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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
*/
/**
* 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>
*
* Chapter 17 of
* <cite>The Java™ Language Specification</cite>
* defines the
* <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
*/