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 31 import static java.lang.invoke.MethodHandleStatics.*; 32 33 /** 34 * A method handle is a typed, directly executable reference to an underlying method, 35 * constructor, field, or similar low-level operation, with optional 36 * transformations of arguments or return values. 37 * These transformations are quite general, and include such patterns as 38 * {@linkplain #asType conversion}, 39 * {@linkplain #bindTo insertion}, 40 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion}, 41 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}. 42 * 43 * <h1>Method handle contents</h1> 44 * Method handles are dynamically and strongly typed according to their parameter and return types. 45 * They are not distinguished by the name or the defining class of their underlying methods. 46 * A method handle must be invoked using a symbolic type descriptor which matches 47 * the method handle's own {@linkplain #type type descriptor}. 48 * <p> 49 * Every method handle reports its type descriptor via the {@link #type type} accessor. 50 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object, 51 * whose structure is a series of classes, one of which is 52 * the return type of the method (or {@code void.class} if none). 53 * <p> 54 * A method handle's type controls the types of invocations it accepts, 55 * and the kinds of transformations that apply to it. 56 * <p> 57 * A method handle contains a pair of special invoker methods 58 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}. 59 * Both invoker methods provide direct access to the method handle's 60 * underlying method, constructor, field, or other operation, 61 * as modified by transformations of arguments and return values. 62 * Both invokers accept calls which exactly match the method handle's own type. 63 * The plain, inexact invoker also accepts a range of other call types. 64 * <p> 65 * Method handles are immutable and have no visible state. 66 * Of course, they can be bound to underlying methods or data which exhibit state. 67 * With respect to the Java Memory Model, any method handle will behave 68 * as if all of its (internal) fields are final variables. This means that any method 69 * handle made visible to the application will always be fully formed. 70 * This is true even if the method handle is published through a shared 71 * variable in a data race. 72 * <p> 73 * Method handles cannot be subclassed by the user. 74 * Implementations may (or may not) create internal subclasses of {@code MethodHandle} 75 * which may be visible via the {@link java.lang.Object#getClass Object.getClass} 76 * operation. The programmer should not draw conclusions about a method handle 77 * from its specific class, as the method handle class hierarchy (if any) 78 * may change from time to time or across implementations from different vendors. 79 * 80 * <h1>Method handle compilation</h1> 81 * A Java method call expression naming {@code invokeExact} or {@code invoke} 82 * can invoke a method handle from Java source code. 83 * From the viewpoint of source code, these methods can take any arguments 84 * and their result can be cast to any return type. 85 * Formally this is accomplished by giving the invoker methods 86 * {@code Object} return types and variable arity {@code Object} arguments, 87 * but they have an additional quality called <em>signature polymorphism</em> 88 * which connects this freedom of invocation directly to the JVM execution stack. 89 * <p> 90 * As is usual with virtual methods, source-level calls to {@code invokeExact} 91 * and {@code invoke} compile to an {@code invokevirtual} instruction. 92 * More unusually, the compiler must record the actual argument types, 93 * and may not perform method invocation conversions on the arguments. 94 * Instead, it must push them on the stack according to their own unconverted types. 95 * The method handle object itself is pushed on the stack before the arguments. 96 * The compiler then calls the method handle with a symbolic type descriptor which 97 * describes the argument and return types. 98 * <p> 99 * To issue a complete symbolic type descriptor, the compiler must also determine 100 * the return type. This is based on a cast on the method invocation expression, 101 * if there is one, or else {@code Object} if the invocation is an expression 102 * or else {@code void} if the invocation is a statement. 103 * The cast may be to a primitive type (but not {@code void}). 104 * <p> 105 * As a corner case, an uncasted {@code null} argument is given 106 * a symbolic type descriptor of {@code java.lang.Void}. 107 * The ambiguity with the type {@code Void} is harmless, since there are no references of type 108 * {@code Void} except the null reference. 109 * 110 * <h1>Method handle invocation</h1> 111 * The first time a {@code invokevirtual} instruction is executed 112 * it is linked, by symbolically resolving the names in the instruction 113 * and verifying that the method call is statically legal. 114 * This is true of calls to {@code invokeExact} and {@code invoke}. 115 * In this case, the symbolic type descriptor emitted by the compiler is checked for 116 * correct syntax and names it contains are resolved. 117 * Thus, an {@code invokevirtual} instruction which invokes 118 * a method handle will always link, as long 119 * as the symbolic type descriptor is syntactically well-formed 120 * and the types exist. 121 * <p> 122 * When the {@code invokevirtual} is executed after linking, 123 * the receiving method handle's type is first checked by the JVM 124 * to ensure that it matches the symbolic type descriptor. 125 * If the type match fails, it means that the method which the 126 * caller is invoking is not present on the individual 127 * method handle being invoked. 128 * <p> 129 * In the case of {@code invokeExact}, the type descriptor of the invocation 130 * (after resolving symbolic type names) must exactly match the method type 131 * of the receiving method handle. 132 * In the case of plain, inexact {@code invoke}, the resolved type descriptor 133 * must be a valid argument to the receiver's {@link #asType asType} method. 134 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}. 135 * <p> 136 * After type matching, a call to {@code invokeExact} directly 137 * and immediately invoke the method handle's underlying method 138 * (or other behavior, as the case may be). 139 * <p> 140 * A call to plain {@code invoke} works the same as a call to 141 * {@code invokeExact}, if the symbolic type descriptor specified by the caller 142 * exactly matches the method handle's own type. 143 * If there is a type mismatch, {@code invoke} attempts 144 * to adjust the type of the receiving method handle, 145 * as if by a call to {@link #asType asType}, 146 * to obtain an exactly invokable method handle {@code M2}. 147 * This allows a more powerful negotiation of method type 148 * between caller and callee. 149 * <p> 150 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable, 151 * and implementations are therefore not required to materialize it.) 152 * 153 * <h1>Invocation checking</h1> 154 * In typical programs, method handle type matching will usually succeed. 155 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, 156 * either directly (in the case of {@code invokeExact}) or indirectly as if 157 * by a failed call to {@code asType} (in the case of {@code invoke}). 158 * <p> 159 * Thus, a method type mismatch which might show up as a linkage error 160 * in a statically typed program can show up as 161 * a dynamic {@code WrongMethodTypeException} 162 * in a program which uses method handles. 163 * <p> 164 * Because method types contain "live" {@code Class} objects, 165 * method type matching takes into account both types names and class loaders. 166 * Thus, even if a method handle {@code M} is created in one 167 * class loader {@code L1} and used in another {@code L2}, 168 * method handle calls are type-safe, because the caller's symbolic type 169 * descriptor, as resolved in {@code L2}, 170 * is matched against the original callee method's symbolic type descriptor, 171 * as resolved in {@code L1}. 172 * The resolution in {@code L1} happens when {@code M} is created 173 * and its type is assigned, while the resolution in {@code L2} happens 174 * when the {@code invokevirtual} instruction is linked. 175 * <p> 176 * Apart from the checking of type descriptors, 177 * a method handle's capability to call its underlying method is unrestricted. 178 * If a method handle is formed on a non-public method by a class 179 * that has access to that method, the resulting handle can be used 180 * in any place by any caller who receives a reference to it. 181 * <p> 182 * Unlike with the Core Reflection API, where access is checked every time 183 * a reflective method is invoked, 184 * method handle access checking is performed 185 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>. 186 * In the case of {@code ldc} (see below), access checking is performed as part of linking 187 * the constant pool entry underlying the constant method handle. 188 * <p> 189 * Thus, handles to non-public methods, or to methods in non-public classes, 190 * should generally be kept secret. 191 * They should not be passed to untrusted code unless their use from 192 * the untrusted code would be harmless. 193 * 194 * <h1>Method handle creation</h1> 195 * Java code can create a method handle that directly accesses 196 * any method, constructor, or field that is accessible to that code. 197 * This is done via a reflective, capability-based API called 198 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup} 199 * For example, a static method handle can be obtained 200 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}. 201 * There are also conversion methods from Core Reflection API objects, 202 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 203 * <p> 204 * Like classes and strings, method handles that correspond to accessible 205 * fields, methods, and constructors can also be represented directly 206 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes. 207 * A new type of constant pool entry, {@code CONSTANT_MethodHandle}, 208 * refers directly to an associated {@code CONSTANT_Methodref}, 209 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref} 210 * constant pool entry. 211 * (For full details on method handle constants, 212 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.) 213 * <p> 214 * Method handles produced by lookups or constant loads from methods or 215 * constructors with the variable arity modifier bit ({@code 0x0080}) 216 * have a corresponding variable arity, as if they were defined with 217 * the help of {@link #asVarargsCollector asVarargsCollector}. 218 * <p> 219 * A method reference may refer either to a static or non-static method. 220 * In the non-static case, the method handle type includes an explicit 221 * receiver argument, prepended before any other arguments. 222 * In the method handle's type, the initial receiver argument is typed 223 * according to the class under which the method was initially requested. 224 * (E.g., if a non-static method handle is obtained via {@code ldc}, 225 * the type of the receiver is the class named in the constant pool entry.) 226 * <p> 227 * Method handle constants are subject to the same link-time access checks 228 * their corresponding bytecode instructions, and the {@code ldc} instruction 229 * will throw corresponding linkage errors if the bytecode behaviors would 230 * throw such errors. 231 * <p> 232 * As a corollary of this, access to protected members is restricted 233 * to receivers only of the accessing class, or one of its subclasses, 234 * and the accessing class must in turn be a subclass (or package sibling) 235 * of the protected member's defining class. 236 * If a method reference refers to a protected non-static method or field 237 * of a class outside the current package, the receiver argument will 238 * be narrowed to the type of the accessing class. 239 * <p> 240 * When a method handle to a virtual method is invoked, the method is 241 * always looked up in the receiver (that is, the first argument). 242 * <p> 243 * A non-virtual method handle to a specific virtual method implementation 244 * can also be created. These do not perform virtual lookup based on 245 * receiver type. Such a method handle simulates the effect of 246 * an {@code invokespecial} instruction to the same method. 247 * 248 * <h1>Usage examples</h1> 249 * Here are some examples of usage: 250 * <blockquote><pre>{@code 251 Object x, y; String s; int i; 252 MethodType mt; MethodHandle mh; 253 MethodHandles.Lookup lookup = MethodHandles.lookup(); 254 // mt is (char,char)String 255 mt = MethodType.methodType(String.class, char.class, char.class); 256 mh = lookup.findVirtual(String.class, "replace", mt); 257 s = (String) mh.invokeExact("daddy",'d','n'); 258 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String; 259 assertEquals(s, "nanny"); 260 // weakly typed invocation (using MHs.invoke) 261 s = (String) mh.invokeWithArguments("sappy", 'p', 'v'); 262 assertEquals(s, "savvy"); 263 // mt is (Object[])List 264 mt = MethodType.methodType(java.util.List.class, Object[].class); 265 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt); 266 assert(mh.isVarargsCollector()); 267 x = mh.invoke("one", "two"); 268 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object; 269 assertEquals(x, java.util.Arrays.asList("one","two")); 270 // mt is (Object,Object,Object)Object 271 mt = MethodType.genericMethodType(3); 272 mh = mh.asType(mt); 273 x = mh.invokeExact((Object)1, (Object)2, (Object)3); 274 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object; 275 assertEquals(x, java.util.Arrays.asList(1,2,3)); 276 // mt is ()int 277 mt = MethodType.methodType(int.class); 278 mh = lookup.findVirtual(java.util.List.class, "size", mt); 279 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3)); 280 // invokeExact(Ljava/util/List;)I 281 assert(i == 3); 282 mt = MethodType.methodType(void.class, String.class); 283 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt); 284 mh.invokeExact(System.out, "Hello, world."); 285 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V 286 * }</pre></blockquote> 287 * Each of the above calls to {@code invokeExact} or plain {@code invoke} 288 * generates a single invokevirtual instruction with 289 * the symbolic type descriptor indicated in the following comment. 290 * In these examples, the helper method {@code assertEquals} is assumed to 291 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals} 292 * on its arguments, and asserts that the result is true. 293 * 294 * <h1>Exceptions</h1> 295 * The methods {@code invokeExact} and {@code invoke} are declared 296 * to throw {@link java.lang.Throwable Throwable}, 297 * which is to say that there is no static restriction on what a method handle 298 * can throw. Since the JVM does not distinguish between checked 299 * and unchecked exceptions (other than by their class, of course), 300 * there is no particular effect on bytecode shape from ascribing 301 * checked exceptions to method handle invocations. But in Java source 302 * code, methods which perform method handle calls must either explicitly 303 * throw {@code Throwable}, or else must catch all 304 * throwables locally, rethrowing only those which are legal in the context, 305 * and wrapping ones which are illegal. 306 * 307 * <h1><a name="sigpoly"></a>Signature polymorphism</h1> 308 * The unusual compilation and linkage behavior of 309 * {@code invokeExact} and plain {@code invoke} 310 * is referenced by the term <em>signature polymorphism</em>. 311 * As defined in the Java Language Specification, 312 * a signature polymorphic method is one which can operate with 313 * any of a wide range of call signatures and return types. 314 * <p> 315 * In source code, a call to a signature polymorphic method will 316 * compile, regardless of the requested symbolic type descriptor. 317 * As usual, the Java compiler emits an {@code invokevirtual} 318 * instruction with the given symbolic type descriptor against the named method. 319 * The unusual part is that the symbolic type descriptor is derived from 320 * the actual argument and return types, not from the method declaration. 321 * <p> 322 * When the JVM processes bytecode containing signature polymorphic calls, 323 * it will successfully link any such call, regardless of its symbolic type descriptor. 324 * (In order to retain type safety, the JVM will guard such calls with suitable 325 * dynamic type checks, as described elsewhere.) 326 * <p> 327 * Bytecode generators, including the compiler back end, are required to emit 328 * untransformed symbolic type descriptors for these methods. 329 * Tools which determine symbolic linkage are required to accept such 330 * untransformed descriptors, without reporting linkage errors. 331 * 332 * <h1>Interoperation between method handles and the Core Reflection API</h1> 333 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, 334 * any class member represented by a Core Reflection API object 335 * can be converted to a behaviorally equivalent method handle. 336 * For example, a reflective {@link java.lang.reflect.Method Method} can 337 * be converted to a method handle using 338 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 339 * The resulting method handles generally provide more direct and efficient 340 * access to the underlying class members. 341 * <p> 342 * As a special case, 343 * when the Core Reflection API is used to view the signature polymorphic 344 * methods {@code invokeExact} or plain {@code invoke} in this class, 345 * they appear as ordinary non-polymorphic methods. 346 * Their reflective appearance, as viewed by 347 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, 348 * is unaffected by their special status in this API. 349 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} 350 * will report exactly those modifier bits required for any similarly 351 * declared method, including in this case {@code native} and {@code varargs} bits. 352 * <p> 353 * As with any reflected method, these methods (when reflected) may be 354 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}. 355 * However, such reflective calls do not result in method handle invocations. 356 * Such a call, if passed the required argument 357 * (a single one, of type {@code Object[]}), will ignore the argument and 358 * will throw an {@code UnsupportedOperationException}. 359 * <p> 360 * Since {@code invokevirtual} instructions can natively 361 * invoke method handles under any symbolic type descriptor, this reflective view conflicts 362 * with the normal presentation of these methods via bytecodes. 363 * Thus, these two native methods, when reflectively viewed by 364 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only. 365 * <p> 366 * In order to obtain an invoker method for a particular type descriptor, 367 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker}, 368 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}. 369 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual} 370 * API is also able to return a method handle 371 * to call {@code invokeExact} or plain {@code invoke}, 372 * for any specified type descriptor . 373 * 374 * <h1>Interoperation between method handles and Java generics</h1> 375 * A method handle can be obtained on a method, constructor, or field 376 * which is declared with Java generic types. 377 * As with the Core Reflection API, the type of the method handle 378 * will constructed from the erasure of the source-level type. 379 * When a method handle is invoked, the types of its arguments 380 * or the return value cast type may be generic types or type instances. 381 * If this occurs, the compiler will replace those 382 * types by their erasures when it constructs the symbolic type descriptor 383 * for the {@code invokevirtual} instruction. 384 * <p> 385 * Method handles do not represent 386 * their function-like types in terms of Java parameterized (generic) types, 387 * because there are three mismatches between function-like types and parameterized 388 * Java types. 389 * <ul> 390 * <li>Method types range over all possible arities, 391 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments. 392 * Generics are not variadic, and so cannot represent this.</li> 393 * <li>Method types can specify arguments of primitive types, 394 * which Java generic types cannot range over.</li> 395 * <li>Higher order functions over method handles (combinators) are 396 * often generic across a wide range of function types, including 397 * those of multiple arities. It is impossible to represent such 398 * genericity with a Java type parameter.</li> 399 * </ul> 400 * 401 * <h1><a name="maxarity"></a>Arity limits</h1> 402 * The JVM imposes on all methods and constructors of any kind an absolute 403 * limit of 255 stacked arguments. This limit can appear more restrictive 404 * in certain cases: 405 * <ul> 406 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots. 407 * <li>A non-static method consumes an extra argument for the object on which the method is called. 408 * <li>A constructor consumes an extra argument for the object which is being constructed. 409 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual, 410 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object. 411 * </ul> 412 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments. 413 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it. 414 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}. 415 * In particular, a method handle’s type must not have an arity of the exact maximum 255. 416 * 417 * @see MethodType 418 * @see MethodHandles 419 * @author John Rose, JSR 292 EG 420 */ 421 public abstract class MethodHandle { 422 static { MethodHandleImpl.initStatics(); } 423 424 /** 425 * Internal marker interface which distinguishes (to the Java compiler) 426 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>. 427 */ 428 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD}) 429 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME) 430 @interface PolymorphicSignature { } 431 432 private final MethodType type; 433 /*private*/ final LambdaForm form; 434 // form is not private so that invokers can easily fetch it 435 /*private*/ MethodHandle asTypeCache; 436 // asTypeCache is not private so that invokers can easily fetch it 437 438 /** 439 * Reports the type of this method handle. 440 * Every invocation of this method handle via {@code invokeExact} must exactly match this type. 441 * @return the method handle type 442 */ 443 public MethodType type() { 444 return type; 445 } 446 447 /** 448 * Package-private constructor for the method handle implementation hierarchy. 449 * Method handle inheritance will be contained completely within 450 * the {@code java.lang.invoke} package. 451 */ 452 // @param type type (permanently assigned) of the new method handle 453 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) { 454 type.getClass(); // explicit NPE 455 form.getClass(); // explicit NPE 456 this.type = type; 457 this.form = form; 458 459 form.prepare(); // TO DO: Try to delay this step until just before invocation. 460 } 461 462 /** 463 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match. 464 * The symbolic type descriptor at the call site of {@code invokeExact} must 465 * exactly match this method handle's {@link #type type}. 466 * No conversions are allowed on arguments or return values. 467 * <p> 468 * When this method is observed via the Core Reflection API, 469 * it will appear as a single native method, taking an object array and returning an object. 470 * If this native method is invoked directly via 471 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 472 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 473 * it will throw an {@code UnsupportedOperationException}. 474 * @param args the signature-polymorphic parameter list, statically represented using varargs 475 * @return the signature-polymorphic result, statically represented using {@code Object} 476 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor 477 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 478 */ 479 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable; 480 481 /** 482 * Invokes the method handle, allowing any caller type descriptor, 483 * and optionally performing conversions on arguments and return values. 484 * <p> 485 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type}, 486 * the call proceeds as if by {@link #invokeExact invokeExact}. 487 * <p> 488 * Otherwise, the call proceeds as if this method handle were first 489 * adjusted by calling {@link #asType asType} to adjust this method handle 490 * to the required type, and then the call proceeds as if by 491 * {@link #invokeExact invokeExact} on the adjusted method handle. 492 * <p> 493 * There is no guarantee that the {@code asType} call is actually made. 494 * If the JVM can predict the results of making the call, it may perform 495 * adaptations directly on the caller's arguments, 496 * and call the target method handle according to its own exact type. 497 * <p> 498 * The resolved type descriptor at the call site of {@code invoke} must 499 * be a valid argument to the receivers {@code asType} method. 500 * In particular, the caller must specify the same argument arity 501 * as the callee's type, 502 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}. 503 * <p> 504 * When this method is observed via the Core Reflection API, 505 * it will appear as a single native method, taking an object array and returning an object. 506 * If this native method is invoked directly via 507 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 508 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 509 * it will throw an {@code UnsupportedOperationException}. 510 * @param args the signature-polymorphic parameter list, statically represented using varargs 511 * @return the signature-polymorphic result, statically represented using {@code Object} 512 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor 513 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails 514 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 515 */ 516 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable; 517 518 /** 519 * Private method for trusted invocation of a method handle respecting simplified signatures. 520 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM. 521 * <p> 522 * The caller signature is restricted to the following basic types: 523 * Object, int, long, float, double, and void return. 524 * <p> 525 * The caller is responsible for maintaining type correctness by ensuring 526 * that the each outgoing argument value is a member of the range of the corresponding 527 * callee argument type. 528 * (The caller should therefore issue appropriate casts and integer narrowing 529 * operations on outgoing argument values.) 530 * The caller can assume that the incoming result value is part of the range 531 * of the callee's return type. 532 * @param args the signature-polymorphic parameter list, statically represented using varargs 533 * @return the signature-polymorphic result, statically represented using {@code Object} 534 */ 535 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; 536 537 /** 538 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}. 539 * The caller signature is restricted to basic types as with {@code invokeBasic}. 540 * The trailing (not leading) argument must be a MemberName. 541 * @param args the signature-polymorphic parameter list, statically represented using varargs 542 * @return the signature-polymorphic result, statically represented using {@code Object} 543 */ 544 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable; 545 546 /** 547 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}. 548 * The caller signature is restricted to basic types as with {@code invokeBasic}. 549 * The trailing (not leading) argument must be a MemberName. 550 * @param args the signature-polymorphic parameter list, statically represented using varargs 551 * @return the signature-polymorphic result, statically represented using {@code Object} 552 */ 553 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable; 554 555 /** 556 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}. 557 * The caller signature is restricted to basic types as with {@code invokeBasic}. 558 * The trailing (not leading) argument must be a MemberName. 559 * @param args the signature-polymorphic parameter list, statically represented using varargs 560 * @return the signature-polymorphic result, statically represented using {@code Object} 561 */ 562 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable; 563 564 /** 565 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}. 566 * The caller signature is restricted to basic types as with {@code invokeBasic}. 567 * The trailing (not leading) argument must be a MemberName. 568 * @param args the signature-polymorphic parameter list, statically represented using varargs 569 * @return the signature-polymorphic result, statically represented using {@code Object} 570 */ 571 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; 572 573 /** 574 * Performs a variable arity invocation, passing the arguments in the given list 575 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 576 * which mentions only the type {@code Object}, and whose arity is the length 577 * of the argument list. 578 * <p> 579 * Specifically, execution proceeds as if by the following steps, 580 * although the methods are not guaranteed to be called if the JVM 581 * can predict their effects. 582 * <ul> 583 * <li>Determine the length of the argument array as {@code N}. 584 * For a null reference, {@code N=0}. </li> 585 * <li>Determine the general type {@code TN} of {@code N} arguments as 586 * as {@code TN=MethodType.genericMethodType(N)}.</li> 587 * <li>Force the original target method handle {@code MH0} to the 588 * required type, as {@code MH1 = MH0.asType(TN)}. </li> 589 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li> 590 * <li>Invoke the type-adjusted method handle on the unpacked arguments: 591 * MH1.invokeExact(A0, ...). </li> 592 * <li>Take the return value as an {@code Object} reference. </li> 593 * </ul> 594 * <p> 595 * Because of the action of the {@code asType} step, the following argument 596 * conversions are applied as necessary: 597 * <ul> 598 * <li>reference casting 599 * <li>unboxing 600 * <li>widening primitive conversions 601 * </ul> 602 * <p> 603 * The result returned by the call is boxed if it is a primitive, 604 * or forced to null if the return type is void. 605 * <p> 606 * This call is equivalent to the following code: 607 * <blockquote><pre>{@code 608 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0); 609 * Object result = invoker.invokeExact(this, arguments); 610 * }</pre></blockquote> 611 * <p> 612 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke}, 613 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI. 614 * It can therefore be used as a bridge between native or reflective code and method handles. 615 * 616 * @param arguments the arguments to pass to the target 617 * @return the result returned by the target 618 * @throws ClassCastException if an argument cannot be converted by reference casting 619 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 620 * @throws Throwable anything thrown by the target method invocation 621 * @see MethodHandles#spreadInvoker 622 */ 623 public Object invokeWithArguments(Object... arguments) throws Throwable { 624 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length); 625 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments); 626 } 627 628 /** 629 * Performs a variable arity invocation, passing the arguments in the given array 630 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 631 * which mentions only the type {@code Object}, and whose arity is the length 632 * of the argument array. 633 * <p> 634 * This method is also equivalent to the following code: 635 * <blockquote><pre>{@code 636 * invokeWithArguments(arguments.toArray() 637 * }</pre></blockquote> 638 * 639 * @param arguments the arguments to pass to the target 640 * @return the result returned by the target 641 * @throws NullPointerException if {@code arguments} is a null reference 642 * @throws ClassCastException if an argument cannot be converted by reference casting 643 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 644 * @throws Throwable anything thrown by the target method invocation 645 */ 646 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { 647 return invokeWithArguments(arguments.toArray()); 648 } 649 650 /** 651 * Produces an adapter method handle which adapts the type of the 652 * current method handle to a new type. 653 * The resulting method handle is guaranteed to report a type 654 * which is equal to the desired new type. 655 * <p> 656 * If the original type and new type are equal, returns {@code this}. 657 * <p> 658 * The new method handle, when invoked, will perform the following 659 * steps: 660 * <ul> 661 * <li>Convert the incoming argument list to match the original 662 * method handle's argument list. 663 * <li>Invoke the original method handle on the converted argument list. 664 * <li>Convert any result returned by the original method handle 665 * to the return type of new method handle. 666 * </ul> 667 * <p> 668 * This method provides the crucial behavioral difference between 669 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. 670 * The two methods 671 * perform the same steps when the caller's type descriptor exactly m atches 672 * the callee's, but when the types differ, plain {@link #invoke invoke} 673 * also calls {@code asType} (or some internal equivalent) in order 674 * to match up the caller's and callee's types. 675 * <p> 676 * If the current method is a variable arity method handle 677 * argument list conversion may involve the conversion and collection 678 * of several arguments into an array, as 679 * {@linkplain #asVarargsCollector described elsewhere}. 680 * In every other case, all conversions are applied <em>pairwise</em>, 681 * which means that each argument or return value is converted to 682 * exactly one argument or return value (or no return value). 683 * The applied conversions are defined by consulting the 684 * the corresponding component types of the old and new 685 * method handle types. 686 * <p> 687 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, 688 * or old and new return types. Specifically, for some valid index {@code i}, let 689 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. 690 * Or else, going the other way for return values, let 691 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. 692 * If the types are the same, the new method handle makes no change 693 * to the corresponding argument or return value (if any). 694 * Otherwise, one of the following conversions is applied 695 * if possible: 696 * <ul> 697 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. 698 * (The types do not need to be related in any particular way. 699 * This is because a dynamic value of null can convert to any reference type.) 700 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation 701 * conversion (JLS 5.3) is applied, if one exists. 702 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) 703 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, 704 * a Java casting conversion (JLS 5.5) is applied if one exists. 705 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, 706 * which is then widened as needed to <em>T1</em>.) 707 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing 708 * conversion will be applied at runtime, possibly followed 709 * by a Java method invocation conversion (JLS 5.3) 710 * on the primitive value. (These are the primitive widening conversions.) 711 * <em>T0</em> must be a wrapper class or a supertype of one. 712 * (In the case where <em>T0</em> is Object, these are the conversions 713 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) 714 * The unboxing conversion must have a possibility of success, which means that 715 * if <em>T0</em> is not itself a wrapper class, there must exist at least one 716 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed 717 * primitive value can be widened to <em>T1</em>. 718 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded 719 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. 720 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, 721 * a zero value is introduced. 722 * </ul> 723 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, 724 * because neither corresponds specifically to the <em>dynamic type</em> of any 725 * actual argument or return value.) 726 * <p> 727 * The method handle conversion cannot be made if any one of the required 728 * pairwise conversions cannot be made. 729 * <p> 730 * At runtime, the conversions applied to reference arguments 731 * or return values may require additional runtime checks which can fail. 732 * An unboxing operation may fail because the original reference is null, 733 * causing a {@link java.lang.NullPointerException NullPointerException}. 734 * An unboxing operation or a reference cast may also fail on a reference 735 * to an object of the wrong type, 736 * causing a {@link java.lang.ClassCastException ClassCastException}. 737 * Although an unboxing operation may accept several kinds of wrappers, 738 * if none are available, a {@code ClassCastException} will be thrown. 739 * 740 * @param newType the expected type of the new method handle 741 * @return a method handle which delegates to {@code this} after performing 742 * any necessary argument conversions, and arranges for any 743 * necessary return value conversions 744 * @throws NullPointerException if {@code newType} is a null reference 745 * @throws WrongMethodTypeException if the conversion cannot be made 746 * @see MethodHandles#explicitCastArguments 747 */ 748 public MethodHandle asType(MethodType newType) { 749 // Fast path alternative to a heavyweight {@code asType} call. 750 // Return 'this' if the conversion will be a no-op. 751 if (newType == type) { 752 return this; 753 } 754 // Return 'this.asTypeCache' if the conversion is already memoized. 755 MethodHandle atc = asTypeCache; 756 if (atc != null && newType == atc.type) { 757 return atc; 758 } 759 return asTypeUncached(newType); 760 } 761 762 /** Override this to change asType behavior. */ 763 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) { 764 if (!type.isConvertibleTo(newType)) 765 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType); 766 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, 1); 767 } 768 769 /** 770 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument 771 * and spreads its elements as positional arguments. 772 * The new method handle adapts, as its <i>target</i>, 773 * the current method handle. The type of the adapter will be 774 * the same as the type of the target, except that the final 775 * {@code arrayLength} parameters of the target's type are replaced 776 * by a single array parameter of type {@code arrayType}. 777 * <p> 778 * If the array element type differs from any of the corresponding 779 * argument types on the original target, 780 * the original target is adapted to take the array elements directly, 781 * as if by a call to {@link #asType asType}. 782 * <p> 783 * When called, the adapter replaces a trailing array argument 784 * by the array's elements, each as its own argument to the target. 785 * (The order of the arguments is preserved.) 786 * They are converted pairwise by casting and/or unboxing 787 * to the types of the trailing parameters of the target. 788 * Finally the target is called. 789 * What the target eventually returns is returned unchanged by the adapter. 790 * <p> 791 * Before calling the target, the adapter verifies that the array 792 * contains exactly enough elements to provide a correct argument count 793 * to the target method handle. 794 * (The array may also be null when zero elements are required.) 795 * <p> 796 * If, when the adapter is called, the supplied array argument does 797 * not have the correct number of elements, the adapter will throw 798 * an {@link IllegalArgumentException} instead of invoking the target. 799 * <p> 800 * Here are some simple examples of array-spreading method handles: 801 * <blockquote><pre>{@code 802 MethodHandle equals = publicLookup() 803 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class)); 804 assert( (boolean) equals.invokeExact("me", (Object)"me")); 805 assert(!(boolean) equals.invokeExact("me", (Object)"thee")); 806 // spread both arguments from a 2-array: 807 MethodHandle eq2 = equals.asSpreader(Object[].class, 2); 808 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" })); 809 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" })); 810 // try to spread from anything but a 2-array: 811 for (int n = 0; n <= 10; n++) { 812 Object[] badArityArgs = (n == 2 ? null : new Object[n]); 813 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); } 814 catch (IllegalArgumentException ex) { } // OK 815 } 816 // spread both arguments from a String array: 817 MethodHandle eq2s = equals.asSpreader(String[].class, 2); 818 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" })); 819 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" })); 820 // spread second arguments from a 1-array: 821 MethodHandle eq1 = equals.asSpreader(Object[].class, 1); 822 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" })); 823 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" })); 824 // spread no arguments from a 0-array or null: 825 MethodHandle eq0 = equals.asSpreader(Object[].class, 0); 826 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0])); 827 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null)); 828 // asSpreader and asCollector are approximate inverses: 829 for (int n = 0; n <= 2; n++) { 830 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) { 831 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n); 832 assert( (boolean) equals2.invokeWithArguments("me", "me")); 833 assert(!(boolean) equals2.invokeWithArguments("me", "thee")); 834 } 835 } 836 MethodHandle caToString = publicLookup() 837 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class)); 838 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray())); 839 MethodHandle caString3 = caToString.asCollector(char[].class, 3); 840 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C')); 841 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2); 842 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray())); 843 * }</pre></blockquote> 844 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 845 * @param arrayLength the number of arguments to spread from an incoming array argument 846 * @return a new method handle which spreads its final array argument, 847 * before calling the original method handle 848 * @throws NullPointerException if {@code arrayType} is a null reference 849 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 850 * or if target does not have at least 851 * {@code arrayLength} parameter types, 852 * or if {@code arrayLength} is negative, 853 * or if the resulting method handle's type would have 854 * <a href="MethodHandle.html#maxarity">too many parameters</a> 855 * @throws WrongMethodTypeException if the implied {@code asType} call fails 856 * @see #asCollector 857 */ 858 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) { 859 asSpreaderChecks(arrayType, arrayLength); 860 int spreadArgPos = type.parameterCount() - arrayLength; 861 return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength); 862 } 863 864 private void asSpreaderChecks(Class<?> arrayType, int arrayLength) { 865 spreadArrayChecks(arrayType, arrayLength); 866 int nargs = type().parameterCount(); 867 if (nargs < arrayLength || arrayLength < 0) 868 throw newIllegalArgumentException("bad spread array length"); 869 if (arrayType != Object[].class && arrayLength != 0) { 870 boolean sawProblem = false; 871 Class<?> arrayElement = arrayType.getComponentType(); 872 for (int i = nargs - arrayLength; i < nargs; i++) { 873 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) { 874 sawProblem = true; 875 break; 876 } 877 } 878 if (sawProblem) { 879 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList()); 880 for (int i = nargs - arrayLength; i < nargs; i++) { 881 ptypes.set(i, arrayElement); 882 } 883 // elicit an error: 884 this.asType(MethodType.methodType(type().returnType(), ptypes)); 885 } 886 } 887 } 888 889 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) { 890 Class<?> arrayElement = arrayType.getComponentType(); 891 if (arrayElement == null) 892 throw newIllegalArgumentException("not an array type", arrayType); 893 if ((arrayLength & 0x7F) != arrayLength) { 894 if ((arrayLength & 0xFF) != arrayLength) 895 throw newIllegalArgumentException("array length is not legal", arrayLength); 896 assert(arrayLength >= 128); 897 if (arrayElement == long.class || 898 arrayElement == double.class) 899 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength); 900 } 901 } 902 903 /** 904 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing 905 * positional arguments and collects them into an array argument. 906 * The new method handle adapts, as its <i>target</i>, 907 * the current method handle. The type of the adapter will be 908 * the same as the type of the target, except that a single trailing 909 * parameter (usually of type {@code arrayType}) is replaced by 910 * {@code arrayLength} parameters whose type is element type of {@code arrayType}. 911 * <p> 912 * If the array type differs from the final argument type on the original target, 913 * the original target is adapted to take the array type directly, 914 * as if by a call to {@link #asType asType}. 915 * <p> 916 * When called, the adapter replaces its trailing {@code arrayLength} 917 * arguments by a single new array of type {@code arrayType}, whose elements 918 * comprise (in order) the replaced arguments. 919 * Finally the target is called. 920 * What the target eventually returns is returned unchanged by the adapter. 921 * <p> 922 * (The array may also be a shared constant when {@code arrayLength} is zero.) 923 * <p> 924 * (<em>Note:</em> The {@code arrayType} is often identical to the last 925 * parameter type of the original target. 926 * It is an explicit argument for symmetry with {@code asSpreader}, and also 927 * to allow the target to use a simple {@code Object} as its last parameter type.) 928 * <p> 929 * In order to create a collecting adapter which is not restricted to a particular 930 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead. 931 * <p> 932 * Here are some examples of array-collecting method handles: 933 * <blockquote><pre>{@code 934 MethodHandle deepToString = publicLookup() 935 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 936 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"})); 937 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1); 938 assertEquals(methodType(String.class, Object.class), ts1.type()); 939 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL 940 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"})); 941 // arrayType can be a subtype of Object[] 942 MethodHandle ts2 = deepToString.asCollector(String[].class, 2); 943 assertEquals(methodType(String.class, String.class, String.class), ts2.type()); 944 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too")); 945 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0); 946 assertEquals("[]", (String) ts0.invokeExact()); 947 // collectors can be nested, Lisp-style 948 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2); 949 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D"))); 950 // arrayType can be any primitive array type 951 MethodHandle bytesToString = publicLookup() 952 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class)) 953 .asCollector(byte[].class, 3); 954 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3)); 955 MethodHandle longsToString = publicLookup() 956 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class)) 957 .asCollector(long[].class, 1); 958 assertEquals("[123]", (String) longsToString.invokeExact((long)123)); 959 * }</pre></blockquote> 960 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 961 * @param arrayLength the number of arguments to collect into a new array argument 962 * @return a new method handle which collects some trailing argument 963 * into an array, before calling the original method handle 964 * @throws NullPointerException if {@code arrayType} is a null reference 965 * @throws IllegalArgumentException if {@code arrayType} is not an array type 966 * or {@code arrayType} is not assignable to this method handle's trailing parameter type, 967 * or {@code arrayLength} is not a legal array size, 968 * or the resulting method handle's type would have 969 * <a href="MethodHandle.html#maxarity">too many parameters</a> 970 * @throws WrongMethodTypeException if the implied {@code asType} call fails 971 * @see #asSpreader 972 * @see #asVarargsCollector 973 */ 974 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) { 975 asCollectorChecks(arrayType, arrayLength); 976 int collectArgPos = type().parameterCount()-1; 977 MethodHandle target = this; 978 if (arrayType != type().parameterType(collectArgPos)) 979 target = MethodHandleImpl.makePairwiseConvert(this, type().changeParameterType(collectArgPos, arrayType), 1); 980 MethodHandle collector = MethodHandleImpl.varargsArray(arrayType, arrayLength); 981 return MethodHandles.collectArguments(target, collectArgPos, collector); 982 } 983 984 /** 985 * See if {@code asCollector} can be validly called with the given arguments. 986 * Return false if the last parameter is not an exact match to arrayType. 987 */ 988 /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) { 989 spreadArrayChecks(arrayType, arrayLength); 990 int nargs = type().parameterCount(); 991 if (nargs != 0) { 992 Class<?> lastParam = type().parameterType(nargs-1); 993 if (lastParam == arrayType) return true; 994 if (lastParam.isAssignableFrom(arrayType)) return false; 995 } 996 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType); 997 } 998 999 /** 1000 * Makes a <em>variable arity</em> adapter which is able to accept 1001 * any number of trailing positional arguments and collect them 1002 * into an array argument. 1003 * <p> 1004 * The type and behavior of the adapter will be the same as 1005 * the type and behavior of the target, except that certain 1006 * {@code invoke} and {@code asType} requests can lead to 1007 * trailing positional arguments being collected into target's 1008 * trailing parameter. 1009 * Also, the last parameter type of the adapter will be 1010 * {@code arrayType}, even if the target has a different 1011 * last parameter type. 1012 * <p> 1013 * This transformation may return {@code this} if the method handle is 1014 * already of variable arity and its trailing parameter type 1015 * is identical to {@code arrayType}. 1016 * <p> 1017 * When called with {@link #invokeExact invokeExact}, the adapter invokes 1018 * the target with no argument changes. 1019 * (<em>Note:</em> This behavior is different from a 1020 * {@linkplain #asCollector fixed arity collector}, 1021 * since it accepts a whole array of indeterminate length, 1022 * rather than a fixed number of arguments.) 1023 * <p> 1024 * When called with plain, inexact {@link #invoke invoke}, if the caller 1025 * type is the same as the adapter, the adapter invokes the target as with 1026 * {@code invokeExact}. 1027 * (This is the normal behavior for {@code invoke} when types match.) 1028 * <p> 1029 * Otherwise, if the caller and adapter arity are the same, and the 1030 * trailing parameter type of the caller is a reference type identical to 1031 * or assignable to the trailing parameter type of the adapter, 1032 * the arguments and return values are converted pairwise, 1033 * as if by {@link #asType asType} on a fixed arity 1034 * method handle. 1035 * <p> 1036 * Otherwise, the arities differ, or the adapter's trailing parameter 1037 * type is not assignable from the corresponding caller type. 1038 * In this case, the adapter replaces all trailing arguments from 1039 * the original trailing argument position onward, by 1040 * a new array of type {@code arrayType}, whose elements 1041 * comprise (in order) the replaced arguments. 1042 * <p> 1043 * The caller type must provides as least enough arguments, 1044 * and of the correct type, to satisfy the target's requirement for 1045 * positional arguments before the trailing array argument. 1046 * Thus, the caller must supply, at a minimum, {@code N-1} arguments, 1047 * where {@code N} is the arity of the target. 1048 * Also, there must exist conversions from the incoming arguments 1049 * to the target's arguments. 1050 * As with other uses of plain {@code invoke}, if these basic 1051 * requirements are not fulfilled, a {@code WrongMethodTypeException} 1052 * may be thrown. 1053 * <p> 1054 * In all cases, what the target eventually returns is returned unchanged by the adapter. 1055 * <p> 1056 * In the final case, it is exactly as if the target method handle were 1057 * temporarily adapted with a {@linkplain #asCollector fixed arity collector} 1058 * to the arity required by the caller type. 1059 * (As with {@code asCollector}, if the array length is zero, 1060 * a shared constant may be used instead of a new array. 1061 * If the implied call to {@code asCollector} would throw 1062 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, 1063 * the call to the variable arity adapter must throw 1064 * {@code WrongMethodTypeException}.) 1065 * <p> 1066 * The behavior of {@link #asType asType} is also specialized for 1067 * variable arity adapters, to maintain the invariant that 1068 * plain, inexact {@code invoke} is always equivalent to an {@code asType} 1069 * call to adjust the target type, followed by {@code invokeExact}. 1070 * Therefore, a variable arity adapter responds 1071 * to an {@code asType} request by building a fixed arity collector, 1072 * if and only if the adapter and requested type differ either 1073 * in arity or trailing argument type. 1074 * The resulting fixed arity collector has its type further adjusted 1075 * (if necessary) to the requested type by pairwise conversion, 1076 * as if by another application of {@code asType}. 1077 * <p> 1078 * When a method handle is obtained by executing an {@code ldc} instruction 1079 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked 1080 * as a variable arity method (with the modifier bit {@code 0x0080}), 1081 * the method handle will accept multiple arities, as if the method handle 1082 * constant were created by means of a call to {@code asVarargsCollector}. 1083 * <p> 1084 * In order to create a collecting adapter which collects a predetermined 1085 * number of arguments, and whose type reflects this predetermined number, 1086 * use {@link #asCollector asCollector} instead. 1087 * <p> 1088 * No method handle transformations produce new method handles with 1089 * variable arity, unless they are documented as doing so. 1090 * Therefore, besides {@code asVarargsCollector}, 1091 * all methods in {@code MethodHandle} and {@code MethodHandles} 1092 * will return a method handle with fixed arity, 1093 * except in the cases where they are specified to return their original 1094 * operand (e.g., {@code asType} of the method handle's own type). 1095 * <p> 1096 * Calling {@code asVarargsCollector} on a method handle which is already 1097 * of variable arity will produce a method handle with the same type and behavior. 1098 * It may (or may not) return the original variable arity method handle. 1099 * <p> 1100 * Here is an example, of a list-making variable arity method handle: 1101 * <blockquote><pre>{@code 1102 MethodHandle deepToString = publicLookup() 1103 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1104 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); 1105 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); 1106 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); 1107 assertEquals("[won]", (String) ts1.invoke( "won" )); 1108 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); 1109 // findStatic of Arrays.asList(...) produces a variable arity method handle: 1110 MethodHandle asList = publicLookup() 1111 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); 1112 assertEquals(methodType(List.class, Object[].class), asList.type()); 1113 assert(asList.isVarargsCollector()); 1114 assertEquals("[]", asList.invoke().toString()); 1115 assertEquals("[1]", asList.invoke(1).toString()); 1116 assertEquals("[two, too]", asList.invoke("two", "too").toString()); 1117 String[] argv = { "three", "thee", "tee" }; 1118 assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); 1119 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); 1120 List ls = (List) asList.invoke((Object)argv); 1121 assertEquals(1, ls.size()); 1122 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); 1123 * }</pre></blockquote> 1124 * <p style="font-size:smaller;"> 1125 * <em>Discussion:</em> 1126 * These rules are designed as a dynamically-typed variation 1127 * of the Java rules for variable arity methods. 1128 * In both cases, callers to a variable arity method or method handle 1129 * can either pass zero or more positional arguments, or else pass 1130 * pre-collected arrays of any length. Users should be aware of the 1131 * special role of the final argument, and of the effect of a 1132 * type match on that final argument, which determines whether 1133 * or not a single trailing argument is interpreted as a whole 1134 * array or a single element of an array to be collected. 1135 * Note that the dynamic type of the trailing argument has no 1136 * effect on this decision, only a comparison between the symbolic 1137 * type descriptor of the call site and the type descriptor of the method handle.) 1138 * 1139 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1140 * @return a new method handle which can collect any number of trailing arguments 1141 * into an array, before calling the original method handle 1142 * @throws NullPointerException if {@code arrayType} is a null reference 1143 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1144 * or {@code arrayType} is not assignable to this method handle's trailing parameter type 1145 * @see #asCollector 1146 * @see #isVarargsCollector 1147 * @see #asFixedArity 1148 */ 1149 public MethodHandle asVarargsCollector(Class<?> arrayType) { 1150 arrayType.getClass(); // explicit NPE 1151 boolean lastMatch = asCollectorChecks(arrayType, 0); 1152 if (isVarargsCollector() && lastMatch) 1153 return this; 1154 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1155 } 1156 1157 /** 1158 * Determines if this method handle 1159 * supports {@linkplain #asVarargsCollector variable arity} calls. 1160 * Such method handles arise from the following sources: 1161 * <ul> 1162 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} 1163 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} 1164 * which resolves to a variable arity Java method or constructor 1165 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} 1166 * which resolves to a variable arity Java method or constructor 1167 * </ul> 1168 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls 1169 * @see #asVarargsCollector 1170 * @see #asFixedArity 1171 */ 1172 public boolean isVarargsCollector() { 1173 return false; 1174 } 1175 1176 /** 1177 * Makes a <em>fixed arity</em> method handle which is otherwise 1178 * equivalent to the current method handle. 1179 * <p> 1180 * If the current method handle is not of 1181 * {@linkplain #asVarargsCollector variable arity}, 1182 * the current method handle is returned. 1183 * This is true even if the current method handle 1184 * could not be a valid input to {@code asVarargsCollector}. 1185 * <p> 1186 * Otherwise, the resulting fixed-arity method handle has the same 1187 * type and behavior of the current method handle, 1188 * except that {@link #isVarargsCollector isVarargsCollector} 1189 * will be false. 1190 * The fixed-arity method handle may (or may not) be the 1191 * a previous argument to {@code asVarargsCollector}. 1192 * <p> 1193 * Here is an example, of a list-making variable arity method handle: 1194 * <blockquote><pre>{@code 1195 MethodHandle asListVar = publicLookup() 1196 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) 1197 .asVarargsCollector(Object[].class); 1198 MethodHandle asListFix = asListVar.asFixedArity(); 1199 assertEquals("[1]", asListVar.invoke(1).toString()); 1200 Exception caught = null; 1201 try { asListFix.invoke((Object)1); } 1202 catch (Exception ex) { caught = ex; } 1203 assert(caught instanceof ClassCastException); 1204 assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); 1205 try { asListFix.invoke("two", "too"); } 1206 catch (Exception ex) { caught = ex; } 1207 assert(caught instanceof WrongMethodTypeException); 1208 Object[] argv = { "three", "thee", "tee" }; 1209 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); 1210 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); 1211 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); 1212 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); 1213 * }</pre></blockquote> 1214 * 1215 * @return a new method handle which accepts only a fixed number of arguments 1216 * @see #asVarargsCollector 1217 * @see #isVarargsCollector 1218 */ 1219 public MethodHandle asFixedArity() { 1220 assert(!isVarargsCollector()); 1221 return this; 1222 } 1223 1224 /** 1225 * Binds a value {@code x} to the first argument of a method handle, without invoking it. 1226 * The new method handle adapts, as its <i>target</i>, 1227 * the current method handle by binding it to the given argument. 1228 * The type of the bound handle will be 1229 * the same as the type of the target, except that a single leading 1230 * reference parameter will be omitted. 1231 * <p> 1232 * When called, the bound handle inserts the given value {@code x} 1233 * as a new leading argument to the target. The other arguments are 1234 * also passed unchanged. 1235 * What the target eventually returns is returned unchanged by the bound handle. 1236 * <p> 1237 * The reference {@code x} must be convertible to the first parameter 1238 * type of the target. 1239 * <p> 1240 * (<em>Note:</em> Because method handles are immutable, the target method handle 1241 * retains its original type and behavior.) 1242 * @param x the value to bind to the first argument of the target 1243 * @return a new method handle which prepends the given value to the incoming 1244 * argument list, before calling the original method handle 1245 * @throws IllegalArgumentException if the target does not have a 1246 * leading parameter type that is a reference type 1247 * @throws ClassCastException if {@code x} cannot be converted 1248 * to the leading parameter type of the target 1249 * @see MethodHandles#insertArguments 1250 */ 1251 public MethodHandle bindTo(Object x) { 1252 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed 1253 return bindArgumentL(0, x); 1254 } 1255 1256 /** 1257 * Returns a string representation of the method handle, 1258 * starting with the string {@code "MethodHandle"} and 1259 * ending with the string representation of the method handle's type. 1260 * In other words, this method returns a string equal to the value of: 1261 * <blockquote><pre>{@code 1262 * "MethodHandle" + type().toString() 1263 * }</pre></blockquote> 1264 * <p> 1265 * (<em>Note:</em> Future releases of this API may add further information 1266 * to the string representation. 1267 * Therefore, the present syntax should not be parsed by applications.) 1268 * 1269 * @return a string representation of the method handle 1270 */ 1271 @Override 1272 public String toString() { 1273 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString(); 1274 return standardString(); 1275 } 1276 String standardString() { 1277 return "MethodHandle"+type; 1278 } 1279 /** Return a string with a several lines describing the method handle structure. 1280 * This string would be suitable for display in an IDE debugger. 1281 */ 1282 String debugString() { 1283 return type+" : "+internalForm()+internalProperties(); 1284 } 1285 1286 //// Implementation methods. 1287 //// Sub-classes can override these default implementations. 1288 //// All these methods assume arguments are already validated. 1289 1290 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch 1291 1292 BoundMethodHandle bindArgumentL(int pos, Object value) { 1293 return rebind().bindArgumentL(pos, value); 1294 } 1295 1296 /*non-public*/ 1297 MethodHandle setVarargs(MemberName member) throws IllegalAccessException { 1298 if (!member.isVarargs()) return this; 1299 Class<?> arrayType = type().lastParameterType(); 1300 if (arrayType.isArray()) { 1301 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1302 } 1303 throw member.makeAccessException("cannot make variable arity", null); 1304 } 1305 1306 /*non-public*/ 1307 MethodHandle viewAsType(MethodType newType, boolean strict) { 1308 // No actual conversions, just a new view of the same method. 1309 // Note that this operation must not produce a DirectMethodHandle, 1310 // because retyped DMHs, like any transformed MHs, 1311 // cannot be cracked into MethodHandleInfo. 1312 assert viewAsTypeChecks(newType, strict); 1313 BoundMethodHandle mh = rebind(); 1314 assert(!((MethodHandle)mh instanceof DirectMethodHandle)); 1315 return mh.copyWith(newType, mh.form); 1316 } 1317 1318 /*non-public*/ 1319 boolean viewAsTypeChecks(MethodType newType, boolean strict) { 1320 if (strict) { 1321 assert(type().isViewableAs(newType, true)) 1322 : Arrays.asList(this, newType); 1323 } else { 1324 assert(type().basicType().isViewableAs(newType.basicType(), true)) 1325 : Arrays.asList(this, newType); 1326 } 1327 return true; 1328 } 1329 1330 // Decoding 1331 1332 /*non-public*/ 1333 LambdaForm internalForm() { 1334 return form; 1335 } 1336 1337 /*non-public*/ 1338 MemberName internalMemberName() { 1339 return null; // DMH returns DMH.member 1340 } 1341 1342 /*non-public*/ 1343 Class<?> internalCallerClass() { 1344 return null; // caller-bound MH for @CallerSensitive method returns caller 1345 } 1346 1347 /*non-public*/ 1348 MethodHandleImpl.Intrinsic intrinsicName() { 1349 // no special intrinsic meaning to most MHs 1350 return MethodHandleImpl.Intrinsic.NONE; 1351 } 1352 1353 /*non-public*/ 1354 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) { 1355 if (member != null) { 1356 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial); 1357 } else if (internalMemberName() == null) { 1358 // The required internaMemberName is null, and this MH (like most) doesn't have one. 1359 return this; 1360 } else { 1361 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. 1362 MethodHandle result = rebind(); 1363 assert (result.internalMemberName() == null); 1364 return result; 1365 } 1366 } 1367 1368 /*non-public*/ 1369 boolean isInvokeSpecial() { 1370 return false; // DMH.Special returns true 1371 } 1372 1373 /*non-public*/ 1374 Object internalValues() { 1375 return null; 1376 } 1377 1378 /*non-public*/ 1379 Object internalProperties() { 1380 // Override to something to follow this.form, like "\n& FOO=bar" 1381 return ""; 1382 } 1383 1384 //// Method handle implementation methods. 1385 //// Sub-classes can override these default implementations. 1386 //// All these methods assume arguments are already validated. 1387 1388 /*non-public*/ 1389 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf); 1390 1391 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH. 1392 * Many transforms are implemented only for BMHs. 1393 * @return a behaviorally equivalent BMH 1394 */ 1395 abstract BoundMethodHandle rebind(); 1396 1397 /** 1398 * Replace the old lambda form of this method handle with a new one. 1399 * The new one must be functionally equivalent to the old one. 1400 * Threads may continue running the old form indefinitely, 1401 * but it is likely that the new one will be preferred for new executions. 1402 * Use with discretion. 1403 */ 1404 /*non-public*/ 1405 void updateForm(LambdaForm newForm) { 1406 if (form == newForm) return; 1407 assert(this instanceof DirectMethodHandle && this.internalMemberName().isStatic()); 1408 // ISSUE: Should we have a memory fence here? 1409 UNSAFE.putObject(this, FORM_OFFSET, newForm); 1410 this.form.prepare(); // as in MethodHandle.<init> 1411 } 1412 1413 private static final long FORM_OFFSET; 1414 static { 1415 try { 1416 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form")); 1417 } catch (ReflectiveOperationException ex) { 1418 throw newInternalError(ex); 1419 } 1420 } 1421 }