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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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JCP 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> Package java.util.concurrent contains utility classes commonly |
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useful in concurrent programming. Like package java.util, it includes |
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a few small standardized extensible frameworks, as well as some |
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classes that provide useful functionality and are otherwise tedious or |
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difficult to implement. JSR166 also includes a few changes and |
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additions in packages outside of java.util.concurrent: java.lang, to |
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address uncaught exceptions, and java.util to better integrate with |
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collections. Since the target release is JDK1.5, many APIs use |
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generics to parameterize on types. Here are brief descriptions of the |
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main components. |
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<h2>Executors</h2> |
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{@link java.util.concurrent.Executor} is a simple standardized |
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interface for defining custom thread-like subsystems, including thread |
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pools, asynch-IO, and lightweight task frameworks. Depending on which |
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concrete Executor class is being used, tasks may execute in a newly |
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created thread, an existing task-execution thread, or the thread |
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calling <tt>execute()</tt>, and may execute sequentially or |
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concurrently. Executors also standardize ways of calling threads that |
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compute functions returning results, via a {@link |
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java.util.concurrent.Future}. This is supported in part by defining |
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interface {@link java.util.concurrent.Callable}, the argument/result |
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analog of Runnable. |
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<p> {@link java.util.concurrent.ExecutorService} provides a more |
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complete framework for executing Runnables. An ExecutorService |
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manages queueing and scheduling of tasks, and allows controlled |
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shutdown. The two primary implementations of ExecutorService are |
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{@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and |
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flexible thread pool and {@link |
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java.util.concurrent.ScheduledExecutor}, which adds support for |
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delayed and periodic task execution. These, and other Executors can |
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be used in conjunction with a {@link java.util.concurrent.FutureTask} |
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to asynchronously |
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start a potentially long-running computation and query the FutureTask |
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to determine if its execution has completed, or cancel it. |
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<p> The {@link java.util.concurrent.Executors} class provides factory |
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methods for the most common kinds and styles of Executors, as well as |
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a few utilities methods for using them. |
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This is the proposed final draft JSR166 specification. To check for |
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further updates, or join a mailing list discussing JSR-166, 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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|
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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) {@link java.util.Queue} interface extending |
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java.util.Collection is introduced into java.util. Existing class |
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java.util.LinkedList is adapted to support Queue, and a new |
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non-thread-safe {@link java.util.PriorityQueue} is added. The |
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java.util.concurrent {@link |
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java.util.concurrent.ConcurrentLinkedQueue} class supplies an |
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efficient sclable thread-safe non-blocking FIFO queue, and {@link |
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java.util.concurrent.ConcurrentLinkedStack} provides a similar |
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non-blocking LIFO stack. |
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<p> Five implementations in java.util.concurrent support the extended |
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{@link java.util.concurrent.BlockingQueue} interface, that defines |
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blocking versions of put and take: {@link |
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java.util.concurrent.LinkedBlockingQueue}, {@link |
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java.util.concurrent.ArrayBlockingQueue}, {@link |
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java.util.concurrent.SynchronousQueue}, {@link |
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java.util.concurrent.PriorityBlockingQueue}, and |
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{@link java.util.concurrent.DelayQueue}. |
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<h2>Locks</h2> |
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The {@link java.util.concurrent.Lock} interface supports locking |
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disciplines that differ in semantics (reentrant, fair, etc), and that |
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can be used in non-block-structured contexts including hand-over-hand |
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and lock reordering algorithms. This flexibility comes at the price of |
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more awkward syntax. Implementations include {@link |
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java.util.concurrent.ReentrantLock} and {@link |
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java.util.concurrent.FairReentrantLock}. |
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<p> The {@link java.util.concurrent.Locks} class additionally supports |
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some common trylock-designs using builtin locks. |
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<p> The {@link java.util.concurrent.ReadWriteLock} interface similarly |
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defines locks that may be shared among readers but are exclusive to |
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writers. Only a single implementation, {@link |
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java.util.concurrent.ReentrantReadWriteLock}, is provided, 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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The {@link java.util.concurrent.Condition} interface describes the |
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kinds of condition variables associated with monitors in other |
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concurrent languages, as well as pthreads-style condvars. Their |
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support reduces the need for tricky and/or inefficient solutions to |
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many classic concurrent problems. To avoid compatibility problems, |
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the names of Condition methods are different than Object versions. |
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<h2>Atomics</h2> |
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The atomic subpackage includes a small library of classes, including |
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AtomicInteger, AtomicLong, and AtomicReference that support |
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compareAndSet (CAS) and related atomic operations. |
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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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The {@link java.util.concurrent.TimeUnit} class provides multiple |
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granularities (including nanoseconds) for both accessing time and |
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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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{@link java.util.concurrent.Semaphore} and {@link |
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java.util.concurrent.FairSemaphore} are classic concurrency tools. |
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{@link java.util.concurrent.CountDownLatch} is very simple yet very |
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common utility for blocking until a single signal, event, or condition |
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holds. A {@link java.util.concurrent.CyclicBarrier} is a resettable multiway |
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synchronization point common in some styles of parallel |
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programming. An {@link java.util.concurrent.Exchanger} allows two |
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threads to exchange objects at a rendezvous point. |
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<h2>Concurrent Collections</h2> |
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Besides Queues, this package supplies a few Collection implementations |
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designed for use in multithreaded contexts: {@link |
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java.util.concurrent.ConcurrentHashMap}, {@link |
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java.util.concurrent.CopyOnWriteArrayList}, and {@link |
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java.util.concurrent.CopyOnWriteArraySet}. |
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<p>The "Concurrent" prefix for classes is a shorthand |
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indicating several differences from similar "synchronized" |
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classes. For example <tt>java.util.Hashtable</tt> and |
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<tt>Collections.synchronizedMap(new HashMap())</tt> are |
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synchronized. But {@link |
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java.util.concurrent.ConcurrentHashMap} is "concurrent". |
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A concurrent collection (among other kinds of classes) is |
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thread-safe, but not governed by a single exclusion lock. So, in the |
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particular case of ConcurrentHashMap, it safely permits any number of |
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concurrent reads as well as a tunable number of concurrent writes. |
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There may still be a role for "synchronized" classes in some |
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multithreaded programs -- they can sometimes be useful when you need |
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to prevent ALL access to a collection via a single lock, at the |
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expense of much poor scalability. In all other cases, "concurrent" |
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versions are normally preferable. |
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<p> Most concurrent Collection implementations (including most Queues) |
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also differ from the usual java.util conventions in that their Iterators |
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provide <em>weakly consistent</em> rather than fast-fail traversal. A |
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weakly consistent iterator is thread-safe, but does not necessarily |
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freeze the collection while iterating, so it may (or may not) reflect |
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any updates since the iterator was created. |
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<h2>Uncaught Exception Handlers</h2> |
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The java.lang.Thread class is 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 |
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combination of features in JSR166 make ThreadGroups even less likely |
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to be used in most programs. Perhaps they will eventually be |
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deprecated.) |
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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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<p> Additionally, java.lang.ThreadLocal now supports a means to remove |
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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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