util/concurrent/package.html

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<p> 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 <tt>locks</tt> and
<tt>atomic</tt> packages.

<h2>Executors</h2>

<b>Interfaces.</b> {@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 <tt>execute()</tt>, 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 <tt>run</tt> method that upon execution,
sets its results.

<p>

<b>Implementations.</b> 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.

<h2>Queues</h2>

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
<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based)
operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
provides an implementation.


<h2>Timing</h2>

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 &quot;best effort&quot; 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 <tt>Long.MAX_VALUE</tt>.

<h2>Synchronizers</h2>

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.

<h2>Concurrent Collections</h2>

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 <tt>ConcurrentHashMap</tt> is normally preferable to
a synchronized <tt>HashMap</tt>, and a
<tt>ConcurrentSkipListMap</tt> is normally preferable
to a synchronized <tt>TreeMap</tt>. A
<tt>CopyOnWriteArrayList</tt> is preferable to
a synchronized <tt>ArrayList</tt> when the expected number of reads
and traversals greatly outnumber the number of updates to a list.

<p>The "Concurrent" prefix used with some classes in this package is a
shorthand indicating several differences from similar "synchronized"
classes. For example <tt>java.util.Hashtable</tt> and
<tt>Collections.synchronizedMap(new HashMap())</tt> 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.

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

<a name="MemoryVisibility">
<h2> Memory Consistency Properties </h2>

<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">
Chapter 17 of the Java Language Specification</a> defines the
<i>happens-before</i> 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 <i>happens-before</i> the read operation.  The
{@code synchronized} and {@code volatile} constructs, as well as the
{@code Thread.start()} and {@code Thread.join()} methods, can form
<i>happens-before</i> relationships. In particular:

<ul>
  <li>Each action in a thread <i>happens-before</i> every action in that
  thread that comes later in the program's order.

  <li>An unlock ({@code synchronized} block or method exit) of a
  monitor <i>happens-before</i> every subsequent lock ({@code synchronized}
  block or method entry) of that same monitor. And because
  the <i>happens-before</i> relation is transitive, all actions
  of a thread prior to unlocking <i>happen-before</i> all actions
  subsequent to any thread locking that monitor.

  <li>A write to a {@code volatile} field <i>happens-before</i> 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 <em>not</em> entail
  mutual exclusion locking.

  <li>A call to {@code start} on a thread <i>happens-before</i> any action in the
  started thread.

  <li>All actions in a thread <i>happen-before</i> any other thread
  successfully returns from a {@code join} on that thread.

</ul>


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

<ul>

  <li>Actions in a thread prior to placing an object into any concurrent
  collection <i>happen-before</i> actions subsequent to the access or
  removal of that element from the collection in another thread.

  <li>Actions in a thread prior to the submission of a {@code Runnable}
  to an {@code Executor} <i>happen-before</i> its execution begins.
  Similarly for {@code Callables} submitted to an {@code ExecutorService}.

  <li>Actions taken by the asynchronous computation represented by a
  {@code Future} <i>happen-before</i> actions subsequent to the
  retrieval of the result via {@code Future.get()} in another thread.

  <li>Actions prior to "releasing" synchronizer methods such as
  {@code Lock.unlock}, {@code Semaphore.release}, and
  {@code CountDownLatch.countDown} <i>happen-before</i> 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.

  <li>For each pair of threads that successfully exchange objects via
  an {@code Exchanger}, actions prior to the {@code exchange()}
  in each thread <i>happen-before</i> those subsequent to the
  corresponding {@code exchange()} in another thread.

  <li>Actions prior to calling {@code CyclicBarrier.await}
  <i>happen-before</i> actions performed by the barrier action, and
  actions performed by the barrier action <i>happen-before</i> actions
  subsequent to a successful return from the corresponding {@code await}
  in other threads.

</ul>

@since 1.5

</body> </html>
Revision:	1.32
Committed:	Fri Apr 27 00:38:47 2007 UTC (17 years, 1 month ago) by jsr166
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#	User	Rev	Content
1	dl	1.1	<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
2			<html> <head>
3			<title>Concurrency Utilities</title>
4	jsr166	1.32	<!--
5			Written by Doug Lea with assistance from members of JCP JSR-166
6			Expert Group and released to the public domain, as explained at
7			http://creativecommons.org/licenses/publicdomain
8			-->
9	dl	1.1	</head>
10
11			<body>
12
13	dl	1.3	<p> Utility classes commonly useful in concurrent programming. This
14			package includes a few small standardized extensible frameworks, as
15			well as some classes that provide useful functionality and are
16			otherwise tedious or difficult to implement. Here are brief
17			descriptions of the main components. See also the <tt>locks</tt> and
18			<tt>atomic</tt> packages.
19	dl	1.1
20			<h2>Executors</h2>
21
22	dl	1.12	<b>Interfaces.</b> {@link java.util.concurrent.Executor} is a simple
23			standardized interface for defining custom thread-like subsystems,
24			including thread pools, asynchronous IO, and lightweight task
25			frameworks. Depending on which concrete Executor class is being used,
26			tasks may execute in a newly created thread, an existing
27			task-execution thread, or the thread calling <tt>execute()</tt>, and
28			may execute sequentially or concurrently. {@link
29			java.util.concurrent.ExecutorService} provides a more complete
30			asynchronous task execution framework. An ExecutorService manages
31	dl	1.14	queuing and scheduling of tasks, and allows controlled shutdown. The
32	dl	1.12	{@link java.util.concurrent.ScheduledExecutorService} subinterface
33	dl	1.19	and associated interfaces add support for delayed and periodic task execution.
34	dl	1.12	ExecutorServices provide methods arranging asynchronous execution of
35			any function expressed as {@link java.util.concurrent.Callable}, the
36			result-bearing analog of {@link java.lang.Runnable}. A {@link
37			java.util.concurrent.Future} returns the results of a function, allows
38	dl	1.16	determination of whether execution has completed, and provides a means to
39	jsr166	1.20	cancel execution. A {@link java.util.concurrent.RunnableFuture} is
40	dl	1.19	a Future that possesses a <tt>run</tt> method that upon execution,
41			sets its results.
42	dl	1.12
43			<p>
44
45	dl	1.16	<b>Implementations.</b> Classes {@link
46	dl	1.12	java.util.concurrent.ThreadPoolExecutor} and {@link
47	dl	1.16	java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable,
48			flexible thread pools. The {@link java.util.concurrent.Executors}
49			class provides factory methods for the most common kinds and
50			configurations of Executors, as well as a few utility methods for
51			using them. Other utilities based on Executors include the concrete
52			class {@link java.util.concurrent.FutureTask} providing a common
53			extensible implementation of Futures, and {@link
54	dl	1.12	java.util.concurrent.ExecutorCompletionService}, that assists in
55			coordinating the processing of groups of asynchronous tasks.
56	dl	1.1
57			<h2>Queues</h2>
58
59	dl	1.6	The java.util.concurrent {@link
60	dl	1.1	java.util.concurrent.ConcurrentLinkedQueue} class supplies an
61	dl	1.6	efficient scalable thread-safe non-blocking FIFO queue. Five
62			implementations in java.util.concurrent support the extended {@link
63			java.util.concurrent.BlockingQueue} interface, that defines blocking
64			versions of put and take: {@link
65	dl	1.1	java.util.concurrent.LinkedBlockingQueue}, {@link
66			java.util.concurrent.ArrayBlockingQueue}, {@link
67			java.util.concurrent.SynchronousQueue}, {@link
68			java.util.concurrent.PriorityBlockingQueue}, and {@link
69	dl	1.2	java.util.concurrent.DelayQueue}. The different classes cover the most
70			common usage contexts for producer-consumer, messaging, parallel
71	dl	1.17	tasking, and related concurrent designs. The {@link
72			java.util.concurrent.BlockingDeque} interface extends
73			<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based)
74			operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
75			provides an implementation.
76	dl	1.1
77
78			<h2>Timing</h2>
79
80			The {@link java.util.concurrent.TimeUnit} class provides multiple
81	dl	1.2	granularities (including nanoseconds) for specifying and controlling
82	jsr166	1.24	time-out based operations. Most classes in the package contain
83	dl	1.9	operations based on time-outs in addition to indefinite waits. In all
84			cases that time-outs are used, the time-out specifies the minimum time
85			that the method should wait before indicating that it
86	jsr166	1.24	timed-out. Implementations make a "best effort" to detect
87			time-outs as soon as possible after they occur. However, an indefinite
88	dl	1.9	amount of time may elapse between a time-out being detected and a
89	jsr166	1.24	thread actually executing again after that time-out. All methods
90	dl	1.18	that accept timeout parameters treat values less than or equal to
91	jsr166	1.24	zero to mean not to wait at all. To wait "forever", you can use
92	dl	1.18	a value of <tt>Long.MAX_VALUE</tt>.
93	dholmes	1.4
94	dl	1.1	<h2>Synchronizers</h2>
95
96	dl	1.13	Four classes aid common special-purpose synchronization idioms.
97	dl	1.10	{@link java.util.concurrent.Semaphore} is a classic concurrency tool.
98	dl	1.16	{@link java.util.concurrent.CountDownLatch} is a very simple yet very
99	dl	1.11	common utility for blocking until a given number of signals, events,
100			or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a
101	dl	1.13	resettable multiway synchronization point useful in some styles of
102	dl	1.11	parallel programming. An {@link java.util.concurrent.Exchanger} allows
103	dl	1.13	two threads to exchange objects at a rendezvous point, and is useful
104			in several pipeline designs.
105	dl	1.1
106			<h2>Concurrent Collections</h2>
107
108	dl	1.17	Besides Queues, this package supplies Collection implementations
109			designed for use in multithreaded contexts:
110			{@link java.util.concurrent.ConcurrentHashMap},
111			{@link java.util.concurrent.ConcurrentSkipListMap},
112			{@link java.util.concurrent.ConcurrentSkipListSet},
113			{@link java.util.concurrent.CopyOnWriteArrayList}, and
114			{@link java.util.concurrent.CopyOnWriteArraySet}.
115			When many threads are expected to access a given collection,
116			a <tt>ConcurrentHashMap</tt> is normally preferable to
117			a synchronized <tt>HashMap</tt>, and a
118			<tt>ConcurrentSkipListMap</tt> is normally preferable
119			to a synchronized <tt>TreeMap</tt>. A
120			<tt>CopyOnWriteArrayList</tt> is preferable to
121			a synchronized <tt>ArrayList</tt> when the expected number of reads
122			and traversals greatly outnumber the number of updates to a list.
123	dl	1.1
124	dl	1.6	<p>The "Concurrent" prefix used with some classes in this package is a
125			shorthand indicating several differences from similar "synchronized"
126	dl	1.1	classes. For example <tt>java.util.Hashtable</tt> and
127			<tt>Collections.synchronizedMap(new HashMap())</tt> are
128	dl	1.6	synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is
129			"concurrent". A concurrent collection is thread-safe, but not
130			governed by a single exclusion lock. In the particular case of
131			ConcurrentHashMap, it safely permits any number of concurrent reads as
132	dl	1.9	well as a tunable number of concurrent writes. "Synchronized" classes
133			can be useful when you need to prevent all access to a collection via
134			a single lock, at the expense of poorer scalability. In other cases in
135			which multiple threads are expected to access a common collection,
136			"concurrent" versions are normally preferable. And unsynchronized
137			collections are preferable when either collections are unshared, or
138			are accessible only when holding other locks.
139	dl	1.1
140			<p> Most concurrent Collection implementations (including most Queues)
141			also differ from the usual java.util conventions in that their Iterators
142			provide <em>weakly consistent</em> rather than fast-fail traversal. A
143			weakly consistent iterator is thread-safe, but does not necessarily
144			freeze the collection while iterating, so it may (or may not) reflect
145			any updates since the iterator was created.
146
147	brian	1.26	<a name="MemoryVisibility">
148	dl	1.21	<h2> Memory Consistency Properties </h2>
149
150			<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">
151			Chapter 17 of the Java Language Specification</a> defines the
152			<i>happens-before</i> relation on memory operations such as reads and
153			writes of shared variables. The results of a write by one thread are
154			guaranteed to be visible to a read by another thread only if the write
155			operation <i>happens-before</i> the read operation. The
156	jsr166	1.31	{@code synchronized} and {@code volatile} constructs, as well as the
157			{@code Thread.start()} and {@code Thread.join()} methods, can form
158	dl	1.21	<i>happens-before</i> relationships. In particular:
159
160			<ul>
161	brian	1.26	<li>Each action in a thread <i>happens-before</i> every action in that
162	dl	1.21	thread that comes later in the program's order.
163
164	jsr166	1.31	<li>An unlock ({@code synchronized} block or method exit) of a
165			monitor <i>happens-before</i> every subsequent lock ({@code synchronized}
166	dl	1.23	block or method entry) of that same monitor. And because
167			the <i>happens-before</i> relation is transitive, all actions
168	brian	1.27	of a thread prior to unlocking <i>happen-before</i> all actions
169	dl	1.23	subsequent to any thread locking that monitor.
170	dl	1.21
171	jsr166	1.31	<li>A write to a {@code volatile} field <i>happens-before</i> every
172	dl	1.23	subsequent read of that same field. Writes and reads of
173	jsr166	1.31	{@code volatile} fields have similar memory consistency effects
174	dl	1.23	as entering and exiting monitors, but do <em>not</em> entail
175			mutual exclusion locking.
176	dl	1.21
177	jsr166	1.31	<li>A call to {@code start} on a thread <i>happens-before</i> any action in the
178	dl	1.21	started thread.
179
180	brian	1.26	<li>All actions in a thread <i>happen-before</i> any other thread
181	jsr166	1.31	successfully returns from a {@code join} on that thread.
182	dl	1.21
183			</ul>
184
185	dl	1.23
186	jsr166	1.31	The methods of all classes in {@code java.util.concurrent} and its
187	dl	1.21	subpackages extend these guarantees to higher-level
188			synchronization. In particular:
189
190			<ul>
191
192	jsr166	1.31	<li>Actions in a thread prior to placing an object into any concurrent
193			collection <i>happen-before</i> actions subsequent to the access or
194			removal of that element from the collection in another thread.
195
196			<li>Actions in a thread prior to the submission of a {@code Runnable}
197			to an {@code Executor} <i>happen-before</i> its execution begins.
198			Similarly for {@code Callables} submitted to an {@code ExecutorService}.
199
200			<li>Actions taken by the asynchronous computation represented by a
201			{@code Future} <i>happen-before</i> actions subsequent to the
202			retrieval of the result via {@code Future.get()} in another thread.
203	dl	1.21
204			<li>Actions prior to "releasing" synchronizer methods such as
205	jsr166	1.31	{@code Lock.unlock}, {@code Semaphore.release}, and
206			{@code CountDownLatch.countDown} <i>happen-before</i> actions
207			subsequent to a successful "acquiring" method such as
208			{@code Lock.lock}, {@code Semaphore.acquire},
209			{@code Condition.await}, and {@code CountDownLatch.await} on the
210			same synchronizer object in another thread.
211	dl	1.21
212	brian	1.27	<li>For each pair of threads that successfully exchange objects via
213	jsr166	1.31	an {@code Exchanger}, actions prior to the {@code exchange()}
214	brian	1.27	in each thread <i>happen-before</i> those subsequent to the
215	jsr166	1.31	corresponding {@code exchange()} in another thread.
216	brian	1.26
217	jsr166	1.31	<li>Actions prior to calling {@code CyclicBarrier.await}
218	brian	1.26	<i>happen-before</i> actions performed by the barrier action, and
219			actions performed by the barrier action <i>happen-before</i> actions
220	jsr166	1.31	subsequent to a successful return from the corresponding {@code await}
221	brian	1.27	in other threads.
222	dl	1.21
223			</ul>
224
225	dl	1.15	@since 1.5
226
227	dl	1.1	</body> </html>