Executoris 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
execute, and may execute sequentially or concurrently.
ExecutorServiceprovides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The
ScheduledExecutorServicesubinterface 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
Futurereturns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A
Futurethat possesses a
runmethod that upon execution, sets its results.
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.
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.
ConcurrentLinkedQueueclass supplies an efficient scalable thread-safe non-blocking FIFO queue. The
ConcurrentLinkedDequeclass is similar, but additionally supports the
Five implementations in
java.util.concurrent support the extended
BlockingQueue interface, that defines blocking versions of put and take:
DelayQueue. The different classes cover the most common usage contexts for producer-consumer, messaging, parallel tasking, and related concurrent designs.
TransferQueue, and implementation
LinkedTransferQueue introduce a synchronous
transfer method (along with related features) in which a producer may optionally block awaiting its consumer.
TimeUnitclass 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
SynchronizersFive classes aid common special-purpose synchronization idioms.
Semaphoreis a classic concurrency tool.
CountDownLatchis a very simple yet very common utility for blocking until a given number of signals, events, or conditions hold.
CyclicBarrieris a resettable multiway synchronization point useful in some styles of parallel programming.
Phaserprovides a more flexible form of barrier that may be used to control phased computation among multiple threads.
Exchangerallows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.
Concurrent CollectionsBesides Queues, this package supplies Collection implementations designed for use in multithreaded contexts:
CopyOnWriteArraySet. When many threads are expected to access a given collection, a
ConcurrentHashMapis normally preferable to a synchronized
HashMap, and a
ConcurrentSkipListMapis normally preferable to a synchronized
CopyOnWriteArrayListis preferable to a synchronized
ArrayListwhen 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
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 large 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 and Spliterators provide weakly consistent rather than fast-fail traversal:
- they may proceed concurrently with other operations
- they will never throw
- they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction.
Memory Consistency PropertiesChapter 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
volatileconstructs, as well as the
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 (
synchronizedblock or method exit) of a monitor happens-before every subsequent lock (
synchronizedblock 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
volatilefield happens-before every subsequent read of that same field. Writes and reads of
volatilefields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
- A call to
starton a thread happens-before any action in the started thread.
- All actions in a thread happen-before any other thread successfully returns from a
joinon that thread.
java.util.concurrentand 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
Executorhappen-before its execution begins. Similarly for
Callablessubmitted to an
- 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
CountDownLatch.countDownhappen-before actions subsequent to a successful "acquiring" method such as
CountDownLatch.awaiton 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
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
awaitin other threads.
Interface Summary Interface Description BlockingDeque<E>A
Dequethat 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.
Queuethat 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. CompletableFuture.AsynchronousCompletionTaskA marker interface identifying asynchronous tasks produced by
CompletionService<V>A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. CompletionStage<T>A stage of a possibly asynchronous computation, that performs an action or computes a value when another CompletionStage completes. ConcurrentMap<K,V>A
Mapproviding thread safety and atomicity guarantees.
ConcurrentNavigableMap<K,V> DelayedA mix-in style interface for marking objects that should be acted upon after a given delay. ExecutorAn object that executes submitted
ExecutorService Flow.Processor<T,R>A component that acts as both a Subscriber and Publisher. Flow.Publisher<T>A producer of items (and related control messages) received by Subscribers. Flow.Subscriber<T>A receiver of messages. Flow.Subscription ForkJoinPool.ForkJoinWorkerThreadFactoryFactory for creating new
ForkJoinPool.ManagedBlockerInterface for extending managed parallelism for tasks running in
Futurerepresents the result of an asynchronous computation.
RejectedExecutionHandlerA handler for tasks that cannot be executed by a
RunnableFuture<V> RunnableScheduledFuture<V> ScheduledExecutorServiceAn
ExecutorServicethat can schedule commands to run after a given delay, or to execute periodically.
ScheduledFuture<V>A delayed result-bearing action that can be cancelled. ThreadFactoryAn object that creates new threads on demand. TransferQueue<E>A
BlockingQueuein which producers may wait for consumers to receive elements.
Class Summary Class Description AbstractExecutorServiceProvides default implementations of
ArrayBlockingQueue<E>A bounded blocking queue backed by an array. CompletableFuture<T> ConcurrentHashMap<K,V>A hash table supporting full concurrency of retrievals and high expected concurrency for updates. ConcurrentHashMap.KeySetView<K,V>A view of a ConcurrentHashMap as a
Setof keys, in which additions may optionally be enabled by mapping to a common value.
ConcurrentLinkedDeque<E>An unbounded concurrent deque based on linked nodes. ConcurrentLinkedQueue<E>An unbounded thread-safe queue based on linked nodes. ConcurrentSkipListMap<K,V>A scalable concurrent
ConcurrentSkipListSet<E> CopyOnWriteArrayList<E>A thread-safe variant of
ArrayListin which all mutative operations (
set, and so on) are implemented by making a fresh copy of the underlying array.
CopyOnWriteArraySet<E> CountDownLatchA synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. CountedCompleter<T>A
ForkJoinTaskwith a completion action performed when triggered and there are no remaining pending actions.
CyclicBarrierA 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
Delayedelements, 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> Executors Flow ForkJoinPool ForkJoinTask<V>Abstract base class for tasks that run within a
ForkJoinWorkerThread 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. LinkedTransferQueue<E>An unbounded
TransferQueuebased on linked nodes.
Phaser PriorityBlockingQueue<E> RecursiveActionA recursive resultless
RecursiveTask<V>A recursive result-bearing
ThreadPoolExecutorthat can additionally schedule commands to run after a given delay, or to execute periodically.
SemaphoreA counting semaphore. SubmissionPublisher<T>A
Flow.Publisherthat asynchronously issues submitted (non-null) items to current subscribers until it is closed.
SynchronousQueue<E>A blocking queue 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. ThreadPoolExecutor ThreadPoolExecutor.AbortPolicyA handler for rejected tasks that throws a
ThreadPoolExecutor.CallerRunsPolicyA handler for rejected tasks that runs the rejected task directly in the calling thread of the
executemethod, 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.
Enum Summary Enum Description TimeUnitA
TimeUnitrepresents 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 Exception Description 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.
CompletionExceptionException thrown when an error or other exception is encountered in the course of completing a result or task. ExecutionExceptionException thrown when attempting to retrieve the result of a task that aborted by throwing an exception. RejectedExecutionExceptionException thrown by an
Executorwhen a task cannot be accepted for execution.
TimeoutExceptionException thrown when a blocking operation times out.