public class ForkJoinPool extends AbstractExecutorService
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. 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.
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).
SecurityManager
is present, then the common pool uses a
factory supplying threads that have no Permissions
enabled.
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
.
This implementation rejects submitted tasks (that is, by throwing
RejectedExecutionException
) only when the pool is shut down or internal resources have
been exhausted.
Modifier and Type | Class and Description |
---|---|
static class |
ForkJoinPool.AlluxioForkJoinWorkerThreadFactory
Default ForkJoinWorkerThreadFactory implementation; creates a new ForkJoinWorkerThread using
the system class loader as the thread context class loader.
|
static interface |
ForkJoinPool.ForkJoinWorkerThreadFactory
Factory for creating new
ForkJoinWorkerThread s. |
static interface |
ForkJoinPool.ManagedBlocker
Interface for extending managed parallelism for tasks running in
ForkJoinPool s. |
Modifier and Type | Field and Description |
---|---|
static ForkJoinPool.ForkJoinWorkerThreadFactory |
defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread.
|
Modifier and Type | Method and Description |
---|---|
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)
Runs the given possibly blocking task.
|
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, invokeAny
public static final ForkJoinPool.ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory
public ForkJoinPool()
ForkJoinPool
with parallelism equal to
Runtime.availableProcessors()
, using defaults for all other parameters (see
ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)
).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, using defaults for all
other parameters (see
ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)
).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 (using defaults for others -- see
ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)
).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 ForkJoinPool(int parallelism, ForkJoinPool.ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode, int corePoolSize, int maximumPoolSize, int minimumRunnable, java.util.function.Predicate<? super ForkJoinPool> saturate, long keepAliveTime, TimeUnit unit)
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
.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
argumentIllegalArgumentException
- 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 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
, before exit.public static int getCommonPoolParallelism()
public static void managedBlock(ForkJoinPool.ManagedBlocker blocker) throws InterruptedException
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()
.blocker
- the blocker taskInterruptedException
- if blocker.block()
did sopublic <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.T
- the type of the task's resulttask
- 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)
T
- the type of the task's resulttask
- 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
in interface ExecutorService
submit
in class AbstractExecutorService
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic <T> ForkJoinTask<T> submit(Runnable task, T result)
submit
in interface ExecutorService
submit
in class AbstractExecutorService
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic ForkJoinTask<?> submit(Runnable task)
submit
in interface ExecutorService
submit
in class AbstractExecutorService
NullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
invokeAll
in interface ExecutorService
invokeAll
in class AbstractExecutorService
NullPointerException
RejectedExecutionException
public ForkJoinPool.ForkJoinWorkerThreadFactory getFactory()
public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
null
if nonepublic int getParallelism()
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, 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.protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
newTaskFor
in class AbstractExecutorService
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
newTaskFor
in class AbstractExecutorService
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