Java.Util.Concurrent Namespace

Implementations. Classes Java.Util.Concurrent.ThreadPoolExecutor and Java.Util.Concurrent.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.Java.Util.Concurrent.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 Java.Util.Concurrent.IExecutor.Execute(Java.Lang.IRunnable), and may execute sequentially or concurrently. Java.Util.Concurrent.IExecutorService provides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The Java.Util.Concurrent.IScheduledExecutorService subinterface and associated interfaces add support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as Java.Util.Concurrent.ICallable, the result-bearing analog of Java.Lang.IRunnable. A Java.Util.Concurrent.IFuture returns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A Java.Util.Concurrent.IRunnableFuture is a Future that possesses a run method that upon execution, sets its results.

Implementations. Classes Java.Util.Concurrent.ThreadPoolExecutor and Java.Util.Concurrent.ScheduledThreadPoolExecutor provide tunable, flexible thread pools. The Java.Util.Concurrent.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 Java.Util.Concurrent.FutureTask providing a common extensible implementation of Futures, and Java.Util.Concurrent.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 Java.Util.Concurrent.IBlockingQueue interface, that defines blocking versions of put and take: Java.Util.Concurrent.LinkedBlockingQueue, Java.Util.Concurrent.ArrayBlockingQueue, Java.Util.Concurrent.SynchronousQueue, Java.Util.Concurrent.PriorityBlockingQueue, and Java.Util.Concurrent.DelayQueue. The different classes cover the most common usage contexts for producer-consumer, messaging, parallel tasking, and related concurrent designs.

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


The Java.Util.Concurrent.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.

Concurrent Collections

Besides Queues, this package supplies Collection implementations designed for use in multithreaded contexts: Java.Util.Concurrent.ConcurrentHashMap, Java.Util.Concurrent.ConcurrentSkipListMap, NoType:java/util/concurrent/ConcurrentSkipListSet;Href=../../../../reference/java/util/concurrent/ConcurrentSkipListSet.html, Java.Util.Concurrent.CopyOnWriteArrayList, and Java.Util.Concurrent.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 Java.Util.Concurrent.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: The methods of all classes in java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization. In particular:


AbstractExecutorServiceProvides default implementations of Java.Util.Concurrent.IExecutorService execution methods.
ArrayBlockingQueueA bounded Java.Util.Concurrent.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 Java.Util.Concurrent.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 Java.Util.IDeque based on linked nodes.
ConcurrentLinkedQueueAn unbounded thread-safe Java.Util.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 Java.Util.ISet that uses an internal Java.Util.Concurrent.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 Java.Util.Concurrent.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 Java.Util.Concurrent.ICompletionService that uses a supplied Java.Util.Concurrent.IExecutor to execute tasks.
ExecutorsFactory and utility methods for Java.Util.Concurrent.IExecutor, Java.Util.Concurrent.IExecutorService, Java.Util.Concurrent.IScheduledExecutorService, Java.Util.Concurrent.IThreadFactory, and Java.Util.Concurrent.ICallable classes defined in this package.
ForkJoinPoolAn Java.Util.Concurrent.IExecutorService for running Java.Util.Concurrent.ForkJoinTasks.
ForkJoinPool+IForkJoinWorkerThreadFactoryDocumentation for this section has not yet been entered.
ForkJoinPool+IManagedBlockerDocumentation for this section has not yet been entered.
ForkJoinTaskAbstract base class for tasks that run within a Java.Util.Concurrent.ForkJoinPool.
ForkJoinWorkerThreadA thread managed by a Java.Util.Concurrent.ForkJoinPool, which executes Java.Util.Concurrent.ForkJoinTasks.
FutureTaskA cancellable asynchronous computation.
IBlockingDequeA Java.Util.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.
IBlockingDequeExtensionsDocumentation for this section has not yet been entered.
IBlockingQueueA Java.Util.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.
IBlockingQueueExtensionsDocumentation for this section has not yet been entered.
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 Java.Util.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 Java.Lang.IRunnable tasks.
IExecutorServiceAn Java.Util.Concurrent.IExecutor that provides methods to manage termination and methods that can produce a Java.Util.Concurrent.IFuture for tracking progress of one or more asynchronous tasks.
IExecutorServiceExtensionsDocumentation for this section has not yet been entered.
IFutureA Future represents the result of an asynchronous computation.
IFutureExtensionsDocumentation for this section has not yet been entered.
IRejectedExecutionHandlerA handler for tasks that cannot be executed by a Java.Util.Concurrent.ThreadPoolExecutor.
IRunnableFutureA Java.Util.Concurrent.IFuture that is Java.Lang.IRunnable.
IRunnableScheduledFutureA Java.Util.Concurrent.IScheduledFuture that is Java.Lang.IRunnable.
IScheduledExecutorServiceAn Java.Util.Concurrent.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 Java.Util.Concurrent.IBlockingQueue in which producers may wait for consumers to receive elements.
LinkedBlockingDequeAn optionally-bounded Java.Util.Concurrent.IBlockingDeque based on linked nodes.
LinkedBlockingQueueAn optionally-bounded Java.Util.Concurrent.IBlockingQueue based on linked nodes.
LinkedTransferQueueAn unbounded Java.Util.Concurrent.ITransferQueue based on linked nodes.
PhaserA reusable synchronization barrier, similar in functionality to Java.Util.Concurrent.CyclicBarrier and Java.Util.Concurrent.CountDownLatch but supporting more flexible usage.
PriorityBlockingQueueAn unbounded Java.Util.Concurrent.IBlockingQueue that uses the same ordering rules as class Java.Util.PriorityQueue and supplies blocking retrieval operations.
RecursiveActionA recursive resultless Java.Util.Concurrent.ForkJoinTask.
RecursiveTaskA recursive result-bearing Java.Util.Concurrent.ForkJoinTask.
RejectedExecutionExceptionException thrown by an Java.Util.Concurrent.IExecutor when a task cannot be accepted for execution.
ScheduledThreadPoolExecutorA Java.Util.Concurrent.ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.
SemaphoreA counting semaphore.
SynchronousQueueA Java.Util.Concurrent.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 Java.Util.Concurrent.IExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Java.Util.Concurrent.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.