Package java.util.concurrent
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.
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.
A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks.
A stage of a possibly asynchronous computation, that performs an action or computes a value when another CompletionStage completes.
An Executor that provides methods to manage termination and methods that can produce a Future for tracking progress of one or more asynchronous tasks.
An ExecutorService that can schedule commands to run after a given delay, or to execute periodically.
A 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.
A view of a ConcurrentHashMap as a Set of keys, in which additions may optionally be enabled by mapping to a common value.
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.
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
A ForkJoinTask with a completion action performed when triggered and there are no remaining pending actions.
A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
An unbounded blocking queue of Delayed elements, in which an element can only be taken when its delay has expired.
Factory and utility methods for Executor , ExecutorService , ScheduledExecutorService , ThreadFactory , and Callable classes defined in this package.
A reusable synchronization barrier, similar in functionality to CyclicBarrier and CountDownLatch but supporting more flexible usage.
An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.
A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.
A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.
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.
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.
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 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.
Exception indicating that the result of a value-producing task, such as a FutureTask , cannot be retrieved because the task was cancelled.
Exception thrown when an error or other exception is encountered in the course of completing a result or task.
Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.
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 java.util.concurrent.locks and java.util.concurrent.atomic packages.
Executors
Interfaces. 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.
Queues
The 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.
Timing
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 .
Synchronizers
- 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
- they may proceed concurrently with other operations
- they will never throw ConcurrentModificationException
- 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 Properties
- 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.
- 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 Executor happen-before its execution begins. Similarly for Callables submitted to an ExecutorService .
- Actions taken by the asynchronous computation represented by a Future happen-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.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.
- 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.
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For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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