[ViewVC] Annotation of: jsr166/jsr166/intro.html

Revision:	1.2
Committed:	Fri Oct 25 22:55:44 2002 UTC (21 years, 6 months ago) by dl
Content type:	text/html
Branch:	MAIN
Changes since 1.1:	+11 -0 lines
Log Message:	Added disclaimer to intro.html

File Contents

#	User	Rev	Content
1	jsr166	1.1	<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
2			<html>
3			<head>
4			<title>JSR 166 Snapshot Introduction.</title>
5			</head>
6
7			<body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
8			<h1>JSR 166 Snapshot Introduction.</h1>
9
10			by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>
11			<p>
12
13			To join a mailing list discussing this JSR, go to:
14			<A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .
15
16			<p>
17	dl	1.2	<em>
18			Disclaimer - This prototype is experimental code developed as part of
19			JSR166 and made available to the developer community for use
20			as-is. It is not a supported product. Use it at your own risk. The
21			specification, language and implementation are subject to change as a
22			result of your feedback. Because these features have not yet been
23			approved for addition to the Java language, there is no schedule for
24			their inclusion in a product.
25			</em>
26
27			<p>
28	jsr166	1.1	Package java.util.concurrent contains utility classes that are
29			commonly useful in concurrent programming. Like package java.util, it
30			includes a few small standardized extensible frameworks, as well as
31			some classes that provide useful functionality and are otherwise
32			tedious or difficult to implement. In this JSR, we have been
33			conservative in selecting only those APIs and implementations that are
34			useful enough to encourage nearly all concurrent programmers to use
35			routinely. JSR 166 also includes a few changes and additions in
36			packages outside of java.util.concurrent: java.lang, to address timing
37			and uncaught exceptions, and java.util to better integrate queues, and
38			to make Timers conform to new frameworks. The API covers:
39
40			<ul>
41			<li> Queues
42			<li> Executors
43			<li> Locks
44			<li> Condition variables
45			<li> Atomic variables
46			<li> Timing
47			<li> Barriers
48			<li> Concurrent Collections
49			<li> Uncaught Exception Handlers
50			</ul>
51
52
53			The main rationale for JSR 166 is that threading primitives, such as
54			synchronized blocks, Object.wait and Object.notify, are insufficient
55			for many programming tasks. Currently, developers can use only the
56			concurrency control constructs provided in the Java language
57			itself. These are too low level for some applications, and are
58			incomplete for others. As a result, application programmers are often
59			forced to implement their own concurrency facilities, resulting in
60			enormous duplication of effort creating facilities that are
61			notoriously hard to get right and even harder to optimize. Offering a
62			standard set of concurrency utilities will ease the task of writing a
63			wide variety of multithreaded applications and generally improve the
64			quality of the applications that use them.
65
66			<p>
67			Here are brief descriptions and rationales of the main components.
68			For details see the javadocs at <a
69			href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a>
70
71
72			<h2>Queues</h2>
73
74			A basic (nonblocking) Queue interface that is compatatible with
75			java.util.Collections will be introduced into java.util. Also,
76			although it is at the borders of being in scope of JSR-166,
77			java.util.LinkedList will be adapted to support Queue, and
78			a new non-thread-safe java.util.HeapPriorityQueue will be added.
79
80			<p>
81			Four implementations in java.util.concurrent support the extended
82			BlockingQueue interface, that defines blocking versions of put and
83			take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, and
84			PriorityBlockingQueue. Additionally, java.util.concurrent.LinkedQueue
85			supplies an efficient thread-safe non-blocking queue.
86			<p>
87			Since the target release is JDK1.5, and generics are slated to be in
88			1.5, Queues should be parametrized on element type. (Also some others
89			below.) We are ignoring this for now.
90
91
92			<h2>Executors</h2>
93
94			Executors provide a simple standardized interface for defining custom
95			thread-like subsystems, including thread pools, asynch-IO, and
96			lightweight task frameworks. Executors also standardize ways of
97			calling threads that compute functions returning results, via
98			Futures. This is supported in part by defining java.lang.Callable, the
99			argument/result analog of Runnable.
100
101			<p> While the Executor framework is intended to be extensible (so
102			includes for example, AbstractExecutor that simplifies construction of
103			new implementations), the most commonly used Executor will be
104			ThreadExecutor, which can be configured to act as all sorts of thread
105			pools, background threads, etc. The class is designed to be general
106			enough to suffice for the vast majority of usages, even sophisticated
107			ones, yet also includes methods and functionality that simplify
108			routine usage.
109
110			<p>
111			A few methods will also be added to the java.util.Timer to support
112			Futures, and address other requests for enhancement.
113
114			<h2>Locks</h2>
115
116			The Lock interface supports locking disciplines that differ in
117			semantics (reentrant, semaphore-based, etc), and that can be used in
118			non-block-structured contexts including hand-over-hand and lock
119			reordering algorithms. This flexibility comes at the price of more
120			awkward syntax. Implementations include Semaphore, ReentrantMutex
121			FIFOSemaphore, and CountDownLatch.
122
123			<p>
124			The Locks class additionally supports trylock-designs using builtin
125			locks without needing to use Lock classes. This requires adding new
126			capabilities to builtin locks inside JVMs.
127
128			<p>
129			A ReadWriteLock interface similarly defines locks that may be shared
130			among readers but are exclusive to writers. For this release, only a
131			single implementation, ReentrantReadWriteLock, is planned, since it
132			covers all standard usage contexts. But programmers may create their
133			own implementations to cover nonstandard requirements.
134
135			<h2>Conditions</h2>
136
137			A Condition class provides the kinds of condition variables associated
138			with monitors in other cocurrent languages, as well as pthreads
139			condvars. Their support reduces the need for tricky and/or
140			inefficient solutions to many classic concurrent problems. Conditions
141			also address the annoying problem that Object.wait(msecs) does not
142			return an indication of whether the wait timed out. This leads to
143			error-prone code. Since this method is in class Object, the problem is
144			basically unfixable.
145			<p>
146			To avoid compatibility problems, the names of Condition methods need
147			to be different than Object versions. The downside of this is that
148			people can make the mistake of calling cond.notify instead of
149			cond.signal. However, they will get IllegalMonitorState exceptions if
150			they do, so they can detect the error if they ever run the code.
151			<p>
152			The implementation requires VM magic to atomically suspend and release
153			lock. But it is unlikely to be very challenging for JVM providers,
154			since most layer Java monitors on top of posix condvars or similar
155			low-level functionality anyway.
156
157			<h2>Atomic variables</h2>
158
159			Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and
160			AtomicReference provide simple scalar variables supporting
161			compareAndSwap (CAS) and related atomic operations. These are
162			desparately needed by those performing low-level concurrent system
163			programming, but much less commonly useful in higher-level frameworks.
164
165
166			<h2>Timing</h2>
167
168			Java has always supported sub-millisecond versions of several native
169			time-out-based methods (such as Object.wait), but not methods to
170			actually perform timing in finer-grained units. We address this by
171			introducing java.lang.Clock, which provides multiple granularities for
172			both accessing time and performing time-out based operations.
173
174
175			<h2>Barriers</h2>
176
177			Barriers (multiway synchronization points) are very common in some
178			styles of parallel programming, yet tricky to get right. The two most
179			useful flavors (CyclicBarriers and Exchangers) don't have much of an
180			interface in common, and only have one standard implementation each,
181			so these are simply defined as public classes rather than interfaces
182			and implementations.
183
184
185			<h2>Concurrent Collections</h2>
186
187			There are no new interfaces, but JSR 166 will supply a few Collection
188			implementations designed for use in multithreaded contexts:
189			ConcurrentHashTable, CopyOnWriteArrayList, and CopyOnWriteArraySet.
190
191			<h2>Uncaught Exception Handlers</h2>
192
193			The java.lang.Thread class will be modified to allow per-thread
194			installation of handlers for uncaught exceptions. Ths optionally
195			disassociates these handlers from ThreadGroups, which has proven to be
196			too inflexible in many multithreaded programs. (Note that the combination
197			of features in JSR 166 make ThreadGroups even less likely to
198			be used in most programs. Perhaps they will eventually be deprecated.)
199			<p>
200			Additionally, Threads and ThreadLocals will now support a means to
201			clear and remove ThreadLocals, which is needed in some thread-pool and
202			worker-thread designs.
203
204			<hr>
205			<address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
206			</body>
207			</html>