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 locks and
atomic packages.
Executors
{@link java.util.concurrent.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. Executors also standardize ways of calling threads that
compute functions returning results, via a {@link
java.util.concurrent.Future}. This is supported in part by defining
interface {@link java.util.concurrent.Callable}, the argument/result
analog of Runnable.
{@link java.util.concurrent.ExecutorService} provides a more
complete framework for executing Runnables. An ExecutorService
manages queueing and scheduling of tasks, and allows controlled
shutdown. The two primary implementations of ExecutorService are
{@link java.util.concurrent.ThreadPoolExecutor}, a tunable and
flexible thread pool and {@link
java.util.concurrent.ScheduledExecutor}, which adds support for
delayed and periodic task execution. These, and other Executors can
be used in conjunction with a {@link
java.util.concurrent.CancellableTask} or {@link
java.util.concurrent.FutureTask} to asynchronously start a potentially
long-running computation and query to determine if its execution has
completed, or cancel it.
The {@link java.util.concurrent.Executors} class provides factory
methods for the most common kinds and configurations of Executors, as
well as a few utility methods for using them.
Queues
The java.util.concurrent {@link
java.util.concurrent.ConcurrentLinkedQueue} class supplies an
efficient scalable thread-safe non-blocking FIFO queue. Five
implementations in java.util.concurrent support the extended {@link
java.util.concurrent.BlockingQueue} interface, that defines blocking
versions of put and take: {@link
java.util.concurrent.LinkedBlockingQueue}, {@link
java.util.concurrent.ArrayBlockingQueue}, {@link
java.util.concurrent.SynchronousQueue}, {@link
java.util.concurrent.PriorityBlockingQueue}, and {@link
java.util.concurrent.DelayQueue}. The different classes cover the most
common usage contexts for producer-consumer, messaging, parallel
tasking, and related concurrent designs.
Timing
The {@link java.util.concurrent.TimeUnit} class provides multiple
granularities (including nanoseconds) for specifying and controlling
time-out based operations. Nearly all other 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. The virtual machine should make a "best effort"
to detect time-outs as soon as possible after they occur. Regardless
of the efforts of the virtual machine, the normal scheduling
mechanisms, and the need to re-acquire locks in many cases, can lead
to an indefinite amount of time elapsing between a time-out being
detected and a thread actually executing again after that time-out.
Synchronizers
Five classes aid common special-purpose synchronization idioms.
{@link java.util.concurrent.Semaphore} and {@link
java.util.concurrent.FairSemaphore} are classic concurrency tools.
{@link java.util.concurrent.CountDownLatch} is very simple yet very
common utility for blocking until a single signal, event, or condition
holds. A {@link java.util.concurrent.CyclicBarrier} is a resettable multiway
synchronization point common in some styles of parallel
programming. An {@link java.util.concurrent.Exchanger} allows two
threads to exchange objects at a rendezvous point.
Concurrent Collections
Besides Queues, this package supplies a few Collection implementations
designed for use in multithreaded contexts: {@link
java.util.concurrent.ConcurrentHashMap}, {@link
java.util.concurrent.CopyOnWriteArrayList}, and {@link
java.util.concurrent.CopyOnWriteArraySet}.
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 {@link java.util.concurrent.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. There may still be a
role for "synchronized" classes in some multithreaded programs -- they
can sometimes be useful when you need to prevent all access to a
collection via a single lock, at the expense of much poor
scalability. In all other cases, "concurrent" versions are normally
preferable.
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.