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<title>JSR 166 Snapshot Introduction.</title> |
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<h1>JSR 166 Snapshot 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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|
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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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<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} |
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is added. The java.util.concurrent {@link |
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java.util.concurrent.LinkedQueue} class supplies an efficient |
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thread-safe non-blocking queue. |
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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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<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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This package supplies a few Collection implementations designed for |
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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> Most concurrent Collection implementations (including most Queues) |
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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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<p> Additionally, java.lang.ThreadLocal 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> |
