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<title>JSR 166 Introduction.</title> |
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<body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000"> |
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<h1>JSR 166 Introduction.</h1> |
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by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a> |
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<p> |
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This is an updated version of the specification submitted for JCP |
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Community Draft review. To check for further updates, access a |
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preliminary prototype release of main functionality, or join a mailing |
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list discussing this JSR, go to: <A |
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HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> |
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http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> |
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. <p> |
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<em> <b>Disclaimer</b>. The prototype implementation is experimental |
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code developed as part of JCP JSR-166 is made available to the |
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developer community for use as-is. It is not a supported product. Use |
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it at your own risk. The specification, language and implementation |
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are subject to change as a result of your feedback. Because these |
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features have not yet been approved for addition to the Java language, |
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there is no schedule for their inclusion in a product. </em> |
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<p> <em> <b>Disclaimer</b>. This draft specification was produced |
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using JDK1.4 tools plus some preprocessing. The resulting javadocs do |
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not yet correctly render other planned JDK1.5 constructs on which |
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JSR-166 relies, most notably the use of generic types. We are |
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releasing this version now (before the availability of JDK1.5-based |
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tools) because, even though they are misformatted and sometimes lack |
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proper cross-referencing, they otherwise convey the intended |
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specifications. </em> |
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<p> JSR-166 introduces package <tt>java.util.concurrent</tt> |
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containing utility classes commonly useful in concurrent |
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programming. Like package <tt>java.util</tt>, it includes a few small |
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standardized extensible frameworks, as well as some classes that |
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provide useful functionality and are otherwise tedious or difficult to |
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implement. |
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<p>JSR-166 focusses on breadth, providing critical functionality |
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useful across a wide range of concurrent programming styles and |
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applications, ranging from low-level atomic operations, to |
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customizable locks and synchronization aids, to various concurrent |
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data structures, to high-level execution agents including thread |
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pools. This diversity reflects the range of contexts in which |
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developers of concurrent programs have been found to require or desire |
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support not previously available in J2SE, which also keeping the |
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resulting package small; providing only that minimial support for |
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which it makes sense to standardize. |
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<p>Descriptions and brief motivations for the main components may be |
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found in the associated package documentation. JSR-166 also includes |
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a few changes and additions in packages outside of |
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java.util.concurrent. Here are brief descriptions. |
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<h2>Queues</h2> |
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A basic (nonblocking) {@link java.util.Queue} interface extending |
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{@link java.util.Collection} is introduced into |
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<tt>java.util</tt>. Existing class {@link java.util.LinkedList} is |
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adapted to support Queue, and a new non-thread-safe {@link |
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java.util.PriorityQueue} is added. |
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<h2>Threads</h2> |
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Two minor changes are introduced to the {@link java.lang.Thread} |
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class: It now allows per-thread installation of handlers for uncaught |
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exceptions. Ths optionally disassociates handlers from ThreadGroups, |
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which has proven to be too inflexible. (Note that the combination of |
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features in JSR-166 make ThreadGroups even less likely to be used in |
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most programs. Perhaps they will eventually be deprecated.) Secondly, |
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access checks are no longer required when a Thread interrupts |
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<em>itself</em>. The <tt>interrupt</tt> method is the only way to |
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re-assert a thread's interruption status (and in the case of |
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self-interruption has no other effect than this). The check here |
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previously caused unjustifiable and uncontrollable failures when |
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restricted code invoked library code that must reassert interruption |
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to correctly propagate status when encountering some |
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<tt>InterruptedExceptions</tt>. |
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<h2>Timing</h2> |
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Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It |
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provides a high-precision timing facility that is distinct from and |
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uncoordinated with <tt>System.currentTimeMillis</tt>. |
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<h2>Removing ThreadLocals</h2> |
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The {@link java.lang.ThreadLocal} class now supports a means to remove |
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a ThreadLocal, which is needed in some thread-pool and worker-thread |
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designs. |
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<hr> |
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<address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address> |
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</body> |
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</html> |
