util/concurrent/package.html

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<title>Concurrency Utilities</title>
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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.

<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
<tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the
<tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form
<i>happens-before</i> relationships. In particular:

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

  <li>An unlock (<tt>synchronized</tt> block or method exit) of a
  monitor happens before every subsequent lock (<tt>synchronized</tt>
  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 happen before all actions
  subsequent to any thread locking that monitor.

  <li>A write to a <tt>volatile</tt> field happens before every
  subsequent read of that same field. Writes and reads of
  <tt>volatile</tt> fields have similar memory consistency effects
  as entering and exiting monitors, but do <em>not</em> entail
  mutual exclusion locking.

  <li>A call to <tt>start</tt> on a thread happens before any action in the
  started thread.

  <li>All actions in a thread happen before any other thread
  successfully returns from a <tt>join</tt> on that thread.

</ul>


The methods of all classes in <tt>java.util.concurrent</tt> and its
subpackages extend these guarantees to higher-level
synchronization. In particular:

<ul>

  <li>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.

  <li>State changes to a <tt>Runnable</tt> object made prior to
  submission to an <tt>Executor</tt> happen before its execution. And
  similarly for a <tt>Callable</tt> object submitted to an
  <tt>ExecutorService</tt>.

  <li>State changes to a <tt>Future</tt> made prior to it becoming
  available happen before access via <tt>Future.get()</tt>.

  <li>Actions prior to "releasing" synchronizer methods such as
  <tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>,
  <tt>CountDownLatch.countDown<tt> and <tt>Condition.signal</tt>
  happen before actions subsequent to "acquiring" methods such as
  <tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, and
  <tt>CountDownLatch.await</tt> on the same synchronizer object.

  <li>Actions prior to symmetric synchronizer methods such as
  <tt>CyclicBarrier.await</tt> and <tt>Exchanger.exchange</tt> happen
  before those subsequent to the matching actions in other threads.

</ul>

@since 1.5

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Revision:	1.24
Committed:	Mon Aug 15 20:35:44 2005 UTC (18 years, 9 months 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			</head>
5
6			<body>
7
8	dl	1.3	<p> Utility classes commonly useful in concurrent programming. This
9			package includes a few small standardized extensible frameworks, as
10			well as some classes that provide useful functionality and are
11			otherwise tedious or difficult to implement. Here are brief
12			descriptions of the main components. See also the <tt>locks</tt> and
13			<tt>atomic</tt> packages.
14	dl	1.1
15			<h2>Executors</h2>
16
17	dl	1.12	<b>Interfaces.</b> {@link java.util.concurrent.Executor} is a simple
18			standardized interface for defining custom thread-like subsystems,
19			including thread pools, asynchronous IO, and lightweight task
20			frameworks. Depending on which concrete Executor class is being used,
21			tasks may execute in a newly created thread, an existing
22			task-execution thread, or the thread calling <tt>execute()</tt>, and
23			may execute sequentially or concurrently. {@link
24			java.util.concurrent.ExecutorService} provides a more complete
25			asynchronous task execution framework. An ExecutorService manages
26	dl	1.14	queuing and scheduling of tasks, and allows controlled shutdown. The
27	dl	1.12	{@link java.util.concurrent.ScheduledExecutorService} subinterface
28	dl	1.19	and associated interfaces add support for delayed and periodic task execution.
29	dl	1.12	ExecutorServices provide methods arranging asynchronous execution of
30			any function expressed as {@link java.util.concurrent.Callable}, the
31			result-bearing analog of {@link java.lang.Runnable}. A {@link
32			java.util.concurrent.Future} returns the results of a function, allows
33	dl	1.16	determination of whether execution has completed, and provides a means to
34	jsr166	1.20	cancel execution. A {@link java.util.concurrent.RunnableFuture} is
35	dl	1.19	a Future that possesses a <tt>run</tt> method that upon execution,
36			sets its results.
37	dl	1.12
38			<p>
39
40	dl	1.16	<b>Implementations.</b> Classes {@link
41	dl	1.12	java.util.concurrent.ThreadPoolExecutor} and {@link
42	dl	1.16	java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable,
43			flexible thread pools. The {@link java.util.concurrent.Executors}
44			class provides factory methods for the most common kinds and
45			configurations of Executors, as well as a few utility methods for
46			using them. Other utilities based on Executors include the concrete
47			class {@link java.util.concurrent.FutureTask} providing a common
48			extensible implementation of Futures, and {@link
49	dl	1.12	java.util.concurrent.ExecutorCompletionService}, that assists in
50			coordinating the processing of groups of asynchronous tasks.
51	dl	1.1
52			<h2>Queues</h2>
53
54	dl	1.6	The java.util.concurrent {@link
55	dl	1.1	java.util.concurrent.ConcurrentLinkedQueue} class supplies an
56	dl	1.6	efficient scalable thread-safe non-blocking FIFO queue. Five
57			implementations in java.util.concurrent support the extended {@link
58			java.util.concurrent.BlockingQueue} interface, that defines blocking
59			versions of put and take: {@link
60	dl	1.1	java.util.concurrent.LinkedBlockingQueue}, {@link
61			java.util.concurrent.ArrayBlockingQueue}, {@link
62			java.util.concurrent.SynchronousQueue}, {@link
63			java.util.concurrent.PriorityBlockingQueue}, and {@link
64	dl	1.2	java.util.concurrent.DelayQueue}. The different classes cover the most
65			common usage contexts for producer-consumer, messaging, parallel
66	dl	1.17	tasking, and related concurrent designs. The {@link
67			java.util.concurrent.BlockingDeque} interface extends
68			<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based)
69			operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
70			provides an implementation.
71	dl	1.1
72
73			<h2>Timing</h2>
74
75			The {@link java.util.concurrent.TimeUnit} class provides multiple
76	dl	1.2	granularities (including nanoseconds) for specifying and controlling
77	jsr166	1.24	time-out based operations. Most classes in the package contain
78	dl	1.9	operations based on time-outs in addition to indefinite waits. In all
79			cases that time-outs are used, the time-out specifies the minimum time
80			that the method should wait before indicating that it
81	jsr166	1.24	timed-out. Implementations make a "best effort" to detect
82			time-outs as soon as possible after they occur. However, an indefinite
83	dl	1.9	amount of time may elapse between a time-out being detected and a
84	jsr166	1.24	thread actually executing again after that time-out. All methods
85	dl	1.18	that accept timeout parameters treat values less than or equal to
86	jsr166	1.24	zero to mean not to wait at all. To wait "forever", you can use
87	dl	1.18	a value of <tt>Long.MAX_VALUE</tt>.
88	dholmes	1.4
89	dl	1.1	<h2>Synchronizers</h2>
90
91	dl	1.13	Four classes aid common special-purpose synchronization idioms.
92	dl	1.10	{@link java.util.concurrent.Semaphore} is a classic concurrency tool.
93	dl	1.16	{@link java.util.concurrent.CountDownLatch} is a very simple yet very
94	dl	1.11	common utility for blocking until a given number of signals, events,
95			or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a
96	dl	1.13	resettable multiway synchronization point useful in some styles of
97	dl	1.11	parallel programming. An {@link java.util.concurrent.Exchanger} allows
98	dl	1.13	two threads to exchange objects at a rendezvous point, and is useful
99			in several pipeline designs.
100	dl	1.1
101			<h2>Concurrent Collections</h2>
102
103	dl	1.17	Besides Queues, this package supplies Collection implementations
104			designed for use in multithreaded contexts:
105			{@link java.util.concurrent.ConcurrentHashMap},
106			{@link java.util.concurrent.ConcurrentSkipListMap},
107			{@link java.util.concurrent.ConcurrentSkipListSet},
108			{@link java.util.concurrent.CopyOnWriteArrayList}, and
109			{@link java.util.concurrent.CopyOnWriteArraySet}.
110			When many threads are expected to access a given collection,
111			a <tt>ConcurrentHashMap</tt> is normally preferable to
112			a synchronized <tt>HashMap</tt>, and a
113			<tt>ConcurrentSkipListMap</tt> is normally preferable
114			to a synchronized <tt>TreeMap</tt>. A
115			<tt>CopyOnWriteArrayList</tt> is preferable to
116			a synchronized <tt>ArrayList</tt> when the expected number of reads
117			and traversals greatly outnumber the number of updates to a list.
118	dl	1.1
119	dl	1.6	<p>The "Concurrent" prefix used with some classes in this package is a
120			shorthand indicating several differences from similar "synchronized"
121	dl	1.1	classes. For example <tt>java.util.Hashtable</tt> and
122			<tt>Collections.synchronizedMap(new HashMap())</tt> are
123	dl	1.6	synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is
124			"concurrent". A concurrent collection is thread-safe, but not
125			governed by a single exclusion lock. In the particular case of
126			ConcurrentHashMap, it safely permits any number of concurrent reads as
127	dl	1.9	well as a tunable number of concurrent writes. "Synchronized" classes
128			can be useful when you need to prevent all access to a collection via
129			a single lock, at the expense of poorer scalability. In other cases in
130			which multiple threads are expected to access a common collection,
131			"concurrent" versions are normally preferable. And unsynchronized
132			collections are preferable when either collections are unshared, or
133			are accessible only when holding other locks.
134	dl	1.1
135			<p> Most concurrent Collection implementations (including most Queues)
136			also differ from the usual java.util conventions in that their Iterators
137			provide <em>weakly consistent</em> rather than fast-fail traversal. A
138			weakly consistent iterator is thread-safe, but does not necessarily
139			freeze the collection while iterating, so it may (or may not) reflect
140			any updates since the iterator was created.
141
142	dl	1.21	<h2> Memory Consistency Properties </h2>
143
144			<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">
145			Chapter 17 of the Java Language Specification</a> defines the
146			<i>happens-before</i> relation on memory operations such as reads and
147			writes of shared variables. The results of a write by one thread are
148			guaranteed to be visible to a read by another thread only if the write
149			operation <i>happens-before</i> the read operation. The
150			<tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the
151			<tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form
152			<i>happens-before</i> relationships. In particular:
153
154			<ul>
155			<li>Each action in a thread happens before every action in that
156			thread that comes later in the program's order.
157
158	jsr166	1.24	<li>An unlock (<tt>synchronized</tt> block or method exit) of a
159	dl	1.21	monitor happens before every subsequent lock (<tt>synchronized</tt>
160	dl	1.23	block or method entry) of that same monitor. And because
161			the <i>happens-before</i> relation is transitive, all actions
162			of a thread prior to unlocking happen before all actions
163			subsequent to any thread locking that monitor.
164	dl	1.21
165			<li>A write to a <tt>volatile</tt> field happens before every
166	dl	1.23	subsequent read of that same field. Writes and reads of
167			<tt>volatile</tt> fields have similar memory consistency effects
168			as entering and exiting monitors, but do <em>not</em> entail
169			mutual exclusion locking.
170	dl	1.21
171	jsr166	1.22	<li>A call to <tt>start</tt> on a thread happens before any action in the
172	dl	1.21	started thread.
173
174			<li>All actions in a thread happen before any other thread
175			successfully returns from a <tt>join</tt> on that thread.
176
177			</ul>
178
179	dl	1.23
180	dl	1.21	The methods of all classes in <tt>java.util.concurrent</tt> and its
181			subpackages extend these guarantees to higher-level
182			synchronization. In particular:
183
184			<ul>
185
186			<li>State changes to any object made prior to placement into any
187			concurrent collection happen before this element is accessed via or
188			removed from that collection.
189
190			<li>State changes to a <tt>Runnable</tt> object made prior to
191			submission to an <tt>Executor</tt> happen before its execution. And
192			similarly for a <tt>Callable</tt> object submitted to an
193			<tt>ExecutorService</tt>.
194
195			<li>State changes to a <tt>Future</tt> made prior to it becoming
196			available happen before access via <tt>Future.get()</tt>.
197
198			<li>Actions prior to "releasing" synchronizer methods such as
199			<tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>,
200			<tt>CountDownLatch.countDown<tt> and <tt>Condition.signal</tt>
201			happen before actions subsequent to "acquiring" methods such as
202			<tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, and
203			<tt>CountDownLatch.await</tt> on the same synchronizer object.
204
205			<li>Actions prior to symmetric synchronizer methods such as
206			<tt>CyclicBarrier.await</tt> and <tt>Exchanger.exchange</tt> happen
207			before those subsequent to the matching actions in other threads.
208
209			</ul>
210
211	dl	1.15	@since 1.5
212
213	dl	1.1	</body> </html>