Class ForkJoinPool
- All Implemented Interfaces:
Executor
,ExecutorService
ExecutorService
for running ForkJoinTask
s.
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
ForkJoinTask
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 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
-
Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionstatic interface
Factory for creating newForkJoinWorkerThread
s.static interface
Interface for extending managed parallelism for tasks running inForkJoinPool
s. -
Field Summary
FieldsModifier and TypeFieldDescriptionstatic final ForkJoinPool.ForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread. -
Constructor Summary
ConstructorsConstructorDescriptionCreates 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
Modifier and TypeMethodDescriptionboolean
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
void
execute
(ForkJoinTask<?> task) Arranges for (asynchronous) execution of the given task.int
Returns an estimate of the number of threads that are currently stealing or executing tasks.boolean
Returnstrue
if this pool uses local first-in-first-out scheduling mode for forked tasks that are never joined.Returns the factory used for constructing new workers.int
Returns the targeted parallelism level of this pool.int
Returns the number of worker threads that have started but not yet terminated.int
Returns an estimate of the number of tasks submitted to this pool that have not yet begun executing.long
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
Returns an estimate of the number of worker threads that are not blocked waiting to join tasks or for other managed synchronization.long
Returns an estimate of the total number of tasks stolen from one thread's work queue by another.Returns the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks.boolean
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.invokeAll
(Collection<? extends Callable<T>> tasks) boolean
Returnstrue
if all worker threads are currently idle.boolean
Returnstrue
if this pool has been shut down.boolean
Returnstrue
if all tasks have completed following shut down.boolean
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
(Runnable runnable, T value) protected <T> RunnableFuture<T>
newTaskFor
(Callable<T> callable) protected ForkJoinTask<?>
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.Attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks.ForkJoinTask<?>
<T> ForkJoinTask<T>
<T> ForkJoinTask<T>
<T> ForkJoinTask<T>
submit
(ForkJoinTask<T> task) Submits a ForkJoinTask for execution.toString()
Returns a string identifying this pool, as well as its state, including indications of run state, parallelism level, and worker and task counts.Methods inherited from class java.util.concurrent.AbstractExecutorService
invokeAll, invokeAny, invokeAny
-
Field Details
-
defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.
-
-
Constructor Details
-
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 Details
-
invoke
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
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
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for execution
-
submit
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
- Specified by:
submit
in interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for execution
-
submit
- Specified by:
submit
in interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for execution
-
submit
- Specified by:
submit
in interfaceExecutorService
- Overrides:
submit
in classAbstractExecutorService
- Throws:
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for execution
-
invokeAll
- Specified by:
invokeAll
in interfaceExecutorService
- Overrides:
invokeAll
in classAbstractExecutorService
- Throws:
NullPointerException
RejectedExecutionException
-
getFactory
Returns the factory used for constructing new workers.- Returns:
- the factory used for constructing new workers
-
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
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
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
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
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
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
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
- Overrides:
newTaskFor
in classAbstractExecutorService
-
newTaskFor
- Overrides:
newTaskFor
in classAbstractExecutorService
-