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2 <html>
3 <head>
4 <title>JSR 166 Introduction.</title>
5 </head>
7 <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
8 <h1>JSR 166 Introduction.</h1>
10 by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>
11 <p>
13 This is an updated version of the specification submitted for JCP
14 Community Draft review. To check for further updates, access a
15 preliminary prototype release of main functionality, or join a mailing
16 list discussing this JSR, go to: <A
17 HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest">
18 http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A>
19 . <p>
21 <em> <b>Disclaimer</b>. The prototype implementation is experimental
22 code developed as part of JCP JSR-166 is made available to the
23 developer community for use as-is. It is not a supported product. Use
24 it at your own risk. The specification, language and implementation
25 are subject to change as a result of your feedback. Because these
26 features have not yet been approved for addition to the Java language,
27 there is no schedule for their inclusion in a product. </em>
29 <p> <em> <b>Disclaimer</b>. This draft specification was produced
30 using JDK1.4 tools plus some preprocessing. The resulting javadocs do
31 not yet correctly render other planned JDK1.5 constructs on which
32 JSR-166 relies, most notably the use of generic types. We are
33 releasing this version now (before the availability of JDK1.5-based
34 tools) because, even though they are misformatted and sometimes lack
35 proper cross-referencing, they otherwise convey the intended
36 specifications. </em>
38 <p> JSR-166 introduces package <tt>java.util.concurrent</tt>
39 containing utility classes commonly useful in concurrent
40 programming. Like package <tt>java.util</tt>, it includes a few small
41 standardized extensible frameworks, as well as some classes that
42 provide useful functionality and are otherwise tedious or difficult to
43 implement.
45 <p>JSR-166 focusses on breadth, providing critical functionality
46 useful across a wide range of concurrent programming styles and
47 applications, ranging from low-level atomic operations, to
48 customizable locks and synchronization aids, to various concurrent
49 data structures, to high-level execution agents including thread
50 pools. This diversity reflects the range of contexts in which
51 developers of concurrent programs have been found to require or desire
52 support not previously available in J2SE, which also keeping the
53 resulting package small; providing only that minimial support for
54 which it makes sense to standardize.
56 <p>Descriptions and brief motivations for the main components may be
57 found in the associated package documentation. JSR-166 also includes
58 a few changes and additions in packages outside of
59 java.util.concurrent. Here are brief descriptions.
61 <h2>Queues</h2>
63 A basic (nonblocking) {@link java.util.Queue} interface extending
64 {@link java.util.Collection} is introduced into
65 <tt>java.util</tt>. Existing class {@link java.util.LinkedList} is
66 adapted to support Queue, and a new non-thread-safe {@link
67 java.util.PriorityQueue} is added.
69 <h2>Threads</h2>
71 Three minor changes are introduced to the {@link java.lang.Thread}
72 class:
73 <ul>
74 <li> It now allows per-thread installation of handlers for uncaught
75 exceptions. Ths optionally disassociates handlers from ThreadGroups,
76 which has proven to be too inflexible. (Note that the combination of
77 features in JSR-166 make ThreadGroups even less likely to be used in
78 most programs. Perhaps they will eventually be deprecated.)
80 <li> Access checks are no longer required when a Thread interrupts
81 <em>itself</em>. The <tt>interrupt</tt> method is the only way to
82 re-assert a thread's interruption status (and in the case of
83 self-interruption has no other effect than this). The check here
84 previously caused unjustifiable and uncontrollable failures when
85 restricted code invoked library code that must reassert interruption
86 to correctly propagate status when encountering some
87 <tt>InterruptedExceptions</tt>.
88 <li> The <tt>destroy</tt> method, which has never been implemented,
89 has finally been deprecated. This is just a spec change, reflecting
90 the fact that that the reason it has never been implmented is that
91 it was undesirable and unworkable.
92 </ul>
94 <h2>Timing</h2>
96 Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It
97 provides a high-precision timing facility that is distinct from and
98 uncoordinated with <tt>System.currentTimeMillis</tt>.
100 <h2>Removing ThreadLocals</h2>
102 The {@link java.lang.ThreadLocal} class now supports a means to remove
103 a ThreadLocal, which is needed in some thread-pool and worker-thread
104 designs.
108 <hr>
109 <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
110 </body>
111 </html>

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