Package java.util.concurrent
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
TheConcurrentLinkedQueue 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
TheTimeUnit 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
Five classes aid common special-purpose synchronization idioms.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.- A
CyclicBarrieris a resettable multiway synchronization point useful in some styles of parallel programming. - A
Phaserprovides a more flexible form of barrier that may be used to control phased computation among multiple threads. - An
Exchangerallows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.
Concurrent Collections
Besides Queues, this package supplies Collection implementations designed for use in multithreaded contexts: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 large 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:
- 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
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. Thesynchronized and volatile constructs, as well as the
Thread.start() and Thread.join() methods, can form
happens-before relationships. In particular:
- Each action in a thread happens-before every action in that thread that comes later in the program's order.
- An unlock (
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. - A write to a
volatilefield happens-before every subsequent read of that same field. Writes and reads ofvolatilefields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking. - A call to
starton a thread happens-before any action in the started thread. - All actions in a thread happen-before any other thread
successfully returns from a
joinon that thread.
java.util.concurrent and its
subpackages extend these guarantees to higher-level
synchronization. In particular:
- 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
Runnableto anExecutorhappen-before its execution begins. Similarly forCallablessubmitted to anExecutorService. - Actions taken by the asynchronous computation represented by a
Futurehappen-before actions subsequent to the retrieval of the result viaFuture.get()in another thread. - Actions prior to "releasing" synchronizer methods such as
Lock.unlock,Semaphore.release, andCountDownLatch.countDownhappen-before actions subsequent to a successful "acquiring" method such asLock.lock,Semaphore.acquire,Condition.await, andCountDownLatch.awaiton the same synchronizer object in another thread. - For each pair of threads that successfully exchange objects via
an
Exchanger, actions prior to theexchange()in each thread happen-before those subsequent to the correspondingexchange()in another thread. - Actions prior to calling
CyclicBarrier.awaitandPhaser.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 correspondingawaitin other threads.
- See Java Language Specification:
-
17.4.5 Happens-before Order
- Since:
- 1.5
-
ClassDescriptionProvides default implementations of
ExecutorServiceexecution methods.A bounded blocking queue backed by an array.ADequethat 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.AQueuethat 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.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.Callable<V>A task that returns a result and may throw an exception.Exception indicating that the result of a value-producing task, such as aFutureTask, cannot be retrieved because the task was cancelled.AFuturethat may be explicitly completed (setting its value and status), and may be used as aCompletionStage, supporting dependent functions and actions that trigger upon its completion.A marker interface identifying asynchronous tasks produced byasyncmethods.Exception thrown when an error or other exception is encountered in the course of completing a result or task.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.ConcurrentHashMap<K,V> A hash table supporting full concurrency of retrievals and high expected concurrency for updates.A view of a ConcurrentHashMap as aSetof keys, in which additions may optionally be enabled by mapping to a common value.An unbounded concurrent deque based on linked nodes.An unbounded thread-safe queue based on linked nodes.ConcurrentMap<K,V> AMapproviding thread safety and atomicity guarantees.AConcurrentMapsupportingNavigableMapoperations, and recursively so for its navigable sub-maps.A scalable concurrentConcurrentNavigableMapimplementation.A scalable concurrentNavigableSetimplementation based on aConcurrentSkipListMap.A thread-safe variant ofArrayListin which all mutative operations (add,set, and so on) are implemented by making a fresh copy of the underlying array.ASetthat uses an internalCopyOnWriteArrayListfor all of its operations.A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.AForkJoinTaskwith 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.A mix-in style interface for marking objects that should be acted upon after a given delay.DelayQueue<E extends Delayed>An unbounded blocking queue ofDelayedelements, in which an element generally becomes eligible for removal when its delay has expired.Exchanger<V>A synchronization point at which threads can pair and swap elements within pairs.Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.An object that executes submittedRunnabletasks.ACompletionServicethat uses a suppliedExecutorto execute tasks.Factory and utility methods forExecutor,ExecutorService,ScheduledExecutorService,ThreadFactory, andCallableclasses defined in this package.Interrelated interfaces and static methods for establishing flow-controlled components in whichPublishersproduce items consumed by one or moreSubscribers, each managed by aSubscription.Flow.Processor<T,R> A component that acts as both a Subscriber and Publisher.A producer of items (and related control messages) received by Subscribers.A receiver of messages.Message control linking aFlow.PublisherandFlow.Subscriber.AnExecutorServicefor runningForkJoinTasks.Factory for creating newForkJoinWorkerThreads.Interface for extending managed parallelism for tasks running inForkJoinPools.ForkJoinTask<V>Abstract base class for tasks that run within aForkJoinPool.A thread managed by aForkJoinPool, which executesForkJoinTasks.Future<V>AFuturerepresents the result of an asynchronous computation.Represents the computation state.FutureTask<V>A cancellable asynchronous computation.An optionally-bounded blocking deque based on linked nodes.An optionally-bounded blocking queue based on linked nodes.An unboundedTransferQueuebased on linked nodes.A reusable synchronization barrier, similar in functionality toCyclicBarrierandCountDownLatchbut supporting more flexible usage.An unbounded blocking queue that uses the same ordering rules as classPriorityQueueand supplies blocking retrieval operations.A recursive resultlessForkJoinTask.A recursive result-bearingForkJoinTask.Exception thrown by anExecutorwhen a task cannot be accepted for execution.A handler for tasks that cannot be executed by aThreadPoolExecutor.AScheduledFuturethat isRunnable.AnExecutorServicethat can schedule commands to run after a given delay, or to execute periodically.A delayed result-bearing action that can be cancelled.AThreadPoolExecutorthat can additionally schedule commands to run after a given delay, or to execute periodically.A counting semaphore.Preview.A basic API for structured concurrency.Preview.AStructuredTaskScopethat captures the result of the first subtask to complete successfullyPREVIEW.Preview.Represents a subtask forked withStructuredTaskScope.fork(Callable)PREVIEW.Preview.Represents the state of a subtask.Preview.Thrown when a structure violation is detected.AFlow.Publisherthat asynchronously issues submitted (non-null) items to current subscribers until it is closed.A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.An object that creates new threads on demand.A random number generator (with period 264) isolated to the current thread.AnExecutorServicethat executes each submitted task using one of possibly several pooled threads, normally configured usingExecutorsfactory methods.A handler for rejected tasks that throws aRejectedExecutionException.A handler for rejected tasks that runs the rejected task directly in the calling thread of theexecutemethod, 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 retriesexecute, unless the executor is shut down, in which case the task is discarded.A handler for rejected tasks that silently discards the rejected task.Exception thrown when a blocking operation times out.ATimeUnitrepresents 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.ABlockingQueuein which producers may wait for consumers to receive elements.