further information 1524 * to the string representation. 1525 * Therefore, the present syntax should not be parsed by applications.) 1526 * 1527 * @return a string representation of the method handle 1528 */ 1529 @Override 1530 public String toString() { 1531 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString(); 1532 return standardString(); 1533 } 1534 String standardString() { 1535 return "MethodHandle"+type; 1536 } 1537 /** Return a string with a several lines describing the method handle structure. 1538 * This string would be suitable for display in an IDE debugger. 1539 */ 1540 String debugString() { 1541 return type+" : "+internalForm()+internalProperties(); 1542 } 1543 1544 //// Implementation methods. 1545 //// Sub-classes can override these default implementations. 1546 //// All these methods assume arguments are already validated. 1547 1548 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch 1549 1550 BoundMethodHandle bindArgumentL(int pos, Object value) { 1551 return rebind().bindArgumentL(pos, value); 1552 } 1553 1554 /*non-public*/ 1555 MethodHandle setVarargs(MemberName member) throws IllegalAccessException { 1556 if (!member.isVarargs()) return this; 1557 try { 1558 return this.withVarargs(true); 1559 } catch (IllegalArgumentException ex) { 1560 throw member.makeAccessException("cannot make variable arity", null); 1561 } 1562 } 1563 1564 /*non-public*/ 1565 MethodHandle viewAsType(MethodType newType, boolean strict) { 1566 // No actual conversions, just a new view of the same method. 1567 // Note that this operation must not produce a DirectMethodHandle, 1568 // because retyped DMHs, like any transformed MHs, 1569 // cannot be cracked into MethodHandleInfo. 1570 assert viewAsTypeChecks(newType, strict); 1571 BoundMethodHandle mh = rebind(); 1572 return mh.copyWith(newType, mh.form); 1573 } 1574 1575 /*non-public*/ 1576 boolean viewAsTypeChecks(MethodType newType, boolean strict) { 1577 if (strict) { 1578 assert(type().isViewableAs(newType, true)) 1579 : Arrays.asList(this, newType); 1580 } else { 1581 assert(type().basicType().isViewableAs(newType.basicType(), true)) 1582 : Arrays.asList(this, newType); 1583 } 1584 return true; 1585 } 1586 1587 // Decoding 1588 1589 /*non-public*/ 1590 LambdaForm internalForm() { 1591 return form; 1592 } 1593 1594 /*non-public*/ 1595 MemberName internalMemberName() { 1596 return null; // DMH returns DMH.member 1597 } 1598 1599 /*non-public*/ 1600 Class<?> internalCallerClass() { 1601 return null; // caller-bound MH for @CallerSensitive method returns caller 1602 } 1603 1604 /*non-public*/ 1605 MethodHandleImpl.Intrinsic intrinsicName() { 1606 // no special intrinsic meaning to most MHs 1607 return MethodHandleImpl.Intrinsic.NONE; 1608 } 1609 1610 /*non-public*/ 1611 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) { 1612 if (member != null) { 1613 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial); 1614 } else if (internalMemberName() == null) { 1615 // The required internaMemberName is null, and this MH (like most) doesn't have one. 1616 return this; 1617 } else { 1618 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. 1619 MethodHandle result = rebind(); 1620 assert (result.internalMemberName() == null); 1621 return result; 1622 } 1623 } 1624 1625 /*non-public*/ 1626 boolean isInvokeSpecial() { 1627 return false; // DMH.Special returns true 1628 } 1629 1630 /*non-public*/ 1631 Object internalValues() { 1632 return null; 1633 } 1634 1635 /*non-public*/ 1636 Object internalProperties() { 1637 // Override to something to follow this.form, like "\n& FOO=bar" 1638 return ""; 1639 } 1640 1641 //// Method handle implementation methods. 1642 //// Sub-classes can override these default implementations. 1643 //// All these methods assume arguments are already validated. 1644 1645 /*non-public*/ 1646 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf); 1647 1648 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH. 1649 * Many transforms are implemented only for BMHs. 1650 * @return a behaviorally equivalent BMH 1651 */ 1652 abstract BoundMethodHandle rebind(); 1653 1654 /** 1655 * Replace the old lambda form of this method handle with a new one. 1656 * The new one must be functionally equivalent to the old one. 1657 * Threads may continue running the old form indefinitely, 1658 * but it is likely that the new one will be preferred for new executions. 1659 * Use with discretion. 1660 */ 1661 /*non-public*/ 1662 void updateForm(LambdaForm newForm) { 1663 assert(newForm.customized == null || newForm.customized == this); 1664 if (form == newForm) return; 1665 newForm.prepare(); // as in MethodHandle.<init> 1666 UNSAFE.putObject(this, FORM_OFFSET, newForm); 1667 UNSAFE.fullFence(); 1668 } 1669 1670 /** Craft a LambdaForm customized for this particular MethodHandle */ 1671 /*non-public*/ 1672 void customize() { 1673 final LambdaForm form = this.form; 1674 if (form.customized == null) { 1675 LambdaForm newForm = form.customize(this); 1676 updateForm(newForm); 1677 } else { 1678 assert(form.customized == this); 1679 } 1680 } 1681 1682 private static final long FORM_OFFSET 1683 = UNSAFE.objectFieldOffset(MethodHandle.class, "form"); 1684 }