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 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.
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.
ConcurrentLinkedQueue
class 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: 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.
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. 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 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.
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. BlockingDeque<E> | 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. |
BlockingQueue<E> | 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. |
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 operations, and recursively so for its navigable sub-maps. |
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. |
AbstractExecutorService | Provides default implementations of ExecutorService execution methods. |
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> | A scalable concurrent ConcurrentNavigableMap implementation. |
ConcurrentSkipListSet<E> | A scalable concurrent NavigableSet implementation based on a ConcurrentSkipListMap . |
CopyOnWriteArrayList<E> | A thread-safe random-access list. |
CopyOnWriteArraySet<E> | A Set that uses an internal CopyOnWriteArrayList for all of its operations. |
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 threads can pair and swap elements within pairs. |
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 based on linked nodes. |
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 in which each insert operation must wait for a corresponding remove operation by another thread, 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. |
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 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. |