public class ForkJoinPool extends AbstractExecutorService
ExecutorService for running ForkJoinTasks. A
ForkJoinPool provides the entry point for submissions from
non-ForkJoinTask clients, as well as management and monitoring
operations.
A ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing
work-stealing: all threads in the pool attempt to find and execute
tasks submitted to the pool and/or created by other active tasks (eventually
blocking waiting for work if none exist). This enables efficient processing
when most tasks spawn other subtasks (as do most ForkJoinTasks), as
well as when many small tasks are submitted to the pool from external
clients. Especially when setting asyncMode to true in constructors, ForkJoinPools may also be appropriate for use with event-style tasks that
are never joined.
A static commonPool() is available and appropriate for most
applications. The common pool is used by any ForkJoinTask that is not
explicitly submitted to a specified pool. Using the common pool normally
reduces resource usage (its threads are slowly reclaimed during periods of
non-use, and reinstated upon subsequent use).
For applications that require separate or custom pools, a ForkJoinPool may be constructed with a given target parallelism level; by
default, equal to the number of available processors. The pool attempts to
maintain enough active (or available) threads by dynamically adding,
suspending, or resuming internal worker threads, even if some tasks are
stalled waiting to join others. However, no such adjustments are guaranteed
in the face of blocked I/O or other unmanaged synchronization. The nested ForkJoinPool.ManagedBlocker interface enables extension of the kinds of synchronization
accommodated.
In addition to execution and lifecycle control methods, this class
provides status check methods (for example getStealCount()) that are
intended to aid in developing, tuning, and monitoring fork/join applications.
Also, method toString() returns indications of pool state in a
convenient form for informal monitoring.
As is the case with other ExecutorServices, there are three main task
execution methods summarized in the following table. These are designed to be
used primarily by clients not already engaged in fork/join computations in
the current pool. The main forms of these methods accept instances of
ForkJoinTask, but overloaded forms also allow mixed execution of
plain Runnable- or Callable- based activities as well. However, tasks that
are already executing in a pool should normally instead use the
within-computation forms listed in the table unless using async event-style
tasks that are not usually joined, in which case there is little difference
among choice of methods.
| 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 common pool is by default constructed with default parameters, but
these may be controlled by setting three system properties with prefix java.util.concurrent.ForkJoinPool.common: parallelism -- an integer
greater than zero, threadFactory -- the class name of a
ForkJoinPool.ForkJoinWorkerThreadFactory, and exceptionHandler -- the class name of a Thread.UncaughtExceptionHandler. Upon any error in establishing these
settings, default parameters are used.
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.
| 限定符和类型 | 类和说明 |
|---|---|
static interface |
ForkJoinPool.ForkJoinWorkerThreadFactory
Factory for creating new
ForkJoinWorkerThreads. |
static interface |
ForkJoinPool.ManagedBlocker
Interface for extending managed parallelism for tasks running in
ForkJoinPools. |
| 限定符和类型 | 字段和说明 |
|---|---|
static ForkJoinPool.ForkJoinWorkerThreadFactory |
defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread.
|
| 构造器和说明 |
|---|
ForkJoinPool()
Creates a
ForkJoinPool with parallelism equal to Runtime.availableProcessors(), using the default thread factory, no
UncaughtExceptionHandler, and non-async LIFO processing mode. |
ForkJoinPool(int parallelism)
Creates a
ForkJoinPool 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 a
ForkJoinPool with the given parameters. |
| 限定符和类型 | 方法和说明 |
|---|---|
boolean |
awaitQuiescence(long timeout,
TimeUnit unit)
If called by a ForkJoinTask operating in this pool, equivalent in effect
to
ForkJoinTask.helpQuiesce(). |
boolean |
awaitTermination(long timeout,
TimeUnit unit)
Blocks until all tasks have completed execution after a shutdown request,
or the timeout occurs, or the current thread is interrupted, whichever
happens first.
|
static ForkJoinPool |
commonPool()
Returns the common pool instance.
|
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) |
int |
getActiveThreadCount()
Returns an estimate of the number of threads that are currently stealing
or executing tasks.
|
boolean |
getAsyncMode()
Returns
true if this pool uses local first-in-first-out
scheduling mode for forked tasks that are never joined. |
static int |
getCommonPoolParallelism()
Returns the targeted parallelism level of the common pool.
|
ForkJoinPool.ForkJoinWorkerThreadFactory |
getFactory()
Returns the factory used for constructing new workers.
|
int |
getParallelism()
Returns the targeted parallelism level of this pool.
|
int |
getPoolSize()
Returns the number of worker threads that have started but not yet
terminated.
|
int |
getQueuedSubmissionCount()
Returns an estimate of the number of tasks submitted to this pool that
have not yet begun executing.
|
long |
getQueuedTaskCount()
Returns an estimate of the total number of tasks currently held in queues
by worker threads (but not including tasks submitted to the pool that
have not begun executing).
|
int |
getRunningThreadCount()
Returns an estimate of the number of worker threads that are not blocked
waiting to join tasks or for other managed synchronization.
|
long |
getStealCount()
Returns an estimate of the total number of tasks stolen from one thread's
work queue by another.
|
Thread.UncaughtExceptionHandler |
getUncaughtExceptionHandler()
Returns the handler for internal worker threads that terminate due to
unrecoverable errors encountered while executing tasks.
|
boolean |
hasQueuedSubmissions()
Returns
true if there are any tasks submitted to this pool that
have not yet begun executing. |
<T> T |
invoke(ForkJoinTask<T> task)
Performs the given task, returning its result upon completion.
|
<T> List<Future<T>> |
invokeAll(Collection<? extends Callable<T>> tasks) |
boolean |
isQuiescent()
Returns
true if all worker threads are currently idle. |
boolean |
isShutdown()
Returns
true if this pool has been shut down. |
boolean |
isTerminated()
Returns
true if all tasks have completed following shut down. |
boolean |
isTerminating()
Returns
true if the process of termination has commenced but not
yet completed. |
static void |
managedBlock(ForkJoinPool.ManagedBlocker blocker)
Blocks in accord with the given blocker.
|
protected <T> RunnableFuture<T> |
newTaskFor(Callable<T> callable) |
protected <T> RunnableFuture<T> |
newTaskFor(Runnable runnable,
T value) |
protected ForkJoinTask<?> |
pollSubmission()
Removes and returns the next unexecuted submission if one is available.
|
void |
shutdown()
Possibly initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted.
|
List<Runnable> |
shutdownNow()
Possibly attempts to cancel and/or stop all tasks, and reject all
subsequently submitted tasks.
|
<T> ForkJoinTask<T> |
submit(Callable<T> task) |
<T> ForkJoinTask<T> |
submit(ForkJoinTask<T> task)
Submits a ForkJoinTask for execution.
|
ForkJoinTask<?> |
submit(Runnable task) |
<T> ForkJoinTask<T> |
submit(Runnable task,
T result) |
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.
|
invokeAll, invokeAny, invokeAnypublic static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory
public ForkJoinPool()
ForkJoinPool with parallelism equal to Runtime.availableProcessors(), using the default thread factory, no
UncaughtExceptionHandler, and non-async LIFO processing mode.SecurityException - if a security manager exists and the caller is
not permitted to modify threads because it does not hold RuntimePermission("modifyThread")public ForkJoinPool(int parallelism)
ForkJoinPool with the indicated parallelism level, the default thread factory, no
UncaughtExceptionHandler, and non-async LIFO processing mode.parallelism - the parallelism levelIllegalArgumentException - 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 hold RuntimePermission("modifyThread")public ForkJoinPool(int parallelism,
ForkJoinPool.ForkJoinWorkerThreadFactory factory,
Thread.UncaughtExceptionHandler handler,
boolean asyncMode)
ForkJoinPool with the given 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.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 hold RuntimePermission("modifyThread")public static ForkJoinPool commonPool()
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(long, java.util.concurrent.TimeUnit), before exit.public <T> T invoke(ForkJoinTask<T> task)
ex.printStackTrace()) of both the
current thread as well as the thread actually encountering the exception;
minimally only the latter.task - the taskNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic void execute(ForkJoinTask<?> task)
task - the taskNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic void execute(Runnable task)
NullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic <T> ForkJoinTask<T> submit(ForkJoinTask<T> task)
task - the task to submitNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic <T> ForkJoinTask<T> submit(Callable<T> task)
submit 在接口中 ExecutorServicesubmit 在类中 AbstractExecutorServiceNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic <T> ForkJoinTask<T> submit(Runnable task, T result)
submit 在接口中 ExecutorServicesubmit 在类中 AbstractExecutorServiceNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic ForkJoinTask<?> submit(Runnable task)
submit 在接口中 ExecutorServicesubmit 在类中 AbstractExecutorServiceNullPointerException - if the task is nullRejectedExecutionException - if the task cannot be scheduled for
executionpublic <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
invokeAll 在接口中 ExecutorServiceinvokeAll 在类中 AbstractExecutorServiceNullPointerExceptionRejectedExecutionExceptionpublic ForkJoinPool.ForkJoinWorkerThreadFactory getFactory()
public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
null if nonepublic int getParallelism()
public static int getCommonPoolParallelism()
public int getPoolSize()
getParallelism() when threads are created to maintain parallelism
when others are cooperatively blocked.public boolean getAsyncMode()
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 modepublic int getRunningThreadCount()
public int getActiveThreadCount()
public boolean isQuiescent()
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 idlepublic long getStealCount()
public long getQueuedTaskCount()
public int getQueuedSubmissionCount()
public boolean hasQueuedSubmissions()
true if there are any tasks submitted to this pool that
have not yet begun executing.true if there are any queued submissionsprotected ForkJoinTask<?> pollSubmission()
null if noneprotected int drainTasksTo(Collection<? super ForkJoinTask<?>> c)
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 intopublic String toString()
public void shutdown()
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.SecurityException - if a security manager exists and the caller is
not permitted to modify threads because it does not hold RuntimePermission("modifyThread")public List<Runnable> shutdownNow()
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).SecurityException - if a security manager exists and the caller is
not permitted to modify threads because it does not hold RuntimePermission("modifyThread")public boolean isTerminated()
true if all tasks have completed following shut down.true if all tasks have completed following shut downpublic boolean isTerminating()
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 terminatedpublic boolean isShutdown()
true if this pool has been shut down.true if this pool has been shut downpublic boolean awaitTermination(long timeout,
TimeUnit unit)
throws InterruptedException
commonPool() never terminates until program shutdown, when applied to
the common pool, this method is equivalent to awaitQuiescence(long, java.util.concurrent.TimeUnit) but always returns false.timeout - the maximum time to waitunit - the time unit of the timeout argumenttrue if this executor terminated and false if the
timeout elapsed before terminationInterruptedException - if interrupted while waitingpublic boolean awaitQuiescence(long timeout,
TimeUnit unit)
ForkJoinTask.helpQuiesce(). Otherwise, waits and/or attempts to
assist performing tasks until this pool isQuiescent() or the
indicated timeout elapses.timeout - the maximum time to waitunit - the time unit of the timeout argumenttrue if quiescent; false if the timeout elapsed.public static void managedBlock(ForkJoinPool.ManagedBlocker blocker) throws InterruptedException
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 blockerInterruptedException - if blocker.block did soprotected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
newTaskFor 在类中 AbstractExecutorServiceprotected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
newTaskFor 在类中 AbstractExecutorServiceCopyright © 2013 The Skfiy Open Association. All Rights Reserved.