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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

/**

 * 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

 * {@link java.util.concurrent.locks} and

 * {@link java.util.concurrent.atomic} packages.

 *

 * <h2>Executors</h2>

 *

 * <b>Interfaces.</b>

 *

 * {@link java.util.concurrent.Executor} is 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 {@link java.util.concurrent.Executor#execute

 * execute}, and may execute sequentially or concurrently.

 *

 * {@link java.util.concurrent.ExecutorService} provides a more

 * complete asynchronous task execution framework.  An

 * ExecutorService manages queuing and scheduling of tasks,

 * and allows controlled shutdown.

 *

 * The {@link java.util.concurrent.ScheduledExecutorService}

 * subinterface and associated interfaces add support for

 * delayed and periodic task execution.  ExecutorServices

 * provide methods arranging asynchronous execution of any

 * function expressed as {@link java.util.concurrent.Callable},

 * the result-bearing analog of {@link java.lang.Runnable}.

 *

 * A {@link java.util.concurrent.Future} returns the results of

 * a function, allows determination of whether execution has

 * completed, and provides a means to cancel execution.

 *

 * A {@link java.util.concurrent.RunnableFuture} is a {@code Future}

 * that possesses a {@code run} method that upon execution,

 * sets its results.

 *

 * <p>

 *

 * <b>Implementations.</b>

 *

 * Classes {@link java.util.concurrent.ThreadPoolExecutor} and

 * {@link java.util.concurrent.ScheduledThreadPoolExecutor}

 * provide tunable, flexible thread pools.

 *

 * The {@link java.util.concurrent.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 {@code Executors} include the

 * concrete class {@link java.util.concurrent.FutureTask}

 * providing a common extensible implementation of Futures, and

 * {@link java.util.concurrent.ExecutorCompletionService}, that

 * assists in coordinating the processing of groups of

 * asynchronous tasks.

 *

 * <p>Class {@link java.util.concurrent.ForkJoinPool} provides an

 * Executor primarily designed for processing instances of {@link

 * java.util.concurrent.ForkJoinTask} and its subclasses.  These

 * classes employ a work-stealing scheduler that attains high

 * throughput for tasks conforming to restrictions that often hold in

 * computation-intensive parallel processing.

 *

 * <h2>Queues</h2>

 *

 * The {@link java.util.concurrent.ConcurrentLinkedQueue} class

 * supplies an efficient scalable thread-safe non-blocking FIFO

 * queue.

 *

 * <p>Five implementations in {@code java.util.concurrent} support

 * the extended {@link java.util.concurrent.BlockingQueue}

 * interface, that defines blocking versions of put and take:

 * {@link java.util.concurrent.LinkedBlockingQueue},

 * {@link java.util.concurrent.ArrayBlockingQueue},

 * {@link java.util.concurrent.SynchronousQueue},

 * {@link java.util.concurrent.PriorityBlockingQueue}, and

 * {@link java.util.concurrent.DelayQueue}.

 * The different classes cover the most common usage contexts

 * for producer-consumer, messaging, parallel tasking, and

 * related concurrent designs.

 *

 * <p> Extended interface {@link java.util.concurrent.TransferQueue},

 * and implementation {@link java.util.concurrent.LinkedTransferQueue}

 * introduce a synchronous {@code transfer} method (along with related

 * features) in which a producer may optionally block awaiting its

 * consumer.

 *

 * <p>The {@link java.util.concurrent.BlockingDeque} interface

 * extends {@code BlockingQueue} to support both FIFO and LIFO

 * (stack-based) operations.

 * Class {@link java.util.concurrent.LinkedBlockingDeque}

 * provides an implementation.

 *

 * <h2>Timing</h2>

 *

 * The {@link java.util.concurrent.TimeUnit} class 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 {@code Long.MAX_VALUE}.

 *

 * <h2>Synchronizers</h2>

 *

 * Five classes aid common special-purpose synchronization idioms.

 * <ul>

 *

 * <li>{@link java.util.concurrent.Semaphore} is a classic concurrency tool.

 *

 * <li>{@link java.util.concurrent.CountDownLatch} is a very simple yet

 * very common utility for blocking until a given number of signals,

 * events, or conditions hold.

 *

 * <li>A {@link java.util.concurrent.CyclicBarrier} is a resettable

 * multiway synchronization point useful in some styles of parallel

 * programming.

 *

 * <li>A {@link java.util.concurrent.Phaser} provides

 * a more flexible form of barrier that may be used to control phased

 * computation among multiple threads.

 *

 * <li>An {@link java.util.concurrent.Exchanger} allows two threads to

 * exchange objects at a rendezvous point, and is useful in several

 * pipeline designs.

 *

 * </ul>

 *

 * <h2>Concurrent Collections</h2>

 *

 * Besides Queues, this package supplies Collection implementations

 * designed for use in multithreaded contexts:

 * {@link java.util.concurrent.ConcurrentHashMap},

 * {@link java.util.concurrent.ConcurrentSkipListMap},

 * {@link java.util.concurrent.ConcurrentSkipListSet},

 * {@link java.util.concurrent.CopyOnWriteArrayList}, and

 * {@link java.util.concurrent.CopyOnWriteArraySet}.

 * When many threads are expected to access a given collection, a

 * {@code ConcurrentHashMap} is normally preferable to a synchronized

 * {@code HashMap}, and a {@code ConcurrentSkipListMap} is normally

 * preferable to a synchronized {@code TreeMap}.

 * A {@code CopyOnWriteArrayList} is preferable to a synchronized

 * {@code ArrayList} when the expected number of reads and traversals

 * greatly outnumber the number of updates to a list.

 * <p>The "Concurrent" prefix used with some classes in this package

 * is a shorthand indicating several differences from similar

 * "synchronized" classes.  For example {@code java.util.Hashtable} and

 * {@code Collections.synchronizedMap(new HashMap())} are

 * synchronized.  But {@link

 * java.util.concurrent.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.

 *

 * <p>Most concurrent Collection implementations (including most

 * Queues) also differ from the usual java.util conventions in that

 * their Iterators provide <em>weakly consistent</em> 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.

 *

 * <h2><a name="MemoryVisibility">Memory Consistency Properties</a></h2>

 *

 * <i>happens-before</i> 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 <i>happens-before</i> the read operation.  The

 * {@code synchronized} and {@code volatile} constructs, as well as the

 * {@code Thread.start()} and {@code Thread.join()} methods, can form

 * <i>happens-before</i> relationships.  In particular:

 *

 * <ul>

 *   <li>Each action in a thread <i>happens-before</i> every action in that

 *   thread that comes later in the program's order.

 *

 *   <li>An unlock ({@code synchronized} block or method exit) of a

 *   monitor <i>happens-before</i> every subsequent lock ({@code synchronized}

 *   block or method entry) of that same monitor.  And because

 *   the <i>happens-before</i> relation is transitive, all actions

 *   of a thread prior to unlocking <i>happen-before</i> all actions

 *   subsequent to any thread locking that monitor.

 *

 *   <li>A write to a {@code volatile} field <i>happens-before</i> every

 *   subsequent read of that same field.  Writes and reads of

 *   {@code volatile} fields have similar memory consistency effects

 *   as entering and exiting monitors, but do <em>not</em> entail

 *   mutual exclusion locking.

 *

 *   <li>A call to {@code start} on a thread <i>happens-before</i> any

 *   action in the started thread.

 *

 *   <li>All actions in a thread <i>happen-before</i> any other thread

 *   successfully returns from a {@code join} on that thread.

 *

 * </ul>

 *

 *

 * The methods of all classes in {@code java.util.concurrent} and its

 * subpackages extend these guarantees to higher-level

 * synchronization.  In particular:

 *

 * <ul>

 *

 *   <li>Actions in a thread prior to placing an object into any concurrent

 *   collection <i>happen-before</i> actions subsequent to the access or

 *   removal of that element from the collection in another thread.

 *

 *   <li>Actions in a thread prior to the submission of a {@code Runnable}

 *   to an {@code Executor} <i>happen-before</i> its execution begins.

 *   Similarly for {@code Callables} submitted to an {@code ExecutorService}.

 *

 *   <li>Actions taken by the asynchronous computation represented by a

 *   {@code Future} <i>happen-before</i> actions subsequent to the

 *   retrieval of the result via {@code Future.get()} in another thread.

 *

 *   <li>Actions prior to "releasing" synchronizer methods such as

 *   {@code Lock.unlock}, {@code Semaphore.release}, and

 *   {@code CountDownLatch.countDown} <i>happen-before</i> actions

 *   subsequent to a successful "acquiring" method such as

 *   {@code Lock.lock}, {@code Semaphore.acquire},

 *   {@code Condition.await}, and {@code CountDownLatch.await} on the

 *   same synchronizer object in another thread.

 *

 *   <li>For each pair of threads that successfully exchange objects via

 *   an {@code Exchanger}, actions prior to the {@code exchange()}

 *   in each thread <i>happen-before</i> those subsequent to the

 *   corresponding {@code exchange()} in another thread.

 *

 *   <li>Actions prior to calling {@code CyclicBarrier.await} and

 *   {@code Phaser.awaitAdvance} (as well as its variants)

 *   <i>happen-before</i> actions performed by the barrier action, and

 *   actions performed by the barrier action <i>happen-before</i> actions

 *   subsequent to a successful return from the corresponding {@code await}

 *   in other threads.

 *

 * </ul>

 *

 * @since 1.5

 */

package java.util.concurrent;