Executoris a simple standardized interface for defining custom thread-like subsystems, including thread pools, asynchronous IO, 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.
ConcurrentLinkedQueueclass supplies an efficient scalable thread-safe non-blocking FIFO queue.
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.
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
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.
Exchangerallows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.
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 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.Chapter 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:
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.
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.
starton a thread happens-before any action in the started thread.
joinon that thread.
java.util.concurrentand its subpackages extend these guarantees to higher-level synchronization. In particular:
Executorhappen-before its execution begins. Similarly for
Callablessubmitted to an
Futurehappen-before actions subsequent to the retrieval of the result via
Future.get()in another thread.
CountDownLatch.countDownhappen-before actions subsequent to a successful "acquiring" method such as
CountDownLatch.awaiton 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.awaithappen-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.
|Callable<V>||A task that returns a result and may throw an exception.|
|CompletionService<V>||A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks.|
|Delayed||A mix-in style interface for marking objects that should be acted upon after a given delay.|
|Executor||An object that executes submitted
|Future<V>||A Future represents the result of an asynchronous computation.|
|RejectedExecutionHandler||A handler for tasks that cannot be executed by a
|ScheduledFuture<V>||A delayed result-bearing action that can be cancelled.|
|ThreadFactory||An object that creates new threads on demand.|
|AbstractExecutorService||Provides default implementations of
|ConcurrentHashMap<K, V>||A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.|
|ConcurrentLinkedQueue<E>||An unbounded thread-safe
|ConcurrentSkipListMap<K, V>||A scalable concurrent
|ConcurrentSkipListSet<E>||A scalable concurrent
|CopyOnWriteArrayList<E>||A thread-safe random-access list.|
|CountDownLatch||A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.|
|CyclicBarrier||A 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
|Exchanger<V>||A synchronization point at which threads can pair and swap elements within pairs.|
|Executors||Factory and utility methods for
|FutureTask<V>||A cancellable asynchronous computation.|
|Semaphore||A counting semaphore.|
|ThreadPoolExecutor.AbortPolicy||A handler for rejected tasks that throws a
|ThreadPoolExecutor.CallerRunsPolicy||A handler for rejected tasks that runs the rejected task directly in the calling thread of the
|ThreadPoolExecutor.DiscardOldestPolicy||A handler for rejected tasks that discards the oldest unhandled request and then retries
|ThreadPoolExecutor.DiscardPolicy||A handler for rejected tasks that silently discards the rejected task.|
|TimeUnit||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.|
|BrokenBarrierException||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.|
|CancellationException||Exception indicating that the result of a value-producing task, such as a
|ExecutionException||Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.|
|RejectedExecutionException||Exception thrown by an
|TimeoutException||Exception thrown when a blocking operation times out.|