java.util.concurrent

Package java.util.concurrent

Utility classes commonly useful in concurrent programming.

Interface Summary

BlockingQueue<E> A Queue that additionally supports operations that wait for elements to exist when retrieving them, and wait for space to exist when storing them.

Callable<V> A task that returns a result and may throw an exception.

Cancellable Something, usually a task, that can be cancelled.

ConcurrentMap<K,V> A Map providing additional atomic putIfAbsent and remove methods.

Delayed A mix-in style interface for marking objects that should be acted upon after a given delay.

Executor An object that executes submitted Runnable tasks.

ExecutorService An Executor that provides methods to manage termination.

Future<V> A Future represents the result of an asynchronous computation.

RejectedExecutionHandler A handler for tasks that cannot be executed by a ThreadPoolExecutor.

ScheduledCancellable A delayed or periodic action that can be cancelled.

ScheduledFuture<V> A delayed result-bearing action that can be cancelled.

ThreadFactory An object that creates new threads on demand.

Interface Summary
BlockingQueue<E>	A `Queue` that additionally supports operations that wait for elements to exist when retrieving them, and wait for space to exist when storing them.
Callable<V>	A task that returns a result and may throw an exception.
Cancellable	Something, usually a task, that can be cancelled.
ConcurrentMap<K,V>	A `Map` providing additional atomic `putIfAbsent` and `remove` methods.
Delayed	A mix-in style interface for marking objects that should be acted upon after a given delay.
Executor	An object that executes submitted `Runnable` tasks.
ExecutorService	An `Executor` that provides methods to manage termination.
Future<V>	A `Future` represents the result of an asynchronous computation.
RejectedExecutionHandler	A handler for tasks that cannot be executed by a `ThreadPoolExecutor`.
ScheduledCancellable	A delayed or periodic action that can be cancelled.
ScheduledFuture<V>	A delayed result-bearing action that can be cancelled.
ThreadFactory	An object that creates new threads on demand.

Class Summary

ArrayBlockingQueue<E> A bounded blocking queue backed by an array.

CancellableTask Base class for Cancellable Runnable actions within the Executor framework.

ConcurrentHashMap<K,V> A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.

ConcurrentLinkedQueue<E> An unbounded thread-safe queue based on linked nodes.

CopyOnWriteArrayList<E> A variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

CopyOnWriteArraySet<E> A Set that uses CopyOnWriteArrayList for all of its operations.

CountDownLatch A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

CyclicBarrier A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.

DelayQueue<E extends Delayed> An unbounded blocking queue of Delayed elements, in which an element can only be taken when its delay has expired.

Exchanger<V> An Exchanger provides a synchronization point at which two threads can exchange objects.

Executors Factory and utility methods for Executor, ExecutorService, ThreadFactory, Future, and Cancellable classes defined in this package.

FutureTask<V> A cancellable asynchronous computation.

LinkedBlockingQueue<E> An optionally-bounded blocking queue based on linked nodes.

PriorityBlockingQueue<E> An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.

PrivilegedFutureTask<T> A FutureTask that executes, if allowed, with the current access control context and context class loader of the thread creating the task.

ScheduledExecutor An Executor that can schedule commands to run after a given delay, or to execute periodically.

Semaphore A counting semaphore.

SynchronousQueue<E> A blocking queue in which each put must wait for a take, and vice versa.

ThreadPoolExecutor An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.

ThreadPoolExecutor.AbortPolicy A handler for rejected tasks that throws a RejectedExecutionException.

ThreadPoolExecutor.CallerRunsPolicy A handler for rejected tasks that runs the rejected task directly in the calling thread of the execute method, unless the executor has been shut down, in which case the task is discarded.

ThreadPoolExecutor.DiscardOldestPolicy A handler for rejected tasks that discards the oldest unhandled request and then retries execute, unless the executor is shut down, in which case the task is discarded.

ThreadPoolExecutor.DiscardPolicy A handler for rejected tasks that silently discards the rejected task.

TimeUnit A TimeUnit represents time durations at a given unit of granularity and provides utility methods to convert across units, and to perform timing and delay operations in these units.

Class Summary
ArrayBlockingQueue<E>	A bounded blocking queue backed by an array.
CancellableTask	Base class for `Cancellable` `Runnable` actions within the `Executor` framework.
ConcurrentHashMap<K,V>	A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.
ConcurrentLinkedQueue<E>	An unbounded thread-safe queue based on linked nodes.
CopyOnWriteArrayList<E>	A variant of `ArrayList` in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.
CopyOnWriteArraySet<E>	A `Set` that uses `CopyOnWriteArrayList` for all of its operations.
CountDownLatch	A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
CyclicBarrier	A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
DelayQueue<E extends Delayed>	An unbounded blocking queue of `Delayed` elements, in which an element can only be taken when its delay has expired.
Exchanger<V>	An `Exchanger` provides a synchronization point at which two threads can exchange objects.
Executors	Factory and utility methods for `Executor`, `ExecutorService`, `ThreadFactory`, `Future`, and `Cancellable` classes defined in this package.
FutureTask<V>	A cancellable asynchronous computation.
LinkedBlockingQueue<E>	An optionally-bounded blocking queue based on linked nodes.
PriorityBlockingQueue<E>	An unbounded blocking queue that uses the same ordering rules as class `PriorityQueue` and supplies blocking retrieval operations.
PrivilegedFutureTask<T>	A `FutureTask` that executes, if allowed, with the current access control context and context class loader of the thread creating the task.
ScheduledExecutor	An `Executor` that can schedule commands to run after a given delay, or to execute periodically.
Semaphore	A counting semaphore.
SynchronousQueue<E>	A blocking queue in which each `put` must wait for a `take`, and vice versa.
ThreadPoolExecutor	An `ExecutorService` that executes each submitted task using one of possibly several pooled threads, normally configured using `Executors` factory methods.
ThreadPoolExecutor.AbortPolicy	A handler for rejected tasks that throws a `RejectedExecutionException`.
ThreadPoolExecutor.CallerRunsPolicy	A handler for rejected tasks that runs the rejected task directly in the calling thread of the `execute` method, unless the executor has been shut down, in which case the task is discarded.
ThreadPoolExecutor.DiscardOldestPolicy	A handler for rejected tasks that discards the oldest unhandled request and then retries `execute`, unless the executor is shut down, in which case the task is discarded.
ThreadPoolExecutor.DiscardPolicy	A handler for rejected tasks that silently discards the rejected task.
TimeUnit	A `TimeUnit` represents time durations at a given unit of granularity and provides utility methods to convert across units, and to perform timing and delay operations in these units.

Exception Summary

BrokenBarrierException Exception thrown when a thread tries to wait upon a barrier that is in a broken state, or which enters the broken state while the thread is waiting.

CancellationException Exception indicating that the result of a value-producing Cancellable task, such as a FutureTask, cannot be retrieved because the task was cancelled.

ExecutionException Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.

RejectedExecutionException Exception thrown by an Executor when a task cannot be accepted for execution.

TimeoutException Exception thrown when a blocking operation times out.

Exception Summary
BrokenBarrierException	Exception thrown when a thread tries to wait upon a barrier that is in a broken state, or which enters the broken state while the thread is waiting.
CancellationException	Exception indicating that the result of a value-producing `Cancellable` task, such as a `FutureTask`, cannot be retrieved because the task was cancelled.
ExecutionException	Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.
RejectedExecutionException	Exception thrown by an `Executor` when a task cannot be accepted for execution.
TimeoutException	Exception thrown when a blocking operation times out.

Package java.util.concurrent Description

Utility classes commonly useful in concurrent programming. This package includes a few small standardized extensible frameworks, as well as some classes that provide useful functionality and are otherwise tedious or difficult to implement. Here are brief descriptions of the main components. See also the locks and atomic packages.

Executors

Executor is a simple standardized interface for defining custom thread-like subsystems, including thread pools, asynchronous IO, and lightweight task frameworks. Depending on which concrete Executor class is being used, tasks may execute in a newly created thread, an existing task-execution thread, or the thread calling execute(), and may execute sequentially or concurrently.

ExecutorService provides a more complete framework for executing Runnables. An ExecutorService manages queueing and scheduling of tasks, and allows controlled shutdown. The two primary implementations of ExecutorService are ThreadPoolExecutor, a tunable and flexible thread pool and ScheduledExecutor, which adds support for delayed and periodic task execution. The Executors class provides factory methods for the most common kinds and configurations of Executors, as well as a few utility methods for using them.

Executors may be used with threads that compute functions returning results. A Future returns the results of a Callable, the result-bearing analog of Runnable. Instances of concrete class FutureTask may be submitted to Executors to asynchronously start a potentially long-running computation, query to determine if its execution has completed, or cancel it. The CancellableTask class provides similar control for actions that do not bear results.

Queues

The java.util.concurrent ConcurrentLinkedQueue class supplies an efficient scalable thread-safe non-blocking FIFO queue. Five implementations in java.util.concurrent support the extended BlockingQueue interface, that defines blocking versions of put and take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, PriorityBlockingQueue, and DelayQueue. The different classes cover the most common usage contexts for producer-consumer, messaging, parallel tasking, and related concurrent designs.

Timing

The TimeUnit class provides multiple granularities (including nanoseconds) for specifying and controlling time-out based operations. Most classes in the package contain operations based on time-outs in addition to indefinite waits. In all cases that time-outs are used, the time-out specifies the minimum time that the method should wait before indicating that it timed-out. Implementations make a "best effort" to detect time-outs as soon as possible after they occur. However, an indefinite amount of time may elapse between a time-out being detected and a thread actually executing again after that time-out.

Synchronizers

Five classes aid common special-purpose synchronization idioms. Semaphore is a classic concurrency tool. CountDownLatch is very simple yet very common utility for blocking until a given number of signals, events, or conditions hold. A CyclicBarrier is a resettable multiway synchronization point common in some styles of parallel programming. An Exchanger allows two threads to exchange objects at a rendezvous point.

Concurrent Collections

Besides Queues, this package supplies a few Collection implementations designed for use in multithreaded contexts: ConcurrentHashMap, CopyOnWriteArrayList, and CopyOnWriteArraySet.

The "Concurrent" prefix used with some classes in this package is a shorthand indicating several differences from similar "synchronized" classes. For example java.util.Hashtable and Collections.synchronizedMap(new HashMap()) are synchronized. But ConcurrentHashMap is "concurrent". A concurrent collection is thread-safe, but not governed by a single exclusion lock. In the particular case of ConcurrentHashMap, it safely permits any number of concurrent reads as well as a tunable number of concurrent writes. "Synchronized" classes can be useful when you need to prevent all access to a collection via a single lock, at the expense of poorer scalability. In other cases in which multiple threads are expected to access a common collection, "concurrent" versions are normally preferable. And unsynchronized collections are preferable when either collections are unshared, or are accessible only when holding other locks.

Most concurrent Collection implementations (including most Queues) also differ from the usual java.util conventions in that their Iterators provide weakly consistent rather than fast-fail traversal. A weakly consistent iterator is thread-safe, but does not necessarily freeze the collection while iterating, so it may (or may not) reflect any updates since the iterator was created.