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.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.lang.ref.WeakReference;
  45 import java.lang.ref.ReferenceQueue;
  46 import java.util.concurrent.Callable;
  47 import java.util.concurrent.CancellationException;
  48 import java.util.concurrent.ExecutionException;
  49 import java.util.concurrent.Executor;
  50 import java.util.concurrent.ExecutorService;
  51 import java.util.concurrent.Future;
  52 import java.util.concurrent.RejectedExecutionException;
  53 import java.util.concurrent.RunnableFuture;
  54 import java.util.concurrent.TimeUnit;
  55 import java.util.concurrent.TimeoutException;
  56 import java.util.concurrent.locks.ReentrantLock;
  57 import java.lang.reflect.Constructor;
  58 
  59 /**
  60  * Abstract base class for tasks that run within a {@link ForkJoinPool}.
  61  * A {@code ForkJoinTask} is a thread-like entity that is much
  62  * lighter weight than a normal thread.  Huge numbers of tasks and
  63  * subtasks may be hosted by a small number of actual threads in a
  64  * ForkJoinPool, at the price of some usage limitations.
  65  *
  66  * <p>A "main" {@code ForkJoinTask} begins execution when submitted
  67  * to a {@link ForkJoinPool}.  Once started, it will usually in turn
  68  * start other subtasks.  As indicated by the name of this class,
  69  * many programs using {@code ForkJoinTask} employ only methods
  70  * {@link #fork} and {@link #join}, or derivatives such as {@link
  71  * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
  72  * provides a number of other methods that can come into play in
  73  * advanced usages, as well as extension mechanics that allow
  74  * support of new forms of fork/join processing.
  75  *
  76  * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
  77  * The efficiency of {@code ForkJoinTask}s stems from a set of
  78  * restrictions (that are only partially statically enforceable)
  79  * reflecting their intended use as computational tasks calculating
  80  * pure functions or operating on purely isolated objects.  The
  81  * primary coordination mechanisms are {@link #fork}, that arranges
  82  * asynchronous execution, and {@link #join}, that doesn't proceed
  83  * until the task's result has been computed.  Computations should
  84  * avoid {@code synchronized} methods or blocks, and should minimize
  85  * other blocking synchronization apart from joining other tasks or
  86  * using synchronizers such as Phasers that are advertised to
  87  * cooperate with fork/join scheduling. Tasks should also not perform
  88  * blocking IO, and should ideally access variables that are
  89  * completely independent of those accessed by other running
  90  * tasks. Minor breaches of these restrictions, for example using
  91  * shared output streams, may be tolerable in practice, but frequent
  92  * use may result in poor performance, and the potential to
  93  * indefinitely stall if the number of threads not waiting for IO or
  94  * other external synchronization becomes exhausted. This usage
  95  * restriction is in part enforced by not permitting checked
  96  * exceptions such as {@code IOExceptions} to be thrown. However,
  97  * computations may still encounter unchecked exceptions, that are
  98  * rethrown to callers attempting to join them. These exceptions may
  99  * additionally include {@link RejectedExecutionException} stemming
 100  * from internal resource exhaustion, such as failure to allocate
 101  * internal task queues. Rethrown exceptions behave in the same way as
 102  * regular exceptions, but, when possible, contain stack traces (as
 103  * displayed for example using {@code ex.printStackTrace()}) of both
 104  * the thread that initiated the computation as well as the thread
 105  * actually encountering the exception; minimally only the latter.
 106  *
 107  * <p>The primary method for awaiting completion and extracting
 108  * results of a task is {@link #join}, but there are several variants:
 109  * The {@link Future#get} methods support interruptible and/or timed
 110  * waits for completion and report results using {@code Future}
 111  * conventions. Method {@link #invoke} is semantically
 112  * equivalent to {@code fork(); join()} but always attempts to begin
 113  * execution in the current thread. The "<em>quiet</em>" forms of
 114  * these methods do not extract results or report exceptions. These
 115  * may be useful when a set of tasks are being executed, and you need
 116  * to delay processing of results or exceptions until all complete.
 117  * Method {@code invokeAll} (available in multiple versions)
 118  * performs the most common form of parallel invocation: forking a set
 119  * of tasks and joining them all.
 120  *
 121  * <p>The execution status of tasks may be queried at several levels
 122  * of detail: {@link #isDone} is true if a task completed in any way
 123  * (including the case where a task was cancelled without executing);
 124  * {@link #isCompletedNormally} is true if a task completed without
 125  * cancellation or encountering an exception; {@link #isCancelled} is
 126  * true if the task was cancelled (in which case {@link #getException}
 127  * returns a {@link java.util.concurrent.CancellationException}); and
 128  * {@link #isCompletedAbnormally} is true if a task was either
 129  * cancelled or encountered an exception, in which case {@link
 130  * #getException} will return either the encountered exception or
 131  * {@link java.util.concurrent.CancellationException}.
 132  *
 133  * <p>The ForkJoinTask class is not usually directly subclassed.
 134  * Instead, you subclass one of the abstract classes that support a
 135  * particular style of fork/join processing, typically {@link
 136  * RecursiveAction} for computations that do not return results, or
 137  * {@link RecursiveTask} for those that do.  Normally, a concrete
 138  * ForkJoinTask subclass declares fields comprising its parameters,
 139  * established in a constructor, and then defines a {@code compute}
 140  * method that somehow uses the control methods supplied by this base
 141  * class. While these methods have {@code public} access (to allow
 142  * instances of different task subclasses to call each other's
 143  * methods), some of them may only be called from within other
 144  * ForkJoinTasks (as may be determined using method {@link
 145  * #inForkJoinPool}).  Attempts to invoke them in other contexts
 146  * result in exceptions or errors, possibly including
 147  * {@code ClassCastException}.
 148  *
 149  * <p>Method {@link #join} and its variants are appropriate for use
 150  * only when completion dependencies are acyclic; that is, the
 151  * parallel computation can be described as a directed acyclic graph
 152  * (DAG). Otherwise, executions may encounter a form of deadlock as
 153  * tasks cyclically wait for each other.  However, this framework
 154  * supports other methods and techniques (for example the use of
 155  * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
 156  * may be of use in constructing custom subclasses for problems that
 157  * are not statically structured as DAGs.
 158  *
 159  * <p>Most base support methods are {@code final}, to prevent
 160  * overriding of implementations that are intrinsically tied to the
 161  * underlying lightweight task scheduling framework.  Developers
 162  * creating new basic styles of fork/join processing should minimally
 163  * implement {@code protected} methods {@link #exec}, {@link
 164  * #setRawResult}, and {@link #getRawResult}, while also introducing
 165  * an abstract computational method that can be implemented in its
 166  * subclasses, possibly relying on other {@code protected} methods
 167  * provided by this class.
 168  *
 169  * <p>ForkJoinTasks should perform relatively small amounts of
 170  * computation. Large tasks should be split into smaller subtasks,
 171  * usually via recursive decomposition. As a very rough rule of thumb,
 172  * a task should perform more than 100 and less than 10000 basic
 173  * computational steps, and should avoid indefinite looping. If tasks
 174  * are too big, then parallelism cannot improve throughput. If too
 175  * small, then memory and internal task maintenance overhead may
 176  * overwhelm processing.
 177  *
 178  * <p>This class provides {@code adapt} methods for {@link Runnable}
 179  * and {@link Callable}, that may be of use when mixing execution of
 180  * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
 181  * of this form, consider using a pool constructed in <em>asyncMode</em>.
 182  *
 183  * <p>ForkJoinTasks are {@code Serializable}, which enables them to be
 184  * used in extensions such as remote execution frameworks. It is
 185  * sensible to serialize tasks only before or after, but not during,
 186  * execution. Serialization is not relied on during execution itself.
 187  *
 188  * @since 1.7
 189  * @author Doug Lea
 190  */
 191 public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
 192 
 193     /*
 194      * See the internal documentation of class ForkJoinPool for a
 195      * general implementation overview.  ForkJoinTasks are mainly
 196      * responsible for maintaining their "status" field amidst relays
 197      * to methods in ForkJoinWorkerThread and ForkJoinPool. The
 198      * methods of this class are more-or-less layered into (1) basic
 199      * status maintenance (2) execution and awaiting completion (3)
 200      * user-level methods that additionally report results. This is
 201      * sometimes hard to see because this file orders exported methods
 202      * in a way that flows well in javadocs.
 203      */
 204 
 205     /*
 206      * The status field holds run control status bits packed into a
 207      * single int to minimize footprint and to ensure atomicity (via
 208      * CAS).  Status is initially zero, and takes on nonnegative
 209      * values until completed, upon which status holds value
 210      * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking
 211      * waits by other threads have the SIGNAL bit set.  Completion of
 212      * a stolen task with SIGNAL set awakens any waiters via
 213      * notifyAll. Even though suboptimal for some purposes, we use
 214      * basic builtin wait/notify to take advantage of "monitor
 215      * inflation" in JVMs that we would otherwise need to emulate to
 216      * avoid adding further per-task bookkeeping overhead.  We want
 217      * these monitors to be "fat", i.e., not use biasing or thin-lock
 218      * techniques, so use some odd coding idioms that tend to avoid
 219      * them.
 220      */
 221 
 222     /** The run status of this task */
 223     volatile int status; // accessed directly by pool and workers
 224     private static final int NORMAL      = -1;
 225     private static final int CANCELLED   = -2;
 226     private static final int EXCEPTIONAL = -3;
 227     private static final int SIGNAL      =  1;
 228 
 229     /**
 230      * Marks completion and wakes up threads waiting to join this task,
 231      * also clearing signal request bits.
 232      *
 233      * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
 234      * @return completion status on exit
 235      */
 236     private int setCompletion(int completion) {
 237         for (int s;;) {
 238             if ((s = status) < 0)
 239                 return s;
 240             if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
 241                 if (s != 0)
 242                     synchronized (this) { notifyAll(); }
 243                 return completion;
 244             }
 245         }
 246     }
 247 
 248     /**
 249      * Tries to block a worker thread until completed or timed out.
 250      * Uses Object.wait time argument conventions.
 251      * May fail on contention or interrupt.
 252      *
 253      * @param millis if > 0, wait time.
 254      */
 255     final void tryAwaitDone(long millis) {
 256         int s;
 257         try {
 258             if (((s = status) > 0 ||
 259                  (s == 0 &&
 260                   UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
 261                 status > 0) {
 262                 synchronized (this) {
 263                     if (status > 0)
 264                         wait(millis);
 265                 }
 266             }
 267         } catch (InterruptedException ie) {
 268             // caller must check termination
 269         }
 270     }
 271 
 272     /**
 273      * Blocks a non-worker-thread until completion.
 274      * @return status upon completion
 275      */
 276     private int externalAwaitDone() {
 277         int s;
 278         if ((s = status) >= 0) {
 279             boolean interrupted = false;
 280             synchronized (this) {
 281                 while ((s = status) >= 0) {
 282                     if (s == 0)
 283                         UNSAFE.compareAndSwapInt(this, statusOffset,
 284                                                  0, SIGNAL);
 285                     else {
 286                         try {
 287                             wait();
 288                         } catch (InterruptedException ie) {
 289                             interrupted = true;
 290                         }
 291                     }
 292                 }
 293             }
 294             if (interrupted)
 295                 Thread.currentThread().interrupt();
 296         }
 297         return s;
 298     }
 299 
 300     /**
 301      * Blocks a non-worker-thread until completion or interruption or timeout.
 302      */
 303     private int externalInterruptibleAwaitDone(long millis)
 304         throws InterruptedException {
 305         int s;
 306         if (Thread.interrupted())
 307             throw new InterruptedException();
 308         if ((s = status) >= 0) {
 309             synchronized (this) {
 310                 while ((s = status) >= 0) {
 311                     if (s == 0)
 312                         UNSAFE.compareAndSwapInt(this, statusOffset,
 313                                                  0, SIGNAL);
 314                     else {
 315                         wait(millis);
 316                         if (millis > 0L)
 317                             break;
 318                     }
 319                 }
 320             }
 321         }
 322         return s;
 323     }
 324 
 325     /**
 326      * Primary execution method for stolen tasks. Unless done, calls
 327      * exec and records status if completed, but doesn't wait for
 328      * completion otherwise.
 329      */
 330     final void doExec() {
 331         if (status >= 0) {
 332             boolean completed;
 333             try {
 334                 completed = exec();
 335             } catch (Throwable rex) {
 336                 setExceptionalCompletion(rex);
 337                 return;
 338             }
 339             if (completed)
 340                 setCompletion(NORMAL); // must be outside try block
 341         }
 342     }
 343 
 344     /**
 345      * Primary mechanics for join, get, quietlyJoin.
 346      * @return status upon completion
 347      */
 348     private int doJoin() {
 349         Thread t; ForkJoinWorkerThread w; int s; boolean completed;
 350         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
 351             if ((s = status) < 0)
 352                 return s;
 353             if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
 354                 try {
 355                     completed = exec();
 356                 } catch (Throwable rex) {
 357                     return setExceptionalCompletion(rex);
 358                 }
 359                 if (completed)
 360                     return setCompletion(NORMAL);
 361             }
 362             return w.joinTask(this);
 363         }
 364         else
 365             return externalAwaitDone();
 366     }
 367 
 368     /**
 369      * Primary mechanics for invoke, quietlyInvoke.
 370      * @return status upon completion
 371      */
 372     private int doInvoke() {
 373         int s; boolean completed;
 374         if ((s = status) < 0)
 375             return s;
 376         try {
 377             completed = exec();
 378         } catch (Throwable rex) {
 379             return setExceptionalCompletion(rex);
 380         }
 381         if (completed)
 382             return setCompletion(NORMAL);
 383         else
 384             return doJoin();
 385     }
 386 
 387     // Exception table support
 388 
 389     /**
 390      * Table of exceptions thrown by tasks, to enable reporting by
 391      * callers. Because exceptions are rare, we don't directly keep
 392      * them with task objects, but instead use a weak ref table.  Note
 393      * that cancellation exceptions don't appear in the table, but are
 394      * instead recorded as status values.
 395      *
 396      * Note: These statics are initialized below in static block.
 397      */
 398     private static final ExceptionNode[] exceptionTable;
 399     private static final ReentrantLock exceptionTableLock;
 400     private static final ReferenceQueue<Object> exceptionTableRefQueue;
 401 
 402     /**
 403      * Fixed capacity for exceptionTable.
 404      */
 405     private static final int EXCEPTION_MAP_CAPACITY = 32;
 406 
 407     /**
 408      * Key-value nodes for exception table.  The chained hash table
 409      * uses identity comparisons, full locking, and weak references
 410      * for keys. The table has a fixed capacity because it only
 411      * maintains task exceptions long enough for joiners to access
 412      * them, so should never become very large for sustained
 413      * periods. However, since we do not know when the last joiner
 414      * completes, we must use weak references and expunge them. We do
 415      * so on each operation (hence full locking). Also, some thread in
 416      * any ForkJoinPool will call helpExpungeStaleExceptions when its
 417      * pool becomes isQuiescent.
 418      */
 419     static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{
 420         final Throwable ex;
 421         ExceptionNode next;
 422         final long thrower;  // use id not ref to avoid weak cycles
 423         ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
 424             super(task, exceptionTableRefQueue);
 425             this.ex = ex;
 426             this.next = next;
 427             this.thrower = Thread.currentThread().getId();
 428         }
 429     }
 430 
 431     /**
 432      * Records exception and sets exceptional completion.
 433      *
 434      * @return status on exit
 435      */
 436     private int setExceptionalCompletion(Throwable ex) {
 437         int h = System.identityHashCode(this);
 438         final ReentrantLock lock = exceptionTableLock;
 439         lock.lock();
 440         try {
 441             expungeStaleExceptions();
 442             ExceptionNode[] t = exceptionTable;
 443             int i = h & (t.length - 1);
 444             for (ExceptionNode e = t[i]; ; e = e.next) {
 445                 if (e == null) {
 446                     t[i] = new ExceptionNode(this, ex, t[i]);
 447                     break;
 448                 }
 449                 if (e.get() == this) // already present
 450                     break;
 451             }
 452         } finally {
 453             lock.unlock();
 454         }
 455         return setCompletion(EXCEPTIONAL);
 456     }
 457 
 458     /**
 459      * Removes exception node and clears status
 460      */
 461     private void clearExceptionalCompletion() {
 462         int h = System.identityHashCode(this);
 463         final ReentrantLock lock = exceptionTableLock;
 464         lock.lock();
 465         try {
 466             ExceptionNode[] t = exceptionTable;
 467             int i = h & (t.length - 1);
 468             ExceptionNode e = t[i];
 469             ExceptionNode pred = null;
 470             while (e != null) {
 471                 ExceptionNode next = e.next;
 472                 if (e.get() == this) {
 473                     if (pred == null)
 474                         t[i] = next;
 475                     else
 476                         pred.next = next;
 477                     break;
 478                 }
 479                 pred = e;
 480                 e = next;
 481             }
 482             expungeStaleExceptions();
 483             status = 0;
 484         } finally {
 485             lock.unlock();
 486         }
 487     }
 488 
 489     /**
 490      * Returns a rethrowable exception for the given task, if
 491      * available. To provide accurate stack traces, if the exception
 492      * was not thrown by the current thread, we try to create a new
 493      * exception of the same type as the one thrown, but with the
 494      * recorded exception as its cause. If there is no such
 495      * constructor, we instead try to use a no-arg constructor,
 496      * followed by initCause, to the same effect. If none of these
 497      * apply, or any fail due to other exceptions, we return the
 498      * recorded exception, which is still correct, although it may
 499      * contain a misleading stack trace.
 500      *
 501      * @return the exception, or null if none
 502      */
 503     private Throwable getThrowableException() {
 504         if (status != EXCEPTIONAL)
 505             return null;
 506         int h = System.identityHashCode(this);
 507         ExceptionNode e;
 508         final ReentrantLock lock = exceptionTableLock;
 509         lock.lock();
 510         try {
 511             expungeStaleExceptions();
 512             ExceptionNode[] t = exceptionTable;
 513             e = t[h & (t.length - 1)];
 514             while (e != null && e.get() != this)
 515                 e = e.next;
 516         } finally {
 517             lock.unlock();
 518         }
 519         Throwable ex;
 520         if (e == null || (ex = e.ex) == null)
 521             return null;
 522         if (e.thrower != Thread.currentThread().getId()) {
 523             Class ec = ex.getClass();
 524             try {
 525                 Constructor<?> noArgCtor = null;
 526                 Constructor<?>[] cs = ec.getConstructors();// public ctors only
 527                 for (int i = 0; i < cs.length; ++i) {
 528                     Constructor<?> c = cs[i];
 529                     Class<?>[] ps = c.getParameterTypes();
 530                     if (ps.length == 0)
 531                         noArgCtor = c;
 532                     else if (ps.length == 1 && ps[0] == Throwable.class)
 533                         return (Throwable)(c.newInstance(ex));
 534                 }
 535                 if (noArgCtor != null) {
 536                     Throwable wx = (Throwable)(noArgCtor.newInstance());
 537                     wx.initCause(ex);
 538                     return wx;
 539                 }
 540             } catch (Exception ignore) {
 541             }
 542         }
 543         return ex;
 544     }
 545 
 546     /**
 547      * Poll stale refs and remove them. Call only while holding lock.
 548      */
 549     private static void expungeStaleExceptions() {
 550         for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
 551             if (x instanceof ExceptionNode) {
 552                 ForkJoinTask<?> key = ((ExceptionNode)x).get();
 553                 ExceptionNode[] t = exceptionTable;
 554                 int i = System.identityHashCode(key) & (t.length - 1);
 555                 ExceptionNode e = t[i];
 556                 ExceptionNode pred = null;
 557                 while (e != null) {
 558                     ExceptionNode next = e.next;
 559                     if (e == x) {
 560                         if (pred == null)
 561                             t[i] = next;
 562                         else
 563                             pred.next = next;
 564                         break;
 565                     }
 566                     pred = e;
 567                     e = next;
 568                 }
 569             }
 570         }
 571     }
 572 
 573     /**
 574      * If lock is available, poll stale refs and remove them.
 575      * Called from ForkJoinPool when pools become quiescent.
 576      */
 577     static final void helpExpungeStaleExceptions() {
 578         final ReentrantLock lock = exceptionTableLock;
 579         if (lock.tryLock()) {
 580             try {
 581                 expungeStaleExceptions();
 582             } finally {
 583                 lock.unlock();
 584             }
 585         }
 586     }
 587 
 588     /**
 589      * Report the result of invoke or join; called only upon
 590      * non-normal return of internal versions.
 591      */
 592     private V reportResult() {
 593         int s; Throwable ex;
 594         if ((s = status) == CANCELLED)
 595             throw new CancellationException();
 596         if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
 597             UNSAFE.throwException(ex);
 598         return getRawResult();
 599     }
 600 
 601     // public methods
 602 
 603     /**
 604      * Arranges to asynchronously execute this task.  While it is not
 605      * necessarily enforced, it is a usage error to fork a task more
 606      * than once unless it has completed and been reinitialized.
 607      * Subsequent modifications to the state of this task or any data
 608      * it operates on are not necessarily consistently observable by
 609      * any thread other than the one executing it unless preceded by a
 610      * call to {@link #join} or related methods, or a call to {@link
 611      * #isDone} returning {@code true}.
 612      *
 613      * <p>This method may be invoked only from within {@code
 614      * ForkJoinPool} computations (as may be determined using method
 615      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
 616      * result in exceptions or errors, possibly including {@code
 617      * ClassCastException}.
 618      *
 619      * @return {@code this}, to simplify usage
 620      */
 621     public final ForkJoinTask<V> fork() {
 622         ((ForkJoinWorkerThread) Thread.currentThread())
 623             .pushTask(this);
 624         return this;
 625     }
 626 
 627     /**
 628      * Returns the result of the computation when it {@link #isDone is
 629      * done}.  This method differs from {@link #get()} in that
 630      * abnormal completion results in {@code RuntimeException} or
 631      * {@code Error}, not {@code ExecutionException}, and that
 632      * interrupts of the calling thread do <em>not</em> cause the
 633      * method to abruptly return by throwing {@code
 634      * InterruptedException}.
 635      *
 636      * @return the computed result
 637      */
 638     public final V join() {
 639         if (doJoin() != NORMAL)
 640             return reportResult();
 641         else
 642             return getRawResult();
 643     }
 644 
 645     /**
 646      * Commences performing this task, awaits its completion if
 647      * necessary, and returns its result, or throws an (unchecked)
 648      * {@code RuntimeException} or {@code Error} if the underlying
 649      * computation did so.
 650      *
 651      * @return the computed result
 652      */
 653     public final V invoke() {
 654         if (doInvoke() != NORMAL)
 655             return reportResult();
 656         else
 657             return getRawResult();
 658     }
 659 
 660     /**
 661      * Forks the given tasks, returning when {@code isDone} holds for
 662      * each task or an (unchecked) exception is encountered, in which
 663      * case the exception is rethrown. If more than one task
 664      * encounters an exception, then this method throws any one of
 665      * these exceptions. If any task encounters an exception, the
 666      * other may be cancelled. However, the execution status of
 667      * individual tasks is not guaranteed upon exceptional return. The
 668      * status of each task may be obtained using {@link
 669      * #getException()} and related methods to check if they have been
 670      * cancelled, completed normally or exceptionally, or left
 671      * unprocessed.
 672      *
 673      * <p>This method may be invoked only from within {@code
 674      * ForkJoinPool} computations (as may be determined using method
 675      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
 676      * result in exceptions or errors, possibly including {@code
 677      * ClassCastException}.
 678      *
 679      * @param t1 the first task
 680      * @param t2 the second task
 681      * @throws NullPointerException if any task is null
 682      */
 683     public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
 684         t2.fork();
 685         t1.invoke();
 686         t2.join();
 687     }
 688 
 689     /**
 690      * Forks the given tasks, returning when {@code isDone} holds for
 691      * each task or an (unchecked) exception is encountered, in which
 692      * case the exception is rethrown. If more than one task
 693      * encounters an exception, then this method throws any one of
 694      * these exceptions. If any task encounters an exception, others
 695      * may be cancelled. However, the execution status of individual
 696      * tasks is not guaranteed upon exceptional return. The status of
 697      * each task may be obtained using {@link #getException()} and
 698      * related methods to check if they have been cancelled, completed
 699      * normally or exceptionally, or left unprocessed.
 700      *
 701      * <p>This method may be invoked only from within {@code
 702      * ForkJoinPool} computations (as may be determined using method
 703      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
 704      * result in exceptions or errors, possibly including {@code
 705      * ClassCastException}.
 706      *
 707      * @param tasks the tasks
 708      * @throws NullPointerException if any task is null
 709      */
 710     public static void invokeAll(ForkJoinTask<?>... tasks) {
 711         Throwable ex = null;
 712         int last = tasks.length - 1;
 713         for (int i = last; i >= 0; --i) {
 714             ForkJoinTask<?> t = tasks[i];
 715             if (t == null) {
 716                 if (ex == null)
 717                     ex = new NullPointerException();
 718             }
 719             else if (i != 0)
 720                 t.fork();
 721             else if (t.doInvoke() < NORMAL && ex == null)
 722                 ex = t.getException();
 723         }
 724         for (int i = 1; i <= last; ++i) {
 725             ForkJoinTask<?> t = tasks[i];
 726             if (t != null) {
 727                 if (ex != null)
 728                     t.cancel(false);
 729                 else if (t.doJoin() < NORMAL && ex == null)
 730                     ex = t.getException();
 731             }
 732         }
 733         if (ex != null)
 734             UNSAFE.throwException(ex);
 735     }
 736 
 737     /**
 738      * Forks all tasks in the specified collection, returning when
 739      * {@code isDone} holds for each task or an (unchecked) exception
 740      * is encountered, in which case the exception is rethrown. If
 741      * more than one task encounters an exception, then this method
 742      * throws any one of these exceptions. If any task encounters an
 743      * exception, others may be cancelled. However, the execution
 744      * status of individual tasks is not guaranteed upon exceptional
 745      * return. The status of each task may be obtained using {@link
 746      * #getException()} and related methods to check if they have been
 747      * cancelled, completed normally or exceptionally, or left
 748      * unprocessed.
 749      *
 750      * <p>This method may be invoked only from within {@code
 751      * ForkJoinPool} computations (as may be determined using method
 752      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
 753      * result in exceptions or errors, possibly including {@code
 754      * ClassCastException}.
 755      *
 756      * @param tasks the collection of tasks
 757      * @return the tasks argument, to simplify usage
 758      * @throws NullPointerException if tasks or any element are null
 759      */
 760     public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
 761         if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
 762             invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
 763             return tasks;
 764         }
 765         @SuppressWarnings("unchecked")
 766         List<? extends ForkJoinTask<?>> ts =
 767             (List<? extends ForkJoinTask<?>>) tasks;
 768         Throwable ex = null;
 769         int last = ts.size() - 1;
 770         for (int i = last; i >= 0; --i) {
 771             ForkJoinTask<?> t = ts.get(i);
 772             if (t == null) {
 773                 if (ex == null)
 774                     ex = new NullPointerException();
 775             }
 776             else if (i != 0)
 777                 t.fork();
 778             else if (t.doInvoke() < NORMAL && ex == null)
 779                 ex = t.getException();
 780         }
 781         for (int i = 1; i <= last; ++i) {
 782             ForkJoinTask<?> t = ts.get(i);
 783             if (t != null) {
 784                 if (ex != null)
 785                     t.cancel(false);
 786                 else if (t.doJoin() < NORMAL && ex == null)
 787                     ex = t.getException();
 788             }
 789         }
 790         if (ex != null)
 791             UNSAFE.throwException(ex);
 792         return tasks;
 793     }
 794 
 795     /**
 796      * Attempts to cancel execution of this task. This attempt will
 797      * fail if the task has already completed or could not be
 798      * cancelled for some other reason. If successful, and this task
 799      * has not started when {@code cancel} is called, execution of
 800      * this task is suppressed. After this method returns
 801      * successfully, unless there is an intervening call to {@link
 802      * #reinitialize}, subsequent calls to {@link #isCancelled},
 803      * {@link #isDone}, and {@code cancel} will return {@code true}
 804      * and calls to {@link #join} and related methods will result in
 805      * {@code CancellationException}.
 806      *
 807      * <p>This method may be overridden in subclasses, but if so, must
 808      * still ensure that these properties hold. In particular, the
 809      * {@code cancel} method itself must not throw exceptions.
 810      *
 811      * <p>This method is designed to be invoked by <em>other</em>
 812      * tasks. To terminate the current task, you can just return or
 813      * throw an unchecked exception from its computation method, or
 814      * invoke {@link #completeExceptionally}.
 815      *
 816      * @param mayInterruptIfRunning this value has no effect in the
 817      * default implementation because interrupts are not used to
 818      * control cancellation.
 819      *
 820      * @return {@code true} if this task is now cancelled
 821      */
 822     public boolean cancel(boolean mayInterruptIfRunning) {
 823         return setCompletion(CANCELLED) == CANCELLED;
 824     }
 825 
 826     /**
 827      * Cancels, ignoring any exceptions thrown by cancel. Used during
 828      * worker and pool shutdown. Cancel is spec'ed not to throw any
 829      * exceptions, but if it does anyway, we have no recourse during
 830      * shutdown, so guard against this case.
 831      */
 832     final void cancelIgnoringExceptions() {
 833         try {
 834             cancel(false);
 835         } catch (Throwable ignore) {
 836         }
 837     }
 838 
 839     public final boolean isDone() {
 840         return status < 0;
 841     }
 842 
 843     public final boolean isCancelled() {
 844         return status == CANCELLED;
 845     }
 846 
 847     /**
 848      * Returns {@code true} if this task threw an exception or was cancelled.
 849      *
 850      * @return {@code true} if this task threw an exception or was cancelled
 851      */
 852     public final boolean isCompletedAbnormally() {
 853         return status < NORMAL;
 854     }
 855 
 856     /**
 857      * Returns {@code true} if this task completed without throwing an
 858      * exception and was not cancelled.
 859      *
 860      * @return {@code true} if this task completed without throwing an
 861      * exception and was not cancelled
 862      */
 863     public final boolean isCompletedNormally() {
 864         return status == NORMAL;
 865     }
 866 
 867     /**
 868      * Returns the exception thrown by the base computation, or a
 869      * {@code CancellationException} if cancelled, or {@code null} if
 870      * none or if the method has not yet completed.
 871      *
 872      * @return the exception, or {@code null} if none
 873      */
 874     public final Throwable getException() {
 875         int s = status;
 876         return ((s >= NORMAL)    ? null :
 877                 (s == CANCELLED) ? new CancellationException() :
 878                 getThrowableException());
 879     }
 880 
 881     /**
 882      * Completes this task abnormally, and if not already aborted or
 883      * cancelled, causes it to throw the given exception upon
 884      * {@code join} and related operations. This method may be used
 885      * to induce exceptions in asynchronous tasks, or to force
 886      * completion of tasks that would not otherwise complete.  Its use
 887      * in other situations is discouraged.  This method is
 888      * overridable, but overridden versions must invoke {@code super}
 889      * implementation to maintain guarantees.
 890      *
 891      * @param ex the exception to throw. If this exception is not a
 892      * {@code RuntimeException} or {@code Error}, the actual exception
 893      * thrown will be a {@code RuntimeException} with cause {@code ex}.
 894      */
 895     public void completeExceptionally(Throwable ex) {
 896         setExceptionalCompletion((ex instanceof RuntimeException) ||
 897                                  (ex instanceof Error) ? ex :
 898                                  new RuntimeException(ex));
 899     }
 900 
 901     /**
 902      * Completes this task, and if not already aborted or cancelled,
 903      * returning the given value as the result of subsequent
 904      * invocations of {@code join} and related operations. This method
 905      * may be used to provide results for asynchronous tasks, or to
 906      * provide alternative handling for tasks that would not otherwise
 907      * complete normally. Its use in other situations is
 908      * discouraged. This method is overridable, but overridden
 909      * versions must invoke {@code super} implementation to maintain
 910      * guarantees.
 911      *
 912      * @param value the result value for this task
 913      */
 914     public void complete(V value) {
 915         try {
 916             setRawResult(value);
 917         } catch (Throwable rex) {
 918             setExceptionalCompletion(rex);
 919             return;
 920         }
 921         setCompletion(NORMAL);
 922     }
 923 
 924     /**
 925      * Waits if necessary for the computation to complete, and then
 926      * retrieves its result.
 927      *
 928      * @return the computed result
 929      * @throws CancellationException if the computation was cancelled
 930      * @throws ExecutionException if the computation threw an
 931      * exception
 932      * @throws InterruptedException if the current thread is not a
 933      * member of a ForkJoinPool and was interrupted while waiting
 934      */
 935     public final V get() throws InterruptedException, ExecutionException {
 936         int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
 937             doJoin() : externalInterruptibleAwaitDone(0L);
 938         Throwable ex;
 939         if (s == CANCELLED)
 940             throw new CancellationException();
 941         if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
 942             throw new ExecutionException(ex);
 943         return getRawResult();
 944     }
 945 
 946     /**
 947      * Waits if necessary for at most the given time for the computation
 948      * to complete, and then retrieves its result, if available.
 949      *
 950      * @param timeout the maximum time to wait
 951      * @param unit the time unit of the timeout argument
 952      * @return the computed result
 953      * @throws CancellationException if the computation was cancelled
 954      * @throws ExecutionException if the computation threw an
 955      * exception
 956      * @throws InterruptedException if the current thread is not a
 957      * member of a ForkJoinPool and was interrupted while waiting
 958      * @throws TimeoutException if the wait timed out
 959      */
 960     public final V get(long timeout, TimeUnit unit)
 961         throws InterruptedException, ExecutionException, TimeoutException {
 962         Thread t = Thread.currentThread();
 963         if (t instanceof ForkJoinWorkerThread) {
 964             ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
 965             long nanos = unit.toNanos(timeout);
 966             if (status >= 0) {
 967                 boolean completed = false;
 968                 if (w.unpushTask(this)) {
 969                     try {
 970                         completed = exec();
 971                     } catch (Throwable rex) {
 972                         setExceptionalCompletion(rex);
 973                     }
 974                 }
 975                 if (completed)
 976                     setCompletion(NORMAL);
 977                 else if (status >= 0 && nanos > 0)
 978                     w.pool.timedAwaitJoin(this, nanos);
 979             }
 980         }
 981         else {
 982             long millis = unit.toMillis(timeout);
 983             if (millis > 0)
 984                 externalInterruptibleAwaitDone(millis);
 985         }
 986         int s = status;
 987         if (s != NORMAL) {
 988             Throwable ex;
 989             if (s == CANCELLED)
 990                 throw new CancellationException();
 991             if (s != EXCEPTIONAL)
 992                 throw new TimeoutException();
 993             if ((ex = getThrowableException()) != null)
 994                 throw new ExecutionException(ex);
 995         }
 996         return getRawResult();
 997     }
 998 
 999     /**
1000      * Joins this task, without returning its result or throwing its
1001      * exception. This method may be useful when processing
1002      * collections of tasks when some have been cancelled or otherwise
1003      * known to have aborted.
1004      */
1005     public final void quietlyJoin() {
1006         doJoin();
1007     }
1008 
1009     /**
1010      * Commences performing this task and awaits its completion if
1011      * necessary, without returning its result or throwing its
1012      * exception.
1013      */
1014     public final void quietlyInvoke() {
1015         doInvoke();
1016     }
1017 
1018     /**
1019      * Possibly executes tasks until the pool hosting the current task
1020      * {@link ForkJoinPool#isQuiescent is quiescent}. This method may
1021      * be of use in designs in which many tasks are forked, but none
1022      * are explicitly joined, instead executing them until all are
1023      * processed.
1024      *
1025      * <p>This method may be invoked only from within {@code
1026      * ForkJoinPool} computations (as may be determined using method
1027      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1028      * result in exceptions or errors, possibly including {@code
1029      * ClassCastException}.
1030      */
1031     public static void helpQuiesce() {
1032         ((ForkJoinWorkerThread) Thread.currentThread())
1033             .helpQuiescePool();
1034     }
1035 
1036     /**
1037      * Resets the internal bookkeeping state of this task, allowing a
1038      * subsequent {@code fork}. This method allows repeated reuse of
1039      * this task, but only if reuse occurs when this task has either
1040      * never been forked, or has been forked, then completed and all
1041      * outstanding joins of this task have also completed. Effects
1042      * under any other usage conditions are not guaranteed.
1043      * This method may be useful when executing
1044      * pre-constructed trees of subtasks in loops.
1045      *
1046      * <p>Upon completion of this method, {@code isDone()} reports
1047      * {@code false}, and {@code getException()} reports {@code
1048      * null}. However, the value returned by {@code getRawResult} is
1049      * unaffected. To clear this value, you can invoke {@code
1050      * setRawResult(null)}.
1051      */
1052     public void reinitialize() {
1053         if (status == EXCEPTIONAL)
1054             clearExceptionalCompletion();
1055         else
1056             status = 0;
1057     }
1058 
1059     /**
1060      * Returns the pool hosting the current task execution, or null
1061      * if this task is executing outside of any ForkJoinPool.
1062      *
1063      * @see #inForkJoinPool
1064      * @return the pool, or {@code null} if none
1065      */
1066     public static ForkJoinPool getPool() {
1067         Thread t = Thread.currentThread();
1068         return (t instanceof ForkJoinWorkerThread) ?
1069             ((ForkJoinWorkerThread) t).pool : null;
1070     }
1071 
1072     /**
1073      * Returns {@code true} if the current thread is a {@link
1074      * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
1075      *
1076      * @return {@code true} if the current thread is a {@link
1077      * ForkJoinWorkerThread} executing as a ForkJoinPool computation,
1078      * or {@code false} otherwise
1079      */
1080     public static boolean inForkJoinPool() {
1081         return Thread.currentThread() instanceof ForkJoinWorkerThread;
1082     }
1083 
1084     /**
1085      * Tries to unschedule this task for execution. This method will
1086      * typically succeed if this task is the most recently forked task
1087      * by the current thread, and has not commenced executing in
1088      * another thread.  This method may be useful when arranging
1089      * alternative local processing of tasks that could have been, but
1090      * were not, stolen.
1091      *
1092      * <p>This method may be invoked only from within {@code
1093      * ForkJoinPool} computations (as may be determined using method
1094      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1095      * result in exceptions or errors, possibly including {@code
1096      * ClassCastException}.
1097      *
1098      * @return {@code true} if unforked
1099      */
1100     public boolean tryUnfork() {
1101         return ((ForkJoinWorkerThread) Thread.currentThread())
1102             .unpushTask(this);
1103     }
1104 
1105     /**
1106      * Returns an estimate of the number of tasks that have been
1107      * forked by the current worker thread but not yet executed. This
1108      * value may be useful for heuristic decisions about whether to
1109      * fork other tasks.
1110      *
1111      * <p>This method may be invoked only from within {@code
1112      * ForkJoinPool} computations (as may be determined using method
1113      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1114      * result in exceptions or errors, possibly including {@code
1115      * ClassCastException}.
1116      *
1117      * @return the number of tasks
1118      */
1119     public static int getQueuedTaskCount() {
1120         return ((ForkJoinWorkerThread) Thread.currentThread())
1121             .getQueueSize();
1122     }
1123 
1124     /**
1125      * Returns an estimate of how many more locally queued tasks are
1126      * held by the current worker thread than there are other worker
1127      * threads that might steal them.  This value may be useful for
1128      * heuristic decisions about whether to fork other tasks. In many
1129      * usages of ForkJoinTasks, at steady state, each worker should
1130      * aim to maintain a small constant surplus (for example, 3) of
1131      * tasks, and to process computations locally if this threshold is
1132      * exceeded.
1133      *
1134      * <p>This method may be invoked only from within {@code
1135      * ForkJoinPool} computations (as may be determined using method
1136      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1137      * result in exceptions or errors, possibly including {@code
1138      * ClassCastException}.
1139      *
1140      * @return the surplus number of tasks, which may be negative
1141      */
1142     public static int getSurplusQueuedTaskCount() {
1143         return ((ForkJoinWorkerThread) Thread.currentThread())
1144             .getEstimatedSurplusTaskCount();
1145     }
1146 
1147     // Extension methods
1148 
1149     /**
1150      * Returns the result that would be returned by {@link #join}, even
1151      * if this task completed abnormally, or {@code null} if this task
1152      * is not known to have been completed.  This method is designed
1153      * to aid debugging, as well as to support extensions. Its use in
1154      * any other context is discouraged.
1155      *
1156      * @return the result, or {@code null} if not completed
1157      */
1158     public abstract V getRawResult();
1159 
1160     /**
1161      * Forces the given value to be returned as a result.  This method
1162      * is designed to support extensions, and should not in general be
1163      * called otherwise.
1164      *
1165      * @param value the value
1166      */
1167     protected abstract void setRawResult(V value);
1168 
1169     /**
1170      * Immediately performs the base action of this task.  This method
1171      * is designed to support extensions, and should not in general be
1172      * called otherwise. The return value controls whether this task
1173      * is considered to be done normally. It may return false in
1174      * asynchronous actions that require explicit invocations of
1175      * {@link #complete} to become joinable. It may also throw an
1176      * (unchecked) exception to indicate abnormal exit.
1177      *
1178      * @return {@code true} if completed normally
1179      */
1180     protected abstract boolean exec();
1181 
1182     /**
1183      * Returns, but does not unschedule or execute, a task queued by
1184      * the current thread but not yet executed, if one is immediately
1185      * available. There is no guarantee that this task will actually
1186      * be polled or executed next. Conversely, this method may return
1187      * null even if a task exists but cannot be accessed without
1188      * contention with other threads.  This method is designed
1189      * primarily to support extensions, and is unlikely to be useful
1190      * otherwise.
1191      *
1192      * <p>This method may be invoked only from within {@code
1193      * ForkJoinPool} computations (as may be determined using method
1194      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1195      * result in exceptions or errors, possibly including {@code
1196      * ClassCastException}.
1197      *
1198      * @return the next task, or {@code null} if none are available
1199      */
1200     protected static ForkJoinTask<?> peekNextLocalTask() {
1201         return ((ForkJoinWorkerThread) Thread.currentThread())
1202             .peekTask();
1203     }
1204 
1205     /**
1206      * Unschedules and returns, without executing, the next task
1207      * queued by the current thread but not yet executed.  This method
1208      * is designed primarily to support extensions, and is unlikely to
1209      * be useful otherwise.
1210      *
1211      * <p>This method may be invoked only from within {@code
1212      * ForkJoinPool} computations (as may be determined using method
1213      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1214      * result in exceptions or errors, possibly including {@code
1215      * ClassCastException}.
1216      *
1217      * @return the next task, or {@code null} if none are available
1218      */
1219     protected static ForkJoinTask<?> pollNextLocalTask() {
1220         return ((ForkJoinWorkerThread) Thread.currentThread())
1221             .pollLocalTask();
1222     }
1223 
1224     /**
1225      * Unschedules and returns, without executing, the next task
1226      * queued by the current thread but not yet executed, if one is
1227      * available, or if not available, a task that was forked by some
1228      * other thread, if available. Availability may be transient, so a
1229      * {@code null} result does not necessarily imply quiescence
1230      * of the pool this task is operating in.  This method is designed
1231      * primarily to support extensions, and is unlikely to be useful
1232      * otherwise.
1233      *
1234      * <p>This method may be invoked only from within {@code
1235      * ForkJoinPool} computations (as may be determined using method
1236      * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1237      * result in exceptions or errors, possibly including {@code
1238      * ClassCastException}.
1239      *
1240      * @return a task, or {@code null} if none are available
1241      */
1242     protected static ForkJoinTask<?> pollTask() {
1243         return ((ForkJoinWorkerThread) Thread.currentThread())
1244             .pollTask();
1245     }
1246 
1247     /**
1248      * Adaptor for Runnables. This implements RunnableFuture
1249      * to be compliant with AbstractExecutorService constraints
1250      * when used in ForkJoinPool.
1251      */
1252     static final class AdaptedRunnable<T> extends ForkJoinTask<T>
1253         implements RunnableFuture<T> {
1254         final Runnable runnable;
1255         final T resultOnCompletion;
1256         T result;
1257         AdaptedRunnable(Runnable runnable, T result) {
1258             if (runnable == null) throw new NullPointerException();
1259             this.runnable = runnable;
1260             this.resultOnCompletion = result;
1261         }
1262         public T getRawResult() { return result; }
1263         public void setRawResult(T v) { result = v; }
1264         public boolean exec() {
1265             runnable.run();
1266             result = resultOnCompletion;
1267             return true;
1268         }
1269         public void run() { invoke(); }
1270         private static final long serialVersionUID = 5232453952276885070L;
1271     }
1272 
1273     /**
1274      * Adaptor for Callables
1275      */
1276     static final class AdaptedCallable<T> extends ForkJoinTask<T>
1277         implements RunnableFuture<T> {
1278         final Callable<? extends T> callable;
1279         T result;
1280         AdaptedCallable(Callable<? extends T> callable) {
1281             if (callable == null) throw new NullPointerException();
1282             this.callable = callable;
1283         }
1284         public T getRawResult() { return result; }
1285         public void setRawResult(T v) { result = v; }
1286         public boolean exec() {
1287             try {
1288                 result = callable.call();
1289                 return true;
1290             } catch (Error err) {
1291                 throw err;
1292             } catch (RuntimeException rex) {
1293                 throw rex;
1294             } catch (Exception ex) {
1295                 throw new RuntimeException(ex);
1296             }
1297         }
1298         public void run() { invoke(); }
1299         private static final long serialVersionUID = 2838392045355241008L;
1300     }
1301 
1302     /**
1303      * Returns a new {@code ForkJoinTask} that performs the {@code run}
1304      * method of the given {@code Runnable} as its action, and returns
1305      * a null result upon {@link #join}.
1306      *
1307      * @param runnable the runnable action
1308      * @return the task
1309      */
1310     public static ForkJoinTask<?> adapt(Runnable runnable) {
1311         return new AdaptedRunnable<Void>(runnable, null);
1312     }
1313 
1314     /**
1315      * Returns a new {@code ForkJoinTask} that performs the {@code run}
1316      * method of the given {@code Runnable} as its action, and returns
1317      * the given result upon {@link #join}.
1318      *
1319      * @param runnable the runnable action
1320      * @param result the result upon completion
1321      * @return the task
1322      */
1323     public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
1324         return new AdaptedRunnable<T>(runnable, result);
1325     }
1326 
1327     /**
1328      * Returns a new {@code ForkJoinTask} that performs the {@code call}
1329      * method of the given {@code Callable} as its action, and returns
1330      * its result upon {@link #join}, translating any checked exceptions
1331      * encountered into {@code RuntimeException}.
1332      *
1333      * @param callable the callable action
1334      * @return the task
1335      */
1336     public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
1337         return new AdaptedCallable<T>(callable);
1338     }
1339 
1340     // Serialization support
1341 
1342     private static final long serialVersionUID = -7721805057305804111L;
1343 
1344     /**
1345      * Saves the state to a stream (that is, serializes it).
1346      *
1347      * @serialData the current run status and the exception thrown
1348      * during execution, or {@code null} if none
1349      * @param s the stream
1350      */
1351     private void writeObject(java.io.ObjectOutputStream s)
1352         throws java.io.IOException {
1353         s.defaultWriteObject();
1354         s.writeObject(getException());
1355     }
1356 
1357     /**
1358      * Reconstitutes the instance from a stream (that is, deserializes it).
1359      *
1360      * @param s the stream
1361      */
1362     private void readObject(java.io.ObjectInputStream s)
1363         throws java.io.IOException, ClassNotFoundException {
1364         s.defaultReadObject();
1365         Object ex = s.readObject();
1366         if (ex != null)
1367             setExceptionalCompletion((Throwable)ex);
1368     }
1369 
1370     // Unsafe mechanics
1371     private static final sun.misc.Unsafe UNSAFE;
1372     private static final long statusOffset;
1373     static {
1374         exceptionTableLock = new ReentrantLock();
1375         exceptionTableRefQueue = new ReferenceQueue<Object>();
1376         exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
1377         try {
1378             UNSAFE = sun.misc.Unsafe.getUnsafe();
1379             statusOffset = UNSAFE.objectFieldOffset
1380                 (ForkJoinTask.class.getDeclaredField("status"));
1381         } catch (Exception e) {
1382             throw new Error(e);
1383         }
1384     }
1385 
1386 }