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# Content
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 the most
102 commonly used Executor will be ThreadExecutor, which can be configured
103 to act as all sorts of thread pools, background threads, etc. The
104 class is designed to be general enough to suffice for the vast
105 majority of usages, even sophisticated ones, yet also includes methods
106 and functionality that simplify routine usage.
107
108 <p>
109 A few methods will also be added to the java.util.Timer to support
110 Futures, and address other requests for enhancement.
111
112 <h2>Locks</h2>
113
114 The Lock interface supports locking disciplines that differ in
115 semantics (reentrant, semaphore-based, etc), and that can be used in
116 non-block-structured contexts including hand-over-hand and lock
117 reordering algorithms. This flexibility comes at the price of more
118 awkward syntax. Implementations include Semaphore, ReentrantMutex
119 FIFOSemaphore, and CountDownLatch.
120
121 <p>
122 The Locks class additionally supports trylock-designs using builtin
123 locks without needing to use Lock classes. This requires adding new
124 capabilities to builtin locks inside JVMs.
125
126 <p>
127 A ReadWriteLock interface similarly defines locks that may be shared
128 among readers but are exclusive to writers. For this release, only a
129 single implementation, ReentrantReadWriteLock, is planned, since it
130 covers all standard usage contexts. But programmers may create their
131 own implementations to cover nonstandard requirements.
132
133 <h2>Conditions</h2>
134
135 A Condition class provides the kinds of condition variables associated
136 with monitors in other cocurrent languages, as well as pthreads
137 condvars. Their support reduces the need for tricky and/or
138 inefficient solutions to many classic concurrent problems. Conditions
139 also address the annoying problem that Object.wait(msecs) does not
140 return an indication of whether the wait timed out. This leads to
141 error-prone code. Since this method is in class Object, the problem is
142 basically unfixable.
143 <p>
144 To avoid compatibility problems, the names of Condition methods need
145 to be different than Object versions. The downside of this is that
146 people can make the mistake of calling cond.notify instead of
147 cond.signal. However, they will get IllegalMonitorState exceptions if
148 they do, so they can detect the error if they ever run the code.
149 <p>
150 The implementation requires VM magic to atomically suspend and release
151 lock. But it is unlikely to be very challenging for JVM providers,
152 since most layer Java monitors on top of posix condvars or similar
153 low-level functionality anyway.
154
155 <h2>Atomic variables</h2>
156
157 Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and
158 AtomicReference provide simple scalar variables supporting
159 compareAndSwap (CAS) and related atomic operations. These are
160 desparately needed by those performing low-level concurrent system
161 programming, but much less commonly useful in higher-level frameworks.
162
163
164 <h2>Timing</h2>
165
166 Java has always supported sub-millisecond versions of several native
167 time-out-based methods (such as Object.wait), but not methods to
168 actually perform timing in finer-grained units. We address this by
169 introducing java.lang.Clock, which provides multiple granularities for
170 both accessing time and performing time-out based operations.
171
172
173 <h2>Barriers</h2>
174
175 Barriers (multiway synchronization points) are very common in some
176 styles of parallel programming, yet tricky to get right. The two most
177 useful flavors (CyclicBarriers and Exchangers) don't have much of an
178 interface in common, and only have one standard implementation each,
179 so these are simply defined as public classes rather than interfaces
180 and implementations.
181
182
183 <h2>Concurrent Collections</h2>
184
185 There are no new interfaces, but JSR 166 will supply a few Collection
186 implementations designed for use in multithreaded contexts:
187 ConcurrentHashTable, CopyOnWriteArrayList, and CopyOnWriteArraySet.
188
189 <h2>Uncaught Exception Handlers</h2>
190
191 The java.lang.Thread class will be modified to allow per-thread
192 installation of handlers for uncaught exceptions. Ths optionally
193 disassociates these handlers from ThreadGroups, which has proven to be
194 too inflexible in many multithreaded programs. (Note that the combination
195 of features in JSR 166 make ThreadGroups even less likely to
196 be used in most programs. Perhaps they will eventually be deprecated.)
197 <p>
198 Additionally, Threads and ThreadLocals will now support a means to
199 clear and remove ThreadLocals, which is needed in some thread-pool and
200 worker-thread designs.
201
202 <hr>
203 <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
204 </body>
205 </html>