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 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
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 Future that possesses a run method that upon execution,
sets its results.
Implementations. 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 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.
Queues
The java.util.concurrent {@link
java.util.concurrent.ConcurrentLinkedQueue} class supplies an
efficient scalable thread-safe non-blocking FIFO queue. Five
implementations in 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. The {@link
java.util.concurrent.BlockingDeque} interface extends
BlockingQueue to support both FIFO and LIFO (stack-based)
operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
provides an implementation.
Timing
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 Long.MAX_VALUE.
Synchronizers
Four classes aid common special-purpose synchronization idioms.
{@link java.util.concurrent.Semaphore} is a classic concurrency tool.
{@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. A {@link java.util.concurrent.CyclicBarrier} is a
resettable multiway synchronization point useful in some styles of
parallel programming. An {@link 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 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 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 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
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:
- State changes to any object made prior to placement into any
concurrent collection happen before this element is accessed via or
removed from that collection.
- State changes to a Runnable object made prior to
submission to an Executor happen-before its execution. And
similarly for a Callable object submitted to an
ExecutorService.
- State changes to a Future made prior to it becoming
available happen-before access via Future.get().
- Actions prior to "releasing" synchronizer methods such as
Lock.unlock, Semaphore.release,
CountDownLatch.countDown and Condition.signal
happen-before actions subsequent to a successful "acquiring" method such as
Lock.lock, Semaphore.acquire, and
CountDownLatch.await on the same synchronizer object.
- Actions prior to calling Exchanger.exchange
happen-before those subsequent to the matching actions in
other threads.
- Actions prior to calling CyclicBarrier.await
happen-before actions performed by the barrier action, and
actions performed by the barrier action happen-before actions
subsequent to the return from await in other threads.
@since 1.5