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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, asynch-IO, and lightweight task frameworks. Depending on which |
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concrete Executor class is being used, tasks may execute in a newly |
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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. Executors also standardize ways of calling threads that |
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compute functions returning results, via a {@link |
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java.util.concurrent.Future}. This is supported in part by defining |
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interface {@link java.util.concurrent.Callable}, the argument/result |
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analog of Runnable. |
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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. These, and other Executors can |
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be used in conjunction with a {@link |
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java.util.concurrent.CancellableTask} or {@link |
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java.util.concurrent.FutureTask} to asynchronously start a potentially |
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long-running computation and query the FutureTask to determine if its |
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execution has completed, or cancel it. |
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<p> The {@link java.util.concurrent.Executors} class provides factory |
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methods for the most common kinds and configurations of Executors, as |
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well as a few utility methods for using them. |
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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. |
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<p> Five implementations in java.util.concurrent support the extended |
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{@link java.util.concurrent.BlockingQueue} interface, that defines |
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blocking 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. Nearly all other classes in the package |
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contain operations based on time-outs in addition to indefinite waits. |
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<p>In all cases that time-outs are used, the time-out specifies the |
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minimum time that the method should wait before indicating that it |
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timed-out. The virtual machine should make a "best effort" |
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to detect time-outs as soon as possible after they occur. Regardless |
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of the efforts of the virtual machine, the normal scheduling |
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mechanisms, and the need to re-acquire locks in many cases, can lead |
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to an indefinite amount of time elapsing between a time-out being |
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detected and a 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} and {@link |
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java.util.concurrent.FairSemaphore} are classic concurrency tools. |
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{@link java.util.concurrent.CountDownLatch} is very simple yet very |
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common utility for blocking until a single signal, event, or condition |
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holds. A {@link java.util.concurrent.CyclicBarrier} is a resettable multiway |
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synchronization point common in some styles of parallel |
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programming. An {@link java.util.concurrent.Exchanger} allows two |
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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 for classes is a shorthand |
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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 |
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java.util.concurrent.ConcurrentHashMap} is "concurrent". |
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A concurrent collection (among other kinds of classes) is |
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thread-safe, but not governed by a single exclusion lock. So, in the |
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particular case of ConcurrentHashMap, it safely permits any number of |
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concurrent reads as well as a tunable number of concurrent writes. |
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There may still be a role for "synchronized" classes in some |
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multithreaded programs -- they can sometimes be useful when you need |
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to prevent ALL access to a collection via a single lock, at the |
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expense of much poor scalability. In all other cases, "concurrent" |
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versions are normally preferable. |
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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> |
