util/concurrent/package.html

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<html> <head>
<title>Concurrency Utilities</title>
</head>

<body>

<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
<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 <i>happens-before</i> 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 <i>happens-before</i> 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 <i>happen-before</i> all actions
  subsequent to any thread locking that monitor.

  <li>A write to a <tt>volatile</tt> field <i>happens-before</i> 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 <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 <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 <i>happen-before</i> this element is accessed via or
  removed from that collection.

  <li>State changes to a <tt>Runnable</tt> object made by the
  submitting thread prior to submission to an <tt>Executor</tt>
  <i>happen-before</i> its execution.  Similarly, state changes to a
  <tt>Callable</tt> object made by the submitting thread before being
  submitted to an <tt>ExecutorService</tt> <i>happen-before</i> its
  execution.

  <li>State changes to a <tt>Future</tt> that
  <i>happen-before</i> it is made available
  <i>happen-before</i> access via <tt>Future.get()</tt>.

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

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

  <li>Actions prior to calling <tt>CyclicBarrier.await</tt>
  <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 <tt>await</tt>
  in other threads.

</ul>

@since 1.5

</body> </html>
Revision:	1.30
Committed:	Sat Sep 10 22:17:52 2005 UTC (18 years, 8 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	brian	1.26	<a name="MemoryVisibility">
143	dl	1.21	<h2> Memory Consistency Properties </h2>
144
145			<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">
146			Chapter 17 of the Java Language Specification</a> defines the
147			<i>happens-before</i> relation on memory operations such as reads and
148			writes of shared variables. The results of a write by one thread are
149			guaranteed to be visible to a read by another thread only if the write
150			operation <i>happens-before</i> the read operation. The
151			<tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the
152			<tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form
153			<i>happens-before</i> relationships. In particular:
154
155			<ul>
156	brian	1.26	<li>Each action in a thread <i>happens-before</i> every action in that
157	dl	1.21	thread that comes later in the program's order.
158
159	jsr166	1.24	<li>An unlock (<tt>synchronized</tt> block or method exit) of a
160	brian	1.26	monitor <i>happens-before</i> every subsequent lock (<tt>synchronized</tt>
161	dl	1.23	block or method entry) of that same monitor. And because
162			the <i>happens-before</i> relation is transitive, all actions
163	brian	1.27	of a thread prior to unlocking <i>happen-before</i> all actions
164	dl	1.23	subsequent to any thread locking that monitor.
165	dl	1.21
166	brian	1.26	<li>A write to a <tt>volatile</tt> field <i>happens-before</i> every
167	dl	1.23	subsequent read of that same field. Writes and reads of
168			<tt>volatile</tt> fields have similar memory consistency effects
169			as entering and exiting monitors, but do <em>not</em> entail
170			mutual exclusion locking.
171	dl	1.21
172	brian	1.26	<li>A call to <tt>start</tt> on a thread <i>happens-before</i> any action in the
173	dl	1.21	started thread.
174
175	brian	1.26	<li>All actions in a thread <i>happen-before</i> any other thread
176	dl	1.21	successfully returns from a <tt>join</tt> on that thread.
177
178			</ul>
179
180	dl	1.23
181	dl	1.21	The methods of all classes in <tt>java.util.concurrent</tt> and its
182			subpackages extend these guarantees to higher-level
183			synchronization. In particular:
184
185			<ul>
186
187			<li>State changes to any object made prior to placement into any
188	brian	1.27	concurrent collection <i>happen-before</i> this element is accessed via or
189	dl	1.21	removed from that collection.
190
191	brian	1.27	<li>State changes to a <tt>Runnable</tt> object made by the
192			submitting thread prior to submission to an <tt>Executor</tt>
193			<i>happen-before</i> its execution. Similarly, state changes to a
194			<tt>Callable</tt> object made by the submitting thread before being
195			submitted to an <tt>ExecutorService</tt> <i>happen-before</i> its
196			execution.
197	dl	1.21
198	dl	1.28	<li>State changes to a <tt>Future</tt> that
199			<i>happen-before</i> it is made available
200			<i>happen-before</i> access via <tt>Future.get()</tt>.
201	dl	1.21
202			<li>Actions prior to "releasing" synchronizer methods such as
203	dl	1.28	<tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>, and
204			<tt>CountDownLatch.countDown</tt>
205	brian	1.26	<i>happen-before</i> actions subsequent to a successful "acquiring" method such as
206	brian	1.27	<tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, <tt>Condition.await</tt>, and
207	jsr166	1.29	<tt>CountDownLatch.await</tt> on the same synchronizer object.
208	dl	1.21
209	brian	1.27	<li>For each pair of threads that successfully exchange objects via
210	jsr166	1.30	an <tt>Exchanger</tt>, actions prior to the <tt>exchange()</tt>
211	brian	1.27	in each thread <i>happen-before</i> those subsequent to the
212			corresponding <tt>exchange()</tt> in the other thread.
213	brian	1.26
214			<li>Actions prior to calling <tt>CyclicBarrier.await</tt>
215			<i>happen-before</i> actions performed by the barrier action, and
216			actions performed by the barrier action <i>happen-before</i> actions
217	jsr166	1.29	subsequent to a successful return from the corresponding <tt>await</tt>
218	brian	1.27	in other threads.
219	dl	1.21
220			</ul>
221
222	dl	1.15	@since 1.5
223
224	dl	1.1	</body> </html>