# Package java.util.concurrent

package java.util.concurrent
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 `java.util.concurrent.locks` and `java.util.concurrent.atomic` packages.

## Executors

Interfaces. `Executor` is a simple standardized interface for defining custom thread-like subsystems, including thread pools, asynchronous I/O, 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. `ExecutorService` provides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The `ScheduledExecutorService` subinterface and associated interfaces add support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as `Callable`, the result-bearing analog of `Runnable`. A `Future` returns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A `RunnableFuture` is a `Future` that possesses a `run` method that upon execution, sets its results.

Implementations. Classes `ThreadPoolExecutor` and `ScheduledThreadPoolExecutor` provide tunable, flexible thread pools. The `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 `FutureTask` providing a common extensible implementation of Futures, and `ExecutorCompletionService`, that assists in coordinating the processing of groups of asynchronous tasks.

Class `ForkJoinPool` provides an Executor primarily designed for processing instances of `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.

## Queues

The `ConcurrentLinkedQueue` class supplies an efficient scalable thread-safe non-blocking FIFO queue. The `ConcurrentLinkedDeque` class is similar, but additionally supports the `Deque` interface.

Five implementations in `java.util.concurrent` support the extended `BlockingQueue` interface, that defines blocking versions of put and take: `LinkedBlockingQueue`, `ArrayBlockingQueue`, `SynchronousQueue`, `PriorityBlockingQueue`, and `DelayQueue`. The different classes cover the most common usage contexts for producer-consumer, messaging, parallel tasking, and related concurrent designs.

Extended interface `TransferQueue`, and implementation `LinkedTransferQueue` introduce a synchronous `transfer` method (along with related features) in which a producer may optionally block awaiting its consumer.

The `BlockingDeque` interface extends `BlockingQueue` to support both FIFO and LIFO (stack-based) operations. Class `LinkedBlockingDeque` provides an implementation.

## Timing

The `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

Five classes aid common special-purpose synchronization idioms.

- `Semaphore` is a classic concurrency tool.
- `CountDownLatch` is a very simple yet very common utility for blocking until a given number of signals, events, or conditions hold.
- A `CyclicBarrier` is a resettable multiway synchronization point useful in some styles of parallel programming.
- A `Phaser` provides a more flexible form of barrier that may be used to control phased computation among multiple threads.
- An `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: `ConcurrentHashMap`, `ConcurrentSkipListMap`, `ConcurrentSkipListSet`, `CopyOnWriteArrayList`, and `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 `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 large 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 and Spliterators provide weakly consistent rather than fast-fail traversal:

- they may proceed concurrently with other operations
- they will never throw `ConcurrentModificationException`
- they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction.

## 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 `synchronized` and `volatile` constructs, as well as the `Thread.start()` and `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 (`synchronized` block or method exit) of a monitor happens-before every subsequent lock (`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 `volatile` field happens-before every subsequent read of that same field. Writes and reads of `volatile` fields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
- A call to `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 `join` on that thread.

The methods of all classes in `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 `Runnable` to an `Executor` happen-before its execution begins. Similarly for `Callables` submitted to an `ExecutorService`.
- Actions taken by the asynchronous computation represented by a `Future` happen-before actions subsequent to the retrieval of the result via `Future.get()` in another thread.
- Actions prior to "releasing" synchronizer methods such as `Lock.unlock`, `Semaphore.release`, and `CountDownLatch.countDown` happen-before actions subsequent to a successful "acquiring" method such as `Lock.lock`, `Semaphore.acquire`, `Condition.await`, and `CountDownLatch.await` on the same synchronizer object in another thread.
- For each pair of threads that successfully exchange objects via an `Exchanger`, actions prior to the `exchange()` in each thread happen-before those subsequent to the corresponding `exchange()` in another thread.
- Actions prior to calling `CyclicBarrier.await` and `Phaser.awaitAdvance` (as well as its variants) 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 `await` in other threads.

Since:
  1.5

- Class
  Description
  StructuredTaskScopePREVIEW<T,R>
  Preview.
  An API for structured concurrency.
  StructuredTaskScope.ConfigurationPREVIEW
  Preview.
  Represents the configuration for a `StructuredTaskScope`.
  StructuredTaskScope.FailedExceptionPREVIEW
  Preview.
  Exception thrown by `StructuredTaskScope.join()`PREVIEW when the outcome is an exception rather than a result.
  StructuredTaskScope.JoinerPREVIEW<T,R>
  Preview.
  An object used with a `StructuredTaskScope`PREVIEW to handle subtask completion and produce the result for the scope owner waiting in the `join`PREVIEW method for subtasks to complete.
  StructuredTaskScope.SubtaskPREVIEW<T>
  Preview.
  Represents a subtask forked with `StructuredTaskScope.fork(Callable)`PREVIEW or `StructuredTaskScope.fork(Runnable)`PREVIEW.
  StructuredTaskScope.Subtask.StatePREVIEW
  Preview.
  Represents the state of a subtask.
  StructuredTaskScope.TimeoutExceptionPREVIEW
  Preview.
  Exception thrown by `StructuredTaskScope.join()`PREVIEW if the scope was created with a timeout and the timeout expired before or while waiting in `join`.