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.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
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 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 {@linkplain java.util.Iterator Iterators}
and {@linkplain java.util.Spliterator 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 (
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 ofvolatile
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.
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
Runnable
to anExecutor
happen-before its execution begins. Similarly forCallables
submitted to anExecutorService
. - Actions taken by the asynchronous computation represented by a
Future
happen-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.countDown
happen-before actions subsequent to a successful "acquiring" method such asLock.lock
,Semaphore.acquire
,Condition.await
, andCountDownLatch.await
on 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.await
andPhaser.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 correspondingawait
in other threads.
Interfaces
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. |
CompletableFuture.AsynchronousCompletionTask | A marker interface identifying asynchronous tasks produced by
async methods. |
CompletionService<V> | A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. |
CompletionStage<T> | A stage of a possibly asynchronous computation, that performs an action or computes a value when another CompletionStage completes. |
ConcurrentMap<K, V> | A Map providing thread safety and atomicity
guarantees. |
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. |
Flow.Processor<T, R> | A component that acts as both a Subscriber and Publisher. |
Flow.Publisher<T> | A producer of items (and related control messages) received by Subscribers. |
Flow.Subscriber<T> | A receiver of messages. |
Flow.Subscription | Message control linking a Flow.Publisher and Flow.Subscriber . |
ForkJoinPool.ForkJoinWorkerThreadFactory | Factory for creating new ForkJoinWorkerThread s. |
ForkJoinPool.ManagedBlocker | Interface for extending managed parallelism for tasks running
in ForkJoinPool s. |
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. |
TransferQueue<E> | A BlockingQueue in which producers may wait for consumers
to receive elements. |
Classes
AbstractExecutorService | Provides default implementations of ExecutorService
execution methods. |
ArrayBlockingQueue<E> | A bounded {@linkplain BlockingQueue blocking queue} backed by an array. |
CompletableFuture<T> | 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. |
ConcurrentHashMap<K, V> | A hash table supporting full concurrency of retrievals and high expected concurrency for updates. |
ConcurrentHashMap.KeySetView<K, V> | A view of a ConcurrentHashMap as a Set of keys, in
which additions may optionally be enabled by mapping to a
common value. |
ConcurrentLinkedDeque<E> | An unbounded concurrent {@linkplain Deque deque} based on linked nodes. |
ConcurrentLinkedQueue<E> | An unbounded thread-safe {@linkplain Queue 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 variant of ArrayList in which all mutative
operations (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
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. |
CountedCompleter<T> | A ForkJoinTask with a completion action performed when
triggered and there are no remaining pending actions. |
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 {@linkplain BlockingQueue 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. |
Flow | Interrelated interfaces and static methods for establishing
flow-controlled components in which Publishers
produce items consumed by one or more Subscribers , each managed by a Subscription . |
ForkJoinPool | An ExecutorService for running ForkJoinTask s. |
ForkJoinTask<V> | Abstract base class for tasks that run within a ForkJoinPool . |
ForkJoinWorkerThread | A thread managed by a ForkJoinPool , which executes
ForkJoinTask s. |
FutureTask<V> | A cancellable asynchronous computation. |
LinkedBlockingDeque<E> | An optionally-bounded {@linkplain BlockingDeque blocking deque} based on linked nodes. |
LinkedBlockingQueue<E> | An optionally-bounded {@linkplain BlockingQueue blocking queue} based on linked nodes. |
LinkedTransferQueue<E> | An unbounded TransferQueue based on linked nodes. |
Phaser | A reusable synchronization barrier, similar in functionality to
CyclicBarrier and
CountDownLatch
but supporting more flexible usage. |
PriorityBlockingQueue<E> | An unbounded {@linkplain BlockingQueue blocking queue} that uses
the same ordering rules as class PriorityQueue and supplies
blocking retrieval operations. |
RecursiveAction | A recursive resultless ForkJoinTask . |
RecursiveTask<V> | A recursive result-bearing ForkJoinTask . |
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 {@linkplain BlockingQueue blocking queue} in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. |
ThreadLocalRandom | A random number generator isolated to the current thread. |
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. |
Enums
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. |
Exceptions
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. |
CompletionException | Exception thrown when an error or other exception is encountered in the course of completing a result or task. |
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. |