jdk.util.concurrent.ForkJoinPool — clojure-interop/java.util.concurrent 1.0.5

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.

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.

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.

Summary of task execution 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 common pool is by default constructed with default parameters, but these may be controlled by setting three 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 java.util.concurrent.ForkJoinPool.common.exceptionHandler
• the class name of a Thread.UncaughtExceptionHandler

If a SecurityManager is present and no factory is specified, then the default pool uses a factory supplying threads that have no Permissions enabled. The system class loader is used to load these classes. 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.

*-default-fork-join-worker-thread-factory clj

Static Constant.

Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool constructors.

type: java.util.concurrent.ForkJoinPool$ForkJoinWorkerThreadFactory

*common-pool clj

(*common-pool)

Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to 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.

returns: the common pool instance - java.util.concurrent.ForkJoinPool

*get-common-pool-parallelism clj

(*get-common-pool-parallelism)

Returns the targeted parallelism level of the common pool.

returns: the targeted parallelism level of the common pool - int

*managed-block clj

(*managed-block blocker)

Runs the given possibly blocking task. When running in a ForkJoinPool, this method possibly arranges for a spare thread to be activated if necessary to ensure sufficient parallelism while the current thread is blocked in 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 task - java.util.concurrent.ForkJoinPool$ManagedBlocker

throws: java.lang.InterruptedException - if blocker.block() did so

->fork-join-pool clj

(->fork-join-pool)

(->fork-join-pool parallelism)

(->fork-join-pool parallelism factory handler async-mode)

Constructor.

Creates a ForkJoinPool with the given parameters.

parallelism - the parallelism level. For default value, use Runtime.availableProcessors(). - int factory - the factory for creating new threads. For default value, use defaultForkJoinWorkerThreadFactory. - java.util.concurrent.ForkJoinPool$ForkJoinWorkerThreadFactory handler - the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks. For default value, use null. - java.lang.Thread$UncaughtExceptionHandler async-mode - 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. - boolean

throws: java.lang.IllegalArgumentException - if parallelism less than or equal to zero, or greater than implementation limit

await-quiescence clj

(await-quiescence this timeout unit)

If called by a ForkJoinTask operating in this pool, equivalent in effect to 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 wait - long unit - the time unit of the timeout argument - java.util.concurrent.TimeUnit

returns: true if quiescent; false if the timeout elapsed. - boolean

await-termination clj

(await-termination this timeout 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. Because the 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 wait - long unit - the time unit of the timeout argument - java.util.concurrent.TimeUnit

returns: true if this executor terminated and false if the timeout elapsed before termination - boolean

throws: java.lang.InterruptedException - if interrupted while waiting

execute clj

(execute this task)

Arranges for (asynchronous) execution of the given task.

task - the task - java.util.concurrent.ForkJoinTask

throws: java.lang.NullPointerException - if the task is null

get-active-thread-count clj

(get-active-thread-count this)

Returns an estimate of the number of threads that are currently stealing or executing tasks. This method may overestimate the number of active threads.

returns: the number of active threads - int

get-async-mode? clj

(get-async-mode? this)

Returns true if this pool uses local first-in-first-out scheduling mode for forked tasks that are never joined.

returns: true if this pool uses async mode - boolean

get-factory clj

(get-factory this)

Returns the factory used for constructing new workers.

returns: the factory used for constructing new workers - java.util.concurrent.ForkJoinPool$ForkJoinWorkerThreadFactory

get-parallelism clj

(get-parallelism this)

Returns the targeted parallelism level of this pool.

returns: the targeted parallelism level of this pool - int

get-pool-size clj

(get-pool-size this)

Returns the number of worker threads that have started but not yet terminated. The result returned by this method may differ from getParallelism() when threads are created to maintain parallelism when others are cooperatively blocked.

returns: the number of worker threads - int

get-queued-submission-count clj

(get-queued-submission-count this)

Returns an estimate of the number of tasks submitted to this pool that have not yet begun executing. This method may take time proportional to the number of submissions.

returns: the number of queued submissions - int

get-queued-task-count clj

(get-queued-task-count this)

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). This value is only an approximation, obtained by iterating across all threads in the pool. This method may be useful for tuning task granularities.

returns: the number of queued tasks - long

get-running-thread-count clj

(get-running-thread-count this)

Returns an estimate of the number of worker threads that are not blocked waiting to join tasks or for other managed synchronization. This method may overestimate the number of running threads.

returns: the number of worker threads - int

get-steal-count clj

(get-steal-count this)

Returns an estimate of the total number of tasks stolen from one thread's work queue by another. The reported value underestimates the actual total number of steals when the pool is not quiescent. This value may be useful for monitoring and tuning fork/join programs: in general, steal counts should be high enough to keep threads busy, but low enough to avoid overhead and contention across threads.

returns: the number of steals - long

get-uncaught-exception-handler clj

(get-uncaught-exception-handler this)

Returns the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks.

returns: the handler, or null if none - java.lang.Thread$UncaughtExceptionHandler

has-queued-submissions? clj

(has-queued-submissions? this)

Returns true if there are any tasks submitted to this pool that have not yet begun executing.

returns: true if there are any queued submissions - boolean

invoke clj

(invoke this task)

Performs the given task, returning its result upon completion. If the computation encounters an unchecked Exception or Error, it is rethrown as the outcome of this invocation. Rethrown exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces (as displayed for example using ex.printStackTrace()) of both the current thread as well as the thread actually encountering the exception; minimally only the latter.

task - the task - java.util.concurrent.ForkJoinTask

returns: the task's result - <T> T

throws: java.lang.NullPointerException - if the task is null

invoke-all clj

(invoke-all this tasks)

Description copied from interface: ExecutorService

tasks - the collection of tasks - java.util.Collection

returns: a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed - <T> java.util.List<java.util.concurrent.Future<T>>

throws: java.lang.NullPointerException - if tasks or any of its elements are null

quiescent? clj

(quiescent? this)

Returns 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.

returns: true if all threads are currently idle - boolean

shutdown clj

(shutdown this)

Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no effect on execution state if this is the 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.

throws: java.lang.SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

shutdown-now clj

(shutdown-now this)

Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks. Invocation has no effect on execution state if this is the 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).

returns: an empty list - java.util.List<java.lang.Runnable>

throws: java.lang.SecurityException - if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")

shutdown? clj

(shutdown? this)

Returns true if this pool has been shut down.

returns: true if this pool has been shut down - boolean

submit clj

(submit this task)

(submit this task result)

Description copied from interface: ExecutorService

task - the task to submit - java.lang.Runnable result - the result to return - T

returns: a Future representing pending completion of the task - <T> java.util.concurrent.ForkJoinTask<T>

throws: java.lang.NullPointerException - if the task is null

terminated? clj

(terminated? this)

Returns true if all tasks have completed following shut down.

returns: true if all tasks have completed following shut down - boolean

terminating? clj

(terminating? this)

Returns 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.)

returns: true if terminating but not yet terminated - boolean

to-string clj

(to-string this)

Returns a string identifying this pool, as well as its state, including indications of run state, parallelism level, and worker and task counts.

returns: a string identifying this pool, as well as its state - java.lang.String