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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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<!-- |
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Written by Doug Lea with assistance from members of JCP JSR-166 |
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Expert Group and released to the public domain, as explained at |
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http://creativecommons.org/licenses/publicdomain |
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--> |
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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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|
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<b>Interfaces.</b> {@link java.util.concurrent.Executor} is a simple |
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standardized interface for defining custom thread-like subsystems, |
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including thread pools, asynchronous IO, and lightweight task |
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frameworks. Depending on which concrete Executor class is being used, |
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tasks may execute in a newly created thread, an existing |
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task-execution thread, or the thread calling <tt>execute()</tt>, and |
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may execute sequentially or concurrently. {@link |
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java.util.concurrent.ExecutorService} provides a more complete |
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asynchronous task execution framework. An ExecutorService manages |
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queuing and scheduling of tasks, and allows controlled shutdown. The |
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{@link java.util.concurrent.ScheduledExecutorService} subinterface |
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and associated interfaces add support for delayed and periodic task execution. |
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ExecutorServices provide methods arranging asynchronous execution of |
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any function expressed as {@link java.util.concurrent.Callable}, the |
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result-bearing analog of {@link java.lang.Runnable}. A {@link |
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java.util.concurrent.Future} returns the results of a function, allows |
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determination of whether execution has completed, and provides a means to |
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cancel execution. A {@link java.util.concurrent.RunnableFuture} is |
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a Future that possesses a <tt>run</tt> method that upon execution, |
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sets its results. |
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<p> |
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|
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<b>Implementations.</b> Classes {@link |
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java.util.concurrent.ThreadPoolExecutor} and {@link |
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java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable, |
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flexible thread pools. The {@link java.util.concurrent.Executors} |
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class provides factory methods for the most common kinds and |
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configurations of Executors, as well as a few utility methods for |
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using them. Other utilities based on Executors include the concrete |
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class {@link java.util.concurrent.FutureTask} providing a common |
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extensible implementation of Futures, and {@link |
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java.util.concurrent.ExecutorCompletionService}, that assists in |
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coordinating the processing of groups of asynchronous tasks. |
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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. The {@link |
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java.util.concurrent.BlockingDeque} interface extends |
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<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based) |
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operations. Class {@link java.util.concurrent.LinkedBlockingDeque} |
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provides an implementation. |
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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. All methods |
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that accept timeout parameters treat values less than or equal to |
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zero to mean not to wait at all. To wait "forever", you can use |
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a value of <tt>Long.MAX_VALUE</tt>. |
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<h2>Synchronizers</h2> |
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Four 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 a 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 useful 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, and is useful |
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in several pipeline designs. |
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<h2>Concurrent Collections</h2> |
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Besides Queues, this package supplies Collection implementations |
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designed for use in multithreaded contexts: |
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{@link java.util.concurrent.ConcurrentHashMap}, |
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{@link java.util.concurrent.ConcurrentSkipListMap}, |
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{@link java.util.concurrent.ConcurrentSkipListSet}, |
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{@link java.util.concurrent.CopyOnWriteArrayList}, and |
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{@link java.util.concurrent.CopyOnWriteArraySet}. |
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When many threads are expected to access a given collection, |
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a <tt>ConcurrentHashMap</tt> is normally preferable to |
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a synchronized <tt>HashMap</tt>, and a |
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<tt>ConcurrentSkipListMap</tt> is normally preferable |
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to a synchronized <tt>TreeMap</tt>. A |
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<tt>CopyOnWriteArrayList</tt> is preferable to |
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a synchronized <tt>ArrayList</tt> when the expected number of reads |
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and traversals greatly outnumber the number of updates to a list. |
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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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<a name="MemoryVisibility"> |
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<h2> Memory Consistency Properties </h2> |
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<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html"> |
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Chapter 17 of the Java Language Specification</a> defines the |
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<i>happens-before</i> relation on memory operations such as reads and |
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writes of shared variables. The results of a write by one thread are |
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guaranteed to be visible to a read by another thread only if the write |
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operation <i>happens-before</i> the read operation. The |
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{@code synchronized} and {@code volatile} constructs, as well as the |
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{@code Thread.start()} and {@code Thread.join()} methods, can form |
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<i>happens-before</i> relationships. In particular: |
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<ul> |
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<li>Each action in a thread <i>happens-before</i> every action in that |
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thread that comes later in the program's order. |
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<li>An unlock ({@code synchronized} block or method exit) of a |
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monitor <i>happens-before</i> every subsequent lock ({@code synchronized} |
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block or method entry) of that same monitor. And because |
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the <i>happens-before</i> relation is transitive, all actions |
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of a thread prior to unlocking <i>happen-before</i> all actions |
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subsequent to any thread locking that monitor. |
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<li>A write to a {@code volatile} field <i>happens-before</i> every |
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subsequent read of that same field. Writes and reads of |
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{@code volatile} fields have similar memory consistency effects |
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as entering and exiting monitors, but do <em>not</em> entail |
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mutual exclusion locking. |
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<li>A call to {@code start} on a thread <i>happens-before</i> any action in the |
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started thread. |
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<li>All actions in a thread <i>happen-before</i> any other thread |
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successfully returns from a {@code join} on that thread. |
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</ul> |
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The methods of all classes in {@code java.util.concurrent} and its |
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subpackages extend these guarantees to higher-level |
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synchronization. In particular: |
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<ul> |
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<li>Actions in a thread prior to placing an object into any concurrent |
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collection <i>happen-before</i> actions subsequent to the access or |
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removal of that element from the collection in another thread. |
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<li>Actions in a thread prior to the submission of a {@code Runnable} |
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to an {@code Executor} <i>happen-before</i> its execution begins. |
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Similarly for {@code Callables} submitted to an {@code ExecutorService}. |
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<li>Actions taken by the asynchronous computation represented by a |
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{@code Future} <i>happen-before</i> actions subsequent to the |
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retrieval of the result via {@code Future.get()} in another thread. |
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<li>Actions prior to "releasing" synchronizer methods such as |
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{@code Lock.unlock}, {@code Semaphore.release}, and |
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{@code CountDownLatch.countDown} <i>happen-before</i> actions |
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subsequent to a successful "acquiring" method such as |
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{@code Lock.lock}, {@code Semaphore.acquire}, |
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{@code Condition.await}, and {@code CountDownLatch.await} on the |
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same synchronizer object in another thread. |
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<li>For each pair of threads that successfully exchange objects via |
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an {@code Exchanger}, actions prior to the {@code exchange()} |
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in each thread <i>happen-before</i> those subsequent to the |
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corresponding {@code exchange()} in another thread. |
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<li>Actions prior to calling {@code CyclicBarrier.await} |
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<i>happen-before</i> actions performed by the barrier action, and |
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actions performed by the barrier action <i>happen-before</i> actions |
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subsequent to a successful return from the corresponding {@code await} |
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in other threads. |
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</ul> |
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@since 1.5 |
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</body> </html> |
