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   // Revision 1.12
   
   /*
    * 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 org.infinispan.commons.util.concurrent.jdk8backported;
  
  import java.util.List;
Abstract base class for tasks that run within a ForkJoinPool. A 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.

A "main" ForkJoinTask begins execution when it is explicitly submitted to a ForkJoinPool, or, if not already engaged in a ForkJoin computation, commenced in the ForkJoinPool.commonPool() via fork(), 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 ForkJoinTask employ only methods fork() and join(), or derivatives such as invokeAll(org.infinispan.commons.util.concurrent.jdk8backported.ForkJoinTask[]). 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.

A ForkJoinTask is a lightweight form of java.util.concurrent.Future. The efficiency of ForkJoinTasks 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 fork(), that arranges asynchronous execution, and join(), that doesn't proceed until the task's result has been computed. Computations should ideally avoid 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 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 java.util.concurrent.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 ex.printStackTrace()) of both the thread that initiated the computation as well as the thread actually encountering the exception; minimally only the latter.

It is possible to define and use ForkJoinTasks that may block, but doing do requires three further considerations: (1) Completion of few if any other 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 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 ForkJoinPool.ManagedBlocker API is used, or the number of possibly blocked tasks is known to be less than the pool's ForkJoinPool.getParallelism() level, the pool cannot guarantee that enough threads will be available to ensure progress or good performance.

The primary method for awaiting completion and extracting results of a task is join(), but there are several variants: The java.util.concurrent.Future.get() methods support interruptible and/or timed waits for completion and report results using Future conventions. Method invoke() is semantically equivalent to fork(); join() but always attempts to begin execution in the current thread. The "quiet" 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 invokeAll (available in multiple versions) performs the most common form of parallel invocation: forking a set of tasks and joining them all.

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, a.fork(); b.fork(); b.join(); a.join(); is likely to be substantially more efficient than joining a before b.

The execution status of tasks may be queried at several levels of detail: isDone() is true if a task completed in any way (including the case where a task was cancelled without executing); isCompletedNormally() is true if a task completed without cancellation or encountering an exception; isCancelled() is true if the task was cancelled (in which case getException() returns a java.util.concurrent.CancellationException); and isCompletedAbnormally() is true if a task was either cancelled or encountered an exception, in which case getException() will return either the encountered exception or java.util.concurrent.CancellationException.

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 RecursiveAction for most computations that do not return results, RecursiveTask for those that do, and 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 compute method that somehow uses the control methods supplied by this base class.

Method 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 java.util.concurrent.Phaser, helpQuiesce(), and complete(java.lang.Object)) 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 tagged with a short value using setForkJoinTaskTag(short) or compareAndSetForkJoinTaskTag(short,short) and checked using getForkJoinTaskTag(). The ForkJoinTask implementation does not use these 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.)

Most base support methods are 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 protected methods exec(), setRawResult(java.lang.Object), and getRawResult(), while also introducing an abstract computational method that can be implemented in its subclasses, possibly relying on other protected methods provided by this class.

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.

This class provides adapt methods for java.lang.Runnable and java.util.concurrent.Callable, that may be of use when mixing execution of ForkJoinTasks with other kinds of tasks. When all tasks are of this form, consider using a pool constructed in asyncMode.

ForkJoinTasks are 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.

Author(s):
Doug Lea
Since:
1.7
 
 @SuppressWarnings("restriction")
 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.

Parameters:
completion one of NORMAL, CANCELLED, EXCEPTIONAL
Returns:
completion status on exit
 
    private int setCompletion(int completion) {
       for (int s;;) {
          if ((s = ) < 0)
             return s;
          if (.compareAndSwapInt(thisss | 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.

Returns:
status on exit from this method
 
    final int doExec() {
       int sboolean completed;
       if ((s = ) >= 0) {
          try {
             completed = exec();
          } catch (Throwable rex) {
             return setExceptionalCompletion(rex);
          }
          if (completed)
             s = setCompletion();
       }
       return s;
    }

   
Tries to set SIGNAL status unless already completed. Used by ForkJoinPool. Other variants are directly incorporated into externalAwaitDone etc.

Returns:
true if successful
 
    final boolean trySetSignal() {
       int s = ;
       return s >= 0 && .compareAndSwapInt(thisss | );
    }

   
Blocks a non-worker-thread until completion.

Returns:
status upon completion
 
    private int externalAwaitDone() {
       int s;
       ForkJoinPool.externalHelpJoin(this);
       boolean interrupted = false;
       while ((s = ) >= 0) {
          if (.compareAndSwapInt(thisss | )) {
             synchronized (this) {
                if ( >= 0) {
                   try {
                      wait();
                   } catch (InterruptedException ie) {
                      interrupted = true;
                   }
                }
                else
                   notifyAll();
             }
          }
       }
       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();
       ForkJoinPool.externalHelpJoin(this);
       while ((s = ) >= 0) {
          if (.compareAndSwapInt(thisss | )) {
             synchronized (this) {
                if ( >= 0)
                   wait();
                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.

Returns:
status upon completion
 
    private int doJoin() {
       int sThread tForkJoinWorkerThread wtForkJoinPool.WorkQueue w;
       return (s = ) < 0 ? s :
             ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                   (w = (wt = (ForkJoinWorkerThread)t).).
                         tryUnpush(this) && (s = doExec()) < 0 ? s :
                         wt.pool.awaitJoin(wthis) :
                   externalAwaitDone();
    }

   
Implementation for invoke, quietlyInvoke.

Returns:
status upon completion
 
    private int doInvoke() {
       int sThread tForkJoinWorkerThread wt;
       return (s = doExec()) < 0 ? s :
             ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                   (wt = (ForkJoinWorkerThread)t)..awaitJoin(wt.workQueuethis) :
                   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<ObjectexceptionTableRefQueue;

   
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
       ExceptionNode(ForkJoinTask<?> taskThrowable exExceptionNode next) {
          super(task);
          this. = ex;
          this. = next;
          this. = Thread.currentThread().getId();
       }
    }

   
Records exception and sets status.

Returns:
status on exit
 
    final int recordExceptionalCompletion(Throwable ex) {
       int s;
       if ((s = ) >= 0) {
          int h = System.identityHashCode(this);
          final ReentrantLock lock = ;
          lock.lock();
          try {
             expungeStaleExceptions();
             ExceptionNode[] t = ;
             int i = h & (t.length - 1);
             for (ExceptionNode e = t[i]; ; e = e.next) {
                if (e == null) {
                   t[i] = new ExceptionNode(thisext[i]);
                   break;
                }
                if (e.get() == this// already present
                   break;
             }
          } finally {
             lock.unlock();
          }
          s = setCompletion();
       }
       return s;
    }

   
Records exception and possibly propagates.

Returns:
status on exit
 
    private int setExceptionalCompletion(Throwable ex) {
       int s = recordExceptionalCompletion(ex);
       if ((s & ) == )
          internalPropagateException(ex);
       return s;
    }

   
Hook for exception propagation support for tasks with completers.
 
    }

   
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 = ;
       lock.lock();
       try {
          ExceptionNode[] t = ;
          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();
           = 0;
       } finally {
          lock.unlock();
       }
    }

   
Returns a rethrowable exception for the given 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.

Returns:
the exception, or null if none
 
    private Throwable getThrowableException() {
       if (( & ) != )
          return null;
       int h = System.identityHashCode(this);
       ExceptionNode e;
       final ReentrantLock lock = ;
       lock.lock();
       try {
          expungeStaleExceptions();
          ExceptionNode[] t = ;
          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 (false && e.thrower != Thread.currentThread().getId()) {
          Class<? extends Throwableec = ex.getClass();
          try {
             Constructor<?> noArgCtor = null;
             Constructor<?>[] cs = ec.getConstructors();// public ctors only
             for (int i = 0; i < cs.length; ++i) {
                Constructor<?> c = cs[i];
                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;
    }

   
Poll stale refs and remove them. Call only while holding lock.
 
    private static void expungeStaleExceptions() {
       for (Object x; (x = .poll()) != null;) {
          if (x instanceof ExceptionNode) {
             ForkJoinTask<?> key = ((ExceptionNode)x).get();
             ExceptionNode[] t = ;
             int i = System.identityHashCode(key) & (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, poll stale refs and remove them. Called from ForkJoinPool when pools become quiescent.
 
    static final void helpExpungeStaleExceptions() {
       final ReentrantLock lock = ;
       if (lock.tryLock()) {
          try {
             expungeStaleExceptions();
          } finally {
             lock.unlock();
          }
       }
    }

   
A version of "sneaky throw" to relay exceptions
 
    static void rethrow(final Throwable ex) {
       if (ex != null) {
          if (ex instanceof Error)
             throw (Error)ex;
          if (ex instanceof RuntimeException)
             throw (RuntimeException)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 tthrows T {
       if (t != null)
          throw (T)t// rely on vacuous cast
    }

   
Throws exception, if any, associated with the given status.
 
    private void reportException(int s) {
       if (s == )
          throw new CancellationException();
       if (s == )
          rethrow(getThrowableException());
    }
 
    // public methods
 
   
Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the ForkJoinPool.commonPool() if not 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 join() or related methods, or a call to isDone() returning true.

Returns:
this, to simplify usage
 
    public final ForkJoinTask<V> fork() {
       Thread t;
       if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ((ForkJoinWorkerThread)t)..push(this);
       else
          ..externalPush(this);
       return this;
    }

   
Returns the result of the computation when it is done. This method differs from get() in that abnormal completion results in RuntimeException or Error, not ExecutionException, and that interrupts of the calling thread do not cause the method to abruptly return by throwing InterruptedException.

Returns:
the computed result
 
    public final V join() {
       int s;
       if ((s = doJoin() & ) != )
          reportException(s);
       return getRawResult();
    }

   
Commences performing this task, awaits its completion if necessary, and returns its result, or throws an (unchecked) RuntimeException or Error if the underlying computation did so.

Returns:
the computed result
 
    public final V invoke() {
       int s;
       if ((s = doInvoke() & ) != )
          reportException(s);
       return getRawResult();
    }

   
Forks the given tasks, returning when 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 getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Parameters:
t1 the first task
t2 the second task
Throws:
java.lang.NullPointerException if any task is null
 
    public static void invokeAll(ForkJoinTask<?> t1ForkJoinTask<?> t2) {
       int s1s2;
       t2.fork();
       if ((s1 = t1.doInvoke() & ) != )
          t1.reportException(s1);
       if ((s2 = t2.doJoin() & ) != )
          t2.reportException(s2);
    }

   
Forks the given tasks, returning when 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 getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Parameters:
tasks the tasks
Throws:
java.lang.NullPointerException if any task is null
 
    public static void invokeAll(ForkJoinTask<?>... tasks) {
       Throwable ex = null;
       int last = tasks.length - 1;
       for (int i = lasti >= 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() <  && 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() < )
                ex = t.getException();
          }
       }
       if (ex != null)
          rethrow(ex);
    }

   
Forks all tasks in the specified collection, returning when 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 getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Parameters:
tasks the collection of tasks
Returns:
the tasks argument, to simplify usage
Throws:
java.lang.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 = lasti >= 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() <  && 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() < )
                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 cancel is called, execution of this task is suppressed. After this method returns successfully, unless there is an intervening call to reinitialize(), subsequent calls to isCancelled(), isDone(), and cancel will return true and calls to join() and related methods will result in CancellationException.

This method may be overridden in subclasses, but if so, must still ensure that these properties hold. In particular, the cancel method itself must not throw exceptions.

This method is designed to be invoked by other tasks. To terminate the current task, you can just return or throw an unchecked exception from its computation method, or invoke completeExceptionally(java.lang.Throwable).

Parameters:
mayInterruptIfRunning this value has no effect in the default implementation because interrupts are not used to control cancellation.
Returns:
true if this task is now cancelled
 
    public boolean cancel(boolean mayInterruptIfRunning) {
       return (setCompletion() & ) == ;
    }
 
    public final boolean isDone() {
       return  < 0;
    }
 
    public final boolean isCancelled() {
       return ( & ) == ;
    }

   
Returns true if this task threw an exception or was cancelled.

Returns:
true if this task threw an exception or was cancelled
 
    public final boolean isCompletedAbnormally() {
       return  < ;
    }

   
Returns true if this task completed without throwing an exception and was not cancelled.

Returns:
true if this task completed without throwing an exception and was not cancelled
 
    public final boolean isCompletedNormally() {
       return ( & ) == ;
    }

   
Returns the exception thrown by the base computation, or a CancellationException if cancelled, or null if none or if the method has not yet completed.

Returns:
the exception, or null if none
 
    public final Throwable getException() {
       int s =  & ;
       return ((s >= )    ? null :
                     (s == ) ? new CancellationException() :
                           getThrowableException());
    }

   
Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon 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 super implementation to maintain guarantees.

Parameters:
ex the exception to throw. If this exception is not a RuntimeException or Error, the actual exception thrown will be a RuntimeException with cause 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 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 super implementation to maintain guarantees.

Parameters:
value the result value for this task
 
    public void complete(V value) {
       try {
          setRawResult(value);
       } catch (Throwable rex) {
          setExceptionalCompletion(rex);
          return;
       }
       setCompletion();
    }

   
Completes this task normally without setting a value. The most recent value established by setRawResult(java.lang.Object) (or null by default) will be returned as the result of subsequent invocations of join and related operations.

Since:
1.8
 
    public final void quietlyComplete() {
       setCompletion();
    }

   
Waits if necessary for the computation to complete, and then retrieves its result.

Returns:
the computed result
Throws:
java.util.concurrent.CancellationException if the computation was cancelled
java.util.concurrent.ExecutionException if the computation threw an exception
java.lang.InterruptedException if the current thread is not a member of a ForkJoinPool and was interrupted while waiting
 
    public final V get() throws InterruptedExceptionExecutionException {
       int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
             doJoin() : externalInterruptibleAwaitDone();
       Throwable ex;
       if ((s &= ) == )
          throw new CancellationException();
       if (s ==  && (ex = getThrowableException()) != null)
          throw new ExecutionException(ex);
       return getRawResult();
    }

   
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Parameters:
timeout the maximum time to wait
unit the time unit of the timeout argument
Returns:
the computed result
Throws:
java.util.concurrent.CancellationException if the computation was cancelled
java.util.concurrent.ExecutionException if the computation threw an exception
java.lang.InterruptedException if the current thread is not a member of a ForkJoinPool and was interrupted while waiting
java.util.concurrent.TimeoutException if the wait timed out
 
    public final V get(long timeoutTimeUnit unit)
       if (Thread.interrupted())
          throw new InterruptedException();
       // Messy in part because we measure in nanosecs, but wait in millisecs
       int slong ms;
       long ns = unit.toNanos(timeout);
       if ((s = ) >= 0 && ns > 0L) {
          long deadline = System.nanoTime() + ns;
          ForkJoinPool p = null;
          ForkJoinPool.WorkQueue w = null;
          Thread t = Thread.currentThread();
          if (t instanceof ForkJoinWorkerThread) {
             ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
             p = wt.pool;
             w = wt.workQueue;
             p.helpJoinOnce(wthis); // no retries on failure
          }
          else
            ForkJoinPool.externalHelpJoin(this);
         boolean canBlock = false;
         boolean interrupted = false;
         try {
            while ((s = ) >= 0) {
               if (w != null && w.qlock < 0)
                  cancelIgnoringExceptions(this);
               else if (!canBlock) {
                  if (p == null || p.tryCompensate())
                     canBlock = true;
               }
               else {
                  if ((ms = ..toMillis(ns)) > 0L &&
                        .compareAndSwapInt(thisss | )) {
                     synchronized (this) {
                        if ( >= 0) {
                           try {
                              wait(ms);
                           } catch (InterruptedException ie) {
                              if (p == null)
                                 interrupted = true;
                           }
                        }
                        else
                           notifyAll();
                     }
                  }
                  if ((s = ) < 0 || interrupted ||
                        (ns = deadline - System.nanoTime()) <= 0L)
                     break;
               }
            }
         } finally {
            if (p != null && canBlock)
               p.incrementActiveCount();
         }
         if (interrupted)
            throw new InterruptedException();
      }
      if ((s &= ) != ) {
         Throwable ex;
         if (s == )
            throw new CancellationException();
         if (s != )
            throw new TimeoutException();
         if ((ex = getThrowableException()) != null)
            throw new ExecutionException(ex);
      }
      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 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 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.

Upon completion of this method, isDone() reports false, and getException() reports null. However, the value returned by getRawResult is unaffected. To clear this value, you can invoke setRawResult(null).

   public void reinitialize() {
      if (( & ) == )
         clearExceptionalCompletion();
      else
          = 0;
   }

   
Returns the pool hosting the current task execution, or null if this task is executing outside of any ForkJoinPool.

Returns:
the pool, or null if none
See also:
inForkJoinPool()
   public static ForkJoinPool getPool() {
      Thread t = Thread.currentThread();
      return (t instanceof ForkJoinWorkerThread) ?
            ((ForkJoinWorkerThreadt). : null;
   }

   
Returns true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation.

Returns:
true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation, or 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.

Returns:
true if unforked
   public boolean tryUnfork() {
      Thread t;
      return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                    ((ForkJoinWorkerThread)t)..tryUnpush(this) :
                    ForkJoinPool.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.

Returns:
the number of tasks
   public static int getQueuedTaskCount() {
      Thread tForkJoinPool.WorkQueue q;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
         q = ((ForkJoinWorkerThread)t).;
      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.

Returns:
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 join(), even if this task completed abnormally, or 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.

Returns:
the result, or 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.

Parameters:
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.

Returns:
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.

Returns:
the next task, or null if none are available
   protected static ForkJoinTask<?> peekNextLocalTask() {
      Thread tForkJoinPool.WorkQueue q;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
         q = ((ForkJoinWorkerThread)t).;
      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.

Returns:
the next task, or null if none are available
   protected static ForkJoinTask<?> pollNextLocalTask() {
      Thread t;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
            ((ForkJoinWorkerThread)t)..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 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.

Returns:
a task, or null if none are available
   protected static ForkJoinTask<?> pollTask() {
      Thread tForkJoinWorkerThread wt;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
            (wt = (ForkJoinWorkerThread)t)..nextTaskFor(wt.workQueue) :
            null;
   }
   // tag operations

   
Returns the tag for this task.

Returns:
the tag for this task
Since:
1.8
   public final short getForkJoinTaskTag() {
      return (short);
   }

   
Atomically sets the tag value for this task.

Parameters:
tag the tag value
Returns:
the previous value of the tag
Since:
1.8
   public final short setForkJoinTaskTag(short tag) {
      for (int s;;) {
         if (.compareAndSwapInt(thiss = ,
               (s & ~) | (tag & )))
            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: if (task.compareAndSetForkJoinTaskTag((short)0, (short)1)) before processing, otherwise exiting because the node has already been visited.

Parameters:
e the expected tag value
tag the new tag value
Returns:
true if successful; i.e., the current value was equal to e and is now tag.
Since:
1.8
   public final boolean compareAndSetForkJoinTaskTag(short eshort tag) {
      for (int s;;) {
         if ((short)(s = ) != e)
            return false;
         if (.compareAndSwapInt(thiss,
               (s & ~) | (tag & )))
            return true;
      }
   }

   
Adaptor 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 == nullthrow new NullPointerException();
         this. = runnable;
         this. = result// OK to set this even before completion
      }
      public final T getRawResult() { return ; }
      public final void setRawResult(T v) {  = v; }
      public final boolean exec() { .run(); return true; }
      public final void run() { invoke(); }
      private static final long serialVersionUID = 5232453952276885070L;
   }

   
Adaptor for Runnables without results
   static final class AdaptedRunnableAction extends ForkJoinTask<Void>
         implements RunnableFuture<Void> {
      final Runnable runnable;
      AdaptedRunnableAction(Runnable runnable) {
         if (runnable == nullthrow new NullPointerException();
         this. = runnable;
      }
      public final Void getRawResult() { return null; }
      public final void setRawResult(Void v) { }
      public final boolean exec() { .run(); return true; }
      public final void run() { invoke(); }
      private static final long serialVersionUID = 5232453952276885070L;
   }

   
Adaptor 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 == nullthrow new NullPointerException();
         this. = callable;
      }
      public final T getRawResult() { return ; }
      public final void setRawResult(T v) {  = v; }
      public final boolean exec() {
         try {
             = .call();
            return true;
         } catch (Error err) {
            throw err;
         } 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 ForkJoinTask that performs the run method of the given Runnable as its action, and returns a null result upon join().

Parameters:
runnable the runnable action
Returns:
the task
   public static ForkJoinTask<?> adapt(Runnable runnable) {
      return new AdaptedRunnableAction(runnable);
   }

   
Returns a new ForkJoinTask that performs the run method of the given Runnable as its action, and returns the given result upon join().

Parameters:
runnable the runnable action
result the result upon completion
Returns:
the task
   public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
      return new AdaptedRunnable<T>(runnableresult);
   }

   
Returns a new ForkJoinTask that performs the call method of the given Callable as its action, and returns its result upon join(), translating any checked exceptions encountered into RuntimeException.

Parameters:
callable the callable action
Returns:
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).

SerialData:
the current run status and the exception thrown during execution, or 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).
   private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOExceptionClassNotFoundException {
      s.defaultReadObject();
      Object ex = s.readObject();
      if (ex != null)
         setExceptionalCompletion((Throwable)ex);
   }
   // Unsafe mechanics
   @SuppressWarnings("internal")
   private static final sun.misc.Unsafe U;
   private static final long STATUS;
   static {
       = new ReentrantLock();
      try {
          = getUnsafe();
         Class<?> k = ForkJoinTask.class;
          = .objectFieldOffset
               (k.getDeclaredField("status"));
      } catch (Exception e) {
         throw new Error(e);
      }
   }

   
Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. Replace with a simple call to Unsafe.getUnsafe when integrating into a jdk.

Returns:
a sun.misc.Unsafe
   private static sun.misc.Unsafe getUnsafe() {
      try {
         return sun.misc.Unsafe.getUnsafe();
      } catch (SecurityException tryReflectionInstead) {}
      try {
         return java.security.AccessController.doPrivileged
               (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                  public sun.misc.Unsafe run() throws Exception {
                     Class<sun.misc.Unsafek = sun.misc.Unsafe.class;
                     for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x))
                           return k.cast(x);
                     }
                     throw new NoSuchFieldError("the Unsafe");
                  }});
      } catch (java.security.PrivilegedActionException e) {
         throw new RuntimeException("Could not initialize intrinsics",
               e.getCause());
      }
   }