content: /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Written by Doug Lea with assistance from members of JCP JSR-166
   * Expert Group and released to the public domain, as explained at
   * http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package java.util.concurrent;
  
  import java.io.Serializable;
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.lang.reflect.Constructor;
  import java.util.Collection;
  import java.util.List;
  import java.util.RandomAccess;
  import java.util.concurrent.locks.ReentrantLock;
  
  // BEGIN android-note
  // removed java 9 code
  // END android-note
  
  /**
   * Abstract base class for tasks that run within a {@link ForkJoinPool}.
   * A {@code ForkJoinTask} is a thread-like entity that is much
   * lighter weight than a normal thread.  Huge numbers of tasks and
   * subtasks may be hosted by a small number of actual threads in a
   * ForkJoinPool, at the price of some usage limitations.
   *
   * <p>A "main" {@code ForkJoinTask} begins execution when it is
   * explicitly submitted to a {@link ForkJoinPool}, or, if not already
   * engaged in a ForkJoin computation, commenced in the {@link
   * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
   * related methods.  Once started, it will usually in turn start other
   * subtasks.  As indicated by the name of this class, many programs
   * using {@code ForkJoinTask} employ only methods {@link #fork} and
   * {@link #join}, or derivatives such as {@link
   * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
   * provides a number of other methods that can come into play in
   * advanced usages, as well as extension mechanics that allow support
   * of new forms of fork/join processing.
   *
   * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
   * The efficiency of {@code ForkJoinTask}s stems from a set of
   * restrictions (that are only partially statically enforceable)
   * reflecting their main use as computational tasks calculating pure
   * functions or operating on purely isolated objects.  The primary
   * coordination mechanisms are {@link #fork}, that arranges
   * asynchronous execution, and {@link #join}, that doesn't proceed
   * until the task's result has been computed.  Computations should
   * ideally avoid {@code synchronized} methods or blocks, and should
   * minimize other blocking synchronization apart from joining other
   * tasks or using synchronizers such as Phasers that are advertised to
   * cooperate with fork/join scheduling. Subdividable tasks should also
   * not perform blocking I/O, and should ideally access variables that
   * are completely independent of those accessed by other running
   * tasks. These guidelines are loosely enforced by not permitting
   * checked exceptions such as {@code IOExceptions} to be
   * thrown. However, computations may still encounter unchecked
   * exceptions, that are rethrown to callers attempting to join
   * them. These exceptions may additionally include {@link
   * RejectedExecutionException} stemming from internal resource
   * exhaustion, such as failure to allocate internal task
   * queues. Rethrown exceptions behave in the same way as regular
   * exceptions, but, when possible, contain stack traces (as displayed
   * for example using {@code ex.printStackTrace()}) of both the thread
   * that initiated the computation as well as the thread actually
   * encountering the exception; minimally only the latter.
   *
   * <p>It is possible to define and use ForkJoinTasks that may block,
   * but doing do requires three further considerations: (1) Completion
   * of few if any <em>other</em> tasks should be dependent on a task
   * that blocks on external synchronization or I/O. Event-style async
   * tasks that are never joined (for example, those subclassing {@link
   * CountedCompleter}) often fall into this category.  (2) To minimize
   * resource impact, tasks should be small; ideally performing only the
   * (possibly) blocking action. (3) Unless the {@link
   * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
   * blocked tasks is known to be less than the pool's {@link
   * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
   * enough threads will be available to ensure progress or good
   * performance.
   *
   * <p>The primary method for awaiting completion and extracting
   * results of a task is {@link #join}, but there are several variants:
   * The {@link Future#get} methods support interruptible and/or timed
   * waits for completion and report results using {@code Future}
   * conventions. Method {@link #invoke} is semantically
   * equivalent to {@code fork(); join()} but always attempts to begin
   * execution in the current thread. The "<em>quiet</em>" forms of
   * these methods do not extract results or report exceptions. These
   * may be useful when a set of tasks are being executed, and you need
   * to delay processing of results or exceptions until all complete.
   * Method {@code invokeAll} (available in multiple versions)
   * performs the most common form of parallel invocation: forking a set
   * of tasks and joining them all.
   *
   * <p>In the most typical usages, a fork-join pair act like a call
   * (fork) and return (join) from a parallel recursive function. As is
   * the case with other forms of recursive calls, returns (joins)
   * should be performed innermost-first. For example, {@code a.fork();
   * b.fork(); b.join(); a.join();} is likely to be substantially more
   * efficient than joining {@code a} before {@code b}.
   *
   * <p>The execution status of tasks may be queried at several levels
   * of detail: {@link #isDone} is true if a task completed in any way
   * (including the case where a task was cancelled without executing);
   * {@link #isCompletedNormally} is true if a task completed without
   * cancellation or encountering an exception; {@link #isCancelled} is
   * true if the task was cancelled (in which case {@link #getException}
   * returns a {@link CancellationException}); and
   * {@link #isCompletedAbnormally} is true if a task was either
   * cancelled or encountered an exception, in which case {@link
   * #getException} will return either the encountered exception or
   * {@link CancellationException}.
   *
   * <p>The ForkJoinTask class is not usually directly subclassed.
   * Instead, you subclass one of the abstract classes that support a
   * particular style of fork/join processing, typically {@link
   * RecursiveAction} for most computations that do not return results,
   * {@link RecursiveTask} for those that do, and {@link
   * CountedCompleter} for those in which completed actions trigger
   * other actions.  Normally, a concrete ForkJoinTask subclass declares
   * fields comprising its parameters, established in a constructor, and
   * then defines a {@code compute} method that somehow uses the control
   * methods supplied by this base class.
   *
   * <p>Method {@link #join} and its variants are appropriate for use
   * only when completion dependencies are acyclic; that is, the
   * parallel computation can be described as a directed acyclic graph
   * (DAG). Otherwise, executions may encounter a form of deadlock as
   * tasks cyclically wait for each other.  However, this framework
   * supports other methods and techniques (for example the use of
   * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
   * may be of use in constructing custom subclasses for problems that
   * are not statically structured as DAGs. To support such usages, a
   * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
   * value using {@link #setForkJoinTaskTag} or {@link
   * #compareAndSetForkJoinTaskTag} and checked using {@link
   * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
   * these {@code protected} methods or tags for any purpose, but they
   * may be of use in the construction of specialized subclasses.  For
   * example, parallel graph traversals can use the supplied methods to
   * avoid revisiting nodes/tasks that have already been processed.
   * (Method names for tagging are bulky in part to encourage definition
   * of methods that reflect their usage patterns.)
   *
   * <p>Most base support methods are {@code final}, to prevent
   * overriding of implementations that are intrinsically tied to the
   * underlying lightweight task scheduling framework.  Developers
   * creating new basic styles of fork/join processing should minimally
   * implement {@code protected} methods {@link #exec}, {@link
   * #setRawResult}, and {@link #getRawResult}, while also introducing
   * an abstract computational method that can be implemented in its
   * subclasses, possibly relying on other {@code protected} methods
   * provided by this class.
   *
   * <p>ForkJoinTasks should perform relatively small amounts of
   * computation. Large tasks should be split into smaller subtasks,
   * usually via recursive decomposition. As a very rough rule of thumb,
   * a task should perform more than 100 and less than 10000 basic
   * computational steps, and should avoid indefinite looping. If tasks
   * are too big, then parallelism cannot improve throughput. If too
   * small, then memory and internal task maintenance overhead may
   * overwhelm processing.
   *
   * <p>This class provides {@code adapt} methods for {@link Runnable}
   * and {@link Callable}, that may be of use when mixing execution of
   * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
   * of this form, consider using a pool constructed in <em>asyncMode</em>.
   *
   * <p>ForkJoinTasks are {@code Serializable}, which enables them to be
   * used in extensions such as remote execution frameworks. It is
   * sensible to serialize tasks only before or after, but not during,
   * execution. Serialization is not relied on during execution itself.
   *
   * @since 1.7
   * @author Doug Lea
   */
  public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
  
      /*
       * See the internal documentation of class ForkJoinPool for a
       * general implementation overview.  ForkJoinTasks are mainly
       * responsible for maintaining their "status" field amidst relays
       * to methods in ForkJoinWorkerThread and ForkJoinPool.
       *
       * The methods of this class are more-or-less layered into
       * (1) basic status maintenance
       * (2) execution and awaiting completion
       * (3) user-level methods that additionally report results.
       * This is sometimes hard to see because this file orders exported
       * methods in a way that flows well in javadocs.
       */
  
      /*
       * The status field holds run control status bits packed into a
       * single int to minimize footprint and to ensure atomicity (via
       * CAS).  Status is initially zero, and takes on nonnegative
       * values until completed, upon which status (anded with
       * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
       * undergoing blocking waits by other threads have the SIGNAL bit
       * set.  Completion of a stolen task with SIGNAL set awakens any
       * waiters via notifyAll. Even though suboptimal for some
       * purposes, we use basic builtin wait/notify to take advantage of
       * "monitor inflation" in JVMs that we would otherwise need to
       * emulate to avoid adding further per-task bookkeeping overhead.
       * We want these monitors to be "fat", i.e., not use biasing or
       * thin-lock techniques, so use some odd coding idioms that tend
       * to avoid them, mainly by arranging that every synchronized
       * block performs a wait, notifyAll or both.
       *
       * These control bits occupy only (some of) the upper half (16
       * bits) of status field. The lower bits are used for user-defined
       * tags.
       */
  
      /** The run status of this task */
      volatile int status; // accessed directly by pool and workers
      static final int DONE_MASK   = 0xf0000000;  // mask out non-completion bits
      static final int NORMAL      = 0xf0000000;  // must be negative
      static final int CANCELLED   = 0xc0000000;  // must be < NORMAL
      static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
      static final int SIGNAL      = 0x00010000;  // must be >= 1 << 16
      static final int SMASK       = 0x0000ffff;  // short bits for tags
  
      /**
       * Marks completion and wakes up threads waiting to join this
       * task.
       *
       * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
       * @return completion status on exit
       */
      private int setCompletion(int completion) {
          for (int s;;) {
              if ((s = status) < 0)
                  return s;
              if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
                  if ((s >>> 16) != 0)
                      synchronized (this) { notifyAll(); }
                  return completion;
              }
          }
      }
  
      /**
       * Primary execution method for stolen tasks. Unless done, calls
       * exec and records status if completed, but doesn't wait for
       * completion otherwise.
       *
       * @return status on exit from this method
       */
      final int doExec() {
          int s; boolean completed;
          if ((s = status) >= 0) {
              try {
                  completed = exec();
              } catch (Throwable rex) {
                  return setExceptionalCompletion(rex);
              }
              if (completed)
                  s = setCompletion(NORMAL);
          }
          return s;
      }
  
      /**
       * If not done, sets SIGNAL status and performs Object.wait(timeout).
       * This task may or may not be done on exit. Ignores interrupts.
       *
       * @param timeout using Object.wait conventions.
       */
      final void internalWait(long timeout) {
          int s;
          if ((s = status) >= 0 && // force completer to issue notify
              U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
              synchronized (this) {
                  if (status >= 0)
                      try { wait(timeout); } catch (InterruptedException ie) { }
                  else
                      notifyAll();
              }
          }
      }
  
      /**
       * Blocks a non-worker-thread until completion.
       * @return status upon completion
       */
      private int externalAwaitDone() {
          int s = ((this instanceof CountedCompleter) ? // try helping
                   ForkJoinPool.common.externalHelpComplete(
                       (CountedCompleter<?>)this, 0) :
                   ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
          if (s >= 0 && (s = status) >= 0) {
              boolean interrupted = false;
              do {
                  if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                      synchronized (this) {
                          if (status >= 0) {
                              try {
                                  wait(0L);
                              } catch (InterruptedException ie) {
                                  interrupted = true;
                              }
                          }
                          else
                              notifyAll();
                      }
                  }
              } while ((s = status) >= 0);
              if (interrupted)
                  Thread.currentThread().interrupt();
          }
          return s;
      }
  
      /**
       * Blocks a non-worker-thread until completion or interruption.
       */
      private int externalInterruptibleAwaitDone() throws InterruptedException {
          int s;
          if (Thread.interrupted())
              throw new InterruptedException();
          if ((s = status) >= 0 &&
              (s = ((this instanceof CountedCompleter) ?
                    ForkJoinPool.common.externalHelpComplete(
                        (CountedCompleter<?>)this, 0) :
                    ForkJoinPool.common.tryExternalUnpush(this) ? doExec() :
                    0)) >= 0) {
              while ((s = status) >= 0) {
                  if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                      synchronized (this) {
                          if (status >= 0)
                              wait(0L);
                          else
                              notifyAll();
                      }
                  }
              }
          }
          return s;
      }
  
      /**
       * Implementation for join, get, quietlyJoin. Directly handles
       * only cases of already-completed, external wait, and
       * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.
       *
       * @return status upon completion
       */
      private int doJoin() {
          int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
          return (s = status) < 0 ? s :
              ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
              (w = (wt = (ForkJoinWorkerThread)t).workQueue).
              tryUnpush(this) && (s = doExec()) < 0 ? s :
              wt.pool.awaitJoin(w, this, 0L) :
              externalAwaitDone();
      }
  
      /**
       * Implementation for invoke, quietlyInvoke.
       *
       * @return status upon completion
       */
      private int doInvoke() {
          int s; Thread t; ForkJoinWorkerThread wt;
          return (s = doExec()) < 0 ? s :
              ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
              (wt = (ForkJoinWorkerThread)t).pool.
              awaitJoin(wt.workQueue, this, 0L) :
              externalAwaitDone();
      }
  
      // Exception table support
  
      /**
       * Table of exceptions thrown by tasks, to enable reporting by
       * callers. Because exceptions are rare, we don't directly keep
       * them with task objects, but instead use a weak ref table.  Note
       * that cancellation exceptions don't appear in the table, but are
       * instead recorded as status values.
       *
       * Note: These statics are initialized below in static block.
       */
      private static final ExceptionNode[] exceptionTable;
      private static final ReentrantLock exceptionTableLock;
      private static final ReferenceQueue<Object> exceptionTableRefQueue;
  
      /**
       * Fixed capacity for exceptionTable.
       */
      private static final int EXCEPTION_MAP_CAPACITY = 32;
  
      /**
       * Key-value nodes for exception table.  The chained hash table
       * uses identity comparisons, full locking, and weak references
       * for keys. The table has a fixed capacity because it only
       * maintains task exceptions long enough for joiners to access
       * them, so should never become very large for sustained
       * periods. However, since we do not know when the last joiner
       * completes, we must use weak references and expunge them. We do
       * so on each operation (hence full locking). Also, some thread in
       * any ForkJoinPool will call helpExpungeStaleExceptions when its
       * pool becomes isQuiescent.
       */
      static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
          final Throwable ex;
          ExceptionNode next;
          final long thrower;  // use id not ref to avoid weak cycles
          final int hashCode;  // store task hashCode before weak ref disappears
          ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next,
                        ReferenceQueue<Object> exceptionTableRefQueue) {
              super(task, exceptionTableRefQueue);
              this.ex = ex;
              this.next = next;
              this.thrower = Thread.currentThread().getId();
              this.hashCode = System.identityHashCode(task);
          }
      }
  
      /**
       * Records exception and sets status.
       *
       * @return status on exit
       */
      final int recordExceptionalCompletion(Throwable ex) {
          int s;
          if ((s = status) >= 0) {
              int h = System.identityHashCode(this);
              final ReentrantLock lock = exceptionTableLock;
              lock.lock();
              try {
                  expungeStaleExceptions();
                  ExceptionNode[] t = exceptionTable;
                  int i = h & (t.length - 1);
                  for (ExceptionNode e = t[i]; ; e = e.next) {
                      if (e == null) {
                          t[i] = new ExceptionNode(this, ex, t[i],
                                                   exceptionTableRefQueue);
                          break;
                      }
                      if (e.get() == this) // already present
                          break;
                  }
              } finally {
                  lock.unlock();
              }
              s = setCompletion(EXCEPTIONAL);
          }
          return s;
      }
  
      /**
       * Records exception and possibly propagates.
       *
       * @return status on exit
       */
      private int setExceptionalCompletion(Throwable ex) {
          int s = recordExceptionalCompletion(ex);
          if ((s & DONE_MASK) == EXCEPTIONAL)
              internalPropagateException(ex);
          return s;
      }
  
      /**
       * Hook for exception propagation support for tasks with completers.
       */
      void internalPropagateException(Throwable ex) {
      }
  
      /**
       * Cancels, ignoring any exceptions thrown by cancel. Used during
       * worker and pool shutdown. Cancel is spec'ed not to throw any
       * exceptions, but if it does anyway, we have no recourse during
       * shutdown, so guard against this case.
       */
      static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
          if (t != null && t.status >= 0) {
              try {
                  t.cancel(false);
              } catch (Throwable ignore) {
              }
          }
      }
  
      /**
       * Removes exception node and clears status.
       */
      private void clearExceptionalCompletion() {
          int h = System.identityHashCode(this);
          final ReentrantLock lock = exceptionTableLock;
          lock.lock();
          try {
              ExceptionNode[] t = exceptionTable;
              int i = h & (t.length - 1);
              ExceptionNode e = t[i];
              ExceptionNode pred = null;
              while (e != null) {
                  ExceptionNode next = e.next;
                  if (e.get() == this) {
                      if (pred == null)
                          t[i] = next;
                      else
                          pred.next = next;
                      break;
                  }
                  pred = e;
                  e = next;
              }
              expungeStaleExceptions();
              status = 0;
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Returns a rethrowable exception for this task, if available.
       * To provide accurate stack traces, if the exception was not
       * thrown by the current thread, we try to create a new exception
       * of the same type as the one thrown, but with the recorded
       * exception as its cause. If there is no such constructor, we
       * instead try to use a no-arg constructor, followed by initCause,
       * to the same effect. If none of these apply, or any fail due to
       * other exceptions, we return the recorded exception, which is
       * still correct, although it may contain a misleading stack
       * trace.
       *
       * @return the exception, or null if none
       */
      private Throwable getThrowableException() {
          int h = System.identityHashCode(this);
          ExceptionNode e;
          final ReentrantLock lock = exceptionTableLock;
          lock.lock();
          try {
              expungeStaleExceptions();
              ExceptionNode[] t = exceptionTable;
              e = t[h & (t.length - 1)];
              while (e != null && e.get() != this)
                  e = e.next;
          } finally {
              lock.unlock();
          }
          Throwable ex;
          if (e == null || (ex = e.ex) == null)
              return null;
          if (e.thrower != Thread.currentThread().getId()) {
              try {
                  Constructor<?> noArgCtor = null;
                  // public ctors only
                  for (Constructor<?> c : ex.getClass().getConstructors()) {
                      Class<?>[] ps = c.getParameterTypes();
                      if (ps.length == 0)
                          noArgCtor = c;
                      else if (ps.length == 1 && ps[0] == Throwable.class)
                          return (Throwable)c.newInstance(ex);
                  }
                  if (noArgCtor != null) {
                      Throwable wx = (Throwable)noArgCtor.newInstance();
                      wx.initCause(ex);
                      return wx;
                  }
              } catch (Exception ignore) {
              }
          }
          return ex;
      }
  
      /**
       * Polls stale refs and removes them. Call only while holding lock.
       */
      private static void expungeStaleExceptions() {
          for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
              if (x instanceof ExceptionNode) {
                  int hashCode = ((ExceptionNode)x).hashCode;
                  ExceptionNode[] t = exceptionTable;
                  int i = hashCode & (t.length - 1);
                  ExceptionNode e = t[i];
                  ExceptionNode pred = null;
                  while (e != null) {
                      ExceptionNode next = e.next;
                      if (e == x) {
                          if (pred == null)
                              t[i] = next;
                          else
                              pred.next = next;
                          break;
                      }
                      pred = e;
                      e = next;
                  }
              }
          }
      }
  
      /**
       * If lock is available, polls stale refs and removes them.
       * Called from ForkJoinPool when pools become quiescent.
       */
      static final void helpExpungeStaleExceptions() {
          final ReentrantLock lock = exceptionTableLock;
          if (lock.tryLock()) {
              try {
                  expungeStaleExceptions();
              } finally {
                  lock.unlock();
              }
          }
      }
  
      /**
       * A version of "sneaky throw" to relay exceptions.
       */
      static void rethrow(Throwable ex) {
          ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
      }
  
      /**
       * The sneaky part of sneaky throw, relying on generics
       * limitations to evade compiler complaints about rethrowing
       * unchecked exceptions.
       */
      @SuppressWarnings("unchecked") static <T extends Throwable>
      void uncheckedThrow(Throwable t) throws T {
          if (t != null)
              throw (T)t; // rely on vacuous cast
          else
              throw new Error("Unknown Exception");
      }
  
      /**
       * Throws exception, if any, associated with the given status.
       */
      private void reportException(int s) {
          if (s == CANCELLED)
              throw new CancellationException();
          if (s == EXCEPTIONAL)
              rethrow(getThrowableException());
      }
  
      // public methods
  
      /**
       * Arranges to asynchronously execute this task in the pool the
       * current task is running in, if applicable, or using the {@link
       * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
       * it is not necessarily enforced, it is a usage error to fork a
       * task more than once unless it has completed and been
       * reinitialized.  Subsequent modifications to the state of this
       * task or any data it operates on are not necessarily
       * consistently observable by any thread other than the one
       * executing it unless preceded by a call to {@link #join} or
       * related methods, or a call to {@link #isDone} returning {@code
       * true}.
       *
       * @return {@code this}, to simplify usage
       */
      public final ForkJoinTask<V> fork() {
          Thread t;
          if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
              ((ForkJoinWorkerThread)t).workQueue.push(this);
          else
              ForkJoinPool.common.externalPush(this);
          return this;
      }
  
      /**
       * Returns the result of the computation when it {@link #isDone is
       * done}.  This method differs from {@link #get()} in that
       * abnormal completion results in {@code RuntimeException} or
       * {@code Error}, not {@code ExecutionException}, and that
       * interrupts of the calling thread do <em>not</em> cause the
       * method to abruptly return by throwing {@code
       * InterruptedException}.
       *
       * @return the computed result
       */
      public final V join() {
          int s;
          if ((s = doJoin() & DONE_MASK) != NORMAL)
              reportException(s);
          return getRawResult();
      }
  
      /**
       * Commences performing this task, awaits its completion if
       * necessary, and returns its result, or throws an (unchecked)
       * {@code RuntimeException} or {@code Error} if the underlying
       * computation did so.
       *
       * @return the computed result
       */
      public final V invoke() {
          int s;
          if ((s = doInvoke() & DONE_MASK) != NORMAL)
              reportException(s);
          return getRawResult();
      }
  
      /**
       * Forks the given tasks, returning when {@code isDone} holds for
       * each task or an (unchecked) exception is encountered, in which
       * case the exception is rethrown. If more than one task
       * encounters an exception, then this method throws any one of
       * these exceptions. If any task encounters an exception, the
       * other may be cancelled. However, the execution status of
       * individual tasks is not guaranteed upon exceptional return. The
       * status of each task may be obtained using {@link
       * #getException()} and related methods to check if they have been
       * cancelled, completed normally or exceptionally, or left
       * unprocessed.
       *
       * @param t1 the first task
       * @param t2 the second task
       * @throws NullPointerException if any task is null
       */
      public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
          int s1, s2;
          t2.fork();
          if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
              t1.reportException(s1);
          if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
              t2.reportException(s2);
      }
  
      /**
       * Forks the given tasks, returning when {@code isDone} holds for
       * each task or an (unchecked) exception is encountered, in which
       * case the exception is rethrown. If more than one task
       * encounters an exception, then this method throws any one of
       * these exceptions. If any task encounters an exception, others
       * may be cancelled. However, the execution status of individual
       * tasks is not guaranteed upon exceptional return. The status of
       * each task may be obtained using {@link #getException()} and
       * related methods to check if they have been cancelled, completed
       * normally or exceptionally, or left unprocessed.
       *
       * @param tasks the tasks
       * @throws NullPointerException if any task is null
       */
      public static void invokeAll(ForkJoinTask<?>... tasks) {
          Throwable ex = null;
          int last = tasks.length - 1;
          for (int i = last; i >= 0; --i) {
              ForkJoinTask<?> t = tasks[i];
              if (t == null) {
                  if (ex == null)
                      ex = new NullPointerException();
              }
              else if (i != 0)
                  t.fork();
              else if (t.doInvoke() < NORMAL && ex == null)
                  ex = t.getException();
          }
          for (int i = 1; i <= last; ++i) {
              ForkJoinTask<?> t = tasks[i];
              if (t != null) {
                  if (ex != null)
                      t.cancel(false);
                  else if (t.doJoin() < NORMAL)
                      ex = t.getException();
              }
          }
          if (ex != null)
              rethrow(ex);
      }
  
      /**
       * Forks all tasks in the specified collection, returning when
       * {@code isDone} holds for each task or an (unchecked) exception
       * is encountered, in which case the exception is rethrown. If
       * more than one task encounters an exception, then this method
       * throws any one of these exceptions. If any task encounters an
       * exception, others may be cancelled. However, the execution
       * status of individual tasks is not guaranteed upon exceptional
       * return. The status of each task may be obtained using {@link
       * #getException()} and related methods to check if they have been
       * cancelled, completed normally or exceptionally, or left
       * unprocessed.
       *
       * @param tasks the collection of tasks
       * @param <T> the type of the values returned from the tasks
       * @return the tasks argument, to simplify usage
       * @throws NullPointerException if tasks or any element are null
       */
      public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
          if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
              invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
              return tasks;
          }
          @SuppressWarnings("unchecked")
          List<? extends ForkJoinTask<?>> ts =
              (List<? extends ForkJoinTask<?>>) tasks;
          Throwable ex = null;
          int last = ts.size() - 1;
          for (int i = last; i >= 0; --i) {
              ForkJoinTask<?> t = ts.get(i);
              if (t == null) {
                  if (ex == null)
                      ex = new NullPointerException();
              }
              else if (i != 0)
                  t.fork();
              else if (t.doInvoke() < NORMAL && ex == null)
                  ex = t.getException();
          }
          for (int i = 1; i <= last; ++i) {
              ForkJoinTask<?> t = ts.get(i);
              if (t != null) {
                  if (ex != null)
                      t.cancel(false);
                  else if (t.doJoin() < NORMAL)
                      ex = t.getException();
              }
          }
          if (ex != null)
              rethrow(ex);
          return tasks;
      }
  
      /**
       * Attempts to cancel execution of this task. This attempt will
       * fail if the task has already completed or could not be
       * cancelled for some other reason. If successful, and this task
       * has not started when {@code cancel} is called, execution of
       * this task is suppressed. After this method returns
       * successfully, unless there is an intervening call to {@link
       * #reinitialize}, subsequent calls to {@link #isCancelled},
       * {@link #isDone}, and {@code cancel} will return {@code true}
       * and calls to {@link #join} and related methods will result in
       * {@code CancellationException}.
       *
       * <p>This method may be overridden in subclasses, but if so, must
       * still ensure that these properties hold. In particular, the
       * {@code cancel} method itself must not throw exceptions.
       *
       * <p>This method is designed to be invoked by <em>other</em>
       * tasks. To terminate the current task, you can just return or
       * throw an unchecked exception from its computation method, or
       * invoke {@link #completeExceptionally(Throwable)}.
       *
       * @param mayInterruptIfRunning this value has no effect in the
       * default implementation because interrupts are not used to
       * control cancellation.
       *
       * @return {@code true} if this task is now cancelled
       */
      public boolean cancel(boolean mayInterruptIfRunning) {
          return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
      }
  
      public final boolean isDone() {
          return status < 0;
      }
  
      public final boolean isCancelled() {
          return (status & DONE_MASK) == CANCELLED;
      }
  
      /**
       * Returns {@code true} if this task threw an exception or was cancelled.
       *
       * @return {@code true} if this task threw an exception or was cancelled
       */
      public final boolean isCompletedAbnormally() {
          return status < NORMAL;
      }
  
      /**
       * Returns {@code true} if this task completed without throwing an
       * exception and was not cancelled.
       *
       * @return {@code true} if this task completed without throwing an
       * exception and was not cancelled
       */
      public final boolean isCompletedNormally() {
          return (status & DONE_MASK) == NORMAL;
      }
  
      /**
       * Returns the exception thrown by the base computation, or a
       * {@code CancellationException} if cancelled, or {@code null} if
       * none or if the method has not yet completed.
       *
       * @return the exception, or {@code null} if none
       */
      public final Throwable getException() {
          int s = status & DONE_MASK;
          return ((s >= NORMAL)    ? null :
                  (s == CANCELLED) ? new CancellationException() :
                  getThrowableException());
      }
  
      /**
       * Completes this task abnormally, and if not already aborted or
       * cancelled, causes it to throw the given exception upon
       * {@code join} and related operations. This method may be used
       * to induce exceptions in asynchronous tasks, or to force
       * completion of tasks that would not otherwise complete.  Its use
       * in other situations is discouraged.  This method is
       * overridable, but overridden versions must invoke {@code super}
       * implementation to maintain guarantees.
       *
       * @param ex the exception to throw. If this exception is not a
       * {@code RuntimeException} or {@code Error}, the actual exception
       * thrown will be a {@code RuntimeException} with cause {@code ex}.
       */
      public void completeExceptionally(Throwable ex) {
          setExceptionalCompletion((ex instanceof RuntimeException) ||
                                   (ex instanceof Error) ? ex :
                                   new RuntimeException(ex));
      }
  
      /**
       * Completes this task, and if not already aborted or cancelled,
       * returning the given value as the result of subsequent
       * invocations of {@code join} and related operations. This method
       * may be used to provide results for asynchronous tasks, or to
       * provide alternative handling for tasks that would not otherwise
       * complete normally. Its use in other situations is
       * discouraged. This method is overridable, but overridden
       * versions must invoke {@code super} implementation to maintain
       * guarantees.
       *
       * @param value the result value for this task
       */
      public void complete(V value) {
          try {
              setRawResult(value);
          } catch (Throwable rex) {
              setExceptionalCompletion(rex);
              return;
          }
          setCompletion(NORMAL);
      }
  
      /**
       * Completes this task normally without setting a value. The most
       * recent value established by {@link #setRawResult} (or {@code
       * null} by default) will be returned as the result of subsequent
       * invocations of {@code join} and related operations.
       *
       * @since 1.8
       */
      public final void quietlyComplete() {
          setCompletion(NORMAL);
      }
  
      /**
       * Waits if necessary for the computation to complete, and then
       * retrieves its result.
       *
       * @return the computed result
       * @throws CancellationException if the computation was cancelled
       * @throws ExecutionException if the computation threw an
       * exception
       * @throws InterruptedException if the current thread is not a
       * member of a ForkJoinPool and was interrupted while waiting
       */
      public final V get() throws InterruptedException, ExecutionException {
          int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
              doJoin() : externalInterruptibleAwaitDone();
          if ((s &= DONE_MASK) == CANCELLED)
              throw new CancellationException();
          if (s == EXCEPTIONAL)
              throw new ExecutionException(getThrowableException());
          return getRawResult();
      }
  
      /**
       * Waits if necessary for at most the given time for the computation
       * to complete, and then retrieves its result, if available.
       *
       * @param timeout the maximum time to wait
       * @param unit the time unit of the timeout argument
       * @return the computed result
       * @throws CancellationException if the computation was cancelled
       * @throws ExecutionException if the computation threw an
       * exception
       * @throws InterruptedException if the current thread is not a
       * member of a ForkJoinPool and was interrupted while waiting
       * @throws TimeoutException if the wait timed out
       */
      public final V get(long timeout, TimeUnit unit)
          throws InterruptedException, ExecutionException, TimeoutException {
          int s;
          long nanos = unit.toNanos(timeout);
          if (Thread.interrupted())
              throw new InterruptedException();
          if ((s = status) >= 0 && nanos > 0L) {
              long d = System.nanoTime() + nanos;
              long deadline = (d == 0L) ? 1L : d; // avoid 0
              Thread t = Thread.currentThread();
              if (t instanceof ForkJoinWorkerThread) {
                  ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
                  s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
              }
              else if ((s = ((this instanceof CountedCompleter) ?
                             ForkJoinPool.common.externalHelpComplete(
                                 (CountedCompleter<?>)this, 0) :
                             ForkJoinPool.common.tryExternalUnpush(this) ?
                             doExec() : 0)) >= 0) {
                  long ns, ms; // measure in nanosecs, but wait in millisecs
                  while ((s = status) >= 0 &&
                         (ns = deadline - System.nanoTime()) > 0L) {
                      if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
                          U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                          synchronized (this) {
                              if (status >= 0)
                                  wait(ms); // OK to throw InterruptedException
                              else
                                  notifyAll();
                          }
                      }
                  }
              }
          }
          if (s >= 0)
              s = status;
          if ((s &= DONE_MASK) != NORMAL) {
              if (s == CANCELLED)
                  throw new CancellationException();
              if (s != EXCEPTIONAL)
                  throw new TimeoutException();
              throw new ExecutionException(getThrowableException());
          }
          return getRawResult();
      }
  
      /**
       * Joins this task, without returning its result or throwing its
       * exception. This method may be useful when processing
       * collections of tasks when some have been cancelled or otherwise
       * known to have aborted.
       */
      public final void quietlyJoin() {
          doJoin();
      }
  
      /**
       * Commences performing this task and awaits its completion if
       * necessary, without returning its result or throwing its
       * exception.
       */
      public final void quietlyInvoke() {
          doInvoke();
      }
  
      /**
       * Possibly executes tasks until the pool hosting the current task
       * {@linkplain ForkJoinPool#isQuiescent is quiescent}.  This
       * method may be of use in designs in which many tasks are forked,
       * but none are explicitly joined, instead executing them until
       * all are processed.
       */
      public static void helpQuiesce() {
          Thread t;
          if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
              ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
              wt.pool.helpQuiescePool(wt.workQueue);
          }
          else
              ForkJoinPool.quiesceCommonPool();
      }
  
      /**
       * Resets the internal bookkeeping state of this task, allowing a
       * subsequent {@code fork}. This method allows repeated reuse of
       * this task, but only if reuse occurs when this task has either
       * never been forked, or has been forked, then completed and all
       * outstanding joins of this task have also completed. Effects
       * under any other usage conditions are not guaranteed.
       * This method may be useful when executing
       * pre-constructed trees of subtasks in loops.
       *
       * <p>Upon completion of this method, {@code isDone()} reports
       * {@code false}, and {@code getException()} reports {@code
       * null}. However, the value returned by {@code getRawResult} is
       * unaffected. To clear this value, you can invoke {@code
       * setRawResult(null)}.
       */
      public void reinitialize() {
          if ((status & DONE_MASK) == EXCEPTIONAL)
              clearExceptionalCompletion();
          else
              status = 0;
      }
  
      /**
       * Returns the pool hosting the current thread, or {@code null}
       * if the current thread is executing outside of any ForkJoinPool.
       *
       * <p>This method returns {@code null} if and only if {@link
       * #inForkJoinPool} returns {@code false}.
       *
       * @return the pool, or {@code null} if none
       */
      public static ForkJoinPool getPool() {
          Thread t = Thread.currentThread();
          return (t instanceof ForkJoinWorkerThread) ?
              ((ForkJoinWorkerThread) t).pool : null;
      }
  
      /**
       * Returns {@code true} if the current thread is a {@link
       * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
       *
       * @return {@code true} if the current thread is a {@link
       * ForkJoinWorkerThread} executing as a ForkJoinPool computation,
       * or {@code false} otherwise
       */
      public static boolean inForkJoinPool() {
          return Thread.currentThread() instanceof ForkJoinWorkerThread;
      }
  
      /**
       * Tries to unschedule this task for execution. This method will
       * typically (but is not guaranteed to) succeed if this task is
       * the most recently forked task by the current thread, and has
       * not commenced executing in another thread.  This method may be
       * useful when arranging alternative local processing of tasks
       * that could have been, but were not, stolen.
       *
       * @return {@code true} if unforked
       */
      public boolean tryUnfork() {
          Thread t;
          return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                  ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
                  ForkJoinPool.common.tryExternalUnpush(this));
      }
  
      /**
       * Returns an estimate of the number of tasks that have been
       * forked by the current worker thread but not yet executed. This
       * value may be useful for heuristic decisions about whether to
       * fork other tasks.
       *
       * @return the number of tasks
       */
      public static int getQueuedTaskCount() {
          Thread t; ForkJoinPool.WorkQueue q;
          if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
              q = ((ForkJoinWorkerThread)t).workQueue;
          else
              q = ForkJoinPool.commonSubmitterQueue();
          return (q == null) ? 0 : q.queueSize();
      }
  
      /**
       * Returns an estimate of how many more locally queued tasks are
       * held by the current worker thread than there are other worker
       * threads that might steal them, or zero if this thread is not
       * operating in a ForkJoinPool. This value may be useful for
       * heuristic decisions about whether to fork other tasks. In many
       * usages of ForkJoinTasks, at steady state, each worker should
       * aim to maintain a small constant surplus (for example, 3) of
       * tasks, and to process computations locally if this threshold is
       * exceeded.
       *
       * @return the surplus number of tasks, which may be negative
       */
      public static int getSurplusQueuedTaskCount() {
          return ForkJoinPool.getSurplusQueuedTaskCount();
      }
  
      // Extension methods
  
      /**
       * Returns the result that would be returned by {@link #join}, even
       * if this task completed abnormally, or {@code null} if this task
       * is not known to have been completed.  This method is designed
       * to aid debugging, as well as to support extensions. Its use in
       * any other context is discouraged.
       *
       * @return the result, or {@code null} if not completed
       */
      public abstract V getRawResult();
  
      /**
       * Forces the given value to be returned as a result.  This method
       * is designed to support extensions, and should not in general be
       * called otherwise.
       *
       * @param value the value
       */
      protected abstract void setRawResult(V value);
  
      /**
       * Immediately performs the base action of this task and returns
       * true if, upon return from this method, this task is guaranteed
       * to have completed normally. This method may return false
       * otherwise, to indicate that this task is not necessarily
       * complete (or is not known to be complete), for example in
       * asynchronous actions that require explicit invocations of
       * completion methods. This method may also throw an (unchecked)
       * exception to indicate abnormal exit. This method is designed to
       * support extensions, and should not in general be called
       * otherwise.
       *
       * @return {@code true} if this task is known to have completed normally
       */
      protected abstract boolean exec();
  
      /**
       * Returns, but does not unschedule or execute, a task queued by
       * the current thread but not yet executed, if one is immediately
       * available. There is no guarantee that this task will actually
       * be polled or executed next. Conversely, this method may return
       * null even if a task exists but cannot be accessed without
       * contention with other threads.  This method is designed
       * primarily to support extensions, and is unlikely to be useful
       * otherwise.
       *
       * @return the next task, or {@code null} if none are available
       */
      protected static ForkJoinTask<?> peekNextLocalTask() {
          Thread t; ForkJoinPool.WorkQueue q;
          if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
              q = ((ForkJoinWorkerThread)t).workQueue;
          else
              q = ForkJoinPool.commonSubmitterQueue();
          return (q == null) ? null : q.peek();
      }
  
      /**
       * Unschedules and returns, without executing, the next task
       * queued by the current thread but not yet executed, if the
       * current thread is operating in a ForkJoinPool.  This method is
       * designed primarily to support extensions, and is unlikely to be
       * useful otherwise.
       *
       * @return the next task, or {@code null} if none are available
       */
      protected static ForkJoinTask<?> pollNextLocalTask() {
          Thread t;
          return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
              ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
              null;
      }
  
      /**
       * If the current thread is operating in a ForkJoinPool,
       * unschedules and returns, without executing, the next task
       * queued by the current thread but not yet executed, if one is
       * available, or if not available, a task that was forked by some
       * other thread, if available. Availability may be transient, so a
       * {@code null} result does not necessarily imply quiescence of
       * the pool this task is operating in.  This method is designed
       * primarily to support extensions, and is unlikely to be useful
       * otherwise.
       *
       * @return a task, or {@code null} if none are available
       */
      protected static ForkJoinTask<?> pollTask() {
          Thread t; ForkJoinWorkerThread wt;
          return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
              (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
              null;
      }
  
      /**
       * If the current thread is operating in a ForkJoinPool,
       * unschedules and returns, without executing, a task externally
       * submitted to the pool, if one is available. Availability may be
       * transient, so a {@code null} result does not necessarily imply
       * quiescence of the pool.  This method is designed primarily to
       * support extensions, and is unlikely to be useful otherwise.
       *
       * @return a task, or {@code null} if none are available
       * @since 9
       * @hide API from OpenJDK 9, not yet exposed on Android.
       */
      protected static ForkJoinTask<?> pollSubmission() {
          Thread t;
          return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
              ((ForkJoinWorkerThread)t).pool.pollSubmission() : null;
      }
  
      // tag operations
  
      /**
       * Returns the tag for this task.
       *
       * @return the tag for this task
       * @since 1.8
       */
      public final short getForkJoinTaskTag() {
          return (short)status;
      }
  
      /**
       * Atomically sets the tag value for this task and returns the old value.
       *
       * @param newValue the new tag value
       * @return the previous value of the tag
       * @since 1.8
       */
      public final short setForkJoinTaskTag(short newValue) {
          for (int s;;) {
              if (U.compareAndSwapInt(this, STATUS, s = status,
                                      (s & ~SMASK) | (newValue & SMASK)))
                  return (short)s;
          }
      }
  
      /**
       * Atomically conditionally sets the tag value for this task.
       * Among other applications, tags can be used as visit markers
       * in tasks operating on graphs, as in methods that check: {@code
       * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
       * before processing, otherwise exiting because the node has
       * already been visited.
       *
       * @param expect the expected tag value
       * @param update the new tag value
       * @return {@code true} if successful; i.e., the current value was
       * equal to {@code expect} and was changed to {@code update}.
       * @since 1.8
       */
      public final boolean compareAndSetForkJoinTaskTag(short expect, short update) {
          for (int s;;) {
              if ((short)(s = status) != expect)
                  return false;
              if (U.compareAndSwapInt(this, STATUS, s,
                                      (s & ~SMASK) | (update & SMASK)))
                  return true;
          }
      }
  
      /**
       * Adapter for Runnables. This implements RunnableFuture
       * to be compliant with AbstractExecutorService constraints
       * when used in ForkJoinPool.
       */
      static final class AdaptedRunnable<T> extends ForkJoinTask<T>
          implements RunnableFuture<T> {
          final Runnable runnable;
          T result;
          AdaptedRunnable(Runnable runnable, T result) {
              if (runnable == null) throw new NullPointerException();
              this.runnable = runnable;
              this.result = result; // OK to set this even before completion
          }
          public final T getRawResult() { return result; }
          public final void setRawResult(T v) { result = v; }
          public final boolean exec() { runnable.run(); return true; }
          public final void run() { invoke(); }
          private static final long serialVersionUID = 5232453952276885070L;
      }
  
      /**
       * Adapter for Runnables without results.
       */
      static final class AdaptedRunnableAction extends ForkJoinTask<Void>
          implements RunnableFuture<Void> {
          final Runnable runnable;
          AdaptedRunnableAction(Runnable runnable) {
              if (runnable == null) throw new NullPointerException();
              this.runnable = runnable;
          }
          public final Void getRawResult() { return null; }
          public final void setRawResult(Void v) { }
          public final boolean exec() { runnable.run(); return true; }
          public final void run() { invoke(); }
          private static final long serialVersionUID = 5232453952276885070L;
      }
  
      /**
       * Adapter for Runnables in which failure forces worker exception.
       */
      static final class RunnableExecuteAction extends ForkJoinTask<Void> {
          final Runnable runnable;
          RunnableExecuteAction(Runnable runnable) {
              if (runnable == null) throw new NullPointerException();
              this.runnable = runnable;
          }
          public final Void getRawResult() { return null; }
          public final void setRawResult(Void v) { }
          public final boolean exec() { runnable.run(); return true; }
          void internalPropagateException(Throwable ex) {
              rethrow(ex); // rethrow outside exec() catches.
          }
          private static final long serialVersionUID = 5232453952276885070L;
      }
  
      /**
       * Adapter for Callables.
       */
      static final class AdaptedCallable<T> extends ForkJoinTask<T>
          implements RunnableFuture<T> {
          final Callable<? extends T> callable;
          T result;
          AdaptedCallable(Callable<? extends T> callable) {
              if (callable == null) throw new NullPointerException();
              this.callable = callable;
          }
          public final T getRawResult() { return result; }
          public final void setRawResult(T v) { result = v; }
          public final boolean exec() {
              try {
                  result = callable.call();
                  return true;
              } catch (RuntimeException rex) {
                  throw rex;
              } catch (Exception ex) {
                  throw new RuntimeException(ex);
              }
          }
          public final void run() { invoke(); }
          private static final long serialVersionUID = 2838392045355241008L;
      }
  
      /**
       * Returns a new {@code ForkJoinTask} that performs the {@code run}
       * method of the given {@code Runnable} as its action, and returns
       * a null result upon {@link #join}.
       *
       * @param runnable the runnable action
       * @return the task
       */
      public static ForkJoinTask<?> adapt(Runnable runnable) {
          return new AdaptedRunnableAction(runnable);
      }
  
      /**
       * Returns a new {@code ForkJoinTask} that performs the {@code run}
       * method of the given {@code Runnable} as its action, and returns
       * the given result upon {@link #join}.
       *
       * @param runnable the runnable action
       * @param result the result upon completion
       * @param <T> the type of the result
       * @return the task
       */
      public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
          return new AdaptedRunnable<T>(runnable, result);
      }
  
      /**
       * Returns a new {@code ForkJoinTask} that performs the {@code call}
       * method of the given {@code Callable} as its action, and returns
       * its result upon {@link #join}, translating any checked exceptions
       * encountered into {@code RuntimeException}.
       *
       * @param callable the callable action
       * @param <T> the type of the callable's result
       * @return the task
       */
      public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
          return new AdaptedCallable<T>(callable);
      }
  
      // Serialization support
  
      private static final long serialVersionUID = -7721805057305804111L;
  
      /**
       * Saves this task to a stream (that is, serializes it).
       *
       * @param s the stream
       * @throws java.io.IOException if an I/O error occurs
       * @serialData the current run status and the exception thrown
       * during execution, or {@code null} if none
       */
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          s.defaultWriteObject();
          s.writeObject(getException());
      }
  
      /**
       * Reconstitutes this task from a stream (that is, deserializes it).
       * @param s the stream
       * @throws ClassNotFoundException if the class of a serialized object
       *         could not be found
       * @throws java.io.IOException if an I/O error occurs
       */
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          s.defaultReadObject();
          Object ex = s.readObject();
          if (ex != null)
              setExceptionalCompletion((Throwable)ex);
      }
  
      // Unsafe mechanics
      private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
      private static final long STATUS;
  
      static {
          exceptionTableLock = new ReentrantLock();
          exceptionTableRefQueue = new ReferenceQueue<Object>();
          exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
          try {
              STATUS = U.objectFieldOffset
                  (ForkJoinTask.class.getDeclaredField("status"));
          } catch (ReflectiveOperationException e) {
              throw new Error(e);
          }
      }
  
  }
  