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Java.Util.Concurrent Namespace

Implementations. Classes ThreadPoolExecutor and ScheduledThreadPoolExecutor provide tunable, flexible thread pools.


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 Java.Util.Concurrent.Locks and Java.Util.Concurrent.Atomic packages.


Interfaces.IExecutor is a simple standardized interface for defining custom thread-like subsystems, including thread pools, asynchronous I/O, 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 IExecutor.Execute(IRunnable), and may execute sequentially or concurrently. IExecutorService provides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The IScheduledExecutorService subinterface and associated interfaces add support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as ICallable, the result-bearing analog of IRunnable. A IFuture returns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A IRunnableFuture 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.

Class ForkJoinPool provides an Executor primarily designed for processing instances of ForkJoinTask and its subclasses. These classes employ a work-stealing scheduler that attains high throughput for tasks conforming to restrictions that often hold in computation-intensive parallel processing.


The ConcurrentLinkedQueue class supplies an efficient scalable thread-safe non-blocking FIFO queue. The ConcurrentLinkedDeque class is similar, but additionally supports the IDeque interface.

Five implementations in java.util.concurrent support the extended IBlockingQueue 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.

Extended interface ITransferQueue, and implementation LinkedTransferQueue introduce a synchronous transfer method (along with related features) in which a producer may optionally block awaiting its consumer.

The IBlockingDeque 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.


Five 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.
  • A Phaser provides a more flexible form of barrier that may be used to control phased computation among multiple threads.
  • 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, NoType:java/util/concurrent/ConcurrentSkipListSet;Href=../../../../reference/java/util/concurrent/ConcurrentSkipListSet.html, 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

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:
  • Each action in a thread happens-before every action in that thread that comes later in the program's order.
  • An unlock (synchronized block or method exit) of a monitor happens-before every subsequent lock (synchronized block or method entry) of that same monitor. And because the happens-before relation is transitive, all actions of a thread prior to unlocking happen-before all actions subsequent to any thread locking that monitor.
  • A write to a volatile field happens-before every subsequent read of that same field. Writes and reads of volatile fields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
  • A call to start on a thread happens-before any action in the started thread.
  • All actions in a thread happen-before any other thread successfully returns from a join on that thread.
The methods of all classes in java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization. In particular:
  • Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread.
  • Actions in a thread prior to the submission of a Runnable to an Executorhappen-before its execution begins. Similarly for Callables submitted to an ExecutorService.
  • Actions taken by the asynchronous computation represented by a Futurehappen-before actions subsequent to the retrieval of the result via Future.get() in another thread.
  • Actions prior to "releasing" synchronizer methods such as Lock.unlock, Semaphore.release, and CountDownLatch.countDownhappen-before actions subsequent to a successful "acquiring" method such as Lock.lock, Semaphore.acquire, Condition.await, and CountDownLatch.await on the same synchronizer object in another thread.
  • For each pair of threads that successfully exchange objects via an Exchanger, actions prior to the exchange() in each thread happen-before those subsequent to the corresponding exchange() in another thread.
  • Actions prior to calling CyclicBarrier.await and Phaser.awaitAdvance (as well as its variants) happen-before actions performed by the barrier action, and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding await in other threads.


AbstractExecutorServiceProvides default implementations of IExecutorService execution methods.
ArrayBlockingQueueA bounded IBlockingQueue backed by an array.
BrokenBarrierExceptionException 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.
CancellationExceptionException indicating that the result of a value-producing task, such as a FutureTask, cannot be retrieved because the task was cancelled.
ConcurrentHashMapA hash table supporting full concurrency of retrievals and high expected concurrency for updates.
ConcurrentLinkedDequeAn unbounded concurrent IDeque based on linked nodes.
ConcurrentLinkedQueueAn unbounded thread-safe IQueue based on linked nodes.
ConcurrentSkipListMapA scalable concurrent NoType:java/util/concurrent/ConcurrentNavigableMap;Href=../../../../reference/java/util/concurrent/ConcurrentNavigableMap.html implementation.
CopyOnWriteArrayListA thread-safe random-access list.
CopyOnWriteArraySetA ISet that uses an internal CopyOnWriteArrayList for all of its operations.
CountDownLatchA synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
CyclicBarrierA synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
DelayQueueAn unbounded IBlockingQueue of Delayed elements, in which an element can only be taken when its delay has expired.
ExchangerA synchronization point at which threads can pair and swap elements within pairs.
ExecutionExceptionException thrown when attempting to retrieve the result of a task that aborted by throwing an exception.
ExecutorCompletionServiceA ICompletionService that uses a supplied IExecutor to execute tasks.
ExecutorsFactory and utility methods for IExecutor, IExecutorService, IScheduledExecutorService, IThreadFactory, and ICallable classes defined in this package.
ForkJoinPoolAn IExecutorService for running ForkJoinTasks.
ForkJoinTaskAbstract base class for tasks that run within a ForkJoinPool.
ForkJoinWorkerThreadA thread managed by a ForkJoinPool, which executes ForkJoinTasks.
FutureTaskA cancellable asynchronous computation.
IBlockingDequeA IDeque 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.
IBlockingQueueA IQueue 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.
ICallableA task that returns a result and may throw an exception.
ICompletionServiceA service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks.
IConcurrentMapA IMap providing additional atomic putIfAbsent, remove, and replace methods.
IDelayedA mix-in style interface for marking objects that should be acted upon after a given delay.
IExecutorAn object that executes submitted IRunnable tasks.
IExecutorServiceAn IExecutor that provides methods to manage termination and methods that can produce a IFuture for tracking progress of one or more asynchronous tasks.
IFutureA Future represents the result of an asynchronous computation.
IRejectedExecutionHandlerA handler for tasks that cannot be executed by a ThreadPoolExecutor.
IRunnableFutureA IFuture that is IRunnable.
IRunnableScheduledFutureA IScheduledFuture that is IRunnable.
IScheduledExecutorServiceAn IExecutorService that can schedule commands to run after a given delay, or to execute periodically.
IScheduledFutureA delayed result-bearing action that can be cancelled.
IThreadFactoryAn object that creates new threads on demand.
ITransferQueueA IBlockingQueue in which producers may wait for consumers to receive elements.
LinkedBlockingDequeAn optionally-bounded IBlockingDeque based on linked nodes.
LinkedBlockingQueueAn optionally-bounded IBlockingQueue based on linked nodes.
LinkedTransferQueueAn unbounded ITransferQueue based on linked nodes.
PhaserA reusable synchronization barrier, similar in functionality to CyclicBarrier and CountDownLatch but supporting more flexible usage.
PriorityBlockingQueueAn unbounded IBlockingQueue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.
RecursiveActionA recursive resultless ForkJoinTask.
RecursiveTaskA recursive result-bearing ForkJoinTask.
RejectedExecutionExceptionException thrown by an IExecutor when a task cannot be accepted for execution.
ScheduledThreadPoolExecutorA ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.
SemaphoreA counting semaphore.
SynchronousQueueA IBlockingQueue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
ThreadLocalRandomA random number generator isolated to the current thread.
ThreadPoolExecutorAn IExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.
ThreadPoolExecutor+AbortPolicyA handler for rejected tasks that throws a RejectedExecutionException.
ThreadPoolExecutor+CallerRunsPolicyA 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+DiscardOldestPolicyA 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+DiscardPolicyA handler for rejected tasks that silently discards the rejected task.
TimeoutExceptionException thrown when a blocking operation times out.
TimeUnitA 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.