See Also: ForkJoinPool Members
An Java.Util.Concurrent.IExecutorService for running Java.Util.Concurrent.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 Java.Util.Concurrent.IExecutorService 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 NoType:java/util/concurrent/ForkJoinPool$ManagedBlocker;Href=../../../../reference/java/util/concurrent/ForkJoinPool.ManagedBlocker.html 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 ForkJoinPool.StealCount) that are intended to aid in developing, tuning, and monitoring fork/join applications. Also, method ForkJoinPool.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 | ForkJoinPool.execute(java.util.concurrent.ForkJoinTask<?>) | ForkJoinTask.Fork |
| Await and obtain result | ForkJoinPool.invoke(java.util.concurrent.ForkJoinTask<T>) | ForkJoinTask.Invoke |
| Arrange exec and obtain Future | ForkJoinPool.submit(java.util.concurrent.ForkJoinTask<T>) | ForkJoinTask.Fork (ForkJoinTasks are Futures) |
The common pool is by default constructed with default parameters, but these may be controlled by setting three Java.Lang.JavaSystem.GetProperty(string):
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 Java.Util.Concurrent.RejectedExecutionException) only when the pool is shut down or internal resources have been exhausted.