Package jsr166y

Class ForkJoinPool

java.lang.Object
java.util.concurrent.AbstractExecutorService
jsr166y.ForkJoinPool
All Implemented Interfaces:
Executor, ExecutorService
public class ForkJoinPool extends AbstractExecutorService
An ExecutorService for running ForkJoinTasks. A ForkJoinPool provides the entry point for submissions from non-ForkJoinTask clients, as well as management and monitoring operations.

A ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing work-stealing: all threads in the pool attempt to find and execute subtasks created by other active tasks (eventually blocking waiting for work if none exist). This enables efficient processing when most tasks spawn other subtasks (as do most ForkJoinTasks). When setting asyncMode to true in constructors, ForkJoinPools may also be appropriate for use with event-style tasks that are never joined.

A ForkJoinPool is constructed with a given target parallelism level; by default, equal to the number of available processors. The pool attempts to maintain enough active (or available) threads by dynamically adding, suspending, or resuming internal worker threads, even if some tasks are stalled waiting to join others. However, no such adjustments are guaranteed in the face of blocked IO or other unmanaged synchronization. The nested ForkJoinPool.ManagedBlocker interface enables extension of the kinds of synchronization accommodated.

In addition to execution and lifecycle control methods, this class provides status check methods (for example getStealCount()) that are intended to aid in developing, tuning, and monitoring fork/join applications. Also, method toString() returns indications of pool state in a convenient form for informal monitoring.

As is the case with other ExecutorServices, there are three main task execution methods summarized in the following table. These are designed to be used by clients not already engaged in fork/join computations in the current pool. The main forms of these methods accept instances of ForkJoinTask, but overloaded forms also allow mixed execution of plain Runnable- or Callable- based activities as well. However, tasks that are already executing in a pool should normally NOT use these pool execution methods, but instead use the within-computation forms listed in the table.

Call from non-fork/join clients Call from within fork/join computations
Arrange async execution execute(ForkJoinTask) ForkJoinTask.fork()
Await and obtain result invoke(ForkJoinTask) ForkJoinTask.invoke()
Arrange exec and obtain Future submit(ForkJoinTask) ForkJoinTask.fork() (ForkJoinTasks are Futures)

Sample Usage. Normally a single ForkJoinPool is used for all parallel task execution in a program or subsystem. Otherwise, use would not usually outweigh the construction and bookkeeping overhead of creating a large set of threads. For example, a common pool could be used for the SortTasks illustrated in RecursiveAction. Because ForkJoinPool uses threads in daemon mode, there is typically no need to explicitly shutdown such a pool upon program exit. 

 static final ForkJoinPool mainPool = new ForkJoinPool();
 ...
 public void sort(long[] array) {
   mainPool.invoke(new SortTask(array, 0, array.length));
 }

Implementation notes: This implementation restricts the maximum number of running threads to 32767. Attempts to create pools with greater than the maximum number result in IllegalArgumentException.

This implementation rejects submitted tasks (that is, by throwing RejectedExecutionException) only when the pool is shut down or internal resources have been exhausted.

Since:
1.7

  • Field Details

    • defaultForkJoinWorkerThreadFactory

      public static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory
      Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.

  • Constructor Details

    • ForkJoinPool

      public ForkJoinPool()
      Creates a ForkJoinPool with parallelism equal to Runtime.availableProcessors(), using the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode.
      Throws:
      SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

    • ForkJoinPool

      public ForkJoinPool(int parallelism)
      Creates a ForkJoinPool with the indicated parallelism level, the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode.
      Parameters:
      parallelism - the parallelism level
      Throws:
      IllegalArgumentException - if parallelism less than or equal to zero, or greater than implementation limit
      SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

    • ForkJoinPool

      public ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode)
      Creates a ForkJoinPool with the given parameters.
      Parameters:
      parallelism - the parallelism level. For default value, use Runtime.availableProcessors().
      factory - the factory for creating new threads. For default value, use defaultForkJoinWorkerThreadFactory.
      handler - the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks. For default value, use null.
      asyncMode - if true, establishes local first-in-first-out scheduling mode for forked tasks that are never joined. This mode may be more appropriate than default locally stack-based mode in applications in which worker threads only process event-style asynchronous tasks. For default value, use false.
      Throws:
      IllegalArgumentException - if parallelism less than or equal to zero, or greater than implementation limit
      NullPointerException - if the factory is null
      SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

  • Method Details

    • invoke

      public <T> T invoke(ForkJoinTask<T> task)
      Performs the given task, returning its result upon completion. If the computation encounters an unchecked Exception or Error, it is rethrown as the outcome of this invocation. 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 current thread as well as the thread actually encountering the exception; minimally only the latter.
      Parameters:
      task - the task
      Returns:
      the task's result
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • execute

      public void execute(ForkJoinTask<?> task)
      Arranges for (asynchronous) execution of the given task.
      Parameters:
      task - the task
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • execute

      public void execute(Runnable task)
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • submit

      public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task)
      Submits a ForkJoinTask for execution.
      Parameters:
      task - the task to submit
      Returns:
      the task
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • submit

      public <T> ForkJoinTask<T> submit(Callable<T> task)
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • submit

      public <T> ForkJoinTask<T> submit(Runnable task, T result)
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • submit

      public ForkJoinTask<?> submit(Runnable task)
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution

    • invokeAll

      public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
      Specified by:
      invokeAll in interface ExecutorService
      Overrides:
      invokeAll in class AbstractExecutorService
      Throws:
      NullPointerException
      RejectedExecutionException

    • getFactory

      public ForkJoinPool.ForkJoinWorkerThreadFactory getFactory()
      Returns the factory used for constructing new workers.
      Returns:
      the factory used for constructing new workers

    • getUncaughtExceptionHandler

      public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
      Returns the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks.
      Returns:
      the handler, or null if none

    • getParallelism

      public int getParallelism()
      Returns the targeted parallelism level of this pool.
      Returns:
      the targeted parallelism level of this pool

    • getPoolSize

      public int getPoolSize()
      Returns the number of worker threads that have started but not yet terminated. The result returned by this method may differ from getParallelism() when threads are created to maintain parallelism when others are cooperatively blocked.
      Returns:
      the number of worker threads

    • getAsyncMode

      public boolean getAsyncMode()
      Returns true if this pool uses local first-in-first-out scheduling mode for forked tasks that are never joined.
      Returns:
      true if this pool uses async mode

    • getRunningThreadCount

      public int getRunningThreadCount()
      Returns an estimate of the number of worker threads that are not blocked waiting to join tasks or for other managed synchronization. This method may overestimate the number of running threads.
      Returns:
      the number of worker threads

    • getActiveThreadCount

      public int getActiveThreadCount()
      Returns an estimate of the number of threads that are currently stealing or executing tasks. This method may overestimate the number of active threads.
      Returns:
      the number of active threads

    • isQuiescent

      public boolean isQuiescent()
      Returns true if all worker threads are currently idle. An idle worker is one that cannot obtain a task to execute because none are available to steal from other threads, and there are no pending submissions to the pool. This method is conservative; it might not return true immediately upon idleness of all threads, but will eventually become true if threads remain inactive.
      Returns:
      true if all threads are currently idle

    • getStealCount

      public long getStealCount()
      Returns an estimate of the total number of tasks stolen from one thread's work queue by another. The reported value underestimates the actual total number of steals when the pool is not quiescent. This value may be useful for monitoring and tuning fork/join programs: in general, steal counts should be high enough to keep threads busy, but low enough to avoid overhead and contention across threads.
      Returns:
      the number of steals

    • getQueuedTaskCount

      public long getQueuedTaskCount()
      Returns an estimate of the total number of tasks currently held in queues by worker threads (but not including tasks submitted to the pool that have not begun executing). This value is only an approximation, obtained by iterating across all threads in the pool. This method may be useful for tuning task granularities.
      Returns:
      the number of queued tasks

    • getQueuedSubmissionCount

      public int getQueuedSubmissionCount()
      Returns an estimate of the number of tasks submitted to this pool that have not yet begun executing. This method may take time proportional to the number of submissions.
      Returns:
      the number of queued submissions

    • hasQueuedSubmissions

      public boolean hasQueuedSubmissions()
      Returns true if there are any tasks submitted to this pool that have not yet begun executing.
      Returns:
      true if there are any queued submissions

    • pollSubmission

      protected ForkJoinTask<?> pollSubmission()
      Removes and returns the next unexecuted submission if one is available. This method may be useful in extensions to this class that re-assign work in systems with multiple pools.
      Returns:
      the next submission, or null if none

    • drainTasksTo

      protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c)
      Removes all available unexecuted submitted and forked tasks from scheduling queues and adds them to the given collection, without altering their execution status. These may include artificially generated or wrapped tasks. This method is designed to be invoked only when the pool is known to be quiescent. Invocations at other times may not remove all tasks. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
      Parameters:
      c - the collection to transfer elements into
      Returns:
      the number of elements transferred

    • toString

      public String toString()
      Returns a string identifying this pool, as well as its state, including indications of run state, parallelism level, and worker and task counts.
      Overrides:
      toString in class Object
      Returns:
      a string identifying this pool, as well as its state

    • shutdown

      public void shutdown()
      Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down. Tasks that are in the process of being submitted concurrently during the course of this method may or may not be rejected.
      Throws:
      SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

    • shutdownNow

      public List<Runnable> shutdownNow()
      Attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks. Tasks that are in the process of being submitted or executed concurrently during the course of this method may or may not be rejected. This method cancels both existing and unexecuted tasks, in order to permit termination in the presence of task dependencies. So the method always returns an empty list (unlike the case for some other Executors).
      Returns:
      an empty list
      Throws:
      SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

    • isTerminated

      public boolean isTerminated()
      Returns true if all tasks have completed following shut down.
      Returns:
      true if all tasks have completed following shut down

    • isTerminating

      public boolean isTerminating()
      Returns true if the process of termination has commenced but not yet completed. This method may be useful for debugging. A return of true reported a sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption, or are waiting for IO, causing this executor not to properly terminate. (See the advisory notes for class ForkJoinTask stating that tasks should not normally entail blocking operations. But if they do, they must abort them on interrupt.)
      Returns:
      true if terminating but not yet terminated

    • isShutdown

      public boolean isShutdown()
      Returns true if this pool has been shut down.
      Returns:
      true if this pool has been shut down

    • awaitTermination

      public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
      Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
      Parameters:
      timeout - the maximum time to wait
      unit - the time unit of the timeout argument
      Returns:
      true if this executor terminated and false if the timeout elapsed before termination
      Throws:
      InterruptedException - if interrupted while waiting

    • managedBlock

      public static void managedBlock(ForkJoinPool.ManagedBlocker blocker) throws InterruptedException
      Blocks in accord with the given blocker. If the current thread is a ForkJoinWorkerThread, this method possibly arranges for a spare thread to be activated if necessary to ensure sufficient parallelism while the current thread is blocked.

      If the caller is not a ForkJoinTask, this method is behaviorally equivalent to 

       
 while (!blocker.isReleasable())
   if (blocker.block())
     return;
      If the caller is a ForkJoinTask, then the pool may first be expanded to ensure parallelism, and later adjusted.
      Parameters:
      blocker - the blocker
      Throws:
      InterruptedException - if blocker.block did so

    • newTaskFor

      protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
      Overrides:
      newTaskFor in class AbstractExecutorService

    • newTaskFor

      protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
      Overrides:
      newTaskFor in class AbstractExecutorService