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/licenses/publicdomain 34 */ 35 36 package java.util.concurrent; 37 38 import java.io.Serializable; 39 import java.util.Collection; 40 import java.util.Collections; 41 import java.util.List; 42 import java.util.RandomAccess; 43 import java.util.Map; 44 import java.util.WeakHashMap; 45 import java.util.concurrent.Callable; 46 import java.util.concurrent.CancellationException; 47 import java.util.concurrent.ExecutionException; 48 import java.util.concurrent.Executor; 49 import java.util.concurrent.ExecutorService; 50 import java.util.concurrent.Future; 51 import java.util.concurrent.RejectedExecutionException; 52 import java.util.concurrent.RunnableFuture; 53 import java.util.concurrent.TimeUnit; 54 import java.util.concurrent.TimeoutException; 55 56 /** 57 * Abstract base class for tasks that run within a {@link ForkJoinPool}. 58 * A {@code ForkJoinTask} is a thread-like entity that is much 59 * lighter weight than a normal thread. Huge numbers of tasks and 60 * subtasks may be hosted by a small number of actual threads in a 61 * ForkJoinPool, at the price of some usage limitations. 62 * 63 * <p>A "main" {@code ForkJoinTask} begins execution when submitted 64 * to a {@link ForkJoinPool}. Once started, it will usually in turn 65 * start other subtasks. As indicated by the name of this class, 66 * many programs using {@code ForkJoinTask} employ only methods 67 * {@link #fork} and {@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 71 * support 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 intended use as computational tasks calculating 77 * pure functions or operating on purely isolated objects. The 78 * primary 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 * avoid {@code synchronized} methods or blocks, and should minimize 82 * other blocking synchronization apart from joining other tasks or 83 * using synchronizers such as Phasers that are advertised to 84 * cooperate with fork/join scheduling. Tasks should also not perform 85 * blocking IO, and should ideally access variables that are 86 * completely independent of those accessed by other running 87 * tasks. Minor breaches of these restrictions, for example using 88 * shared output streams, may be tolerable in practice, but frequent 89 * use may result in poor performance, and the potential to 90 * indefinitely stall if the number of threads not waiting for IO or 91 * other external synchronization becomes exhausted. This usage 92 * restriction is in part enforced by not permitting checked 93 * exceptions such as {@code IOExceptions} to be thrown. However, 94 * computations may still encounter unchecked exceptions, that are 95 * rethrown to callers attempting to join them. These exceptions may 96 * additionally include {@link RejectedExecutionException} stemming 97 * from internal resource exhaustion, such as failure to allocate 98 * internal task queues. 99 * 100 * <p>The primary method for awaiting completion and extracting 101 * results of a task is {@link #join}, but there are several variants: 102 * The {@link Future#get} methods support interruptible and/or timed 103 * waits for completion and report results using {@code Future} 104 * conventions. Method {@link #invoke} is semantically 105 * equivalent to {@code fork(); join()} but always attempts to begin 106 * execution in the current thread. The "<em>quiet</em>" forms of 107 * these methods do not extract results or report exceptions. These 108 * may be useful when a set of tasks are being executed, and you need 109 * to delay processing of results or exceptions until all complete. 110 * Method {@code invokeAll} (available in multiple versions) 111 * performs the most common form of parallel invocation: forking a set 112 * of tasks and joining them all. 113 * 114 * <p>The execution status of tasks may be queried at several levels 115 * of detail: {@link #isDone} is true if a task completed in any way 116 * (including the case where a task was cancelled without executing); 117 * {@link #isCompletedNormally} is true if a task completed without 118 * cancellation or encountering an exception; {@link #isCancelled} is 119 * true if the task was cancelled (in which case {@link #getException} 120 * returns a {@link java.util.concurrent.CancellationException}); and 121 * {@link #isCompletedAbnormally} is true if a task was either 122 * cancelled or encountered an exception, in which case {@link 123 * #getException} will return either the encountered exception or 124 * {@link java.util.concurrent.CancellationException}. 125 * 126 * <p>The ForkJoinTask class is not usually directly subclassed. 127 * Instead, you subclass one of the abstract classes that support a 128 * particular style of fork/join processing, typically {@link 129 * RecursiveAction} for computations that do not return results, or 130 * {@link RecursiveTask} for those that do. Normally, a concrete 131 * ForkJoinTask subclass declares fields comprising its parameters, 132 * established in a constructor, and then defines a {@code compute} 133 * method that somehow uses the control methods supplied by this base 134 * class. While these methods have {@code public} access (to allow 135 * instances of different task subclasses to call each other's 136 * methods), some of them may only be called from within other 137 * ForkJoinTasks (as may be determined using method {@link 138 * #inForkJoinPool}). Attempts to invoke them in other contexts 139 * result in exceptions or errors, possibly including 140 * {@code ClassCastException}. 141 * 142 * <p>Method {@link #join} and its variants are appropriate for use 143 * only when completion dependencies are acyclic; that is, the 144 * parallel computation can be described as a directed acyclic graph 145 * (DAG). Otherwise, executions may encounter a form of deadlock as 146 * tasks cyclically wait for each other. However, this framework 147 * supports other methods and techniques (for example the use of 148 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that 149 * may be of use in constructing custom subclasses for problems that 150 * are not statically structured as DAGs. 151 * 152 * <p>Most base support methods are {@code final}, to prevent 153 * overriding of implementations that are intrinsically tied to the 154 * underlying lightweight task scheduling framework. Developers 155 * creating new basic styles of fork/join processing should minimally 156 * implement {@code protected} methods {@link #exec}, {@link 157 * #setRawResult}, and {@link #getRawResult}, while also introducing 158 * an abstract computational method that can be implemented in its 159 * subclasses, possibly relying on other {@code protected} methods 160 * provided by this class. 161 * 162 * <p>ForkJoinTasks should perform relatively small amounts of 163 * computation. Large tasks should be split into smaller subtasks, 164 * usually via recursive decomposition. As a very rough rule of thumb, 165 * a task should perform more than 100 and less than 10000 basic 166 * computational steps, and should avoid indefinite looping. If tasks 167 * are too big, then parallelism cannot improve throughput. If too 168 * small, then memory and internal task maintenance overhead may 169 * overwhelm processing. 170 * 171 * <p>This class provides {@code adapt} methods for {@link Runnable} 172 * and {@link Callable}, that may be of use when mixing execution of 173 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are 174 * of this form, consider using a pool constructed in <em>asyncMode</em>. 175 * 176 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be 177 * used in extensions such as remote execution frameworks. It is 178 * sensible to serialize tasks only before or after, but not during, 179 * execution. Serialization is not relied on during execution itself. 180 * 181 * @since 1.7 182 * @author Doug Lea 183 */ 184 public abstract class ForkJoinTask<V> implements Future<V>, Serializable { 185 186 /* 187 * See the internal documentation of class ForkJoinPool for a 188 * general implementation overview. ForkJoinTasks are mainly 189 * responsible for maintaining their "status" field amidst relays 190 * to methods in ForkJoinWorkerThread and ForkJoinPool. The 191 * methods of this class are more-or-less layered into (1) basic 192 * status maintenance (2) execution and awaiting completion (3) 193 * user-level methods that additionally report results. This is 194 * sometimes hard to see because this file orders exported methods 195 * in a way that flows well in javadocs. In particular, most 196 * join mechanics are in method quietlyJoin, below. 197 */ 198 199 /* 200 * The status field holds run control status bits packed into a 201 * single int to minimize footprint and to ensure atomicity (via 202 * CAS). Status is initially zero, and takes on nonnegative 203 * values until completed, upon which status holds value 204 * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking 205 * waits by other threads have the SIGNAL bit set. Completion of 206 * a stolen task with SIGNAL set awakens any waiters via 207 * notifyAll. Even though suboptimal for some purposes, we use 208 * basic builtin wait/notify to take advantage of "monitor 209 * inflation" in JVMs that we would otherwise need to emulate to 210 * avoid adding further per-task bookkeeping overhead. We want 211 * these monitors to be "fat", i.e., not use biasing or thin-lock 212 * techniques, so use some odd coding idioms that tend to avoid 213 * them. 214 */ 215 216 /** The run status of this task */ 217 volatile int status; // accessed directly by pool and workers 218 219 private static final int NORMAL = -1; 220 private static final int CANCELLED = -2; 221 private static final int EXCEPTIONAL = -3; 222 private static final int SIGNAL = 1; 223 224 /** 225 * Table of exceptions thrown by tasks, to enable reporting by 226 * callers. Because exceptions are rare, we don't directly keep 227 * them with task objects, but instead use a weak ref table. Note 228 * that cancellation exceptions don't appear in the table, but are 229 * instead recorded as status values. 230 * TODO: Use ConcurrentReferenceHashMap 231 */ 232 static final Map<ForkJoinTask<?>, Throwable> exceptionMap = 233 Collections.synchronizedMap 234 (new WeakHashMap<ForkJoinTask<?>, Throwable>()); 235 236 // Maintaining completion status 237 238 /** 239 * Marks completion and wakes up threads waiting to join this task, 240 * also clearing signal request bits. 241 * 242 * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL 243 */ 244 private void setCompletion(int completion) { 245 int s; 246 while ((s = status) >= 0) { 247 if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) { 248 if (s != 0) 249 synchronized (this) { notifyAll(); } 250 break; 251 } 252 } 253 } 254 255 /** 256 * Records exception and sets exceptional completion. 257 * 258 * @return status on exit 259 */ 260 private void setExceptionalCompletion(Throwable rex) { 261 exceptionMap.put(this, rex); 262 setCompletion(EXCEPTIONAL); 263 } 264 265 /** 266 * Blocks a worker thread until completed or timed out. Called 267 * only by pool. 268 */ 269 final void internalAwaitDone(long millis, int nanos) { 270 int s = status; 271 if ((s == 0 && 272 UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL)) || 273 s > 0) { 274 try { // the odd construction reduces lock bias effects 275 synchronized (this) { 276 if (status > 0) 277 wait(millis, nanos); 278 else 279 notifyAll(); 280 } 281 } catch (InterruptedException ie) { 282 cancelIfTerminating(); 283 } 284 } 285 } 286 287 /** 288 * Blocks a non-worker-thread until completion. 289 */ 290 private void externalAwaitDone() { 291 if (status >= 0) { 292 boolean interrupted = false; 293 synchronized (this) { 294 for (;;) { 295 int s = status; 296 if (s == 0) 297 UNSAFE.compareAndSwapInt(this, statusOffset, 298 0, SIGNAL); 299 else if (s < 0) { 300 notifyAll(); 301 break; 302 } 303 else { 304 try { 305 wait(); 306 } catch (InterruptedException ie) { 307 interrupted = true; 308 } 309 } 310 } 311 } 312 if (interrupted) 313 Thread.currentThread().interrupt(); 314 } 315 } 316 317 /** 318 * Blocks a non-worker-thread until completion or interruption or timeout. 319 */ 320 private void externalInterruptibleAwaitDone(boolean timed, long nanos) 321 throws InterruptedException { 322 if (Thread.interrupted()) 323 throw new InterruptedException(); 324 if (status >= 0) { 325 long startTime = timed ? System.nanoTime() : 0L; 326 synchronized (this) { 327 for (;;) { 328 long nt; 329 int s = status; 330 if (s == 0) 331 UNSAFE.compareAndSwapInt(this, statusOffset, 332 0, SIGNAL); 333 else if (s < 0) { 334 notifyAll(); 335 break; 336 } 337 else if (!timed) 338 wait(); 339 else if ((nt = nanos - (System.nanoTime()-startTime)) > 0L) 340 wait(nt / 1000000, (int)(nt % 1000000)); 341 else 342 break; 343 } 344 } 345 } 346 } 347 348 /** 349 * Unless done, calls exec and records status if completed, but 350 * doesn't wait for completion otherwise. Primary execution method 351 * for ForkJoinWorkerThread. 352 */ 353 final void quietlyExec() { 354 try { 355 if (status < 0 || !exec()) 356 return; 357 } catch (Throwable rex) { 358 setExceptionalCompletion(rex); 359 return; 360 } 361 setCompletion(NORMAL); // must be outside try block 362 } 363 364 // public methods 365 366 /** 367 * Arranges to asynchronously execute this task. While it is not 368 * necessarily enforced, it is a usage error to fork a task more 369 * than once unless it has completed and been reinitialized. 370 * Subsequent modifications to the state of this task or any data 371 * it operates on are not necessarily consistently observable by 372 * any thread other than the one executing it unless preceded by a 373 * call to {@link #join} or related methods, or a call to {@link 374 * #isDone} returning {@code true}. 375 * 376 * <p>This method may be invoked only from within {@code 377 * ForkJoinPool} computations (as may be determined using method 378 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 379 * result in exceptions or errors, possibly including {@code 380 * ClassCastException}. 381 * 382 * @return {@code this}, to simplify usage 383 */ 384 public final ForkJoinTask<V> fork() { 385 ((ForkJoinWorkerThread) Thread.currentThread()) 386 .pushTask(this); 387 return this; 388 } 389 390 /** 391 * Returns the result of the computation when it {@link #isDone is 392 * done}. This method differs from {@link #get()} in that 393 * abnormal completion results in {@code RuntimeException} or 394 * {@code Error}, not {@code ExecutionException}, and that 395 * interrupts of the calling thread do <em>not</em> cause the 396 * method to abruptly return by throwing {@code 397 * InterruptedException}. 398 * 399 * @return the computed result 400 */ 401 public final V join() { 402 quietlyJoin(); 403 Throwable ex; 404 if (status < NORMAL && (ex = getException()) != null) 405 UNSAFE.throwException(ex); 406 return getRawResult(); 407 } 408 409 /** 410 * Commences performing this task, awaits its completion if 411 * necessary, and returns its result, or throws an (unchecked) 412 * {@code RuntimeException} or {@code Error} if the underlying 413 * computation did so. 414 * 415 * @return the computed result 416 */ 417 public final V invoke() { 418 quietlyInvoke(); 419 Throwable ex; 420 if (status < NORMAL && (ex = getException()) != null) 421 UNSAFE.throwException(ex); 422 return getRawResult(); 423 } 424 425 /** 426 * Forks the given tasks, returning when {@code isDone} holds for 427 * each task or an (unchecked) exception is encountered, in which 428 * case the exception is rethrown. If more than one task 429 * encounters an exception, then this method throws any one of 430 * these exceptions. If any task encounters an exception, the 431 * other may be cancelled. However, the execution status of 432 * individual tasks is not guaranteed upon exceptional return. The 433 * status of each task may be obtained using {@link 434 * #getException()} and related methods to check if they have been 435 * cancelled, completed normally or exceptionally, or left 436 * unprocessed. 437 * 438 * <p>This method may be invoked only from within {@code 439 * ForkJoinPool} computations (as may be determined using method 440 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 441 * result in exceptions or errors, possibly including {@code 442 * ClassCastException}. 443 * 444 * @param t1 the first task 445 * @param t2 the second task 446 * @throws NullPointerException if any task is null 447 */ 448 public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) { 449 t2.fork(); 450 t1.invoke(); 451 t2.join(); 452 } 453 454 /** 455 * Forks the given tasks, returning when {@code isDone} holds for 456 * each task or an (unchecked) exception is encountered, in which 457 * case the exception is rethrown. If more than one task 458 * encounters an exception, then this method throws any one of 459 * these exceptions. If any task encounters an exception, others 460 * may be cancelled. However, the execution status of individual 461 * tasks is not guaranteed upon exceptional return. The status of 462 * each task may be obtained using {@link #getException()} and 463 * related methods to check if they have been cancelled, completed 464 * normally or exceptionally, or left unprocessed. 465 * 466 * <p>This method may be invoked only from within {@code 467 * ForkJoinPool} computations (as may be determined using method 468 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 469 * result in exceptions or errors, possibly including {@code 470 * ClassCastException}. 471 * 472 * @param tasks the tasks 473 * @throws NullPointerException if any task is null 474 */ 475 public static void invokeAll(ForkJoinTask<?>... tasks) { 476 Throwable ex = null; 477 int last = tasks.length - 1; 478 for (int i = last; i >= 0; --i) { 479 ForkJoinTask<?> t = tasks[i]; 480 if (t == null) { 481 if (ex == null) 482 ex = new NullPointerException(); 483 } 484 else if (i != 0) 485 t.fork(); 486 else { 487 t.quietlyInvoke(); 488 if (ex == null && t.status < NORMAL) 489 ex = t.getException(); 490 } 491 } 492 for (int i = 1; i <= last; ++i) { 493 ForkJoinTask<?> t = tasks[i]; 494 if (t != null) { 495 if (ex != null) 496 t.cancel(false); 497 else { 498 t.quietlyJoin(); 499 if (ex == null && t.status < NORMAL) 500 ex = t.getException(); 501 } 502 } 503 } 504 if (ex != null) 505 UNSAFE.throwException(ex); 506 } 507 508 /** 509 * Forks all tasks in the specified collection, returning when 510 * {@code isDone} holds for each task or an (unchecked) exception 511 * is encountered, in which case the exception is rethrown. If 512 * more than one task encounters an exception, then this method 513 * throws any one of these exceptions. If any task encounters an 514 * exception, others may be cancelled. However, the execution 515 * status of individual tasks is not guaranteed upon exceptional 516 * return. The status of each task may be obtained using {@link 517 * #getException()} and related methods to check if they have been 518 * cancelled, completed normally or exceptionally, or left 519 * unprocessed. 520 * 521 * <p>This method may be invoked only from within {@code 522 * ForkJoinPool} computations (as may be determined using method 523 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 524 * result in exceptions or errors, possibly including {@code 525 * ClassCastException}. 526 * 527 * @param tasks the collection of tasks 528 * @return the tasks argument, to simplify usage 529 * @throws NullPointerException if tasks or any element are null 530 */ 531 public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) { 532 if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) { 533 invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()])); 534 return tasks; 535 } 536 @SuppressWarnings("unchecked") 537 List<? extends ForkJoinTask<?>> ts = 538 (List<? extends ForkJoinTask<?>>) tasks; 539 Throwable ex = null; 540 int last = ts.size() - 1; 541 for (int i = last; i >= 0; --i) { 542 ForkJoinTask<?> t = ts.get(i); 543 if (t == null) { 544 if (ex == null) 545 ex = new NullPointerException(); 546 } 547 else if (i != 0) 548 t.fork(); 549 else { 550 t.quietlyInvoke(); 551 if (ex == null && t.status < NORMAL) 552 ex = t.getException(); 553 } 554 } 555 for (int i = 1; i <= last; ++i) { 556 ForkJoinTask<?> t = ts.get(i); 557 if (t != null) { 558 if (ex != null) 559 t.cancel(false); 560 else { 561 t.quietlyJoin(); 562 if (ex == null && t.status < NORMAL) 563 ex = t.getException(); 564 } 565 } 566 } 567 if (ex != null) 568 UNSAFE.throwException(ex); 569 return tasks; 570 } 571 572 /** 573 * Attempts to cancel execution of this task. This attempt will 574 * fail if the task has already completed or could not be 575 * cancelled for some other reason. If successful, and this task 576 * has not started when {@code cancel} is called, execution of 577 * this task is suppressed. After this method returns 578 * successfully, unless there is an intervening call to {@link 579 * #reinitialize}, subsequent calls to {@link #isCancelled}, 580 * {@link #isDone}, and {@code cancel} will return {@code true} 581 * and calls to {@link #join} and related methods will result in 582 * {@code CancellationException}. 583 * 584 * <p>This method may be overridden in subclasses, but if so, must 585 * still ensure that these properties hold. In particular, the 586 * {@code cancel} method itself must not throw exceptions. 587 * 588 * <p>This method is designed to be invoked by <em>other</em> 589 * tasks. To terminate the current task, you can just return or 590 * throw an unchecked exception from its computation method, or 591 * invoke {@link #completeExceptionally}. 592 * 593 * @param mayInterruptIfRunning this value has no effect in the 594 * default implementation because interrupts are not used to 595 * control cancellation. 596 * 597 * @return {@code true} if this task is now cancelled 598 */ 599 public boolean cancel(boolean mayInterruptIfRunning) { 600 setCompletion(CANCELLED); 601 return status == CANCELLED; 602 } 603 604 /** 605 * Cancels, ignoring any exceptions thrown by cancel. Used during 606 * worker and pool shutdown. Cancel is spec'ed not to throw any 607 * exceptions, but if it does anyway, we have no recourse during 608 * shutdown, so guard against this case. 609 */ 610 final void cancelIgnoringExceptions() { 611 try { 612 cancel(false); 613 } catch (Throwable ignore) { 614 } 615 } 616 617 /** 618 * Cancels if current thread is a terminating worker thread, 619 * ignoring any exceptions thrown by cancel. 620 */ 621 final void cancelIfTerminating() { 622 Thread t = Thread.currentThread(); 623 if ((t instanceof ForkJoinWorkerThread) && 624 ((ForkJoinWorkerThread) t).isTerminating()) { 625 try { 626 cancel(false); 627 } catch (Throwable ignore) { 628 } 629 } 630 } 631 632 public final boolean isDone() { 633 return status < 0; 634 } 635 636 public final boolean isCancelled() { 637 return status == CANCELLED; 638 } 639 640 /** 641 * Returns {@code true} if this task threw an exception or was cancelled. 642 * 643 * @return {@code true} if this task threw an exception or was cancelled 644 */ 645 public final boolean isCompletedAbnormally() { 646 return status < NORMAL; 647 } 648 649 /** 650 * Returns {@code true} if this task completed without throwing an 651 * exception and was not cancelled. 652 * 653 * @return {@code true} if this task completed without throwing an 654 * exception and was not cancelled 655 */ 656 public final boolean isCompletedNormally() { 657 return status == NORMAL; 658 } 659 660 /** 661 * Returns the exception thrown by the base computation, or a 662 * {@code CancellationException} if cancelled, or {@code null} if 663 * none or if the method has not yet completed. 664 * 665 * @return the exception, or {@code null} if none 666 */ 667 public final Throwable getException() { 668 int s = status; 669 return ((s >= NORMAL) ? null : 670 (s == CANCELLED) ? new CancellationException() : 671 exceptionMap.get(this)); 672 } 673 674 /** 675 * Completes this task abnormally, and if not already aborted or 676 * cancelled, causes it to throw the given exception upon 677 * {@code join} and related operations. This method may be used 678 * to induce exceptions in asynchronous tasks, or to force 679 * completion of tasks that would not otherwise complete. Its use 680 * in other situations is discouraged. This method is 681 * overridable, but overridden versions must invoke {@code super} 682 * implementation to maintain guarantees. 683 * 684 * @param ex the exception to throw. If this exception is not a 685 * {@code RuntimeException} or {@code Error}, the actual exception 686 * thrown will be a {@code RuntimeException} with cause {@code ex}. 687 */ 688 public void completeExceptionally(Throwable ex) { 689 setExceptionalCompletion((ex instanceof RuntimeException) || 690 (ex instanceof Error) ? ex : 691 new RuntimeException(ex)); 692 } 693 694 /** 695 * Completes this task, and if not already aborted or cancelled, 696 * returning the given value as the result of subsequent 697 * invocations of {@code join} and related operations. This method 698 * may be used to provide results for asynchronous tasks, or to 699 * provide alternative handling for tasks that would not otherwise 700 * complete normally. Its use in other situations is 701 * discouraged. This method is overridable, but overridden 702 * versions must invoke {@code super} implementation to maintain 703 * guarantees. 704 * 705 * @param value the result value for this task 706 */ 707 public void complete(V value) { 708 try { 709 setRawResult(value); 710 } catch (Throwable rex) { 711 setExceptionalCompletion(rex); 712 return; 713 } 714 setCompletion(NORMAL); 715 } 716 717 /** 718 * Waits if necessary for the computation to complete, and then 719 * retrieves its result. 720 * 721 * @return the computed result 722 * @throws CancellationException if the computation was cancelled 723 * @throws ExecutionException if the computation threw an 724 * exception 725 * @throws InterruptedException if the current thread is not a 726 * member of a ForkJoinPool and was interrupted while waiting 727 */ 728 public final V get() throws InterruptedException, ExecutionException { 729 Thread t = Thread.currentThread(); 730 if (t instanceof ForkJoinWorkerThread) 731 quietlyJoin(); 732 else 733 externalInterruptibleAwaitDone(false, 0L); 734 int s = status; 735 if (s != NORMAL) { 736 Throwable ex; 737 if (s == CANCELLED) 738 throw new CancellationException(); 739 if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) 740 throw new ExecutionException(ex); 741 } 742 return getRawResult(); 743 } 744 745 /** 746 * Waits if necessary for at most the given time for the computation 747 * to complete, and then retrieves its result, if available. 748 * 749 * @param timeout the maximum time to wait 750 * @param unit the time unit of the timeout argument 751 * @return the computed result 752 * @throws CancellationException if the computation was cancelled 753 * @throws ExecutionException if the computation threw an 754 * exception 755 * @throws InterruptedException if the current thread is not a 756 * member of a ForkJoinPool and was interrupted while waiting 757 * @throws TimeoutException if the wait timed out 758 */ 759 public final V get(long timeout, TimeUnit unit) 760 throws InterruptedException, ExecutionException, TimeoutException { 761 long nanos = unit.toNanos(timeout); 762 Thread t = Thread.currentThread(); 763 if (t instanceof ForkJoinWorkerThread) 764 ((ForkJoinWorkerThread)t).joinTask(this, true, nanos); 765 else 766 externalInterruptibleAwaitDone(true, nanos); 767 int s = status; 768 if (s != NORMAL) { 769 Throwable ex; 770 if (s == CANCELLED) 771 throw new CancellationException(); 772 if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) 773 throw new ExecutionException(ex); 774 throw new TimeoutException(); 775 } 776 return getRawResult(); 777 } 778 779 /** 780 * Joins this task, without returning its result or throwing its 781 * exception. This method may be useful when processing 782 * collections of tasks when some have been cancelled or otherwise 783 * known to have aborted. 784 */ 785 public final void quietlyJoin() { 786 Thread t; 787 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) { 788 ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; 789 if (status >= 0) { 790 if (w.unpushTask(this)) { 791 boolean completed; 792 try { 793 completed = exec(); 794 } catch (Throwable rex) { 795 setExceptionalCompletion(rex); 796 return; 797 } 798 if (completed) { 799 setCompletion(NORMAL); 800 return; 801 } 802 } 803 w.joinTask(this, false, 0L); 804 } 805 } 806 else 807 externalAwaitDone(); 808 } 809 810 /** 811 * Commences performing this task and awaits its completion if 812 * necessary, without returning its result or throwing its 813 * exception. 814 */ 815 public final void quietlyInvoke() { 816 if (status >= 0) { 817 boolean completed; 818 try { 819 completed = exec(); 820 } catch (Throwable rex) { 821 setExceptionalCompletion(rex); 822 return; 823 } 824 if (completed) 825 setCompletion(NORMAL); 826 else 827 quietlyJoin(); 828 } 829 } 830 831 /** 832 * Possibly executes tasks until the pool hosting the current task 833 * {@link ForkJoinPool#isQuiescent is quiescent}. This method may 834 * be of use in designs in which many tasks are forked, but none 835 * are explicitly joined, instead executing them until all are 836 * processed. 837 * 838 * <p>This method may be invoked only from within {@code 839 * ForkJoinPool} computations (as may be determined using method 840 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 841 * result in exceptions or errors, possibly including {@code 842 * ClassCastException}. 843 */ 844 public static void helpQuiesce() { 845 ((ForkJoinWorkerThread) Thread.currentThread()) 846 .helpQuiescePool(); 847 } 848 849 /** 850 * Resets the internal bookkeeping state of this task, allowing a 851 * subsequent {@code fork}. This method allows repeated reuse of 852 * this task, but only if reuse occurs when this task has either 853 * never been forked, or has been forked, then completed and all 854 * outstanding joins of this task have also completed. Effects 855 * under any other usage conditions are not guaranteed. 856 * This method may be useful when executing 857 * pre-constructed trees of subtasks in loops. 858 * 859 * <p>Upon completion of this method, {@code isDone()} reports 860 * {@code false}, and {@code getException()} reports {@code 861 * null}. However, the value returned by {@code getRawResult} is 862 * unaffected. To clear this value, you can invoke {@code 863 * setRawResult(null)}. 864 */ 865 public void reinitialize() { 866 if (status == EXCEPTIONAL) 867 exceptionMap.remove(this); 868 status = 0; 869 } 870 871 /** 872 * Returns the pool hosting the current task execution, or null 873 * if this task is executing outside of any ForkJoinPool. 874 * 875 * @see #inForkJoinPool 876 * @return the pool, or {@code null} if none 877 */ 878 public static ForkJoinPool getPool() { 879 Thread t = Thread.currentThread(); 880 return (t instanceof ForkJoinWorkerThread) ? 881 ((ForkJoinWorkerThread) t).pool : null; 882 } 883 884 /** 885 * Returns {@code true} if the current thread is a {@link 886 * ForkJoinWorkerThread} executing as a ForkJoinPool computation. 887 * 888 * @return {@code true} if the current thread is a {@link 889 * ForkJoinWorkerThread} executing as a ForkJoinPool computation, 890 * or {@code false} otherwise 891 */ 892 public static boolean inForkJoinPool() { 893 return Thread.currentThread() instanceof ForkJoinWorkerThread; 894 } 895 896 /** 897 * Tries to unschedule this task for execution. This method will 898 * typically succeed if this task is the most recently forked task 899 * by the current thread, and has not commenced executing in 900 * another thread. This method may be useful when arranging 901 * alternative local processing of tasks that could have been, but 902 * were not, stolen. 903 * 904 * <p>This method may be invoked only from within {@code 905 * ForkJoinPool} computations (as may be determined using method 906 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 907 * result in exceptions or errors, possibly including {@code 908 * ClassCastException}. 909 * 910 * @return {@code true} if unforked 911 */ 912 public boolean tryUnfork() { 913 return ((ForkJoinWorkerThread) Thread.currentThread()) 914 .unpushTask(this); 915 } 916 917 /** 918 * Returns an estimate of the number of tasks that have been 919 * forked by the current worker thread but not yet executed. This 920 * value may be useful for heuristic decisions about whether to 921 * fork other tasks. 922 * 923 * <p>This method may be invoked only from within {@code 924 * ForkJoinPool} computations (as may be determined using method 925 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 926 * result in exceptions or errors, possibly including {@code 927 * ClassCastException}. 928 * 929 * @return the number of tasks 930 */ 931 public static int getQueuedTaskCount() { 932 return ((ForkJoinWorkerThread) Thread.currentThread()) 933 .getQueueSize(); 934 } 935 936 /** 937 * Returns an estimate of how many more locally queued tasks are 938 * held by the current worker thread than there are other worker 939 * threads that might steal them. This value may be useful for 940 * heuristic decisions about whether to fork other tasks. In many 941 * usages of ForkJoinTasks, at steady state, each worker should 942 * aim to maintain a small constant surplus (for example, 3) of 943 * tasks, and to process computations locally if this threshold is 944 * exceeded. 945 * 946 * <p>This method may be invoked only from within {@code 947 * ForkJoinPool} computations (as may be determined using method 948 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 949 * result in exceptions or errors, possibly including {@code 950 * ClassCastException}. 951 * 952 * @return the surplus number of tasks, which may be negative 953 */ 954 public static int getSurplusQueuedTaskCount() { 955 return ((ForkJoinWorkerThread) Thread.currentThread()) 956 .getEstimatedSurplusTaskCount(); 957 } 958 959 // Extension methods 960 961 /** 962 * Returns the result that would be returned by {@link #join}, even 963 * if this task completed abnormally, or {@code null} if this task 964 * is not known to have been completed. This method is designed 965 * to aid debugging, as well as to support extensions. Its use in 966 * any other context is discouraged. 967 * 968 * @return the result, or {@code null} if not completed 969 */ 970 public abstract V getRawResult(); 971 972 /** 973 * Forces the given value to be returned as a result. This method 974 * is designed to support extensions, and should not in general be 975 * called otherwise. 976 * 977 * @param value the value 978 */ 979 protected abstract void setRawResult(V value); 980 981 /** 982 * Immediately performs the base action of this task. This method 983 * is designed to support extensions, and should not in general be 984 * called otherwise. The return value controls whether this task 985 * is considered to be done normally. It may return false in 986 * asynchronous actions that require explicit invocations of 987 * {@link #complete} to become joinable. It may also throw an 988 * (unchecked) exception to indicate abnormal exit. 989 * 990 * @return {@code true} if completed normally 991 */ 992 protected abstract boolean exec(); 993 994 /** 995 * Returns, but does not unschedule or execute, a task queued by 996 * the current thread but not yet executed, if one is immediately 997 * available. There is no guarantee that this task will actually 998 * be polled or executed next. Conversely, this method may return 999 * null even if a task exists but cannot be accessed without 1000 * contention with other threads. This method is designed 1001 * primarily to support extensions, and is unlikely to be useful 1002 * otherwise. 1003 * 1004 * <p>This method may be invoked only from within {@code 1005 * ForkJoinPool} computations (as may be determined using method 1006 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1007 * result in exceptions or errors, possibly including {@code 1008 * ClassCastException}. 1009 * 1010 * @return the next task, or {@code null} if none are available 1011 */ 1012 protected static ForkJoinTask<?> peekNextLocalTask() { 1013 return ((ForkJoinWorkerThread) Thread.currentThread()) 1014 .peekTask(); 1015 } 1016 1017 /** 1018 * Unschedules and returns, without executing, the next task 1019 * queued by the current thread but not yet executed. This method 1020 * is designed primarily to support extensions, and is unlikely to 1021 * be useful otherwise. 1022 * 1023 * <p>This method may be invoked only from within {@code 1024 * ForkJoinPool} computations (as may be determined using method 1025 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1026 * result in exceptions or errors, possibly including {@code 1027 * ClassCastException}. 1028 * 1029 * @return the next task, or {@code null} if none are available 1030 */ 1031 protected static ForkJoinTask<?> pollNextLocalTask() { 1032 return ((ForkJoinWorkerThread) Thread.currentThread()) 1033 .pollLocalTask(); 1034 } 1035 1036 /** 1037 * Unschedules and returns, without executing, the next task 1038 * queued by the current thread but not yet executed, if one is 1039 * available, or if not available, a task that was forked by some 1040 * other thread, if available. Availability may be transient, so a 1041 * {@code null} result does not necessarily imply quiescence 1042 * of the pool this task is operating in. This method is designed 1043 * primarily to support extensions, and is unlikely to be useful 1044 * otherwise. 1045 * 1046 * <p>This method may be invoked only from within {@code 1047 * ForkJoinPool} computations (as may be determined using method 1048 * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1049 * result in exceptions or errors, possibly including {@code 1050 * ClassCastException}. 1051 * 1052 * @return a task, or {@code null} if none are available 1053 */ 1054 protected static ForkJoinTask<?> pollTask() { 1055 return ((ForkJoinWorkerThread) Thread.currentThread()) 1056 .pollTask(); 1057 } 1058 1059 /** 1060 * Adaptor for Runnables. This implements RunnableFuture 1061 * to be compliant with AbstractExecutorService constraints 1062 * when used in ForkJoinPool. 1063 */ 1064 static final class AdaptedRunnable<T> extends ForkJoinTask<T> 1065 implements RunnableFuture<T> { 1066 final Runnable runnable; 1067 final T resultOnCompletion; 1068 T result; 1069 AdaptedRunnable(Runnable runnable, T result) { 1070 if (runnable == null) throw new NullPointerException(); 1071 this.runnable = runnable; 1072 this.resultOnCompletion = result; 1073 } 1074 public T getRawResult() { return result; } 1075 public void setRawResult(T v) { result = v; } 1076 public boolean exec() { 1077 runnable.run(); 1078 result = resultOnCompletion; 1079 return true; 1080 } 1081 public void run() { invoke(); } 1082 private static final long serialVersionUID = 5232453952276885070L; 1083 } 1084 1085 /** 1086 * Adaptor for Callables 1087 */ 1088 static final class AdaptedCallable<T> extends ForkJoinTask<T> 1089 implements RunnableFuture<T> { 1090 final Callable<? extends T> callable; 1091 T result; 1092 AdaptedCallable(Callable<? extends T> callable) { 1093 if (callable == null) throw new NullPointerException(); 1094 this.callable = callable; 1095 } 1096 public T getRawResult() { return result; } 1097 public void setRawResult(T v) { result = v; } 1098 public boolean exec() { 1099 try { 1100 result = callable.call(); 1101 return true; 1102 } catch (Error err) { 1103 throw err; 1104 } catch (RuntimeException rex) { 1105 throw rex; 1106 } catch (Exception ex) { 1107 throw new RuntimeException(ex); 1108 } 1109 } 1110 public void run() { invoke(); } 1111 private static final long serialVersionUID = 2838392045355241008L; 1112 } 1113 1114 /** 1115 * Returns a new {@code ForkJoinTask} that performs the {@code run} 1116 * method of the given {@code Runnable} as its action, and returns 1117 * a null result upon {@link #join}. 1118 * 1119 * @param runnable the runnable action 1120 * @return the task 1121 */ 1122 public static ForkJoinTask<?> adapt(Runnable runnable) { 1123 return new AdaptedRunnable<Void>(runnable, null); 1124 } 1125 1126 /** 1127 * Returns a new {@code ForkJoinTask} that performs the {@code run} 1128 * method of the given {@code Runnable} as its action, and returns 1129 * the given result upon {@link #join}. 1130 * 1131 * @param runnable the runnable action 1132 * @param result the result upon completion 1133 * @return the task 1134 */ 1135 public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { 1136 return new AdaptedRunnable<T>(runnable, result); 1137 } 1138 1139 /** 1140 * Returns a new {@code ForkJoinTask} that performs the {@code call} 1141 * method of the given {@code Callable} as its action, and returns 1142 * its result upon {@link #join}, translating any checked exceptions 1143 * encountered into {@code RuntimeException}. 1144 * 1145 * @param callable the callable action 1146 * @return the task 1147 */ 1148 public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { 1149 return new AdaptedCallable<T>(callable); 1150 } 1151 1152 // Serialization support 1153 1154 private static final long serialVersionUID = -7721805057305804111L; 1155 1156 /** 1157 * Saves the state to a stream (that is, serializes it). 1158 * 1159 * @serialData the current run status and the exception thrown 1160 * during execution, or {@code null} if none 1161 * @param s the stream 1162 */ 1163 private void writeObject(java.io.ObjectOutputStream s) 1164 throws java.io.IOException { 1165 s.defaultWriteObject(); 1166 s.writeObject(getException()); 1167 } 1168 1169 /** 1170 * Reconstitutes the instance from a stream (that is, deserializes it). 1171 * 1172 * @param s the stream 1173 */ 1174 private void readObject(java.io.ObjectInputStream s) 1175 throws java.io.IOException, ClassNotFoundException { 1176 s.defaultReadObject(); 1177 Object ex = s.readObject(); 1178 if (ex != null) 1179 setExceptionalCompletion((Throwable) ex); 1180 } 1181 1182 // Unsafe mechanics 1183 1184 private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); 1185 private static final long statusOffset = 1186 objectFieldOffset("status", ForkJoinTask.class); 1187 1188 private static long objectFieldOffset(String field, Class<?> klazz) { 1189 try { 1190 return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); 1191 } catch (NoSuchFieldException e) { 1192 // Convert Exception to corresponding Error 1193 NoSuchFieldError error = new NoSuchFieldError(field); 1194 error.initCause(e); 1195 throw error; 1196 } 1197 } 1198 }