- java.lang.Object
-
- java.util.concurrent.AbstractExecutorService
-
- java.util.concurrent.ForkJoinPool
-
- All Implemented Interfaces:
- Executor, ExecutorService
public class ForkJoinPool extends AbstractExecutorService
AnExecutorService
for runningForkJoinTask
s. AForkJoinPool
provides the entry point for submissions from non-ForkJoinTask
clients, as well as management and monitoring operations.A
ForkJoinPool
differs from other kinds ofExecutorService
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 mostForkJoinTask
s). When setting asyncMode to true in constructors,ForkJoinPool
s 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 nestedForkJoinPool.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, methodtoString()
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 plainRunnable
- orCallable
- 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 theSortTasks
illustrated inRecursiveAction
. BecauseForkJoinPool
uses threads in daemon mode, there is typically no need to explicitlyshutdown
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
-
-
Nested Class Summary
Nested Classes Modifier and Type Class and Description static interface
ForkJoinPool.ForkJoinWorkerThreadFactory
Factory for creating newForkJoinWorkerThread
s.static interface
ForkJoinPool.ManagedBlocker
Interface for extending managed parallelism for tasks running inForkJoinPool
s.
-
Field Summary
Fields Modifier and Type Field and Description static ForkJoinPool.ForkJoinWorkerThreadFactory
defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread.
-
Constructor Summary
Constructors Constructor and Description ForkJoinPool()
Creates aForkJoinPool
with parallelism equal toRuntime.availableProcessors()
, using the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode.ForkJoinPool(int parallelism)
Creates aForkJoinPool
with the indicated parallelism level, the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode.ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode)
Creates aForkJoinPool
with the given parameters.
-
Method Summary
Methods Modifier and Type Method and Description 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.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.void
execute(ForkJoinTask<?> task)
Arranges for (asynchronous) execution of the given task.void
execute(Runnable task)
Executes the given command at some time in the future.int
getActiveThreadCount()
Returns an estimate of the number of threads that are currently stealing or executing tasks.boolean
getAsyncMode()
Returnstrue
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()
Returnstrue
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()
Returnstrue
if all worker threads are currently idle.boolean
isShutdown()
Returnstrue
if this pool has been shut down.boolean
isTerminated()
Returnstrue
if all tasks have completed following shut down.boolean
isTerminating()
Returnstrue
if the process of termination has commenced but not yet completed.static void
managedBlock(ForkJoinPool.ManagedBlocker blocker)
Blocks in accord with the given blocker.protected <T> RunnableFuture<T>
newTaskFor(Callable<T> callable)
Returns a RunnableFuture for the given callable task.protected <T> RunnableFuture<T>
newTaskFor(Runnable runnable, T value)
Returns a RunnableFuture for the given runnable and default value.protected ForkJoinTask<?>
pollSubmission()
Removes and returns the next unexecuted submission if one is available.void
shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.List<Runnable>
shutdownNow()
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.
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-
-
Field Detail
-
defaultForkJoinWorkerThreadFactory
public static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.
-
-
Constructor Detail
-
ForkJoinPool
public ForkJoinPool()
Creates aForkJoinPool
with parallelism equal toRuntime.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 holdRuntimePermission
("modifyThread")
-
ForkJoinPool
public ForkJoinPool(int parallelism)
Creates aForkJoinPool
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 limitSecurityException
- if a security manager exists and the caller is not permitted to modify threads because it does not holdRuntimePermission
("modifyThread")
-
ForkJoinPool
public ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode)
Creates aForkJoinPool
with the given parameters.- Parameters:
parallelism
- the parallelism level. For default value, useRuntime.availableProcessors()
.factory
- the factory for creating new threads. For default value, usedefaultForkJoinWorkerThreadFactory
.handler
- the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks. For default value, usenull
.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, usefalse
.- Throws:
IllegalArgumentException
- if parallelism less than or equal to zero, or greater than implementation limitNullPointerException
- if the factory is nullSecurityException
- if a security manager exists and the caller is not permitted to modify threads because it does not holdRuntimePermission
("modifyThread")
-
-
Method Detail
-
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 usingex.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 nullRejectedExecutionException
- 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 nullRejectedExecutionException
- 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 nullRejectedExecutionException
- 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 nullRejectedExecutionException
- 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
toCallable
form so they can be submitted.- Specified by:
submit
in interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Parameters:
task
- the task to submit- Returns:
- a Future representing pending completion of the task
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- 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 interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Parameters:
task
- the task to submitresult
- the result to return- Returns:
- a Future representing pending completion of the task
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- 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 interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Parameters:
task
- the task to submit- Returns:
- a Future representing pending completion of the task
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for execution
-
invokeAll
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
Description copied from interface:ExecutorService
Executes the given tasks, returning a list of Futures holding their status and results when all complete.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.- Specified by:
invokeAll
in interfaceExecutorService
- Overrides:
invokeAll
in classAbstractExecutorService
- 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 nullRejectedExecutionException
- if any task cannot be scheduled for execution
-
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 fromgetParallelism()
when threads are created to maintain parallelism when others are cooperatively blocked.- Returns:
- the number of worker threads
-
getAsyncMode
public boolean getAsyncMode()
Returnstrue
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()
Returnstrue
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 returntrue
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()
Returnstrue
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 collectionc
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.
-
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 holdRuntimePermission
("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 holdRuntimePermission
("modifyThread")
-
isTerminated
public boolean isTerminated()
Returnstrue
if all tasks have completed following shut down.- Returns:
true
if all tasks have completed following shut down
-
isTerminating
public boolean isTerminating()
Returnstrue
if the process of termination has commenced but not yet completed. This method may be useful for debugging. A return oftrue
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 classForkJoinTask
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()
Returnstrue
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 waitunit
- the time unit of the timeout argument- Returns:
true
if this executor terminated andfalse
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 aForkJoinWorkerThread
, 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 towhile (!blocker.isReleasable()) if (blocker.block()) return;
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)
Description copied from class:AbstractExecutorService
Returns a RunnableFuture for the given runnable and default value.- Overrides:
newTaskFor
in classAbstractExecutorService
- Parameters:
runnable
- the runnable task being wrappedvalue
- 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.
-
newTaskFor
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
Description copied from class:AbstractExecutorService
Returns a RunnableFuture for the given callable task.- Overrides:
newTaskFor
in classAbstractExecutorService
- 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.
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Bij voorbaat dank.
Document heeft de 11/06/2005 gemaakt, de laatste keer de 04/03/2020 gewijzigd
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