ForkJoinPool - JRE Emulation | JRE Emulation
public class

ForkJoinPool

extends AbstractExecutorService
java.lang.Object
   ↳ java.util.concurrent.AbstractExecutorService
     ↳ java.util.concurrent.ForkJoinPool

Class Overview

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 tasks submitted to the pool and/or 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), as well as when many small tasks are submitted to the pool from external clients. Especially when setting asyncMode to true in constructors, ForkJoinPools may also be appropriate for use with event-style tasks that are never joined.

A static commonPool() is available and appropriate for most applications. The common pool is used by any ForkJoinTask that is not explicitly submitted to a specified pool. Using the common pool normally reduces resource usage (its threads are slowly reclaimed during periods of non-use, and reinstated upon subsequent use).

For applications that require separate or custom pools, a ForkJoinPool may be 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 I/O 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 primarily 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 instead use the within-computation forms listed in the table unless using async event-style tasks that are not usually joined, in which case there is little difference among choice of methods.
Summary of task execution methods
Call from non-fork/join clients Call from within fork/join computations
Arrange async execution execute(ForkJoinTask) fork()
Await and obtain result invoke(ForkJoinTask) invoke()
Arrange exec and obtain Future submit(ForkJoinTask) fork() (ForkJoinTasks are Futures)

The common pool is by default constructed with default parameters, but these may be controlled by setting three {@linkplain System#getProperty system properties}:

The system class loader is used to load these classes. Upon any error in establishing these settings, default parameters are used. It is possible to disable or limit the use of threads in the common pool by setting the parallelism property to zero, and/or using a factory that may return null.

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.

Summary

Nested Classes
interface ForkJoinPool.ForkJoinWorkerThreadFactory Factory for creating new ForkJoinWorkerThreads. 
interface ForkJoinPool.ManagedBlocker Interface for extending managed parallelism for tasks running in ForkJoinPools. 
Fields
public static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory Creates a new ForkJoinWorkerThread.
Public Constructors
ForkJoinPool()
Creates a ForkJoinPool with parallelism equal to availableProcessors(), using the {@linkplain #defaultForkJoinWorkerThreadFactory default thread factory}, no UncaughtExceptionHandler, and non-async LIFO processing mode.
ForkJoinPool(int parallelism)
Creates a ForkJoinPool with the indicated parallelism level, the {@linkplain #defaultForkJoinWorkerThreadFactory default thread factory}, no UncaughtExceptionHandler, and non-async LIFO processing mode.
ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode)
Creates a ForkJoinPool with the given parameters.
Public Methods
boolean awaitQuiescence(long timeout, TimeUnit unit)
If called by a ForkJoinTask operating in this pool, equivalent in effect to helpQuiesce().
boolean awaitTermination(long timeout, TimeUnit unit)
Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
void execute(Runnable task)
void execute(ForkJoinTask<?> task)
Arranges for (asynchronous) execution of the given task.
int getActiveThreadCount()
Returns an estimate of the number of threads that are currently stealing or executing tasks.
boolean getAsyncMode()
Returns true if this pool uses local first-in-first-out scheduling mode for forked tasks that are never joined.
ForkJoinPool.ForkJoinWorkerThreadFactory getFactory()
Returns the factory used for constructing new workers.
int getParallelism()
Returns the targeted parallelism level of this pool.
int getPoolSize()
Returns the number of worker threads that have started but not yet terminated.
int getQueuedSubmissionCount()
Returns an estimate of the number of tasks submitted to this pool that have not yet begun executing.
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).
int getRunningThreadCount()
Returns an estimate of the number of worker threads that are not blocked waiting to join tasks or for other managed synchronization.
long getStealCount()
Returns an estimate of the total number of tasks stolen from one thread's work queue by another.
Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
Returns the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks.
boolean hasQueuedSubmissions()
Returns true if there are any tasks submitted to this pool that have not yet begun executing.
<T> T invoke(ForkJoinTask<T> task)
Performs the given task, returning its result upon completion.
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
boolean isQuiescent()
Returns true if all worker threads are currently idle.
boolean isShutdown()
Returns true if this pool has been shut down.
boolean isTerminated()
Returns true if all tasks have completed following shut down.
boolean isTerminating()
Returns true if the process of termination has commenced but not yet completed.
static void managedBlock(ForkJoinPool.ManagedBlocker blocker)
Blocks in accord with the given blocker.
void shutdown()
Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
List<Runnable> shutdownNow()
Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks.
<T> ForkJoinTask<T> submit(Callable<T> task)
Submits a value-returning task for execution and returns a Future representing the pending results of the task.
<T> ForkJoinTask<T> submit(ForkJoinTask<T> task)
Submits a ForkJoinTask for execution.
ForkJoinTask<?> submit(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.
<T> ForkJoinTask<T> submit(Runnable task, T result)
Submits a Runnable task for execution and returns a Future representing that task.
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.
Protected Methods
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.
<T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
Returns a RunnableFuture for the given runnable and default value.
<T> RunnableFuture<T> newTaskFor(Callable<T> callable)
Returns a RunnableFuture for the given callable task.
ForkJoinTask<?> pollSubmission()
Removes and returns the next unexecuted submission if one is available.
Inherited Methods
[Expand]
From class java.util.concurrent.AbstractExecutorService
From class java.lang.Object
From interface java.util.concurrent.ExecutorService
From interface java.util.concurrent.Executor

Fields

public static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory

Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.

Public Constructors

public ForkJoinPool ()

Creates a ForkJoinPool with parallelism equal to availableProcessors(), using the {@linkplain #defaultForkJoinWorkerThreadFactory default thread factory}, no UncaughtExceptionHandler, and non-async LIFO processing mode.

public ForkJoinPool (int parallelism)

Creates a ForkJoinPool with the indicated parallelism level, the {@linkplain #defaultForkJoinWorkerThreadFactory 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

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 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

Public Methods

public boolean awaitQuiescence (long timeout, TimeUnit unit)

If called by a ForkJoinTask operating in this pool, equivalent in effect to helpQuiesce(). Otherwise, waits and/or attempts to assist performing tasks until this pool isQuiescent() or the indicated timeout elapses.

Parameters
timeout the maximum time to wait
unit the time unit of the timeout argument
Returns
  • true if quiescent; false if the timeout elapsed.

public boolean awaitTermination (long timeout, TimeUnit unit)

Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first. Because the commonPool() never terminates until program shutdown, when applied to the common pool, this method is equivalent to awaitQuiescence(long, TimeUnit) but always returns false.

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

public void execute (Runnable task)

Throws
NullPointerException if the task is null
RejectedExecutionException if the task cannot be scheduled for execution

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

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

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

public ForkJoinPool.ForkJoinWorkerThreadFactory getFactory ()

Returns the factory used for constructing new workers.

Returns
  • the factory used for constructing new workers

public int getParallelism ()

Returns the targeted parallelism level of this pool.

Returns
  • the targeted parallelism level of this pool

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

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

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

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

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

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

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

public 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

public List<Future<T>> invokeAll (Collection<? extends Callable<T>> tasks)

Executes the given tasks, returning a list of Futures holding their status and results when all complete. isDone() is true for each element of the returned list. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.

Parameters
tasks the collection of tasks
Returns
  • a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed

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

public boolean isShutdown ()

Returns true if this pool has been shut down.

Returns
  • true if this pool has been shut down

public boolean isTerminated ()

Returns true if all tasks have completed following shut down.

Returns
  • true if all tasks have completed following shut down

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 I/O, 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

public static void managedBlock (ForkJoinPool.ManagedBlocker blocker)

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

public void shutdown ()

Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no effect on execution state if this is the commonPool(), and 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.

public List<Runnable> shutdownNow ()

Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks. Invocation has no effect on execution state if this is the commonPool(), and no additional effect if already shut down. Otherwise, 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

public ForkJoinTask<T> submit (Callable<T> task)

Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's get method will return the task's result upon successful completion.

If you would like to immediately block waiting for a task, you can use constructions of the form result = exec.submit(aCallable).get();

Note: The Executors class includes a set of methods that can convert some other common closure-like objects, for example, PrivilegedAction to Callable form so they can be submitted.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task
Throws
NullPointerException if the task is null
RejectedExecutionException if the task cannot be scheduled for execution

public 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

public ForkJoinTask<?> submit (Runnable task)

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task
Throws
NullPointerException if the task is null
RejectedExecutionException if the task cannot be scheduled for execution

public ForkJoinTask<T> submit (Runnable task, T result)

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return the given result upon successful completion.

Parameters
task the task to submit
result the result to return
Returns
  • a Future representing pending completion of the task
Throws
NullPointerException if the task is null
RejectedExecutionException if the task cannot be scheduled for execution

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.

Returns
  • a string identifying this pool, as well as its state

Protected Methods

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

protected RunnableFuture<T> newTaskFor (Runnable runnable, T value)

Returns a RunnableFuture for the given runnable and default value.

Parameters
runnable the runnable task being wrapped
value the default value for the returned future
Returns
  • a RunnableFuture which, when run, will run the underlying runnable and which, as a Future, will yield the given value as its result and provide for cancellation of the underlying task

protected RunnableFuture<T> newTaskFor (Callable<T> callable)

Returns a RunnableFuture for the given callable task.

Parameters
callable the callable task being wrapped
Returns
  • a RunnableFuture which, when run, will call the underlying callable and which, as a Future, will yield the callable's result as its result and provide for cancellation of the underlying task

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