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4     <title>JSR 166 Snapshot Introduction.</title>     <title>JSR 166 Introduction.</title>
5    </head>    </head>
7    <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">    <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
8    <h1>JSR 166 Snapshot Introduction.</h1>    <h1>JSR 166 Introduction.</h1>
10    by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>    by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>
11    <p>    <p>
13  To join a mailing list discussing this JSR, go to:  This is maintenance repository of JSR-166 specifications.  For further
14  <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .  information, go to: <A
15    HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest">
16  <p>  http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A>.
17  <em>  
18   Disclaimer - This prototype is experimental code developed as part of  <p>JSR-166 introduces package <code>java.util.concurrent</code>
19   JSR166 and made available to the developer community for use  containing utility classes commonly useful in concurrent
20   as-is. It is not a supported product. Use it at your own risk. The  programming. Like package <code>java.util</code>, it includes a few small
21   specification, language and implementation are subject to change as a  standardized extensible frameworks, as well as other classes that
22   result of your feedback. Because these features have not yet been  provide useful functionality and are otherwise tedious or difficult to
23   approved for addition to the Java language, there is no schedule for  implement.
24   their inclusion in a product.  
25  </em>  <p>JSR-166 focuses on breadth, providing critical functionality
26    useful across a wide range of concurrent programming styles and
27  <p>  applications, ranging from low-level atomic operations, to
28  Package java.util.concurrent contains utility classes that are  customizable locks and synchronization aids, to various concurrent
29  commonly useful in concurrent programming. Like package java.util, it  data structures, to high-level execution agents including thread
30  includes a few small standardized extensible frameworks, as well as  pools. This diversity reflects the range of contexts in which
31  some classes that provide useful functionality and are otherwise  developers of concurrent programs have been found to require or desire
32  tedious or difficult to implement.  In this JSR, we have been  support not previously available in J2SE, while also keeping the
33  conservative in selecting only those APIs and implementations that are  resulting package small; providing only functionality that has been
34  useful enough to encourage nearly all concurrent programmers to use  found to be worthwhile to standardize.
35  routinely.  JSR 166 also includes a few changes and additions in  
36  packages outside of java.util.concurrent: java.lang, to address  <p>Descriptions and brief motivations for the main components may be
37  uncaught exceptions, and java.util to better integrate queues.  found in the associated package documentation.  JSR-166 also includes
38  The API covers:  a few changes and additions in packages outside of
39    java.util.concurrent.  Here are brief descriptions.
     <li> Queues  
     <li> Executors  
     <li> Locks  
     <li> Condition variables  
     <li> Atomic variables  
     <li> Timing  
     <li> Synchronizers  
     <li> Concurrent Collections  
     <li> Uncaught Exception Handlers  
 The main rationale for JSR 166 is that threading primitives, such as  
 synchronized blocks, Object.wait and Object.notify, are insufficient  
 for many programming tasks.  Currently, developers can use only the  
 concurrency control constructs provided in the Java language  
 itself. These are too low level for some applications, and are  
 incomplete for others.  As a result, application programmers are often  
 forced to implement their own concurrency facilities, resulting in  
 enormous duplication of effort creating facilities that are  
 notoriously hard to get right and even harder to optimize.  Offering a  
 standard set of concurrency utilities will ease the task of writing a  
 wide variety of multithreaded applications and generally improve the  
 quality of the applications that use them.  
 Here are brief descriptions and rationales of the main components.  
 For details see the javadocs at <a  
41  <h2>Queues</h2>  <h2>Queues</h2>
43  A basic (nonblocking) Queue interface that is compatatible with  A basic (nonblocking) {@link java.util.Queue} interface extending
44  java.util.Collections will be introduced into java.util. Also,  {@link java.util.Collection} is introduced into
45  although it is at the borders of being in scope of JSR-166,  <code>java.util</code>. Existing class {@link java.util.LinkedList} is
46  java.util.LinkedList will be adapted to support Queue, and  adapted to support Queue, and a new non-thread-safe {@link
47  a new non-thread-safe java.util.HeapPriorityQueue will be added.  java.util.PriorityQueue} is added.
49  <p> Five implementations in java.util.concurrent support the extended  <h2>Threads</h2>
50  BlockingQueue interface, that defines blocking versions of put and  
51  take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue,  Three minor changes are introduced to the {@link java.lang.Thread}
52  PriorityBlockingQueue, and DelayQueue. Additionally,  class:
53  java.util.concurrent.LinkedQueue supplies an efficient thread-safe  <ul>
54  non-blocking queue.    <li> It now allows per-thread installation of handlers for uncaught
55      exceptions. This optionally disassociates handlers from ThreadGroups,
56  <p> Since the target release is JDK1.5, and generics are slated to be    which has proven to be too inflexible. (Note that the combination of
57  in 1.5, Queues are parametrized on element type. (Also some others    features in JSR-166 make ThreadGroups even less likely to be used in
58  below.)    most programs. Perhaps they will eventually be deprecated.)
60      <li> Access checks are no longer required when a Thread interrupts
61  <h2>Executors</h2>    <em>itself</em>.  The <code>interrupt</code> method is the only way to
62      re-assert a thread's interruption status (and in the case of
63  Executors provide a simple standardized interface for defining custom    self-interruption has no other effect than this).  The check here
64  thread-like subsystems, including thread pools, asynch-IO, and    previously caused unjustifiable and uncontrollable failures when
65  lightweight task frameworks.  Executors also standardize ways of    restricted code invoked library code that must reassert interruption
66  calling threads that compute functions returning results, via    to correctly propagate status when encountering some
67  Futures. This is supported in part by defining interface Callable, the    <code>InterruptedExceptions</code>.
68  argument/result analog of Runnable.    <li> The <code>destroy</code> method, which has never been implemented,
69      has finally been deprecated. This is just a spec change, reflecting
70  <p> While the Executor framework is intended to be extensible the most    the fact that the reason it has never been implemented is that
71  commonly used Executor will be ThreadExecutor, which can be configured    it was undesirable and unworkable.
72  to act as all sorts of thread pools, background threads, etc. The  </ul>
 class is designed to be general enough to suffice for the vast  
 majority of usages, even sophisticated ones, yet also includes methods  
 and functionality that simplify routine usage.  
 The Lock interface supports locking disciplines that differ in  
 semantics (reentrant, semaphore-based, etc), and that can be used in  
 non-block-structured contexts including hand-over-hand and lock  
 reordering algorithms. This flexibility comes at the price of more  
 awkward syntax.  Implementations include Semaphore, ReentrantMutex  
 FIFOSemaphore, and CountDownLatch.  
 The Locks class additionally supports trylock-designs using builtin  
 locks without needing to use Lock classes.  This requires adding new  
 capabilities to builtin locks inside JVMs.  
 A ReadWriteLock interface similarly defines locks that may be shared  
 among readers but are exclusive to writers. For this release, only a  
 single implementation, ReentrantReadWriteLock, is planned, since it  
 covers all standard usage contexts. But programmers may create their  
 own implementations to cover nonstandard requirements.  
 A Condition class provides the kinds of condition variables associated  
 with monitors in other cocurrent languages, as well as pthreads  
 condvars.  Their support reduces the need for tricky and/or  
 inefficient solutions to many classic concurrent problems.  Conditions  
 also address the annoying problem that Object.wait(msecs) does not  
 return an indication of whether the wait timed out. This leads to  
 error-prone code. Since this method is in class Object, the problem is  
 basically unfixable.  
 To avoid compatibility problems, the names of Condition methods need  
 to be different than Object versions. The downside of this is that  
 people can make the mistake of calling cond.notify instead of  
 cond.signal. However, they will get IllegalMonitorState exceptions if  
 they do, so they can detect the error if they ever run the code.  
 The implementation requires VM magic to atomically suspend and release  
 lock. But it is unlikely to be very challenging for JVM providers,  
 since most layer Java monitors on top of posix condvars or similar  
 low-level functionality anyway.  
 <h2>Atomic variables</h2>  
 Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and  
 AtomicReference provide simple scalar variables supporting  
 compareAndSwap (CAS) and related atomic operations. These are  
 desparately needed by those performing low-level concurrent system  
 programming, but much less commonly useful in higher-level frameworks.  
74  <h2>Timing</h2>  <h2>Timing</h2>
76  Java has always supported sub-millisecond versions of several native  Method <code>nanoTime</code> is added to {@link java.lang.System}. It
77  time-out-based methods (such as Object.wait), but not methods to  provides a high-precision timing facility that is distinct from and
78  actually perform timing in finer-grained units. We address this by  uncoordinated with <code>System.currentTimeMillis</code>.
79  introducing class Clock, which provides multiple granularities for  
80  both accessing time and performing time-out based operations.  <h2>Removing ThreadLocals</h2>
82    The {@link java.lang.ThreadLocal} class now supports a means to remove
83  <h2>Synchronizers</h2>  a ThreadLocal, which is needed in some thread-pool and worker-thread
84    designs.
85  Five classes aid common special-purpose synchronization idioms.  
86  Semaphores and FifoSemaphores are classic concurrency tools.  Latches  
 are very simple yet very common objects useful for blocking until a  
 single signal, event, or condition holds.  CyclicBarriers are  
 resettable multiway synchronization points very common in some styles  
 of parallel programming. Exchangers allow two threads to exchange  
 objects at a rendezvous point.  
 <h2>Concurrent Collections</h2>  
 JSR 166 will supply a few Collection implementations designed for use  
 in multithreaded contexts: ConcurrentHashTable, CopyOnWriteArrayList,  
 and CopyOnWriteArraySet.  
 <h2>Uncaught Exception Handlers</h2>  
 The java.lang.Thread class will be modified to allow per-thread  
 installation of handlers for uncaught exceptions. Ths optionally  
 disassociates these handlers from ThreadGroups, which has proven to be  
 too inflexible in many multithreaded programs. (Note that the combination  
 of features in JSR 166 make ThreadGroups even less likely to  
 be used in most programs. Perhaps they will eventually be deprecated.)  
 Additionally,  ThreadLocals will now support a means to  
 remove a ThreadLocals, which is needed in some thread-pool and  
 worker-thread designs.  
88    <hr>    <hr>
   <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>  
89   </body>   </body>
90  </html>  </html>

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