1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 38 import java.io.Serializable; 39 import java.lang.invoke.MethodHandles; 40 import java.lang.invoke.VarHandle; 41 import java.lang.ref.ReferenceQueue; 42 import java.lang.ref.WeakReference; 43 import java.lang.reflect.Constructor; 44 import java.util.Collection; 45 import java.util.List; 46 import java.util.RandomAccess; 47 import java.util.concurrent.locks.ReentrantLock; 48 49 // BEGIN android-note 50 // removed java 9 code 51 // END android-note 52 53 /** 54 * Abstract base class for tasks that run within a {@link ForkJoinPool}. 55 * A {@code ForkJoinTask} is a thread-like entity that is much 56 * lighter weight than a normal thread. Huge numbers of tasks and 57 * subtasks may be hosted by a small number of actual threads in a 58 * ForkJoinPool, at the price of some usage limitations. 59 * 60 * <p>A "main" {@code ForkJoinTask} begins execution when it is 61 * explicitly submitted to a {@link ForkJoinPool}, or, if not already 62 * engaged in a ForkJoin computation, commenced in the {@link 63 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or 64 * related methods. Once started, it will usually in turn start other 65 * subtasks. As indicated by the name of this class, many programs 66 * using {@code ForkJoinTask} employ only methods {@link #fork} and 67 * {@link #join}, or derivatives such as {@link 68 * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also 69 * provides a number of other methods that can come into play in 70 * advanced usages, as well as extension mechanics that allow support 71 * of new forms of fork/join processing. 72 * 73 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. 74 * The efficiency of {@code ForkJoinTask}s stems from a set of 75 * restrictions (that are only partially statically enforceable) 76 * reflecting their main use as computational tasks calculating pure 77 * functions or operating on purely isolated objects. The primary 78 * coordination mechanisms are {@link #fork}, that arranges 79 * asynchronous execution, and {@link #join}, that doesn't proceed 80 * until the task's result has been computed. Computations should 81 * ideally avoid {@code synchronized} methods or blocks, and should 82 * minimize other blocking synchronization apart from joining other 83 * tasks or using synchronizers such as Phasers that are advertised to 84 * cooperate with fork/join scheduling. Subdividable tasks should also 85 * not perform blocking I/O, and should ideally access variables that 86 * are completely independent of those accessed by other running 87 * tasks. These guidelines are loosely enforced by not permitting 88 * checked exceptions such as {@code IOExceptions} to be 89 * thrown. However, computations may still encounter unchecked 90 * exceptions, that are rethrown to callers attempting to join 91 * them. These exceptions may additionally include {@link 92 * RejectedExecutionException} stemming from internal resource 93 * exhaustion, such as failure to allocate internal task 94 * queues. Rethrown exceptions behave in the same way as regular 95 * exceptions, but, when possible, contain stack traces (as displayed 96 * for example using {@code ex.printStackTrace()}) of both the thread 97 * that initiated the computation as well as the thread actually 98 * encountering the exception; minimally only the latter. 99 * 100 * <p>It is possible to define and use ForkJoinTasks that may block, 101 * but doing so requires three further considerations: (1) Completion 102 * of few if any <em>other</em> tasks should be dependent on a task 103 * that blocks on external synchronization or I/O. Event-style async 104 * tasks that are never joined (for example, those subclassing {@link 105 * CountedCompleter}) often fall into this category. (2) To minimize 106 * resource impact, tasks should be small; ideally performing only the 107 * (possibly) blocking action. (3) Unless the {@link 108 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly 109 * blocked tasks is known to be less than the pool's {@link 110 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that 111 * enough threads will be available to ensure progress or good 112 * performance. 113 * 114 * <p>The primary method for awaiting completion and extracting 115 * results of a task is {@link #join}, but there are several variants: 116 * The {@link Future#get} methods support interruptible and/or timed 117 * waits for completion and report results using {@code Future} 118 * conventions. Method {@link #invoke} is semantically 119 * equivalent to {@code fork(); join()} but always attempts to begin 120 * execution in the current thread. The "<em>quiet</em>" forms of 121 * these methods do not extract results or report exceptions. These 122 * may be useful when a set of tasks are being executed, and you need 123 * to delay processing of results or exceptions until all complete. 124 * Method {@code invokeAll} (available in multiple versions) 125 * performs the most common form of parallel invocation: forking a set 126 * of tasks and joining them all. 127 * 128 * <p>In the most typical usages, a fork-join pair act like a call 129 * (fork) and return (join) from a parallel recursive function. As is 130 * the case with other forms of recursive calls, returns (joins) 131 * should be performed innermost-first. For example, {@code a.fork(); 132 * b.fork(); b.join(); a.join();} is likely to be substantially more 133 * efficient than joining {@code a} before {@code b}. 134 * 135 * <p>The execution status of tasks may be queried at several levels 136 * of detail: {@link #isDone} is true if a task completed in any way 137 * (including the case where a task was cancelled without executing); 138 * {@link #isCompletedNormally} is true if a task completed without 139 * cancellation or encountering an exception; {@link #isCancelled} is 140 * true if the task was cancelled (in which case {@link #getException} 141 * returns a {@link CancellationException}); and 142 * {@link #isCompletedAbnormally} is true if a task was either 143 * cancelled or encountered an exception, in which case {@link 144 * #getException} will return either the encountered exception or 145 * {@link CancellationException}. 146 * 147 * <p>The ForkJoinTask class is not usually directly subclassed. 148 * Instead, you subclass one of the abstract classes that support a 149 * particular style of fork/join processing, typically {@link 150 * RecursiveAction} for most computations that do not return results, 151 * {@link RecursiveTask} for those that do, and {@link 152 * CountedCompleter} for those in which completed actions trigger 153 * other actions. Normally, a concrete ForkJoinTask subclass declares 154 * fields comprising its parameters, established in a constructor, and 155 * then defines a {@code compute} method that somehow uses the control 156 * methods supplied by this base class. 157 * 158 * <p>Method {@link #join} and its variants are appropriate for use 159 * only when completion dependencies are acyclic; that is, the 160 * parallel computation can be described as a directed acyclic graph 161 * (DAG). Otherwise, executions may encounter a form of deadlock as 162 * tasks cyclically wait for each other. However, this framework 163 * supports other methods and techniques (for example the use of 164 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that 165 * may be of use in constructing custom subclasses for problems that 166 * are not statically structured as DAGs. To support such usages, a 167 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short} 168 * value using {@link #setForkJoinTaskTag} or {@link 169 * #compareAndSetForkJoinTaskTag} and checked using {@link 170 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use 171 * these {@code protected} methods or tags for any purpose, but they 172 * may be of use in the construction of specialized subclasses. For 173 * example, parallel graph traversals can use the supplied methods to 174 * avoid revisiting nodes/tasks that have already been processed. 175 * (Method names for tagging are bulky in part to encourage definition 176 * of methods that reflect their usage patterns.) 177 * 178 * <p>Most base support methods are {@code final}, to prevent 179 * overriding of implementations that are intrinsically tied to the 180 * underlying lightweight task scheduling framework. Developers 181 * creating new basic styles of fork/join processing should minimally 182 * implement {@code protected} methods {@link #exec}, {@link 183 * #setRawResult}, and {@link #getRawResult}, while also introducing 184 * an abstract computational method that can be implemented in its 185 * subclasses, possibly relying on other {@code protected} methods 186 * provided by this class. 187 * 188 * <p>ForkJoinTasks should perform relatively small amounts of 189 * computation. Large tasks should be split into smaller subtasks, 190 * usually via recursive decomposition. As a very rough rule of thumb, 191 * a task should perform more than 100 and less than 10000 basic 192 * computational steps, and should avoid indefinite looping. If tasks 193 * are too big, then parallelism cannot improve throughput. If too 194 * small, then memory and internal task maintenance overhead may 195 * overwhelm processing. 196 * 197 * <p>This class provides {@code adapt} methods for {@link Runnable} 198 * and {@link Callable}, that may be of use when mixing execution of 199 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are 200 * of this form, consider using a pool constructed in <em>asyncMode</em>. 201 * 202 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be 203 * used in extensions such as remote execution frameworks. It is 204 * sensible to serialize tasks only before or after, but not during, 205 * execution. Serialization is not relied on during execution itself. 206 * 207 * @since 1.7 208 * @author Doug Lea 209 */ 210 public abstract class ForkJoinTask<V> implements Future<V>, Serializable { 211 212 /* 213 * See the internal documentation of class ForkJoinPool for a 214 * general implementation overview. ForkJoinTasks are mainly 215 * responsible for maintaining their "status" field amidst relays 216 * to methods in ForkJoinWorkerThread and ForkJoinPool. 217 * 218 * The methods of this class are more-or-less layered into 219 * (1) basic status maintenance 220 * (2) execution and awaiting completion 221 * (3) user-level methods that additionally report results. 222 * This is sometimes hard to see because this file orders exported 223 * methods in a way that flows well in javadocs. 224 */ 225 226 /** 227 * The status field holds run control status bits packed into a 228 * single int to ensure atomicity. Status is initially zero, and 229 * takes on nonnegative values until completed, upon which it 230 * holds (sign bit) DONE, possibly with ABNORMAL (cancelled or 231 * exceptional) and THROWN (in which case an exception has been 232 * stored). Tasks with dependent blocked waiting joiners have the 233 * SIGNAL bit set. Completion of a task with SIGNAL set awakens 234 * any waiters via notifyAll. (Waiters also help signal others 235 * upon completion.) 236 * 237 * These control bits occupy only (some of) the upper half (16 238 * bits) of status field. The lower bits are used for user-defined 239 * tags. 240 */ 241 volatile int status; // accessed directly by pool and workers 242 243 private static final int DONE = 1 << 31; // must be negative 244 private static final int ABNORMAL = 1 << 18; // set atomically with DONE 245 private static final int THROWN = 1 << 17; // set atomically with ABNORMAL 246 private static final int SIGNAL = 1 << 16; // true if joiner waiting 247 private static final int SMASK = 0xffff; // short bits for tags 248 isExceptionalStatus(int s)249 static boolean isExceptionalStatus(int s) { // needed by subclasses 250 return (s & THROWN) != 0; 251 } 252 253 /** 254 * Sets DONE status and wakes up threads waiting to join this task. 255 * 256 * @return status on exit 257 */ setDone()258 private int setDone() { 259 int s; 260 if (((s = (int)STATUS.getAndBitwiseOr(this, DONE)) & SIGNAL) != 0) 261 synchronized (this) { notifyAll(); } 262 return s | DONE; 263 } 264 265 /** 266 * Marks cancelled or exceptional completion unless already done. 267 * 268 * @param completion must be DONE | ABNORMAL, ORed with THROWN if exceptional 269 * @return status on exit 270 */ abnormalCompletion(int completion)271 private int abnormalCompletion(int completion) { 272 for (int s, ns;;) { 273 if ((s = status) < 0) 274 return s; 275 else if (STATUS.weakCompareAndSet(this, s, ns = s | completion)) { 276 if ((s & SIGNAL) != 0) 277 synchronized (this) { notifyAll(); } 278 return ns; 279 } 280 } 281 } 282 283 /** 284 * Primary execution method for stolen tasks. Unless done, calls 285 * exec and records status if completed, but doesn't wait for 286 * completion otherwise. 287 * 288 * @return status on exit from this method 289 */ doExec()290 final int doExec() { 291 int s; boolean completed; 292 if ((s = status) >= 0) { 293 try { 294 completed = exec(); 295 } catch (Throwable rex) { 296 completed = false; 297 s = setExceptionalCompletion(rex); 298 } 299 if (completed) 300 s = setDone(); 301 } 302 return s; 303 } 304 305 /** 306 * If not done, sets SIGNAL status and performs Object.wait(timeout). 307 * This task may or may not be done on exit. Ignores interrupts. 308 * 309 * @param timeout using Object.wait conventions. 310 */ internalWait(long timeout)311 final void internalWait(long timeout) { 312 if ((int)STATUS.getAndBitwiseOr(this, SIGNAL) >= 0) { 313 synchronized (this) { 314 if (status >= 0) 315 try { wait(timeout); } catch (InterruptedException ie) { } 316 else 317 notifyAll(); 318 } 319 } 320 } 321 322 /** 323 * Blocks a non-worker-thread until completion. 324 * @return status upon completion 325 */ externalAwaitDone()326 private int externalAwaitDone() { 327 int s = tryExternalHelp(); 328 if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) { 329 boolean interrupted = false; 330 synchronized (this) { 331 for (;;) { 332 if ((s = status) >= 0) { 333 try { 334 wait(0L); 335 } catch (InterruptedException ie) { 336 interrupted = true; 337 } 338 } 339 else { 340 notifyAll(); 341 break; 342 } 343 } 344 } 345 if (interrupted) 346 Thread.currentThread().interrupt(); 347 } 348 return s; 349 } 350 351 /** 352 * Blocks a non-worker-thread until completion or interruption. 353 */ externalInterruptibleAwaitDone()354 private int externalInterruptibleAwaitDone() throws InterruptedException { 355 int s = tryExternalHelp(); 356 if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) { 357 synchronized (this) { 358 for (;;) { 359 if ((s = status) >= 0) 360 wait(0L); 361 else { 362 notifyAll(); 363 break; 364 } 365 } 366 } 367 } 368 else if (Thread.interrupted()) 369 throw new InterruptedException(); 370 return s; 371 } 372 373 /** 374 * Tries to help with tasks allowed for external callers. 375 * 376 * @return current status 377 */ tryExternalHelp()378 private int tryExternalHelp() { 379 int s; 380 return ((s = status) < 0 ? s: 381 (this instanceof CountedCompleter) ? 382 ForkJoinPool.common.externalHelpComplete( 383 (CountedCompleter<?>)this, 0) : 384 ForkJoinPool.common.tryExternalUnpush(this) ? 385 doExec() : 0); 386 } 387 388 /** 389 * Implementation for join, get, quietlyJoin. Directly handles 390 * only cases of already-completed, external wait, and 391 * unfork+exec. Others are relayed to ForkJoinPool.awaitJoin. 392 * 393 * @return status upon completion 394 */ doJoin()395 private int doJoin() { 396 int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w; 397 return (s = status) < 0 ? s : 398 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 399 (w = (wt = (ForkJoinWorkerThread)t).workQueue). 400 tryUnpush(this) && (s = doExec()) < 0 ? s : 401 wt.pool.awaitJoin(w, this, 0L) : 402 externalAwaitDone(); 403 } 404 405 /** 406 * Implementation for invoke, quietlyInvoke. 407 * 408 * @return status upon completion 409 */ doInvoke()410 private int doInvoke() { 411 int s; Thread t; ForkJoinWorkerThread wt; 412 return (s = doExec()) < 0 ? s : 413 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 414 (wt = (ForkJoinWorkerThread)t).pool. 415 awaitJoin(wt.workQueue, this, 0L) : 416 externalAwaitDone(); 417 } 418 419 // Exception table support 420 421 /** 422 * Hash table of exceptions thrown by tasks, to enable reporting 423 * by callers. Because exceptions are rare, we don't directly keep 424 * them with task objects, but instead use a weak ref table. Note 425 * that cancellation exceptions don't appear in the table, but are 426 * instead recorded as status values. 427 * 428 * The exception table has a fixed capacity. 429 */ 430 private static final ExceptionNode[] exceptionTable 431 = new ExceptionNode[32]; 432 433 /** Lock protecting access to exceptionTable. */ 434 private static final ReentrantLock exceptionTableLock 435 = new ReentrantLock(); 436 437 /** Reference queue of stale exceptionally completed tasks. */ 438 private static final ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue 439 = new ReferenceQueue<>(); 440 441 /** 442 * Key-value nodes for exception table. The chained hash table 443 * uses identity comparisons, full locking, and weak references 444 * for keys. The table has a fixed capacity because it only 445 * maintains task exceptions long enough for joiners to access 446 * them, so should never become very large for sustained 447 * periods. However, since we do not know when the last joiner 448 * completes, we must use weak references and expunge them. We do 449 * so on each operation (hence full locking). Also, some thread in 450 * any ForkJoinPool will call helpExpungeStaleExceptions when its 451 * pool becomes isQuiescent. 452 */ 453 static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> { 454 final Throwable ex; 455 ExceptionNode next; 456 final long thrower; // use id not ref to avoid weak cycles 457 final int hashCode; // store task hashCode before weak ref disappears ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next, ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue)458 ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next, 459 ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue) { 460 super(task, exceptionTableRefQueue); 461 this.ex = ex; 462 this.next = next; 463 this.thrower = Thread.currentThread().getId(); 464 this.hashCode = System.identityHashCode(task); 465 } 466 } 467 468 /** 469 * Records exception and sets status. 470 * 471 * @return status on exit 472 */ recordExceptionalCompletion(Throwable ex)473 final int recordExceptionalCompletion(Throwable ex) { 474 int s; 475 if ((s = status) >= 0) { 476 int h = System.identityHashCode(this); 477 final ReentrantLock lock = exceptionTableLock; 478 lock.lock(); 479 try { 480 expungeStaleExceptions(); 481 ExceptionNode[] t = exceptionTable; 482 int i = h & (t.length - 1); 483 for (ExceptionNode e = t[i]; ; e = e.next) { 484 if (e == null) { 485 t[i] = new ExceptionNode(this, ex, t[i], 486 exceptionTableRefQueue); 487 break; 488 } 489 if (e.get() == this) // already present 490 break; 491 } 492 } finally { 493 lock.unlock(); 494 } 495 s = abnormalCompletion(DONE | ABNORMAL | THROWN); 496 } 497 return s; 498 } 499 500 /** 501 * Records exception and possibly propagates. 502 * 503 * @return status on exit 504 */ setExceptionalCompletion(Throwable ex)505 private int setExceptionalCompletion(Throwable ex) { 506 int s = recordExceptionalCompletion(ex); 507 if ((s & THROWN) != 0) 508 internalPropagateException(ex); 509 return s; 510 } 511 512 /** 513 * Hook for exception propagation support for tasks with completers. 514 */ internalPropagateException(Throwable ex)515 void internalPropagateException(Throwable ex) { 516 } 517 518 /** 519 * Cancels, ignoring any exceptions thrown by cancel. Used during 520 * worker and pool shutdown. Cancel is spec'ed not to throw any 521 * exceptions, but if it does anyway, we have no recourse during 522 * shutdown, so guard against this case. 523 */ cancelIgnoringExceptions(ForkJoinTask<?> t)524 static final void cancelIgnoringExceptions(ForkJoinTask<?> t) { 525 if (t != null && t.status >= 0) { 526 try { 527 t.cancel(false); 528 } catch (Throwable ignore) { 529 } 530 } 531 } 532 533 /** 534 * Removes exception node and clears status. 535 */ clearExceptionalCompletion()536 private void clearExceptionalCompletion() { 537 int h = System.identityHashCode(this); 538 final ReentrantLock lock = exceptionTableLock; 539 lock.lock(); 540 try { 541 ExceptionNode[] t = exceptionTable; 542 int i = h & (t.length - 1); 543 ExceptionNode e = t[i]; 544 ExceptionNode pred = null; 545 while (e != null) { 546 ExceptionNode next = e.next; 547 if (e.get() == this) { 548 if (pred == null) 549 t[i] = next; 550 else 551 pred.next = next; 552 break; 553 } 554 pred = e; 555 e = next; 556 } 557 expungeStaleExceptions(); 558 status = 0; 559 } finally { 560 lock.unlock(); 561 } 562 } 563 564 /** 565 * Returns a rethrowable exception for this task, if available. 566 * To provide accurate stack traces, if the exception was not 567 * thrown by the current thread, we try to create a new exception 568 * of the same type as the one thrown, but with the recorded 569 * exception as its cause. If there is no such constructor, we 570 * instead try to use a no-arg constructor, followed by initCause, 571 * to the same effect. If none of these apply, or any fail due to 572 * other exceptions, we return the recorded exception, which is 573 * still correct, although it may contain a misleading stack 574 * trace. 575 * 576 * @return the exception, or null if none 577 */ getThrowableException()578 private Throwable getThrowableException() { 579 int h = System.identityHashCode(this); 580 ExceptionNode e; 581 final ReentrantLock lock = exceptionTableLock; 582 lock.lock(); 583 try { 584 expungeStaleExceptions(); 585 ExceptionNode[] t = exceptionTable; 586 e = t[h & (t.length - 1)]; 587 while (e != null && e.get() != this) 588 e = e.next; 589 } finally { 590 lock.unlock(); 591 } 592 Throwable ex; 593 if (e == null || (ex = e.ex) == null) 594 return null; 595 if (e.thrower != Thread.currentThread().getId()) { 596 try { 597 Constructor<?> noArgCtor = null; 598 // public ctors only 599 for (Constructor<?> c : ex.getClass().getConstructors()) { 600 Class<?>[] ps = c.getParameterTypes(); 601 if (ps.length == 0) 602 noArgCtor = c; 603 else if (ps.length == 1 && ps[0] == Throwable.class) 604 return (Throwable)c.newInstance(ex); 605 } 606 if (noArgCtor != null) { 607 Throwable wx = (Throwable)noArgCtor.newInstance(); 608 wx.initCause(ex); 609 return wx; 610 } 611 } catch (Exception ignore) { 612 } 613 } 614 return ex; 615 } 616 617 /** 618 * Polls stale refs and removes them. Call only while holding lock. 619 */ expungeStaleExceptions()620 private static void expungeStaleExceptions() { 621 for (Object x; (x = exceptionTableRefQueue.poll()) != null;) { 622 if (x instanceof ExceptionNode) { 623 ExceptionNode[] t = exceptionTable; 624 int i = ((ExceptionNode)x).hashCode & (t.length - 1); 625 ExceptionNode e = t[i]; 626 ExceptionNode pred = null; 627 while (e != null) { 628 ExceptionNode next = e.next; 629 if (e == x) { 630 if (pred == null) 631 t[i] = next; 632 else 633 pred.next = next; 634 break; 635 } 636 pred = e; 637 e = next; 638 } 639 } 640 } 641 } 642 643 /** 644 * If lock is available, polls stale refs and removes them. 645 * Called from ForkJoinPool when pools become quiescent. 646 */ helpExpungeStaleExceptions()647 static final void helpExpungeStaleExceptions() { 648 final ReentrantLock lock = exceptionTableLock; 649 if (lock.tryLock()) { 650 try { 651 expungeStaleExceptions(); 652 } finally { 653 lock.unlock(); 654 } 655 } 656 } 657 658 /** 659 * A version of "sneaky throw" to relay exceptions. 660 */ rethrow(Throwable ex)661 static void rethrow(Throwable ex) { 662 ForkJoinTask.<RuntimeException>uncheckedThrow(ex); 663 } 664 665 /** 666 * The sneaky part of sneaky throw, relying on generics 667 * limitations to evade compiler complaints about rethrowing 668 * unchecked exceptions. 669 */ 670 @SuppressWarnings("unchecked") static <T extends Throwable> uncheckedThrow(Throwable t)671 void uncheckedThrow(Throwable t) throws T { 672 if (t != null) 673 throw (T)t; // rely on vacuous cast 674 else 675 throw new Error("Unknown Exception"); 676 } 677 678 /** 679 * Throws exception, if any, associated with the given status. 680 */ reportException(int s)681 private void reportException(int s) { 682 rethrow((s & THROWN) != 0 ? getThrowableException() : 683 new CancellationException()); 684 } 685 686 // public methods 687 688 /** 689 * Arranges to asynchronously execute this task in the pool the 690 * current task is running in, if applicable, or using the {@link 691 * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}. While 692 * it is not necessarily enforced, it is a usage error to fork a 693 * task more than once unless it has completed and been 694 * reinitialized. Subsequent modifications to the state of this 695 * task or any data it operates on are not necessarily 696 * consistently observable by any thread other than the one 697 * executing it unless preceded by a call to {@link #join} or 698 * related methods, or a call to {@link #isDone} returning {@code 699 * true}. 700 * 701 * @return {@code this}, to simplify usage 702 */ fork()703 public final ForkJoinTask<V> fork() { 704 Thread t; 705 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) 706 ((ForkJoinWorkerThread)t).workQueue.push(this); 707 else 708 ForkJoinPool.common.externalPush(this); 709 return this; 710 } 711 712 /** 713 * Returns the result of the computation when it 714 * {@linkplain #isDone is done}. 715 * This method differs from {@link #get()} in that abnormal 716 * completion results in {@code RuntimeException} or {@code Error}, 717 * not {@code ExecutionException}, and that interrupts of the 718 * calling thread do <em>not</em> cause the method to abruptly 719 * return by throwing {@code InterruptedException}. 720 * 721 * @return the computed result 722 */ join()723 public final V join() { 724 int s; 725 if (((s = doJoin()) & ABNORMAL) != 0) 726 reportException(s); 727 return getRawResult(); 728 } 729 730 /** 731 * Commences performing this task, awaits its completion if 732 * necessary, and returns its result, or throws an (unchecked) 733 * {@code RuntimeException} or {@code Error} if the underlying 734 * computation did so. 735 * 736 * @return the computed result 737 */ invoke()738 public final V invoke() { 739 int s; 740 if (((s = doInvoke()) & ABNORMAL) != 0) 741 reportException(s); 742 return getRawResult(); 743 } 744 745 /** 746 * Forks the given tasks, returning when {@code isDone} holds for 747 * each task or an (unchecked) exception is encountered, in which 748 * case the exception is rethrown. If more than one task 749 * encounters an exception, then this method throws any one of 750 * these exceptions. If any task encounters an exception, the 751 * other may be cancelled. However, the execution status of 752 * individual tasks is not guaranteed upon exceptional return. The 753 * status of each task may be obtained using {@link 754 * #getException()} and related methods to check if they have been 755 * cancelled, completed normally or exceptionally, or left 756 * unprocessed. 757 * 758 * @param t1 the first task 759 * @param t2 the second task 760 * @throws NullPointerException if any task is null 761 */ invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2)762 public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) { 763 int s1, s2; 764 t2.fork(); 765 if (((s1 = t1.doInvoke()) & ABNORMAL) != 0) 766 t1.reportException(s1); 767 if (((s2 = t2.doJoin()) & ABNORMAL) != 0) 768 t2.reportException(s2); 769 } 770 771 /** 772 * Forks the given tasks, returning when {@code isDone} holds for 773 * each task or an (unchecked) exception is encountered, in which 774 * case the exception is rethrown. If more than one task 775 * encounters an exception, then this method throws any one of 776 * these exceptions. If any task encounters an exception, others 777 * may be cancelled. However, the execution status of individual 778 * tasks is not guaranteed upon exceptional return. The status of 779 * each task may be obtained using {@link #getException()} and 780 * related methods to check if they have been cancelled, completed 781 * normally or exceptionally, or left unprocessed. 782 * 783 * @param tasks the tasks 784 * @throws NullPointerException if any task is null 785 */ invokeAll(ForkJoinTask<?>.... tasks)786 public static void invokeAll(ForkJoinTask<?>... tasks) { 787 Throwable ex = null; 788 int last = tasks.length - 1; 789 for (int i = last; i >= 0; --i) { 790 ForkJoinTask<?> t = tasks[i]; 791 if (t == null) { 792 if (ex == null) 793 ex = new NullPointerException(); 794 } 795 else if (i != 0) 796 t.fork(); 797 else if ((t.doInvoke() & ABNORMAL) != 0 && ex == null) 798 ex = t.getException(); 799 } 800 for (int i = 1; i <= last; ++i) { 801 ForkJoinTask<?> t = tasks[i]; 802 if (t != null) { 803 if (ex != null) 804 t.cancel(false); 805 else if ((t.doJoin() & ABNORMAL) != 0) 806 ex = t.getException(); 807 } 808 } 809 if (ex != null) 810 rethrow(ex); 811 } 812 813 /** 814 * Forks all tasks in the specified collection, returning when 815 * {@code isDone} holds for each task or an (unchecked) exception 816 * is encountered, in which case the exception is rethrown. If 817 * more than one task encounters an exception, then this method 818 * throws any one of these exceptions. If any task encounters an 819 * exception, others may be cancelled. However, the execution 820 * status of individual tasks is not guaranteed upon exceptional 821 * return. The status of each task may be obtained using {@link 822 * #getException()} and related methods to check if they have been 823 * cancelled, completed normally or exceptionally, or left 824 * unprocessed. 825 * 826 * @param tasks the collection of tasks 827 * @param <T> the type of the values returned from the tasks 828 * @return the tasks argument, to simplify usage 829 * @throws NullPointerException if tasks or any element are null 830 */ invokeAll(Collection<T> tasks)831 public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) { 832 if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) { 833 invokeAll(tasks.toArray(new ForkJoinTask<?>[0])); 834 return tasks; 835 } 836 @SuppressWarnings("unchecked") 837 List<? extends ForkJoinTask<?>> ts = 838 (List<? extends ForkJoinTask<?>>) tasks; 839 Throwable ex = null; 840 int last = ts.size() - 1; 841 for (int i = last; i >= 0; --i) { 842 ForkJoinTask<?> t = ts.get(i); 843 if (t == null) { 844 if (ex == null) 845 ex = new NullPointerException(); 846 } 847 else if (i != 0) 848 t.fork(); 849 else if ((t.doInvoke() & ABNORMAL) != 0 && ex == null) 850 ex = t.getException(); 851 } 852 for (int i = 1; i <= last; ++i) { 853 ForkJoinTask<?> t = ts.get(i); 854 if (t != null) { 855 if (ex != null) 856 t.cancel(false); 857 else if ((t.doJoin() & ABNORMAL) != 0) 858 ex = t.getException(); 859 } 860 } 861 if (ex != null) 862 rethrow(ex); 863 return tasks; 864 } 865 866 /** 867 * Attempts to cancel execution of this task. This attempt will 868 * fail if the task has already completed or could not be 869 * cancelled for some other reason. If successful, and this task 870 * has not started when {@code cancel} is called, execution of 871 * this task is suppressed. After this method returns 872 * successfully, unless there is an intervening call to {@link 873 * #reinitialize}, subsequent calls to {@link #isCancelled}, 874 * {@link #isDone}, and {@code cancel} will return {@code true} 875 * and calls to {@link #join} and related methods will result in 876 * {@code CancellationException}. 877 * 878 * <p>This method may be overridden in subclasses, but if so, must 879 * still ensure that these properties hold. In particular, the 880 * {@code cancel} method itself must not throw exceptions. 881 * 882 * <p>This method is designed to be invoked by <em>other</em> 883 * tasks. To terminate the current task, you can just return or 884 * throw an unchecked exception from its computation method, or 885 * invoke {@link #completeExceptionally(Throwable)}. 886 * 887 * @param mayInterruptIfRunning this value has no effect in the 888 * default implementation because interrupts are not used to 889 * control cancellation. 890 * 891 * @return {@code true} if this task is now cancelled 892 */ cancel(boolean mayInterruptIfRunning)893 public boolean cancel(boolean mayInterruptIfRunning) { 894 int s = abnormalCompletion(DONE | ABNORMAL); 895 return (s & (ABNORMAL | THROWN)) == ABNORMAL; 896 } 897 isDone()898 public final boolean isDone() { 899 return status < 0; 900 } 901 isCancelled()902 public final boolean isCancelled() { 903 return (status & (ABNORMAL | THROWN)) == ABNORMAL; 904 } 905 906 /** 907 * Returns {@code true} if this task threw an exception or was cancelled. 908 * 909 * @return {@code true} if this task threw an exception or was cancelled 910 */ isCompletedAbnormally()911 public final boolean isCompletedAbnormally() { 912 return (status & ABNORMAL) != 0; 913 } 914 915 /** 916 * Returns {@code true} if this task completed without throwing an 917 * exception and was not cancelled. 918 * 919 * @return {@code true} if this task completed without throwing an 920 * exception and was not cancelled 921 */ isCompletedNormally()922 public final boolean isCompletedNormally() { 923 return (status & (DONE | ABNORMAL)) == DONE; 924 } 925 926 /** 927 * Returns the exception thrown by the base computation, or a 928 * {@code CancellationException} if cancelled, or {@code null} if 929 * none or if the method has not yet completed. 930 * 931 * @return the exception, or {@code null} if none 932 */ getException()933 public final Throwable getException() { 934 int s = status; 935 return ((s & ABNORMAL) == 0 ? null : 936 (s & THROWN) == 0 ? new CancellationException() : 937 getThrowableException()); 938 } 939 940 /** 941 * Completes this task abnormally, and if not already aborted or 942 * cancelled, causes it to throw the given exception upon 943 * {@code join} and related operations. This method may be used 944 * to induce exceptions in asynchronous tasks, or to force 945 * completion of tasks that would not otherwise complete. Its use 946 * in other situations is discouraged. This method is 947 * overridable, but overridden versions must invoke {@code super} 948 * implementation to maintain guarantees. 949 * 950 * @param ex the exception to throw. If this exception is not a 951 * {@code RuntimeException} or {@code Error}, the actual exception 952 * thrown will be a {@code RuntimeException} with cause {@code ex}. 953 */ completeExceptionally(Throwable ex)954 public void completeExceptionally(Throwable ex) { 955 setExceptionalCompletion((ex instanceof RuntimeException) || 956 (ex instanceof Error) ? ex : 957 new RuntimeException(ex)); 958 } 959 960 /** 961 * Completes this task, and if not already aborted or cancelled, 962 * returning the given value as the result of subsequent 963 * invocations of {@code join} and related operations. This method 964 * may be used to provide results for asynchronous tasks, or to 965 * provide alternative handling for tasks that would not otherwise 966 * complete normally. Its use in other situations is 967 * discouraged. This method is overridable, but overridden 968 * versions must invoke {@code super} implementation to maintain 969 * guarantees. 970 * 971 * @param value the result value for this task 972 */ complete(V value)973 public void complete(V value) { 974 try { 975 setRawResult(value); 976 } catch (Throwable rex) { 977 setExceptionalCompletion(rex); 978 return; 979 } 980 setDone(); 981 } 982 983 /** 984 * Completes this task normally without setting a value. The most 985 * recent value established by {@link #setRawResult} (or {@code 986 * null} by default) will be returned as the result of subsequent 987 * invocations of {@code join} and related operations. 988 * 989 * @since 1.8 990 */ quietlyComplete()991 public final void quietlyComplete() { 992 setDone(); 993 } 994 995 /** 996 * Waits if necessary for the computation to complete, and then 997 * retrieves its result. 998 * 999 * @return the computed result 1000 * @throws CancellationException if the computation was cancelled 1001 * @throws ExecutionException if the computation threw an 1002 * exception 1003 * @throws InterruptedException if the current thread is not a 1004 * member of a ForkJoinPool and was interrupted while waiting 1005 */ get()1006 public final V get() throws InterruptedException, ExecutionException { 1007 int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? 1008 doJoin() : externalInterruptibleAwaitDone(); 1009 if ((s & THROWN) != 0) 1010 throw new ExecutionException(getThrowableException()); 1011 else if ((s & ABNORMAL) != 0) 1012 throw new CancellationException(); 1013 else 1014 return getRawResult(); 1015 } 1016 1017 /** 1018 * Waits if necessary for at most the given time for the computation 1019 * to complete, and then retrieves its result, if available. 1020 * 1021 * @param timeout the maximum time to wait 1022 * @param unit the time unit of the timeout argument 1023 * @return the computed result 1024 * @throws CancellationException if the computation was cancelled 1025 * @throws ExecutionException if the computation threw an 1026 * exception 1027 * @throws InterruptedException if the current thread is not a 1028 * member of a ForkJoinPool and was interrupted while waiting 1029 * @throws TimeoutException if the wait timed out 1030 */ get(long timeout, TimeUnit unit)1031 public final V get(long timeout, TimeUnit unit) 1032 throws InterruptedException, ExecutionException, TimeoutException { 1033 int s; 1034 long nanos = unit.toNanos(timeout); 1035 if (Thread.interrupted()) 1036 throw new InterruptedException(); 1037 if ((s = status) >= 0 && nanos > 0L) { 1038 long d = System.nanoTime() + nanos; 1039 long deadline = (d == 0L) ? 1L : d; // avoid 0 1040 Thread t = Thread.currentThread(); 1041 if (t instanceof ForkJoinWorkerThread) { 1042 ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t; 1043 s = wt.pool.awaitJoin(wt.workQueue, this, deadline); 1044 } 1045 else if ((s = ((this instanceof CountedCompleter) ? 1046 ForkJoinPool.common.externalHelpComplete( 1047 (CountedCompleter<?>)this, 0) : 1048 ForkJoinPool.common.tryExternalUnpush(this) ? 1049 doExec() : 0)) >= 0) { 1050 long ns, ms; // measure in nanosecs, but wait in millisecs 1051 while ((s = status) >= 0 && 1052 (ns = deadline - System.nanoTime()) > 0L) { 1053 if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L && 1054 (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) { 1055 synchronized (this) { 1056 if (status >= 0) 1057 wait(ms); // OK to throw InterruptedException 1058 else 1059 notifyAll(); 1060 } 1061 } 1062 } 1063 } 1064 } 1065 if (s >= 0) 1066 throw new TimeoutException(); 1067 else if ((s & THROWN) != 0) 1068 throw new ExecutionException(getThrowableException()); 1069 else if ((s & ABNORMAL) != 0) 1070 throw new CancellationException(); 1071 else 1072 return getRawResult(); 1073 } 1074 1075 /** 1076 * Joins this task, without returning its result or throwing its 1077 * exception. This method may be useful when processing 1078 * collections of tasks when some have been cancelled or otherwise 1079 * known to have aborted. 1080 */ quietlyJoin()1081 public final void quietlyJoin() { 1082 doJoin(); 1083 } 1084 1085 /** 1086 * Commences performing this task and awaits its completion if 1087 * necessary, without returning its result or throwing its 1088 * exception. 1089 */ quietlyInvoke()1090 public final void quietlyInvoke() { 1091 doInvoke(); 1092 } 1093 1094 /** 1095 * Possibly executes tasks until the pool hosting the current task 1096 * {@linkplain ForkJoinPool#isQuiescent is quiescent}. This 1097 * method may be of use in designs in which many tasks are forked, 1098 * but none are explicitly joined, instead executing them until 1099 * all are processed. 1100 */ helpQuiesce()1101 public static void helpQuiesce() { 1102 Thread t; 1103 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) { 1104 ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t; 1105 wt.pool.helpQuiescePool(wt.workQueue); 1106 } 1107 else 1108 ForkJoinPool.quiesceCommonPool(); 1109 } 1110 1111 /** 1112 * Resets the internal bookkeeping state of this task, allowing a 1113 * subsequent {@code fork}. This method allows repeated reuse of 1114 * this task, but only if reuse occurs when this task has either 1115 * never been forked, or has been forked, then completed and all 1116 * outstanding joins of this task have also completed. Effects 1117 * under any other usage conditions are not guaranteed. 1118 * This method may be useful when executing 1119 * pre-constructed trees of subtasks in loops. 1120 * 1121 * <p>Upon completion of this method, {@code isDone()} reports 1122 * {@code false}, and {@code getException()} reports {@code 1123 * null}. However, the value returned by {@code getRawResult} is 1124 * unaffected. To clear this value, you can invoke {@code 1125 * setRawResult(null)}. 1126 */ reinitialize()1127 public void reinitialize() { 1128 if ((status & THROWN) != 0) 1129 clearExceptionalCompletion(); 1130 else 1131 status = 0; 1132 } 1133 1134 /** 1135 * Returns the pool hosting the current thread, or {@code null} 1136 * if the current thread is executing outside of any ForkJoinPool. 1137 * 1138 * <p>This method returns {@code null} if and only if {@link 1139 * #inForkJoinPool} returns {@code false}. 1140 * 1141 * @return the pool, or {@code null} if none 1142 */ getPool()1143 public static ForkJoinPool getPool() { 1144 Thread t = Thread.currentThread(); 1145 return (t instanceof ForkJoinWorkerThread) ? 1146 ((ForkJoinWorkerThread) t).pool : null; 1147 } 1148 1149 /** 1150 * Returns {@code true} if the current thread is a {@link 1151 * ForkJoinWorkerThread} executing as a ForkJoinPool computation. 1152 * 1153 * @return {@code true} if the current thread is a {@link 1154 * ForkJoinWorkerThread} executing as a ForkJoinPool computation, 1155 * or {@code false} otherwise 1156 */ inForkJoinPool()1157 public static boolean inForkJoinPool() { 1158 return Thread.currentThread() instanceof ForkJoinWorkerThread; 1159 } 1160 1161 /** 1162 * Tries to unschedule this task for execution. This method will 1163 * typically (but is not guaranteed to) succeed if this task is 1164 * the most recently forked task by the current thread, and has 1165 * not commenced executing in another thread. This method may be 1166 * useful when arranging alternative local processing of tasks 1167 * that could have been, but were not, stolen. 1168 * 1169 * @return {@code true} if unforked 1170 */ tryUnfork()1171 public boolean tryUnfork() { 1172 Thread t; 1173 return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 1174 ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) : 1175 ForkJoinPool.common.tryExternalUnpush(this)); 1176 } 1177 1178 /** 1179 * Returns an estimate of the number of tasks that have been 1180 * forked by the current worker thread but not yet executed. This 1181 * value may be useful for heuristic decisions about whether to 1182 * fork other tasks. 1183 * 1184 * @return the number of tasks 1185 */ getQueuedTaskCount()1186 public static int getQueuedTaskCount() { 1187 Thread t; ForkJoinPool.WorkQueue q; 1188 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) 1189 q = ((ForkJoinWorkerThread)t).workQueue; 1190 else 1191 q = ForkJoinPool.commonSubmitterQueue(); 1192 return (q == null) ? 0 : q.queueSize(); 1193 } 1194 1195 /** 1196 * Returns an estimate of how many more locally queued tasks are 1197 * held by the current worker thread than there are other worker 1198 * threads that might steal them, or zero if this thread is not 1199 * operating in a ForkJoinPool. This value may be useful for 1200 * heuristic decisions about whether to fork other tasks. In many 1201 * usages of ForkJoinTasks, at steady state, each worker should 1202 * aim to maintain a small constant surplus (for example, 3) of 1203 * tasks, and to process computations locally if this threshold is 1204 * exceeded. 1205 * 1206 * @return the surplus number of tasks, which may be negative 1207 */ getSurplusQueuedTaskCount()1208 public static int getSurplusQueuedTaskCount() { 1209 return ForkJoinPool.getSurplusQueuedTaskCount(); 1210 } 1211 1212 // Extension methods 1213 1214 /** 1215 * Returns the result that would be returned by {@link #join}, even 1216 * if this task completed abnormally, or {@code null} if this task 1217 * is not known to have been completed. This method is designed 1218 * to aid debugging, as well as to support extensions. Its use in 1219 * any other context is discouraged. 1220 * 1221 * @return the result, or {@code null} if not completed 1222 */ getRawResult()1223 public abstract V getRawResult(); 1224 1225 /** 1226 * Forces the given value to be returned as a result. This method 1227 * is designed to support extensions, and should not in general be 1228 * called otherwise. 1229 * 1230 * @param value the value 1231 */ setRawResult(V value)1232 protected abstract void setRawResult(V value); 1233 1234 /** 1235 * Immediately performs the base action of this task and returns 1236 * true if, upon return from this method, this task is guaranteed 1237 * to have completed normally. This method may return false 1238 * otherwise, to indicate that this task is not necessarily 1239 * complete (or is not known to be complete), for example in 1240 * asynchronous actions that require explicit invocations of 1241 * completion methods. This method may also throw an (unchecked) 1242 * exception to indicate abnormal exit. This method is designed to 1243 * support extensions, and should not in general be called 1244 * otherwise. 1245 * 1246 * @return {@code true} if this task is known to have completed normally 1247 */ exec()1248 protected abstract boolean exec(); 1249 1250 /** 1251 * Returns, but does not unschedule or execute, a task queued by 1252 * the current thread but not yet executed, if one is immediately 1253 * available. There is no guarantee that this task will actually 1254 * be polled or executed next. Conversely, this method may return 1255 * null even if a task exists but cannot be accessed without 1256 * contention with other threads. This method is designed 1257 * primarily to support extensions, and is unlikely to be useful 1258 * otherwise. 1259 * 1260 * @return the next task, or {@code null} if none are available 1261 */ peekNextLocalTask()1262 protected static ForkJoinTask<?> peekNextLocalTask() { 1263 Thread t; ForkJoinPool.WorkQueue q; 1264 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) 1265 q = ((ForkJoinWorkerThread)t).workQueue; 1266 else 1267 q = ForkJoinPool.commonSubmitterQueue(); 1268 return (q == null) ? null : q.peek(); 1269 } 1270 1271 /** 1272 * Unschedules and returns, without executing, the next task 1273 * queued by the current thread but not yet executed, if the 1274 * current thread is operating in a ForkJoinPool. This method is 1275 * designed primarily to support extensions, and is unlikely to be 1276 * useful otherwise. 1277 * 1278 * @return the next task, or {@code null} if none are available 1279 */ pollNextLocalTask()1280 protected static ForkJoinTask<?> pollNextLocalTask() { 1281 Thread t; 1282 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 1283 ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() : 1284 null; 1285 } 1286 1287 /** 1288 * If the current thread is operating in a ForkJoinPool, 1289 * unschedules and returns, without executing, the next task 1290 * queued by the current thread but not yet executed, if one is 1291 * available, or if not available, a task that was forked by some 1292 * other thread, if available. Availability may be transient, so a 1293 * {@code null} result does not necessarily imply quiescence of 1294 * the pool this task is operating in. This method is designed 1295 * primarily to support extensions, and is unlikely to be useful 1296 * otherwise. 1297 * 1298 * @return a task, or {@code null} if none are available 1299 */ pollTask()1300 protected static ForkJoinTask<?> pollTask() { 1301 Thread t; ForkJoinWorkerThread wt; 1302 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 1303 (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) : 1304 null; 1305 } 1306 1307 /** 1308 * If the current thread is operating in a ForkJoinPool, 1309 * unschedules and returns, without executing, a task externally 1310 * submitted to the pool, if one is available. Availability may be 1311 * transient, so a {@code null} result does not necessarily imply 1312 * quiescence of the pool. This method is designed primarily to 1313 * support extensions, and is unlikely to be useful otherwise. 1314 * 1315 * @return a task, or {@code null} if none are available 1316 * @since 9 1317 * @hide API from OpenJDK 9, not yet exposed on Android. 1318 */ pollSubmission()1319 protected static ForkJoinTask<?> pollSubmission() { 1320 Thread t; 1321 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? 1322 ((ForkJoinWorkerThread)t).pool.pollSubmission() : null; 1323 } 1324 1325 // tag operations 1326 1327 /** 1328 * Returns the tag for this task. 1329 * 1330 * @return the tag for this task 1331 * @since 1.8 1332 */ getForkJoinTaskTag()1333 public final short getForkJoinTaskTag() { 1334 return (short)status; 1335 } 1336 1337 /** 1338 * Atomically sets the tag value for this task and returns the old value. 1339 * 1340 * @param newValue the new tag value 1341 * @return the previous value of the tag 1342 * @since 1.8 1343 */ setForkJoinTaskTag(short newValue)1344 public final short setForkJoinTaskTag(short newValue) { 1345 for (int s;;) { 1346 if (STATUS.weakCompareAndSet(this, s = status, 1347 (s & ~SMASK) | (newValue & SMASK))) 1348 return (short)s; 1349 } 1350 } 1351 1352 /** 1353 * Atomically conditionally sets the tag value for this task. 1354 * Among other applications, tags can be used as visit markers 1355 * in tasks operating on graphs, as in methods that check: {@code 1356 * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} 1357 * before processing, otherwise exiting because the node has 1358 * already been visited. 1359 * 1360 * @param expect the expected tag value 1361 * @param update the new tag value 1362 * @return {@code true} if successful; i.e., the current value was 1363 * equal to {@code expect} and was changed to {@code update}. 1364 * @since 1.8 1365 */ compareAndSetForkJoinTaskTag(short expect, short update)1366 public final boolean compareAndSetForkJoinTaskTag(short expect, short update) { 1367 for (int s;;) { 1368 if ((short)(s = status) != expect) 1369 return false; 1370 if (STATUS.weakCompareAndSet(this, s, 1371 (s & ~SMASK) | (update & SMASK))) 1372 return true; 1373 } 1374 } 1375 1376 /** 1377 * Adapter for Runnables. This implements RunnableFuture 1378 * to be compliant with AbstractExecutorService constraints 1379 * when used in ForkJoinPool. 1380 */ 1381 static final class AdaptedRunnable<T> extends ForkJoinTask<T> 1382 implements RunnableFuture<T> { 1383 final Runnable runnable; 1384 T result; AdaptedRunnable(Runnable runnable, T result)1385 AdaptedRunnable(Runnable runnable, T result) { 1386 if (runnable == null) throw new NullPointerException(); 1387 this.runnable = runnable; 1388 this.result = result; // OK to set this even before completion 1389 } getRawResult()1390 public final T getRawResult() { return result; } setRawResult(T v)1391 public final void setRawResult(T v) { result = v; } exec()1392 public final boolean exec() { runnable.run(); return true; } run()1393 public final void run() { invoke(); } toString()1394 public String toString() { 1395 return super.toString() + "[Wrapped task = " + runnable + "]"; 1396 } 1397 private static final long serialVersionUID = 5232453952276885070L; 1398 } 1399 1400 /** 1401 * Adapter for Runnables without results. 1402 */ 1403 static final class AdaptedRunnableAction extends ForkJoinTask<Void> 1404 implements RunnableFuture<Void> { 1405 final Runnable runnable; AdaptedRunnableAction(Runnable runnable)1406 AdaptedRunnableAction(Runnable runnable) { 1407 if (runnable == null) throw new NullPointerException(); 1408 this.runnable = runnable; 1409 } getRawResult()1410 public final Void getRawResult() { return null; } setRawResult(Void v)1411 public final void setRawResult(Void v) { } exec()1412 public final boolean exec() { runnable.run(); return true; } run()1413 public final void run() { invoke(); } toString()1414 public String toString() { 1415 return super.toString() + "[Wrapped task = " + runnable + "]"; 1416 } 1417 private static final long serialVersionUID = 5232453952276885070L; 1418 } 1419 1420 /** 1421 * Adapter for Runnables in which failure forces worker exception. 1422 */ 1423 static final class RunnableExecuteAction extends ForkJoinTask<Void> { 1424 final Runnable runnable; RunnableExecuteAction(Runnable runnable)1425 RunnableExecuteAction(Runnable runnable) { 1426 if (runnable == null) throw new NullPointerException(); 1427 this.runnable = runnable; 1428 } getRawResult()1429 public final Void getRawResult() { return null; } setRawResult(Void v)1430 public final void setRawResult(Void v) { } exec()1431 public final boolean exec() { runnable.run(); return true; } internalPropagateException(Throwable ex)1432 void internalPropagateException(Throwable ex) { 1433 rethrow(ex); // rethrow outside exec() catches. 1434 } 1435 private static final long serialVersionUID = 5232453952276885070L; 1436 } 1437 1438 /** 1439 * Adapter for Callables. 1440 */ 1441 static final class AdaptedCallable<T> extends ForkJoinTask<T> 1442 implements RunnableFuture<T> { 1443 final Callable<? extends T> callable; 1444 T result; AdaptedCallable(Callable<? extends T> callable)1445 AdaptedCallable(Callable<? extends T> callable) { 1446 if (callable == null) throw new NullPointerException(); 1447 this.callable = callable; 1448 } getRawResult()1449 public final T getRawResult() { return result; } setRawResult(T v)1450 public final void setRawResult(T v) { result = v; } exec()1451 public final boolean exec() { 1452 try { 1453 result = callable.call(); 1454 return true; 1455 } catch (RuntimeException rex) { 1456 throw rex; 1457 } catch (Exception ex) { 1458 throw new RuntimeException(ex); 1459 } 1460 } run()1461 public final void run() { invoke(); } toString()1462 public String toString() { 1463 return super.toString() + "[Wrapped task = " + callable + "]"; 1464 } 1465 private static final long serialVersionUID = 2838392045355241008L; 1466 } 1467 1468 /** 1469 * Returns a new {@code ForkJoinTask} that performs the {@code run} 1470 * method of the given {@code Runnable} as its action, and returns 1471 * a null result upon {@link #join}. 1472 * 1473 * @param runnable the runnable action 1474 * @return the task 1475 */ adapt(Runnable runnable)1476 public static ForkJoinTask<?> adapt(Runnable runnable) { 1477 return new AdaptedRunnableAction(runnable); 1478 } 1479 1480 /** 1481 * Returns a new {@code ForkJoinTask} that performs the {@code run} 1482 * method of the given {@code Runnable} as its action, and returns 1483 * the given result upon {@link #join}. 1484 * 1485 * @param runnable the runnable action 1486 * @param result the result upon completion 1487 * @param <T> the type of the result 1488 * @return the task 1489 */ adapt(Runnable runnable, T result)1490 public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { 1491 return new AdaptedRunnable<T>(runnable, result); 1492 } 1493 1494 /** 1495 * Returns a new {@code ForkJoinTask} that performs the {@code call} 1496 * method of the given {@code Callable} as its action, and returns 1497 * its result upon {@link #join}, translating any checked exceptions 1498 * encountered into {@code RuntimeException}. 1499 * 1500 * @param callable the callable action 1501 * @param <T> the type of the callable's result 1502 * @return the task 1503 */ adapt(Callable<? extends T> callable)1504 public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { 1505 return new AdaptedCallable<T>(callable); 1506 } 1507 1508 // Serialization support 1509 1510 private static final long serialVersionUID = -7721805057305804111L; 1511 1512 /** 1513 * Saves this task to a stream (that is, serializes it). 1514 * 1515 * @param s the stream 1516 * @throws java.io.IOException if an I/O error occurs 1517 * @serialData the current run status and the exception thrown 1518 * during execution, or {@code null} if none 1519 */ writeObject(java.io.ObjectOutputStream s)1520 private void writeObject(java.io.ObjectOutputStream s) 1521 throws java.io.IOException { 1522 s.defaultWriteObject(); 1523 s.writeObject(getException()); 1524 } 1525 1526 /** 1527 * Reconstitutes this task from a stream (that is, deserializes it). 1528 * @param s the stream 1529 * @throws ClassNotFoundException if the class of a serialized object 1530 * could not be found 1531 * @throws java.io.IOException if an I/O error occurs 1532 */ readObject(java.io.ObjectInputStream s)1533 private void readObject(java.io.ObjectInputStream s) 1534 throws java.io.IOException, ClassNotFoundException { 1535 s.defaultReadObject(); 1536 Object ex = s.readObject(); 1537 if (ex != null) 1538 setExceptionalCompletion((Throwable)ex); 1539 } 1540 1541 // VarHandle mechanics 1542 private static final VarHandle STATUS; 1543 static { 1544 try { 1545 MethodHandles.Lookup l = MethodHandles.lookup(); 1546 STATUS = l.findVarHandle(ForkJoinTask.class, "status", int.class); 1547 } catch (ReflectiveOperationException e) { 1548 throw new ExceptionInInitializerError(e); 1549 } 1550 } 1551 1552 } 1553