See: Description
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.
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.
async methods.
Map providing thread safety and atomicity guarantees.
ConcurrentMap supporting
NavigableMap operations, and recursively so for its navigable sub-maps.
ForkJoinWorkerThreads.
ForkJoinPools.
Future represents the result of an asynchronous computation.
ThreadPoolExecutor.
ScheduledFuture that is
Runnable.
ExecutorService that can schedule commands to run after a given delay, or to execute periodically.
BlockingQueue in which producers may wait for consumers to receive elements.
ExecutorService execution methods.
Future that may be explicitly completed (setting its value and status), and may be used as a
CompletionStage, supporting dependent functions and actions that trigger upon its completion.
Set of keys, in which additions may optionally be enabled by mapping to a common value.
ConcurrentNavigableMap implementation.
NavigableSet implementation based on a
ConcurrentSkipListMap.
ArrayList in which all mutative operations (
add,
set, and so on) are implemented by making a fresh copy of the underlying array.
Set that uses an internal
CopyOnWriteArrayList for all of its operations.
ForkJoinTask with a completion action performed when triggered and there are no remaining pending actions.
Delayed elements, in which an element can only be taken when its delay has expired.
CompletionService that uses a supplied
Executor to execute tasks.
Executor,
ExecutorService,
ScheduledExecutorService,
ThreadFactory, and
Callable classes defined in this package.
ExecutorService for running
ForkJoinTasks.
ForkJoinPool.
ForkJoinPool, which executes
ForkJoinTasks.
TransferQueue based on linked nodes.
CyclicBarrier and
CountDownLatch but supporting more flexible usage.
PriorityQueue and supplies blocking retrieval operations.
ForkJoinTask.
ForkJoinTask.
ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.
ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using
Executors factory methods.
RejectedExecutionException.
execute method, unless the executor has been shut down, in which case the task is discarded.
execute, unless the executor is shut down, in which case the task is discarded.
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.
FutureTask, cannot be retrieved because the task was cancelled.
Executor when a task cannot be accepted for execution.
java.util.concurrent.locks and
java.util.concurrent.atomic packages.
Executor 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
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.
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.
ConcurrentLinkedQueue class supplies an efficient scalable thread-safe non-blocking FIFO queue. The
ConcurrentLinkedDeque class is similar, but additionally supports the
Deque interface.
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.
Extended interface TransferQueue, and implementation LinkedTransferQueue introduce a synchronous transfer method (along with related features) in which a producer may optionally block awaiting its consumer.
The BlockingDeque interface extends BlockingQueue to support both FIFO and LIFO (stack-based) operations. Class LinkedBlockingDeque provides an implementation.
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.
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. CyclicBarrier is a resettable multiway synchronization point useful in some styles of parallel programming. Phaser provides a more flexible form of barrier that may be used to control phased computation among multiple threads. Exchanger allows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs. 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 and Spliterators provide weakly consistent rather than fast-fail traversal:
ConcurrentModificationException synchronized and
volatile constructs, as well as the
Thread.start() and
Thread.join() methods, can form
happens-before relationships. In particular:
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. 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. start on a thread happens-before any action in the started thread. join on that thread. java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization. In particular:
Runnable to an Executor happen-before its execution begins. Similarly for Callables submitted to an ExecutorService. Future happen-before actions subsequent to the retrieval of the result via Future.get() in another thread. Lock.unlock, Semaphore.release, and CountDownLatch.countDown happen-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. Exchanger, actions prior to the exchange() in each thread happen-before those subsequent to the corresponding exchange() in another thread. 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.