mirror of git://gcc.gnu.org/git/gcc.git
223 lines
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HTML
223 lines
10 KiB
HTML
<!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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<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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<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>
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