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<title>JSR 166 Snapshot Introduction.</title> |
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<title>JSR 166 Introduction.</title> |
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<h1>JSR 166 Snapshot Introduction.</h1> |
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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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To join a mailing list discussing this JSR, go to: |
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<A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> . |
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<p> |
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<em> |
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Disclaimer - This prototype is experimental code developed as part of |
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JSR166 and made available to the developer community for use |
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as-is. It is not a supported product. Use it at your own risk. The |
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specification, language and implementation are subject to change as a |
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result of your feedback. Because these features have not yet been |
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approved for addition to the Java language, there is no schedule for |
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their inclusion in a product. |
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</em> |
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<p> |
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Package java.util.concurrent contains utility classes that are |
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commonly useful in concurrent programming. Like package java.util, it |
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includes a few small standardized extensible frameworks, as well as |
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some classes that provide useful functionality and are otherwise |
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tedious or difficult to implement. In this JSR, we have been |
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conservative in selecting only those APIs and implementations that are |
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useful enough to encourage nearly all concurrent programmers to use |
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routinely. JSR 166 also includes a few changes and additions in |
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packages outside of java.util.concurrent: java.lang, to address |
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uncaught exceptions, and java.util to better integrate queues. |
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The API covers: |
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<ul> |
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<li> Queues |
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<li> Executors |
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<li> Locks |
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<li> Condition variables |
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<li> Atomic variables |
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<li> Timing |
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<li> Synchronizers |
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<li> Concurrent Collections |
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<li> Uncaught Exception Handlers |
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</ul> |
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The main rationale for JSR 166 is that threading primitives, such as |
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synchronized blocks, Object.wait and Object.notify, are insufficient |
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for many programming tasks. Currently, developers can use only the |
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concurrency control constructs provided in the Java language |
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itself. These are too low level for some applications, and are |
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incomplete for others. As a result, application programmers are often |
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forced to implement their own concurrency facilities, resulting in |
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enormous duplication of effort creating facilities that are |
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notoriously hard to get right and even harder to optimize. Offering a |
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standard set of concurrency utilities will ease the task of writing a |
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wide variety of multithreaded applications and generally improve the |
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quality of the applications that use them. |
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<p> |
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Here are brief descriptions and rationales of the main components. |
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For details see the javadocs at <a |
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href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a> |
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This is the final JSR166 specification submitted to JCP. For further |
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information, 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><em>Note: The javadocs here includes some existing java.util |
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Collection interfaces and classes that are not part of the JSR-166 |
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spec, but are included because JSR-166 methods implement or inherit |
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from their 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 other 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 functionality that has been |
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found to be worthwhile 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) Queue interface that is compatatible with |
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java.util.Collections will be introduced into java.util. Also, |
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although it is at the borders of being in scope of JSR-166, |
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java.util.LinkedList will be adapted to support Queue, and |
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a new non-thread-safe java.util.HeapPriorityQueue will be added. |
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<p> Five implementations in java.util.concurrent support the extended |
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BlockingQueue interface, that defines blocking versions of put and |
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take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, |
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PriorityBlockingQueue, and DelayQueue. Additionally, |
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java.util.concurrent.LinkedQueue supplies an efficient thread-safe |
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non-blocking queue. |
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<p> Since the target release is JDK1.5, and generics are slated to be |
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in 1.5, Queues are parametrized on element type. (Also some others |
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below.) |
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<h2>Executors</h2> |
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Executors provide a simple standardized interface for defining custom |
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thread-like subsystems, including thread pools, asynch-IO, and |
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lightweight task frameworks. Executors also standardize ways of |
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calling threads that compute functions returning results, via |
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Futures. This is supported in part by defining interface Callable, the |
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argument/result analog of Runnable. |
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<p> While the Executor framework is intended to be extensible the most |
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commonly used Executor will be ThreadExecutor, which can be configured |
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to act as all sorts of thread pools, background threads, etc. The |
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class is designed to be general enough to suffice for the vast |
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majority of usages, even sophisticated ones, yet also includes methods |
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and functionality that simplify routine usage. |
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<h2>Locks</h2> |
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The Lock interface supports locking disciplines that differ in |
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semantics (reentrant, semaphore-based, etc), and that can be used in |
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non-block-structured contexts including hand-over-hand and lock |
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reordering algorithms. This flexibility comes at the price of more |
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awkward syntax. Implementations include Semaphore, ReentrantMutex |
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FIFOSemaphore, and CountDownLatch. |
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<p> |
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The Locks class additionally supports trylock-designs using builtin |
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locks without needing to use Lock classes. This requires adding new |
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capabilities to builtin locks inside JVMs. |
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<p> |
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A ReadWriteLock interface similarly defines locks that may be shared |
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among readers but are exclusive to writers. For this release, only a |
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single implementation, ReentrantReadWriteLock, is planned, since it |
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covers all standard usage contexts. But programmers may create their |
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own implementations to cover nonstandard requirements. |
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<h2>Conditions</h2> |
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A Condition class provides the kinds of condition variables associated |
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with monitors in other cocurrent languages, as well as pthreads |
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condvars. Their support reduces the need for tricky and/or |
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inefficient solutions to many classic concurrent problems. Conditions |
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also address the annoying problem that Object.wait(msecs) does not |
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return an indication of whether the wait timed out. This leads to |
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error-prone code. Since this method is in class Object, the problem is |
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basically unfixable. |
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<p> |
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To avoid compatibility problems, the names of Condition methods need |
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to be different than Object versions. The downside of this is that |
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people can make the mistake of calling cond.notify instead of |
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cond.signal. However, they will get IllegalMonitorState exceptions if |
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they do, so they can detect the error if they ever run the code. |
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<p> |
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The implementation requires VM magic to atomically suspend and release |
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lock. But it is unlikely to be very challenging for JVM providers, |
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since most layer Java monitors on top of posix condvars or similar |
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low-level functionality anyway. |
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<h2>Atomic variables</h2> |
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Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and |
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AtomicReference provide simple scalar variables supporting |
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compareAndSwap (CAS) and related atomic operations. These are |
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desparately needed by those performing low-level concurrent system |
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programming, but much less commonly useful in higher-level frameworks. |
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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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Three minor changes are introduced to the {@link java.lang.Thread} |
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class: |
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<ul> |
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<li> 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.) |
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<li> 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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<li> The <tt>destroy</tt> method, which has never been implemented, |
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has finally been deprecated. This is just a spec change, reflecting |
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the fact that that the reason it has never been implemented is that |
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it was undesirable and unworkable. |
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</ul> |
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<h2>Timing</h2> |
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Java has always supported sub-millisecond versions of several native |
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time-out-based methods (such as Object.wait), but not methods to |
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actually perform timing in finer-grained units. We address this by |
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introducing class Clock, which provides multiple granularities for |
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both accessing time and performing time-out based operations. |
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<h2>Synchronizers</h2> |
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Five classes aid common special-purpose synchronization idioms. |
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Semaphores and FifoSemaphores are classic concurrency tools. Latches |
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are very simple yet very common objects useful for blocking until a |
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single signal, event, or condition holds. CyclicBarriers are |
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resettable multiway synchronization points very common in some styles |
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of parallel programming. Exchangers allow two threads to exchange |
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objects at a rendezvous point. |
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<h2>Concurrent Collections</h2> |
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JSR 166 will supply a few Collection implementations designed for use |
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in multithreaded contexts: ConcurrentHashTable, CopyOnWriteArrayList, |
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and CopyOnWriteArraySet. |
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<h2>Uncaught Exception Handlers</h2> |
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The java.lang.Thread class will be modified to allow per-thread |
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installation of handlers for uncaught exceptions. Ths optionally |
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disassociates these handlers from ThreadGroups, which has proven to be |
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too inflexible in many multithreaded programs. (Note that the combination |
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of features in JSR 166 make ThreadGroups even less likely to |
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be used in most programs. Perhaps they will eventually be deprecated.) |
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<p> |
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Additionally, ThreadLocals will now support a means to |
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remove a ThreadLocals, which is needed in some thread-pool and |
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worker-thread designs. |
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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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