AutoCloseable
, Executor
, ExecutorService
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, and are not guaranteed to preserve the values of ThreadLocal
variables across tasks. 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.
IllegalArgumentException
. Also, 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 | Description |
---|---|---|
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 | Description |
---|---|---|
static final ForkJoinPool.ForkJoinWorkerThreadFactory |
defaultForkJoinWorkerThreadFactory |
Creates a new ForkJoinWorkerThread. |
Modifier and Type | Method | Description |
---|---|---|
boolean |
awaitQuiescence |
If called by a ForkJoinTask operating in this pool, equivalent in effect to ForkJoinTask.helpQuiesce() . |
boolean |
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. |
void |
close() |
Unless this is the commonPool() , initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted, and waits until all tasks have completed execution and the executor has terminated. |
static ForkJoinPool |
commonPool() |
Returns the common pool instance. |
protected int |
drainTasksTo |
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 |
Executes the given command at some time in the future. |
void |
execute |
Arranges for (asynchronous) execution of the given task. |
<T> ForkJoinTask |
externalSubmit |
Submits the given task as if submitted from a non- ForkJoinTask client. |
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 completed tasks that were executed by a thread other than their submitter. |
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 |
Performs the given task, returning its result upon completion. |
<T> List |
invokeAll |
Executes the given tasks, returning a list of Futures holding their status and results when all complete. |
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. |
<T> ForkJoinTask |
lazySubmit |
Submits the given task without guaranteeing that it will eventually execute in the absence of available active threads. |
static void |
managedBlock |
Runs the given possibly blocking task. |
protected ForkJoinTask |
pollSubmission() |
Removes and returns the next unexecuted submission if one is available. |
int |
setParallelism |
Changes the target parallelism of this pool, controlling the future creation, use, and termination of worker threads. |
void |
shutdown() |
Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. |
List |
shutdownNow() |
Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks. |
ForkJoinTask |
submit |
Submits a Runnable task for execution and returns a Future representing that task. |
<T> ForkJoinTask |
submit |
Submits a Runnable task for execution and returns a Future representing that task. |
<T> ForkJoinTask |
submit |
Submits a value-returning task for execution and returns a Future representing the pending results of the task. |
<T> ForkJoinTask |
submit |
Submits a ForkJoinTask for execution. |
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, newTaskFor, newTaskFor
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, 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, 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, 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, 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 <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)
Executor
Executor
implementation.task
- the runnable taskNullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic <T> ForkJoinTask<T> submit(ForkJoinTask<T> task)
externalSubmit(ForkJoinTask)
when called from a thread that is not in this pool.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)
ExecutorService
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.
submit
in interface ExecutorService
submit
in class AbstractExecutorService
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(Runnable task, T result)
ExecutorService
get
method will return the given result upon successful completion.submit
in interface ExecutorService
submit
in class AbstractExecutorService
T
- the type of the resulttask
- the task to submitresult
- the result to returnNullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic ForkJoinTask<?> submit(Runnable task)
ExecutorService
get
method will return null
upon successful completion.submit
in interface ExecutorService
submit
in class AbstractExecutorService
task
- the task to submitNullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task)
ForkJoinTask
client. The task is added to a scheduling queue for submissions to the pool even when called from a thread in the pool.submit(ForkJoinTask)
when called from a thread that is not in this pool.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> lazySubmit(ForkJoinTask<T> task)
T
- the type of the task's resulttask
- the taskNullPointerException
- if the task is nullRejectedExecutionException
- if the task cannot be scheduled for executionpublic int setParallelism(int size)
size
- the target parallelism levelIllegalArgumentException
- if size is less than 1 or greater than the maximum supported by this pool.UnsupportedOperationException
- this is thecommonPool()
and parallelism level was set by System property java.util.concurrent.ForkJoinPool.common.parallelism
.SecurityException
- if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission
("modifyThread")
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
ExecutorService
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.T
- the type of the values returned from the taskstasks
- the collection of tasksNullPointerException
- if tasks or any of its elements are null
RejectedExecutionException
- if any task cannot be scheduled for executionpublic 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, 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 void close()
commonPool()
, initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted, and waits until all tasks have completed execution and the executor has terminated. If already terminated, or this is the commonPool()
, this method has no effect on execution, and does not wait. Otherwise, if interrupted while waiting, this method stops all executing tasks as if by invoking shutdownNow()
. It then continues to wait until all actively executing tasks have completed. Tasks that were awaiting execution are not executed. The interrupt status will be re-asserted before this method returns.
SecurityException
- if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold RuntimePermission
("modifyThread")
, or the security manager's checkAccess
method denies access.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 so
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