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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> |
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<html> <head> |
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<title>Concurrency Utilities</title> |
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</head> |
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<body> |
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<p> Utility classes commonly useful in concurrent programming. This |
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package includes a few small standardized extensible frameworks, as |
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well as some classes that provide useful functionality and are |
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otherwise tedious or difficult to implement. Here are brief |
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descriptions of the main components. See also the <tt>locks</tt> and |
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<tt>atomic</tt> packages. |
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<h2>Executors</h2> |
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{@link java.util.concurrent.Executor} is a simple standardized |
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interface for defining custom thread-like subsystems, including thread |
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pools, asynchronous IO, and lightweight task frameworks. Depending on |
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which concrete Executor class is being used, tasks may execute in a |
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newly created thread, an existing task-execution thread, or the thread |
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calling <tt>execute()</tt>, and may execute sequentially or |
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concurrently. |
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<p> {@link java.util.concurrent.ExecutorService} provides a more |
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complete framework for executing Runnables. An ExecutorService |
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manages queueing and scheduling of tasks, and allows controlled |
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shutdown. The two primary implementations of ExecutorService are |
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{@link java.util.concurrent.ThreadPoolExecutor}, a tunable and |
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flexible thread pool and {@link |
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java.util.concurrent.ScheduledExecutor}, which adds support for |
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delayed and periodic task execution. The {@link |
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java.util.concurrent.Executors} class provides factory methods for the |
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most common kinds and configurations of Executors, as well as a few |
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utility methods for using them. |
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<p> Executors may be used with threads that compute functions |
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returning results. A {@link java.util.concurrent.Future} returns the |
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results of a {@link java.util.concurrent.Callable}, the result-bearing |
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analog of {@link java.lang.Runnable}. Instances of concrete class |
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{@link java.util.concurrent.FutureTask} may be submitted to Executors |
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to asynchronously start a potentially long-running computation, query |
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to determine if its execution has completed, or cancel it. The {@link |
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java.util.concurrent.CancellableTask} class provides similar control |
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for actions that do not bear results. |
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<h2>Queues</h2> |
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The java.util.concurrent {@link |
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java.util.concurrent.ConcurrentLinkedQueue} class supplies an |
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efficient scalable thread-safe non-blocking FIFO queue. Five |
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implementations in java.util.concurrent support the extended {@link |
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java.util.concurrent.BlockingQueue} interface, that defines blocking |
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versions of put and take: {@link |
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java.util.concurrent.LinkedBlockingQueue}, {@link |
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java.util.concurrent.ArrayBlockingQueue}, {@link |
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java.util.concurrent.SynchronousQueue}, {@link |
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java.util.concurrent.PriorityBlockingQueue}, and {@link |
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java.util.concurrent.DelayQueue}. The different classes cover the most |
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common usage contexts for producer-consumer, messaging, parallel |
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tasking, and related concurrent designs. |
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<h2>Timing</h2> |
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The {@link java.util.concurrent.TimeUnit} class provides multiple |
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granularities (including nanoseconds) for specifying and controlling |
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time-out based operations. Most classes in the package contain |
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operations based on time-outs in addition to indefinite waits. In all |
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cases that time-outs are used, the time-out specifies the minimum time |
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that the method should wait before indicating that it |
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timed-out. Implementations make a "best effort" to detect |
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time-outs as soon as possible after they occur. However, an indefinite |
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amount of time may elapse between a time-out being detected and a |
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thread actually executing again after that time-out. |
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<h2>Synchronizers</h2> |
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Five classes aid common special-purpose synchronization idioms. |
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{@link java.util.concurrent.Semaphore} is a classic concurrency tool. |
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{@link java.util.concurrent.CountDownLatch} is very simple yet very |
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common utility for blocking until a given number of signals, events, |
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or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a |
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resettable multiway synchronization point common in some styles of |
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parallel programming. An {@link java.util.concurrent.Exchanger} allows |
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two threads to exchange objects at a rendezvous point. |
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<h2>Concurrent Collections</h2> |
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Besides Queues, this package supplies a few Collection implementations |
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designed for use in multithreaded contexts: {@link |
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java.util.concurrent.ConcurrentHashMap}, {@link |
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java.util.concurrent.CopyOnWriteArrayList}, and {@link |
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java.util.concurrent.CopyOnWriteArraySet}. |
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<p>The "Concurrent" prefix used with some classes in this package is a |
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shorthand indicating several differences from similar "synchronized" |
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classes. For example <tt>java.util.Hashtable</tt> and |
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<tt>Collections.synchronizedMap(new HashMap())</tt> are |
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synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is |
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"concurrent". A concurrent collection is thread-safe, but not |
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governed by a single exclusion lock. In the particular case of |
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ConcurrentHashMap, it safely permits any number of concurrent reads as |
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well as a tunable number of concurrent writes. "Synchronized" classes |
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can be useful when you need to prevent all access to a collection via |
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a single lock, at the expense of poorer scalability. In other cases in |
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which multiple threads are expected to access a common collection, |
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"concurrent" versions are normally preferable. And unsynchronized |
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collections are preferable when either collections are unshared, or |
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are accessible only when holding other locks. |
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<p> Most concurrent Collection implementations (including most Queues) |
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also differ from the usual java.util conventions in that their Iterators |
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provide <em>weakly consistent</em> rather than fast-fail traversal. A |
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weakly consistent iterator is thread-safe, but does not necessarily |
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freeze the collection while iterating, so it may (or may not) reflect |
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any updates since the iterator was created. |
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</body> </html> |
