Utility classes commonly useful in concurrent programming. This
package includes a few small standardized extensible frameworks, as
well as some classes that provide useful functionality and are
otherwise tedious or difficult to implement. Here are brief
descriptions of the main components. See also the
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
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.