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## Package java.util.concurrent

Utility classes commonly useful in concurrent programming.

See:
Description

Interface Summary
BlockingQueue<E> | A `Queue` that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.
Callable<V> | A task that returns a result and may throw an exception.
CompletionService<V> | A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks.
ConcurrentMap<K,V> | A `Map` providing additional atomic `putIfAbsent`, `remove`, and `replace` 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 and methods that can produce a `Future` for tracking progress of one or more asynchronous tasks.
Future<V> | A `Future` represents the result of an asynchronous computation.
RejectedExecutionHandler | A handler for tasks that cannot be executed by a `ThreadPoolExecutor`.
ScheduledExecutorService | An `ExecutorService` that can schedule commands to run after a given delay, or to execute periodically.
ScheduledFuture<V> | A delayed result-bearing action that can be cancelled.
ThreadFactory | An object that creates new threads on demand.

Class Summary
AbstractExecutorService | Provides default implementation of `ExecutorService` execution methods.
ArrayBlockingQueue<E> | A bounded blocking queue backed by an array.
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 thread-safe 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> | A synchronization point at which two threads can exchange objects.
ExecutorCompletionService<V> | A `CompletionService` that uses a supplied `Executor` to execute tasks.
Executors | Factory and utility methods for `Executor`, `ExecutorService`, `ScheduledExecutorService`, `ThreadFactory`, and `Callable` 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.
ScheduledThreadPoolExecutor | A `ThreadPoolExecutor` that can additionally 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.

Enum Summary
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 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

Interfaces. `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 asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The `ScheduledExecutorService` subinterface adds support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as `Callable`, the result-bearing analog of `Runnable`. A `Future` returns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution.

Implementations. Classes `ThreadPoolExecutor` and `ScheduledThreadPoolExecutor` provide tunable, flexible thread pools. 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. Other utilities based on Executors include the concrete class `FutureTask` providing a common extensible implementation of Futures, and `ExecutorCompletionService`, that assists in coordinating the processing of groups of asynchronous tasks.

## 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

Four classes aid common special-purpose synchronization idioms. `Semaphore` is a classic concurrency tool. `CountDownLatch` is a 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 useful in some styles of parallel programming. An `Exchanger` allows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.

## 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.

Since:
  1.5

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

Copyright 2004 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.