An 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. 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 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 ForkJoinWorkerThreadFactory.
The system class loader
is used to load this class.
java.util.concurrent.ForkJoinPool.common.exceptionHandler
- the class name of a 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.
extends