Package java.util.concurrent

See:
           Description

Interface Summary
BlockingDeque<E> BlockingQueue<E>	A Deque Queue
BlockingQueue<E>	A Queue
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.
ConcurrentMap<K,V>	A Map providing additional atomic putIfAbsent , remove , and replace methods.
ConcurrentNavigableMap<K,V>	A ConcurrentMap supporting NavigableMap
Delayed	A mix-in style interface for marking objects that should be acted upon after a given delay.
Executor	An object that executes submitted Runnable tasks.
ExecutorService	An Executor that provides methods to manage termination and methods that can produce a Future for tracking progress of one or more asynchronous tasks.
Future<V>	A Future represents the result of an asynchronous computation.
RejectedExecutionHandler	A handler for tasks that cannot be executed by a ThreadPoolExecutor .
RunnableFuture<V>	A Future that is Runnable
RunnableScheduledFuture<V>	A ScheduledFuture that is Runnable
ScheduledExecutorService	An ExecutorService that can schedule commands to run after a given delay, or to execute periodically.
ScheduledFuture<V>	A delayed result-bearing action that can be cancelled.
ThreadFactory	An object that creates new threads on demand.

 

Class Summary
AbstractExecutorService	Provides default implementations implementation of ExecutorService
ArrayBlockingQueue<E>	A bounded blocking queue backed by an array.
ConcurrentHashMap<K,V>	A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.
ConcurrentLinkedQueue<E>	An unbounded thread-safe queue based on linked nodes.
ConcurrentSkipListMap<K,V> CopyOnWriteArrayList<E>	A scalable concurrent ConcurrentNavigableMap thread-safe variant of ArrayList
ConcurrentSkipListSet<E> CopyOnWriteArraySet<E>	A scalable concurrent NavigableSet Set implementation based on a ConcurrentSkipListMap that uses CopyOnWriteArrayList
CopyOnWriteArrayList<E>	A thread-safe variant of ArrayList add , set , and so on) are implemented by making a fresh copy of the underlying array.
CopyOnWriteArraySet<E>	A Set that uses an internal CopyOnWriteArrayList
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 blocking queue of Delayed elements, in which an element can only be taken when its delay has expired.
Exchanger<V>	A synchronization point at which two threads can pair and swap elements within pairs. exchange objects.
ExecutorCompletionService<V>	A CompletionService that uses a supplied Executor to execute tasks.
Executors	Factory and utility methods for Executor , ExecutorService , ScheduledExecutorService , ThreadFactory , and Callable classes defined in this package.
FutureTask<V>	A cancellable asynchronous computation.
LinkedBlockingDeque<E>	An optionally-bounded blocking deque
LinkedBlockingQueue<E>	An optionally-bounded blocking queue based on linked nodes.
PriorityBlockingQueue<E>	An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.
ScheduledThreadPoolExecutor	A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.
Semaphore	A counting semaphore.
SynchronousQueue<E>	A blocking queue put must wait for a take , and vice versa.
ThreadPoolExecutor	An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.
ThreadPoolExecutor.AbortPolicy	A handler for rejected tasks that throws a RejectedExecutionException .
ThreadPoolExecutor.CallerRunsPolicy	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.
ThreadPoolExecutor.DiscardOldestPolicy	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.
ThreadPoolExecutor.DiscardPolicy	A handler for rejected tasks that silently discards the rejected task.

 

Enum Summary
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.

 

Exception Summary
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 FutureTask , cannot be retrieved because the task was cancelled.
ExecutionException	Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.
RejectedExecutionException	Exception thrown by an Executor when a task cannot be accepted for execution.
TimeoutException	Exception thrown when a blocking operation times out.

 

Package java.util.concurrent Description

Utility classes commonly useful in concurrent programming. Thispackage includes a few small standardized extensible frameworks, aswell as some classes that provide useful functionality and areotherwise tedious or difficult to implement. Here are briefdescriptions of the main components. See also the locks and atomic packages.

Executors

run

Implementations. Classes ThreadPoolExecutor and ScheduledThreadPoolExecutor provide tunable,flexible thread pools. The Executors class provides factory methods for the most common kinds andconfigurations of Executors, as well as a few utility methods forusing them. Other utilities based on Executors include the concreteclass FutureTask providing a commonextensible implementation of Futures, and ExecutorCompletionService , that assists incoordinating the processing of groups of asynchronous tasks.

Queues

BlockingQueue

Timing

Long.MAX_VALUE

Synchronizers

Concurrent Collections

ConcurrentHashMap

HashMap

ConcurrentSkipListMap

TreeMap

CopyOnWriteArrayList

ArrayList

The "Concurrent" prefix used with some classes in this package is ashorthand indicating several differences from similar "synchronized"classes. For example java.util.Hashtable and Collections.synchronizedMap(new HashMap()) aresynchronized. But ConcurrentHashMap is"concurrent". A concurrent collection is thread-safe, but notgoverned by a single exclusion lock. In the particular case ofConcurrentHashMap, it safely permits any number of concurrent reads aswell as a tunable number of concurrent writes. "Synchronized" classescan be useful when you need to prevent all access to a collection viaa single lock, at the expense of poorer scalability. In other cases inwhich multiple threads are expected to access a common collection,"concurrent" versions are normally preferable. And unsynchronizedcollections are preferable when either collections are unshared, orare accessible only when holding other locks.

Most concurrent Collection implementations (including most Queues)also differ from the usual java.util conventions in that their Iteratorsprovide weakly consistent rather than fast-fail traversal. Aweakly consistent iterator is thread-safe, but does not necessarilyfreeze the collection while iterating, so it may (or may not) reflectany updates since the iterator was created.

Memory Consistency Properties

happens-before

happens-before

happens-before

• 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 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.

Since:

1.5