Concurrency Utilities

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 `locks` and `atomic` packages.

## Executors

Interfaces. {@link edu.emory.mathcs.backport.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 `execute()`, and may execute sequentially or concurrently. {@link edu.emory.mathcs.backport.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 edu.emory.mathcs.backport.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 edu.emory.mathcs.backport.java.util.concurrent.Callable}, the result-bearing analog of {@link java.lang.Runnable}. A {@link edu.emory.mathcs.backport.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 edu.emory.mathcs.backport.java.util.concurrent.RunnableFuture} is a Future that possesses a `run` method that upon execution, sets its results.

Implementations. Classes {@link edu.emory.mathcs.backport.java.util.concurrent.ThreadPoolExecutor} and {@link edu.emory.mathcs.backport.java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable, flexible thread pools. The {@link edu.emory.mathcs.backport.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 Executors include the concrete class {@link edu.emory.mathcs.backport.java.util.concurrent.FutureTask} providing a common extensible implementation of Futures, and {@link edu.emory.mathcs.backport.java.util.concurrent.ExecutorCompletionService}, that assists in coordinating the processing of groups of asynchronous tasks.

## Queues

The edu.emory.mathcs.backport.java.util.concurrent {@link edu.emory.mathcs.backport.java.util.concurrent.ConcurrentLinkedQueue} class supplies an efficient scalable thread-safe non-blocking FIFO queue. Five implementations in edu.emory.mathcs.backport.java.util.concurrent support the extended {@link edu.emory.mathcs.backport.java.util.concurrent.BlockingQueue} interface, that defines blocking versions of put and take: {@link edu.emory.mathcs.backport.java.util.concurrent.LinkedBlockingQueue}, {@link edu.emory.mathcs.backport.java.util.concurrent.ArrayBlockingQueue}, {@link edu.emory.mathcs.backport.java.util.concurrent.SynchronousQueue}, {@link edu.emory.mathcs.backport.java.util.concurrent.PriorityBlockingQueue}, and {@link edu.emory.mathcs.backport.java.util.concurrent.DelayQueue}. The different classes cover the most common usage contexts for producer-consumer, messaging, parallel tasking, and related concurrent designs. The {@link edu.emory.mathcs.backport.java.util.concurrent.BlockingDeque} interface extends `BlockingQueue` to support both FIFO and LIFO (stack-based) operations. Class {@link edu.emory.mathcs.backport.java.util.concurrent.LinkedBlockingDeque} provides an implementation.

## Timing

The {@link edu.emory.mathcs.backport.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 `Long.MAX_VALUE`.

## Synchronizers

Four classes aid common special-purpose synchronization idioms. {@link edu.emory.mathcs.backport.java.util.concurrent.Semaphore} is a classic concurrency tool. {@link edu.emory.mathcs.backport.java.util.concurrent.CountDownLatch} is a very simple yet very common utility for blocking until a given number of signals, events, or conditions hold. A {@link edu.emory.mathcs.backport.java.util.concurrent.CyclicBarrier} is a resettable multiway synchronization point useful in some styles of parallel programming. An {@link edu.emory.mathcs.backport.java.util.concurrent.Exchanger} allows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.

## Concurrent Collections

Besides Queues, this package supplies Collection implementations designed for use in multithreaded contexts: {@link edu.emory.mathcs.backport.java.util.concurrent.ConcurrentHashMap}, {@link edu.emory.mathcs.backport.java.util.concurrent.ConcurrentSkipListMap}, {@link edu.emory.mathcs.backport.java.util.concurrent.ConcurrentSkipListSet}, {@link edu.emory.mathcs.backport.java.util.concurrent.CopyOnWriteArrayList}, and {@link edu.emory.mathcs.backport.java.util.concurrent.CopyOnWriteArraySet}. When many threads are expected to access a given collection, a `ConcurrentHashMap` is normally preferable to a synchronized `HashMap`, and a `ConcurrentSkipListMap` is normally preferable to a synchronized `TreeMap`. A `CopyOnWriteArrayList` is preferable to a synchronized `ArrayList` when the expected number of reads and traversals greatly outnumber the number of updates to a list.

The "Concurrent" prefix used with some classes in this package is a shorthand indicating several differences from similar "synchronized" classes. For example `java.util.Hashtable` and `Collections.synchronizedMap(new HashMap())` are synchronized. But {@link edu.emory.mathcs.backport.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.

Most concurrent Collection implementations (including most Queues) also differ from the usual java.util conventions in that their Iterators provide weakly consistent 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.

## Memory Consistency Properties

Chapter 17 of the Java Language Specification defines the happens-before 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 happens-before the read operation. The {@code synchronized} and {@code volatile} constructs, as well as the {@code Thread.start()} and {@code Thread.join()} methods, can form happens-before relationships. In particular:

- Each action in a thread happens-before every action in that thread that comes later in the program's order.
- An unlock ({@code synchronized} block or method exit) of a monitor happens-before every subsequent lock ({@code synchronized} block or method entry) of that same monitor. And because the happens-before relation is transitive, all actions of a thread prior to unlocking happen-before all actions subsequent to any thread locking that monitor.
- A write to a {@code volatile} field happens-before 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 not entail mutual exclusion locking.
- A call to {@code start} on a thread happens-before any action in the started thread.
- All actions in a thread happen-before any other thread successfully returns from a {@code join} on that thread.

The methods of all classes in {@code edu.emory.mathcs.backport.java.util.concurrent} and its subpackages extend these guarantees to higher-level synchronization. In particular:

- Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread.
- Actions in a thread prior to the submission of a {@code Runnable} to an {@code Executor} happen-before its execution begins. Similarly for {@code Callables} submitted to an {@code ExecutorService}.
- Actions taken by the asynchronous computation represented by a {@code Future} happen-before actions subsequent to the retrieval of the result via {@code Future.get()} in another thread.
- Actions prior to "releasing" synchronizer methods such as {@code Lock.unlock}, {@code Semaphore.release}, and {@code CountDownLatch.countDown} happen-before 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.
- For each pair of threads that successfully exchange objects via an {@code Exchanger}, actions prior to the {@code exchange()} in each thread happen-before those subsequent to the corresponding {@code exchange()} in another thread.
- Actions prior to calling {@code CyclicBarrier.await} happen-before actions performed by the barrier action, and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding {@code await} in other threads.

@since 1.5