Class ForkJoinPool

java.lang.Object
java.util.concurrent.AbstractExecutorService
java.util.concurrent.ForkJoinPool
All Implemented Interfaces:
AutoCloseable, 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 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. All worker threads are initialized with Thread.isDaemon() set true.

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. The default policies may be overridden using a constructor with parameters corresponding to those documented in class ThreadPoolExecutor.

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) ForkJoinTask.fork()
Await and obtain result invoke(ForkJoinTask) ForkJoinTask.invoke()
Arrange exec and obtain Future submit(ForkJoinTask) ForkJoinTask.fork() (ForkJoinTasks are Futures)

The parameters used to construct the common pool may be controlled by setting the following system properties:

  • java.util.concurrent.ForkJoinPool.common.parallelism - the parallelism level, a non-negative integer
  • java.util.concurrent.ForkJoinPool.common.threadFactory - the class name of a ForkJoinPool.ForkJoinWorkerThreadFactory. The system class loader is used to load this class.
  • java.util.concurrent.ForkJoinPool.common.exceptionHandler - the class name of a Thread.UncaughtExceptionHandler. The system class loader is used to load this class.
  • java.util.concurrent.ForkJoinPool.common.maximumSpares - the maximum number of allowed extra threads to maintain target parallelism (default 256).
If no thread factory is supplied via a system property, then the common pool uses a factory that uses the system class loader as the thread context class loader. 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. However doing so may cause unjoined tasks to never be executed.
Implementation Note:
This implementation restricts the maximum number of running threads to 32767. Attempts to create pools with greater than the maximum number result in IllegalArgumentException. Also, 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 defaults for all other parameters (see ForkJoinPool(int, ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)).

    • ForkJoinPool

      public ForkJoinPool(int parallelism)
      Creates a ForkJoinPool with the indicated parallelism level, using defaults for all other parameters (see ForkJoinPool(int, ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)).
      Parameters:
      parallelism - the parallelism level
      Throws:
      IllegalArgumentException - if parallelism less than or equal to zero, or greater than implementation limit

    • ForkJoinPool

      public ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode)
      Creates a ForkJoinPool with the given parameters (using defaults for others -- see ForkJoinPool(int, ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)).
      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

    • ForkJoinPool

      public ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode, int corePoolSize, int maximumPoolSize, int minimumRunnable, Predicate<? super ForkJoinPool> saturate, long keepAliveTime, TimeUnit unit)
      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.
      corePoolSize - the number of threads to keep in the pool (unless timed out after an elapsed keep-alive). Normally (and by default) this is the same value as the parallelism level, but may be set to a larger value to reduce dynamic overhead if tasks regularly block. Using a smaller value (for example 0) has the same effect as the default.
      maximumPoolSize - the maximum number of threads allowed. When the maximum is reached, attempts to replace blocked threads fail. (However, because creation and termination of different threads may overlap, and may be managed by the given thread factory, this value may be transiently exceeded.) To arrange the same value as is used by default for the common pool, use 256 plus the parallelism level. (By default, the common pool allows a maximum of 256 spare threads.) Using a value (for example Integer.MAX_VALUE) larger than the implementation's total thread limit has the same effect as using this limit (which is the default).
      minimumRunnable - the minimum allowed number of core threads not blocked by a join or ForkJoinPool.ManagedBlocker. To ensure progress, when too few unblocked threads exist and unexecuted tasks may exist, new threads are constructed, up to the given maximumPoolSize. For the default value, use 1, that ensures liveness. A larger value might improve throughput in the presence of blocked activities, but might not, due to increased overhead. A value of zero may be acceptable when submitted tasks cannot have dependencies requiring additional threads.
      saturate - if non-null, a predicate invoked upon attempts to create more than the maximum total allowed threads. By default, when a thread is about to block on a join or ForkJoinPool.ManagedBlocker, but cannot be replaced because the maximumPoolSize would be exceeded, a RejectedExecutionException is thrown. But if this predicate returns true, then no exception is thrown, so the pool continues to operate with fewer than the target number of runnable threads, which might not ensure progress.
      keepAliveTime - the elapsed time since last use before a thread is terminated (and then later replaced if needed). For the default value, use 60, TimeUnit.SECONDS.
      unit - the time unit for the keepAliveTime argument
      Throws:
      IllegalArgumentException - if parallelism is less than or equal to zero, or is greater than implementation limit, or if maximumPoolSize is less than parallelism, of if the keepAliveTime is less than or equal to zero.
      NullPointerException - if the factory is null
      Since:
      9

  • Method Details

    • commonPool

      public static ForkJoinPool commonPool()
      Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to shutdown() or shutdownNow(). However this pool and any ongoing processing are automatically terminated upon program System.exit(int). Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit.
      Returns:
      the common pool instance
      Since:
      1.8

    • 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.
      Type Parameters:
      T - the type of the task's result
      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)
      Description copied from interface: Executor
      Executes the given command at some time in the future. The command may execute in a new thread, in a pooled thread, or in the calling thread, at the discretion of the Executor implementation.
      Parameters:
      task - the 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.
      Implementation Requirements:
      This method is equivalent to externalSubmit(ForkJoinTask) when called from a thread that is not in this pool.
      Type Parameters:
      T - the type of the task's result
      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)
      Description copied from interface: ExecutorService
      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.

      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Type Parameters:
      T - the type of the task's result
      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

    • submit

      public <T> ForkJoinTask<T> submit(Runnable task, T result)
      Description copied from interface: ExecutorService
      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.
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Type Parameters:
      T - the type of the result
      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

    • submit

      public ForkJoinTask<?> submit(Runnable task)
      Description copied from interface: ExecutorService
      Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      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

    • externalSubmit

      public <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task)
      Submits the given task as if submitted from a non-ForkJoinTask client. The task is added to a scheduling queue for submissions to the pool even when called from a thread in the pool.
      Implementation Requirements:
      This method is equivalent to submit(ForkJoinTask) when called from a thread that is not in this pool.
      Type Parameters:
      T - the type of the task's result
      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
      Since:
      20

    • lazySubmit

      public <T> ForkJoinTask<T> lazySubmit(ForkJoinTask<T> task)
      Submits the given task without guaranteeing that it will eventually execute in the absence of available active threads. In some contexts, this method may reduce contention and overhead by relying on context-specific knowledge that existing threads (possibly including the calling thread if operating in this pool) will eventually be available to execute the task.
      Type Parameters:
      T - the type of the task's result
      Parameters:
      task - the task
      Returns:
      the task
      Throws:
      NullPointerException - if the task is null
      RejectedExecutionException - if the task cannot be scheduled for execution
      Since:
      19

    • setParallelism

      public int setParallelism(int size)
      Changes the target parallelism of this pool, controlling the future creation, use, and termination of worker threads. Applications include contexts in which the number of available processors changes over time.
      Implementation Note:
      This implementation restricts the maximum number of running threads to 32767
      Parameters:
      size - the target parallelism level
      Returns:
      the previous parallelism level.
      Throws:
      IllegalArgumentException - if size is less than 1 or greater than the maximum supported by this pool.
      UnsupportedOperationException - this is thecommonPool() and parallelism level was set by System property java.util.concurrent.ForkJoinPool.common.parallelism.
      Since:
      19

    • invokeAllUninterruptibly

      public <T> List<Future<T>> invokeAllUninterruptibly(Collection<? extends Callable<T>> tasks)
      Uninterrupible version of invokeAll. Executes the given tasks, returning a list of Futures holding their status and results when all complete, ignoring interrupts. Future.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.
      API Note:
      This method supports usages that previously relied on an incompatible override of ExecutorService.invokeAll(java.util.Collection).
      Type Parameters:
      T - the type of the values returned from the tasks
      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
      Throws:
      NullPointerException - if tasks or any of its elements are null
      RejectedExecutionException - if any task cannot be scheduled for execution
      Since:
      22

    • 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

    • getCommonPoolParallelism

      public static int getCommonPoolParallelism()
      Returns the targeted parallelism level of the common pool.
      Returns:
      the targeted parallelism level of the common pool
      Since:
      1.8

    • 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 completed tasks that were executed by a thread other than their submitter. 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
      See Also:

    • 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()
      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.

    • shutdownNow

      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

    • 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 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

    • 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. 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

    • awaitQuiescence

      public boolean awaitQuiescence(long timeout, TimeUnit unit)
      If called by a ForkJoinTask operating in this pool, equivalent in effect to ForkJoinTask.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.

    • close

      public void close()
      Unless this is the commonPool(), initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted, and waits until all tasks have completed execution and the executor has terminated.

      If already terminated, or this is the commonPool(), this method has no effect on execution, and does not wait. Otherwise, if interrupted while waiting, this method stops all executing tasks as if by invoking shutdownNow(). It then continues to wait until all actively executing tasks have completed. Tasks that were awaiting execution are not executed. The interrupt status will be re-asserted before this method returns.

      Since:
      19

    • managedBlock

      public static void managedBlock(ForkJoinPool.ManagedBlocker blocker) throws InterruptedException
      Runs the given possibly blocking task. When running in a ForkJoinPool, this method possibly arranges for a spare thread to be activated if necessary to ensure sufficient parallelism while the current thread is blocked in blocker.block().

      This method repeatedly calls blocker.isReleasable() and blocker.block() until either method returns true. Every call to blocker.block() is preceded by a call to blocker.isReleasable() that returned false.

      If not running in a ForkJoinPool, this method is behaviorally equivalent to 

       
 while (!blocker.isReleasable())
   if (blocker.block())
     break;
      If running in a ForkJoinPool, the pool may first be expanded to ensure sufficient parallelism available during the call to blocker.block().
      Parameters:
      blocker - the blocker task
      Throws:
      InterruptedException - if blocker.block() did so