OpenJDK 1.23 · java.util.concurrent · APIdia, the JavaDoc alternative

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. java.util.concurrent.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. 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 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 java.util.concurrent.Callable, the result-bearing analog of java.lang.Runnable. A 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 java.util.concurrent.RunnableFuture is a Future that possesses a run method that upon execution, sets its results.

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

Class java.util.concurrent.ForkJoinPool provides an Executor primarily designed for processing instances of 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.

Queues

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

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

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

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

Timing

The 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

Five classes aid common special-purpose synchronization idioms.

java.util.concurrent.Semaphore is a classic concurrency tool.
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 java.util.concurrent.CyclicBarrier is a resettable multiway synchronization point useful in some styles of parallel programming.
A java.util.concurrent.Phaser provides a more flexible form of barrier that may be used to control phased computation among multiple threads.
An 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: java.util.concurrent.ConcurrentHashMap, java.util.concurrent.ConcurrentSkipListMap, java.util.concurrent.ConcurrentSkipListSet, java.util.concurrent.CopyOnWriteArrayList, and 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 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 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.

Interface Summary

Modifier and Type	Interface and Description
public interface	BlockingDeque< the type of elements held in this deque E> A `Deque` that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.
public interface	BlockingQueue< the type of elements held in this queue E> A `Queue` that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.
public interface	Callable< the result type of method `call` V> A task that returns a result and may throw an exception.
public interface	CompletionService< the type of values the tasks of this service produce and consume V> A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks.
public interface	CompletionStage< the type of values the stage produces or consumes T> A stage of a possibly asynchronous computation, that performs an action or computes a value when another CompletionStage completes.
public interface	ConcurrentMap< the type of keys maintained by this map K, the type of mapped values V> A `Map` providing thread safety and atomicity guarantees.
public interface	ConcurrentNavigableMap< the type of keys maintained by this map K, the type of mapped values V> A `ConcurrentMap` supporting `NavigableMap` operations, and recursively so for its navigable sub-maps.
public interface	Delayed A mix-in style interface for marking objects that should be acted upon after a given delay.
public interface	Executor An object that executes submitted `Runnable` tasks.
public interface	ExecutorService An `Executor` that provides methods to manage termination and methods that can produce a `Future` for tracking progress of one or more asynchronous tasks.
public interface	Future< The result type returned by this Future's `get` method V> A `Future` represents the result of an asynchronous computation.
public interface	RejectedExecutionHandler A handler for tasks that cannot be executed by a `ThreadPoolExecutor`.
public interface	RunnableFuture< The result type returned by this Future's `get` method V> A `Future` that is `Runnable`.
public interface	RunnableScheduledFuture< The result type returned by this Future's `get` method V> A `ScheduledFuture` that is `Runnable`.
public interface	ScheduledExecutorService An `ExecutorService` that can schedule commands to run after a given delay, or to execute periodically.
public interface	ScheduledFuture< The result type returned by this Future V> A delayed result-bearing action that can be cancelled.
public interface	ThreadFactory An object that creates new threads on demand.
public interface	TransferQueue< the type of elements held in this queue E> A `BlockingQueue` in which producers may wait for consumers to receive elements.

Class Summary

Modifier and Type	Class and Description
public abstract class	AbstractExecutorService Provides default implementations of `ExecutorService` execution methods.
public class	ArrayBlockingQueue< the type of elements held in this queue E> A bounded blocking queue backed by an array.
public class	BrokenBarrierException Exception thrown when a thread tries to wait upon a barrier that is in a broken state, or which enters the broken state while the thread is waiting.
public class	CancellationException Exception indicating that the result of a value-producing task, such as a `FutureTask`, cannot be retrieved because the task was cancelled.
public class	CompletableFuture< The result type returned by this future's `join` and `get` methods T> A `Future` that may be explicitly completed (setting its value and status), and may be used as a `CompletionStage`, supporting dependent functions and actions that trigger upon its completion.
public class	CompletionException Exception thrown when an error or other exception is encountered in the course of completing a result or task.
public class	ConcurrentHashMap< the type of keys maintained by this map K, the type of mapped values V> A hash table supporting full concurrency of retrievals and high expected concurrency for updates.
public class	ConcurrentLinkedDeque< the type of elements held in this deque E> An unbounded concurrent deque based on linked nodes.
public class	ConcurrentLinkedQueue< the type of elements held in this queue E> An unbounded thread-safe queue based on linked nodes.
public class	ConcurrentSkipListMap< the type of keys maintained by this map K, the type of mapped values V> A scalable concurrent `ConcurrentNavigableMap` implementation.
public class	ConcurrentSkipListSet< the type of elements maintained by this set E> A scalable concurrent `NavigableSet` implementation based on a `ConcurrentSkipListMap`.
public class	CopyOnWriteArrayList< the type of elements held in this list E> A thread-safe variant of `java.util.ArrayList` in which all mutative operations (`add`, `set`, and so on) are implemented by making a fresh copy of the underlying array.
public class	CopyOnWriteArraySet< the type of elements held in this set E> A `Set` that uses an internal `CopyOnWriteArrayList` for all of its operations.
public class	CountDownLatch A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
public abstract class	CountedCompleter< the type of the result of the completer T> A `ForkJoinTask` with a completion action performed when triggered and there are no remaining pending actions.
public class	CyclicBarrier A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
public class	DelayQueue< the type of elements held in this queue E extends Delayed> An unbounded blocking queue of `Delayed` elements, in which an element generally becomes eligible for removal when its delay has expired.
public class	Exchanger< The type of objects that may be exchanged V> A synchronization point at which threads can pair and swap elements within pairs.
public class	ExecutionException Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception.
public class	ExecutorCompletionService< the type of values the tasks of this service produce and consume V> A `CompletionService` that uses a supplied `Executor` to execute tasks.
public class	Executors Factory and utility methods for `Executor`, `ExecutorService`, `ScheduledExecutorService`, `ThreadFactory`, and `Callable` classes defined in this package.
public class	Flow Interrelated interfaces and static methods for establishing flow-controlled components in which `Publishers` produce items consumed by one or more `Subscribers`, each managed by a `Subscription`.
public class	ForkJoinPool An `ExecutorService` for running `ForkJoinTask`s.
public abstract class	ForkJoinTask< the type of the result of the task V> Abstract base class for tasks that run within a `ForkJoinPool`.
public class	ForkJoinWorkerThread A thread managed by a `ForkJoinPool`, which executes `ForkJoinTask`s.
public class	FutureTask< The result type returned by this FutureTask's `get` methods V> A cancellable asynchronous computation.
pack-priv class	Helpers Shared implementation code for java.util.concurrent.
public class	LinkedBlockingDeque< the type of elements held in this deque E> An optionally-bounded blocking deque based on linked nodes.
public class	LinkedBlockingQueue< the type of elements held in this queue E> An optionally-bounded blocking queue based on linked nodes.
public class	LinkedTransferQueue< the type of elements held in this queue E> An unbounded `TransferQueue` based on linked nodes.
public class	Phaser A reusable synchronization barrier, similar in functionality to `CyclicBarrier` and `CountDownLatch` but supporting more flexible usage.
public class	PriorityBlockingQueue< the type of elements held in this queue E> An unbounded blocking queue that uses the same ordering rules as class `PriorityQueue` and supplies blocking retrieval operations.
public abstract class	RecursiveAction A recursive resultless `ForkJoinTask`.
public abstract class	RecursiveTask< the type of the result of the task V> A recursive result-bearing `ForkJoinTask`.
public class	RejectedExecutionException Exception thrown by an `Executor` when a task cannot be accepted for execution.
public class	ScheduledThreadPoolExecutor A `ThreadPoolExecutor` that can additionally schedule commands to run after a given delay, or to execute periodically.
public class	Semaphore A counting semaphore.
public class	StructuredTaskScope< the result type of tasks executed in the task scope T> Preview Third Preview of Structured Concurrency (JEP 480). A basic API for structured concurrency.
public class	StructureViolationException Preview Third Preview of Structured Concurrency (JEP 480). Thrown when a structure violation is detected.
public class	SubmissionPublisher< the published item type T> A `Flow.Publisher` that asynchronously issues submitted (non-null) items to current subscribers until it is closed.
public class	SynchronousQueue< the type of elements held in this queue E> A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
public class	ThreadLocalRandom A random number generator (with period 2⁶⁴) isolated to the current thread.
pack-priv class	ThreadPerTaskExecutor An ExecutorService that starts a new thread for each task.
public class	ThreadPoolExecutor An `ExecutorService` that executes each submitted task using one of possibly several pooled threads, normally configured using `Executors` factory methods.
public class	TimeoutException Exception thrown when a blocking operation times out.

Package java.util.concurrent

Executors

Queues

Timing

Synchronizers

Concurrent Collections

Memory Consistency Properties

Interface Summary

Class Summary

Enum Summary