java.lang.Object | ||
↳ | java.util.concurrent.AbstractExecutorService | |
↳ | java.util.concurrent.ForkJoinPool |
An 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 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 ForkJoinTask
s), as well as when many small
tasks are submitted to the pool from external clients. Especially
when setting asyncMode to true in constructors, ForkJoinPool
s 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.
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
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
java.util.concurrent.ForkJoinPool.common.exceptionHandler
- the class name of a Thread.UncaughtExceptionHandler
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.
Nested Classes | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
ForkJoinPool.ForkJoinWorkerThreadFactory |
Factory for creating new ForkJoinWorkerThread s.
|
||||||||||
ForkJoinPool.ManagedBlocker |
Interface for extending managed parallelism for tasks running
in ForkJoinPool s.
|
Fields | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
defaultForkJoinWorkerThreadFactory | Creates a new ForkJoinWorkerThread. |
Public Constructors | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Creates a
ForkJoinPool with parallelism equal to availableProcessors() , using the default thread factory ,
no UncaughtExceptionHandler, and non-async LIFO processing mode.
| |||||||||||
Creates a
ForkJoinPool with the indicated parallelism
level, the default thread factory ,
no UncaughtExceptionHandler, and non-async LIFO processing mode.
| |||||||||||
Creates a
ForkJoinPool with the given parameters.
|
Public Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
If called by a ForkJoinTask operating in this pool, equivalent
in effect to
helpQuiesce() .
| |||||||||||
Blocks until all tasks have completed execution after a
shutdown request, or the timeout occurs, or the current thread
is interrupted, whichever happens first.
| |||||||||||
Arranges for (asynchronous) execution of the given task.
| |||||||||||
Returns an estimate of the number of threads that are currently
stealing or executing tasks.
| |||||||||||
Returns
true 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.
| |||||||||||
Returns the targeted parallelism level of this pool.
| |||||||||||
Returns the number of worker threads that have started but not
yet terminated.
| |||||||||||
Returns an estimate of the number of tasks submitted to this
pool that have not yet begun executing.
| |||||||||||
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).
| |||||||||||
Returns an estimate of the number of worker threads that are
not blocked waiting to join tasks or for other managed
synchronization.
| |||||||||||
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.
| |||||||||||
Returns
true if there are any tasks submitted to this
pool that have not yet begun executing.
| |||||||||||
Performs the given task, returning its result upon completion.
| |||||||||||
Executes the given tasks, returning a list of Futures holding
their status and results when all complete.
| |||||||||||
Returns
true if all worker threads are currently idle.
| |||||||||||
Returns
true if this pool has been shut down.
| |||||||||||
Returns
true if all tasks have completed following shut down.
| |||||||||||
Returns
true if the process of termination has
commenced but not yet completed.
| |||||||||||
Blocks in accord with the given blocker.
| |||||||||||
Possibly initiates an orderly shutdown in which previously
submitted tasks are executed, but no new tasks will be
accepted.
| |||||||||||
Possibly attempts to cancel and/or stop all tasks, and reject
all subsequently submitted tasks.
| |||||||||||
Submits a Runnable task for execution and returns a Future
representing that task.
| |||||||||||
Submits a ForkJoinTask for execution.
| |||||||||||
Submits a value-returning task for execution and returns a
Future representing the pending results of the task.
| |||||||||||
Submits a Runnable task for execution and returns a Future
representing that task.
| |||||||||||
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 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Removes all available unexecuted submitted and forked tasks
from scheduling queues and adds them to the given collection,
without altering their execution status.
| |||||||||||
Returns a
RunnableFuture for the given callable task.
| |||||||||||
Returns a
RunnableFuture for the given runnable and default
value.
| |||||||||||
Removes and returns the next unexecuted submission if one is
available.
|
[Expand]
Inherited Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
From class
java.util.concurrent.AbstractExecutorService
| |||||||||||
From class
java.lang.Object
| |||||||||||
From interface
java.util.concurrent.Executor
| |||||||||||
From interface
java.util.concurrent.ExecutorService
|
Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.
Creates a ForkJoinPool
with parallelism equal to availableProcessors()
, using the default thread factory
,
no UncaughtExceptionHandler, and non-async LIFO processing mode.
Creates a ForkJoinPool
with the indicated parallelism
level, the default thread factory
,
no UncaughtExceptionHandler, and non-async LIFO processing mode.
parallelism | the parallelism level |
---|
IllegalArgumentException | if parallelism less than or equal to zero, or greater than implementation limit |
---|
Creates a ForkJoinPool
with the given 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 . |
IllegalArgumentException | if parallelism less than or equal to zero, or greater than implementation limit |
---|---|
NullPointerException | if the factory is null |
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.
timeout | the maximum time to wait |
---|---|
unit | the time unit of the timeout argument |
true
if quiescent; false
if the
timeout elapsed.
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
.
timeout | the maximum time to wait |
---|---|
unit | the time unit of the timeout argument |
true
if this executor terminated and
false
if the timeout elapsed before terminationInterruptedException | if interrupted while waiting |
---|
Arranges for (asynchronous) execution of the given task.
task | the task |
---|
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
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 true
if this pool uses local first-in-first-out
scheduling mode for forked tasks that are never joined.
true
if this pool uses async mode
Returns the factory used for constructing new workers.
Returns the targeted parallelism level of this pool.
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 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 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 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 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 handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks.
null
if none
Returns true
if there are any tasks submitted to this
pool that have not yet begun executing.
true
if there are any queued submissions
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.
task | the task |
---|
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
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.
tasks | the collection of tasks |
---|
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.
true
if all threads are currently idle
Returns true
if this pool has been shut down.
true
if this pool has been shut down
Returns true
if all tasks have completed following shut down.
true
if all tasks have completed following shut down
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.)
true
if terminating but not yet terminated
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.blocker | the blocker |
---|
InterruptedException | if blocker.block did so |
---|
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.
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).
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.
task | the task to submit |
---|---|
result | the result to return |
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
Submits a ForkJoinTask for execution.
task | the task to submit |
---|
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
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.
task | the task to submit |
---|
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
Submits a Runnable task for execution and returns a Future
representing that task. The Future's get
method will
return null
upon successful completion.
task | the task to submit |
---|
NullPointerException | if the task is null |
---|---|
RejectedExecutionException | if the task cannot be scheduled for execution |
Returns a string identifying this pool, as well as its state, including indications of run state, parallelism level, and worker and task counts.
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.
c | the collection to transfer elements into |
---|
Returns a RunnableFuture
for the given callable task.
callable | the callable task being wrapped |
---|
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 taskReturns a RunnableFuture
for the given runnable and default
value.
runnable | the runnable task being wrapped |
---|---|
value | the default value for the returned future |
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 taskRemoves 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.
null
if none