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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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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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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 timing |
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and uncaught exceptions, and java.util to better integrate queues, and |
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to make Timers conform to new frameworks. 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> Barriers |
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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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<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> |
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Four 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, and |
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PriorityBlockingQueue. Additionally, java.util.concurrent.LinkedQueue |
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supplies an efficient thread-safe non-blocking queue. |
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<p> |
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Since the target release is JDK1.5, and generics are slated to be in |
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1.5, Queues should be parametrized on element type. (Also some others |
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below.) We are ignoring this for now. |
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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 java.lang.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 (so |
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includes for example, AbstractExecutor that simplifies construction of |
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new implementations), the most commonly used Executor will be |
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ThreadExecutor, which can be configured to act as all sorts of thread |
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pools, background threads, etc. The class is designed to be general |
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enough to suffice for the vast majority of usages, even sophisticated |
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ones, yet also includes methods and functionality that simplify |
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routine usage. |
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<p> |
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A few methods will also be added to the java.util.Timer to support |
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Futures, and address other requests for enhancement. |
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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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<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 java.lang.Clock, which provides multiple granularities for |
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both accessing time and performing time-out based operations. |
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<h2>Barriers</h2> |
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Barriers (multiway synchronization points) are very common in some |
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styles of parallel programming, yet tricky to get right. The two most |
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useful flavors (CyclicBarriers and Exchangers) don't have much of an |
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interface in common, and only have one standard implementation each, |
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so these are simply defined as public classes rather than interfaces |
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and implementations. |
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<h2>Concurrent Collections</h2> |
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There are no new interfaces, but JSR 166 will supply a few Collection |
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implementations designed for use in multithreaded contexts: |
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ConcurrentHashTable, CopyOnWriteArrayList, 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, Threads and ThreadLocals will now support a means to |
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clear and remove ThreadLocals, which is needed in some thread-pool and |
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worker-thread 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> |
