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.
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 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.
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 ForkJoinPool.ForkJoinWorkerThreadFactory |
defaultForkJoinWorkerThreadFactory |
Creates a new ForkJoinWorkerThread.
|
Constructor | Description |
---|---|
ForkJoinPool() |
Creates a
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) ). |
ForkJoinPool(int parallelism) |
Creates a
ForkJoinPool with the indicated parallelism
level, using defaults for all other parameters (see ForkJoinPool(int, ForkJoinWorkerThreadFactory,
UncaughtExceptionHandler, boolean, int, int, int, Predicate,
long, TimeUnit) ). |
ForkJoinPool(int parallelism,
ForkJoinPool.ForkJoinWorkerThreadFactory factory,
Thread.UncaughtExceptionHandler handler,
boolean asyncMode) |
Creates a
ForkJoinPool with the given parameters (using
defaults for others -- see ForkJoinPool(int,
ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
int, int, int, Predicate, long, TimeUnit) ). |
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) |
Creates a
ForkJoinPool with the given parameters. |
Modifier and Type | Method | 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(Runnable task) |
Executes the given command at some time in the future.
|
void |
execute(ForkJoinTask<?> task) |
Arranges for (asynchronous) execution of the given 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) |
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. |
static void |
managedBlock(ForkJoinPool.ManagedBlocker blocker) |
Runs the given possibly blocking task.
|
protected <T> RunnableFuture<T> |
newTaskFor(Runnable runnable,
T value) |
Returns a
RunnableFuture for the given runnable and default
value. |
protected <T> RunnableFuture<T> |
newTaskFor(Callable<T> callable) |
Returns a
RunnableFuture for the given callable task. |
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.
|
ForkJoinTask<?> |
submit(Runnable task) |
Submits a Runnable task for execution and returns a Future
representing that task.
|
<T> ForkJoinTask<T> |
submit(Runnable task,
T result) |
Submits a Runnable task for execution and returns a Future
representing that task.
|
<T> ForkJoinTask<T> |
submit(Callable<T> task) |
Submits a value-returning task for execution and returns a
Future representing the pending results of the task.
|
<T> ForkJoinTask<T> |
submit(ForkJoinTask<T> task) |
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
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)
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> 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.invokeAll
in interface ExecutorService
invokeAll
in class AbstractExecutorService
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 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 soprotected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
AbstractExecutorService
RunnableFuture
for the given runnable and default
value.newTaskFor
in class AbstractExecutorService
T
- the type of the given valuerunnable
- the runnable task being wrappedvalue
- the default value for the returned futureRunnableFuture
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 taskprotected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
AbstractExecutorService
RunnableFuture
for the given callable task.newTaskFor
in class AbstractExecutorService
T
- the type of the callable's resultcallable
- the callable task being wrappedRunnableFuture
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 task Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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DRAFT 9-internal+0-adhoc.mlchung.jdk9-jdeps