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
Executoris 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.
ExecutorServiceprovides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The
ScheduledExecutorServicesubinterface 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
Futurereturns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A
Futurethat possesses a
runmethod that upon execution, sets its results.
provide tunable, flexible thread pools.
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
providing a common extensible implementation of Futures, and
assists in coordinating the processing of groups of
ConcurrentLinkedQueueclass supplies an efficient scalable thread-safe non-blocking FIFO queue.
Five implementations in
interface, that defines blocking versions of put and take:
The different classes cover the most common usage contexts
for producer-consumer, messaging, parallel tasking, and
related concurrent designs.
TimeUnitclass 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
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.
CyclicBarrieris a resettable multiway synchronization point useful in some styles of parallel programming.
Exchangerallows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.
CopyOnWriteArraySet. When many threads are expected to access a given collection, a
ConcurrentHashMapis normally preferable to a synchronized
HashMap, and a
ConcurrentSkipListMapis normally preferable to a synchronized
CopyOnWriteArrayListis preferable to a synchronized
ArrayListwhen 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
Collections.synchronizedMap(new HashMap()) are
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.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. The
volatileconstructs, as well as the
Thread.join()methods, can form happens-before relationships. In particular:
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.
volatilefield happens-before every subsequent read of that same field. Writes and reads of
volatilefields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
starton a thread happens-before any action in the started thread.
joinon that thread.
java.util.concurrentand its subpackages extend these guarantees to higher-level synchronization. In particular:
Executorhappen-before its execution begins. Similarly for
Callablessubmitted to an
Futurehappen-before actions subsequent to the retrieval of the result via
Future.get()in another thread.
CountDownLatch.countDownhappen-before actions subsequent to a successful "acquiring" method such as
CountDownLatch.awaiton 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.awaithappen-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
awaitin other threads.