Java™ Platform
Standard Ed. 6

Package java.util.concurrent

Utility classes commonly useful in concurrent programming.


Interface Summary
BlockingDeque<E> A Deque that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.
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.
ConcurrentNavigableMap<K,V> A ConcurrentMap supporting NavigableMap operations, and recursively so for its navigable sub-maps.
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.
RunnableFuture<V> A Future that is Runnable.
RunnableScheduledFuture<V> A ScheduledFuture that is Runnable.
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 implementations 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.
ConcurrentSkipListMap<K,V> A scalable concurrent ConcurrentNavigableMap implementation.
ConcurrentSkipListSet<E> A scalable concurrent NavigableSet implementation based on a ConcurrentSkipListMap.
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 an internal 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 threads can pair and swap elements within pairs.
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.
LinkedBlockingDeque<E> An optionally-bounded blocking deque based on linked nodes.
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 insert operation must wait for a corresponding remove operation by another thread, 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.


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 and associated interfaces add 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. A RunnableFuture is a Future that possesses a run method that upon execution, sets its results.

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.


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. The BlockingDeque interface extends BlockingQueue to support both FIFO and LIFO (stack-based) operations. Class LinkedBlockingDeque provides an implementation.


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. All methods that accept timeout parameters treat values less than or equal to zero to mean not to wait at all. To wait "forever", you can use a value of Long.MAX_VALUE.


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 Collection implementations designed for use in multithreaded contexts: ConcurrentHashMap, ConcurrentSkipListMap, ConcurrentSkipListSet, CopyOnWriteArrayList, and CopyOnWriteArraySet. When many threads are expected to access a given collection, a ConcurrentHashMap is normally preferable to a synchronized HashMap, and a ConcurrentSkipListMap is normally preferable to a synchronized TreeMap. A CopyOnWriteArrayList is preferable to a synchronized ArrayList when the expected number of reads and traversals greatly outnumber the number of updates to a list.

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.

Memory Consistency Properties

Chapter 17 of the Java Language Specification defines the happens-before relation on memory operations such as reads and writes of shared variables. The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation. The synchronized and volatile constructs, as well as the Thread.start() and Thread.join() methods, can form happens-before relationships. In particular: The methods of all classes in java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization. In particular:


Java™ Platform
Standard Ed. 6

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