JavaTM 2 Platform Std. Ed. v1.6.0
Utility classes commonly useful in concurrent programming.
||Provides default implementations of
||A bounded blocking queue backed by an
||A hash table supporting full concurrency of retrievals and
adjustable expected concurrency for updates.
||An unbounded thread-safe queue based on linked nodes.
||A scalable concurrent
||A scalable concurrent
NavigableSet implementation based on
||A thread-safe variant of
ArrayList in which all mutative
operations (add, set, and so on) are implemented by
making a fresh copy of the underlying array.
Set that uses an internal
for all of its operations.
||A synchronization aid that allows one or more threads to wait until
a set of operations being performed in other threads completes.
||A synchronization aid that allows a set of threads to all wait for
each other to reach a common barrier point.
|DelayQueue<E extends Delayed>
||An unbounded blocking queue of
Delayed elements, in which an element can only be taken
when its delay has expired.
||A synchronization point at which threads can pair and swap elements
CompletionService that uses a supplied
to execute tasks.
||Factory and utility methods for
Callable classes defined in this
||A cancellable asynchronous computation.
||An optionally-bounded blocking deque based on
||An optionally-bounded blocking queue based on
||An unbounded blocking queue that uses
the same ordering rules as class
PriorityQueue and supplies
blocking retrieval operations.
ThreadPoolExecutor that can additionally schedule
commands to run after a given delay, or to execute
||A counting semaphore.
||A blocking queue in which each insert
operation must wait for a corresponding remove operation by another
thread, and vice versa.
ExecutorService that executes each submitted task using
one of possibly several pooled threads, normally configured
Executors factory methods.
||A handler for rejected tasks that throws a
||A handler for rejected tasks that runs the rejected task
directly in the calling thread of the execute method,
unless the executor has been shut down, in which case the task
||A handler for rejected tasks that discards the oldest unhandled
request and then retries execute, unless the executor
is shut down, in which case the task is discarded.
||A handler for rejected tasks that silently discards the
||A TimeUnit represents time durations at a given unit of
granularity and provides utility methods to convert across units,
and to perform timing and delay operations in these units.
||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
||Exception indicating that the result of a value-producing task,
such as a
FutureTask, cannot be retrieved because the task
||Exception thrown when attempting to retrieve the result of a task
that aborted by throwing an exception.
||Exception thrown by an
Executor when a task cannot be
accepted for execution.
||Exception thrown when a blocking operation times out.
Package java.util.concurrent Description
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
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.
ExecutorService provides a more complete
asynchronous task execution framework. An ExecutorService manages
queuing and scheduling of tasks, and allows controlled shutdown. The
and associated interfaces add support for delayed and periodic task execution.
ExecutorServices provide methods arranging asynchronous execution of
any function expressed as
result-bearing analog of
Future returns the results of a function, allows
determination of whether execution has completed, and provides a means to
cancel execution. A
a Future that possesses a run method that upon execution,
sets its results.
ScheduledThreadPoolExecutor provide tunable,
flexible thread pools. The
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
FutureTask providing a common
extensible implementation of Futures, and
ExecutorCompletionService, that assists in
coordinating the processing of groups of asynchronous tasks.
ConcurrentLinkedQueue class supplies an
efficient scalable thread-safe non-blocking FIFO queue. Five
implementations in java.util.concurrent support the extended
BlockingQueue interface, that defines blocking
versions of put and take:
DelayQueue. The different classes cover the most
common usage contexts for producer-consumer, messaging, parallel
tasking, and related concurrent designs. The
BlockingDeque interface extends
BlockingQueue to support both FIFO and LIFO (stack-based)
provides an implementation.
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.
Four 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. An
two threads to exchange objects at a rendezvous point, and is useful
in several pipeline designs.
Besides Queues, this package supplies Collection implementations
designed for use in multithreaded contexts:
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
"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
volatile constructs, as well as the
Thread.join() methods, can form
happens-before relationships. In particular:
The methods of all classes in
- 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 (
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
- All actions in a thread happen-before any other thread
successfully returns from a
join on that thread.
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
Executor happen-before its execution begins.
Callables submitted to an
- 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
CountDownLatch.countDown happen-before actions
subsequent to a successful "acquiring" method such as
CountDownLatch.await on the
same synchronizer object in another thread.
- For each pair of threads that successfully exchange objects via
Exchanger, actions prior to the
in each thread happen-before those subsequent to the
exchange() in another thread.
- Actions prior to calling
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
in other threads.
Copyright 2003 Sun Microsystems, Inc. All rights reserved