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
adds 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.

<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.

@since 1.5

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Revision:	1.18
Committed:	Sat Jan 1 12:50:34 2005 UTC (19 years, 5 months ago) by dl
Content type:	text/html
Branch:	MAIN
Changes since 1.17:	+4 -1 lines
Log Message:	Clarify that timeout of zero means not to wait
#	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			adds support for delayed and periodic task execution.
29			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	dl	1.12	cancel execution.
35
36			<p>
37
38	dl	1.16	<b>Implementations.</b> Classes {@link
39	dl	1.12	java.util.concurrent.ThreadPoolExecutor} and {@link
40	dl	1.16	java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable,
41			flexible thread pools. The {@link java.util.concurrent.Executors}
42			class provides factory methods for the most common kinds and
43			configurations of Executors, as well as a few utility methods for
44			using them. Other utilities based on Executors include the concrete
45			class {@link java.util.concurrent.FutureTask} providing a common
46			extensible implementation of Futures, and {@link
47	dl	1.12	java.util.concurrent.ExecutorCompletionService}, that assists in
48			coordinating the processing of groups of asynchronous tasks.
49	dl	1.1
50			<h2>Queues</h2>
51
52	dl	1.6	The java.util.concurrent {@link
53	dl	1.1	java.util.concurrent.ConcurrentLinkedQueue} class supplies an
54	dl	1.6	efficient scalable thread-safe non-blocking FIFO queue. Five
55			implementations in java.util.concurrent support the extended {@link
56			java.util.concurrent.BlockingQueue} interface, that defines blocking
57			versions of put and take: {@link
58	dl	1.1	java.util.concurrent.LinkedBlockingQueue}, {@link
59			java.util.concurrent.ArrayBlockingQueue}, {@link
60			java.util.concurrent.SynchronousQueue}, {@link
61			java.util.concurrent.PriorityBlockingQueue}, and {@link
62	dl	1.2	java.util.concurrent.DelayQueue}. The different classes cover the most
63			common usage contexts for producer-consumer, messaging, parallel
64	dl	1.17	tasking, and related concurrent designs. The {@link
65			java.util.concurrent.BlockingDeque} interface extends
66			<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based)
67			operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
68			provides an implementation.
69	dl	1.1
70
71			<h2>Timing</h2>
72
73			The {@link java.util.concurrent.TimeUnit} class provides multiple
74	dl	1.2	granularities (including nanoseconds) for specifying and controlling
75	dl	1.9	time-out based operations. Most classes in the package contain
76			operations based on time-outs in addition to indefinite waits. In all
77			cases that time-outs are used, the time-out specifies the minimum time
78			that the method should wait before indicating that it
79			timed-out. Implementations make a "best effort" to detect
80			time-outs as soon as possible after they occur. However, an indefinite
81			amount of time may elapse between a time-out being detected and a
82	dl	1.18	thread actually executing again after that time-out. All methods
83			that accept timeout parameters treat values less than or equal to
84			zero to mean not to wait at all. To wait "forever", you can use
85			a value of <tt>Long.MAX_VALUE</tt>.
86	dholmes	1.4
87	dl	1.1	<h2>Synchronizers</h2>
88
89	dl	1.13	Four classes aid common special-purpose synchronization idioms.
90	dl	1.10	{@link java.util.concurrent.Semaphore} is a classic concurrency tool.
91	dl	1.16	{@link java.util.concurrent.CountDownLatch} is a very simple yet very
92	dl	1.11	common utility for blocking until a given number of signals, events,
93			or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a
94	dl	1.13	resettable multiway synchronization point useful in some styles of
95	dl	1.11	parallel programming. An {@link java.util.concurrent.Exchanger} allows
96	dl	1.13	two threads to exchange objects at a rendezvous point, and is useful
97			in several pipeline designs.
98	dl	1.1
99			<h2>Concurrent Collections</h2>
100
101	dl	1.17	Besides Queues, this package supplies Collection implementations
102			designed for use in multithreaded contexts:
103			{@link java.util.concurrent.ConcurrentHashMap},
104			{@link java.util.concurrent.ConcurrentSkipListMap},
105			{@link java.util.concurrent.ConcurrentSkipListSet},
106			{@link java.util.concurrent.CopyOnWriteArrayList}, and
107			{@link java.util.concurrent.CopyOnWriteArraySet}.
108			When many threads are expected to access a given collection,
109			a <tt>ConcurrentHashMap</tt> is normally preferable to
110			a synchronized <tt>HashMap</tt>, and a
111			<tt>ConcurrentSkipListMap</tt> is normally preferable
112			to a synchronized <tt>TreeMap</tt>. A
113			<tt>CopyOnWriteArrayList</tt> is preferable to
114			a synchronized <tt>ArrayList</tt> when the expected number of reads
115			and traversals greatly outnumber the number of updates to a list.
116	dl	1.1
117	dl	1.6	<p>The "Concurrent" prefix used with some classes in this package is a
118			shorthand indicating several differences from similar "synchronized"
119	dl	1.1	classes. For example <tt>java.util.Hashtable</tt> and
120			<tt>Collections.synchronizedMap(new HashMap())</tt> are
121	dl	1.6	synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is
122			"concurrent". A concurrent collection is thread-safe, but not
123			governed by a single exclusion lock. In the particular case of
124			ConcurrentHashMap, it safely permits any number of concurrent reads as
125	dl	1.9	well as a tunable number of concurrent writes. "Synchronized" classes
126			can be useful when you need to prevent all access to a collection via
127			a single lock, at the expense of poorer scalability. In other cases in
128			which multiple threads are expected to access a common collection,
129			"concurrent" versions are normally preferable. And unsynchronized
130			collections are preferable when either collections are unshared, or
131			are accessible only when holding other locks.
132	dl	1.1
133			<p> Most concurrent Collection implementations (including most Queues)
134			also differ from the usual java.util conventions in that their Iterators
135			provide <em>weakly consistent</em> rather than fast-fail traversal. A
136			weakly consistent iterator is thread-safe, but does not necessarily
137			freeze the collection while iterating, so it may (or may not) reflect
138			any updates since the iterator was created.
139
140	dl	1.15	@since 1.5
141
142	dl	1.1	</body> </html>