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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 <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 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>

dl@cs.oswego.edu
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