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2  <html>  <html>
3   <head>   <head>
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    http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A>.
18    <p>JSR-166 introduces package <code>java.util.concurrent</code>
19    containing utility classes commonly useful in concurrent
20    programming. Like package <code>java.util</code>, it includes a few small
21    standardized extensible frameworks, as well as other classes that
22    provide useful functionality and are otherwise tedious or difficult to
23    implement.
25    <p>JSR-166 focuses on breadth, providing critical functionality
26    useful across a wide range of concurrent programming styles and
27    applications, ranging from low-level atomic operations, to
28    customizable locks and synchronization aids, to various concurrent
29    data structures, to high-level execution agents including thread
30    pools. This diversity reflects the range of contexts in which
31    developers of concurrent programs have been found to require or desire
32    support not previously available in J2SE, while also keeping the
33    resulting package small; providing only functionality that has been
34    found to be worthwhile to standardize.
36    <p>Descriptions and brief motivations for the main components may be
37    found in the associated package documentation.  JSR-166 also includes
38    a few changes and additions in packages outside of
39    java.util.concurrent.  Here are brief descriptions.
41  <p>  <h2>Queues</h2>
  Disclaimer - This prototype is experimental code developed as part of  
  JSR166 and made available to the developer community for use  
  as-is. It is not a supported product. Use it at your own risk. The  
  specification, language and implementation are subject to change as a  
  result of your feedback. Because these features have not yet been  
  approved for addition to the Java language, there is no schedule for  
  their inclusion in a product.  
43  <p>  A basic (nonblocking) {@link java.util.Queue} interface extending
44  Package java.util.concurrent contains utility classes that are  {@link java.util.Collection} is introduced into
45  commonly useful in concurrent programming. Like package java.util, it  <code>java.util</code>. Existing class {@link java.util.LinkedList} is
46  includes a few small standardized extensible frameworks, as well as  adapted to support Queue, and a new non-thread-safe {@link
47  some classes that provide useful functionality and are otherwise  java.util.PriorityQueue} is added.
 tedious or difficult to implement.  In this JSR, we have been  
 conservative in selecting only those APIs and implementations that are  
 useful enough to encourage nearly all concurrent programmers to use  
 routinely.  JSR 166 also includes a few changes and additions in  
 packages outside of java.util.concurrent: java.lang, to address  
 uncaught exceptions, and java.util to better integrate queues.  
 The API covers:  
49    <h2>Threads</h2>
51    Three minor changes are introduced to the {@link java.lang.Thread}
52    class:
53    <ul>    <ul>
54      <li> Queues    <li> It now allows per-thread installation of handlers for uncaught
55      <li> Executors    exceptions. This optionally disassociates handlers from ThreadGroups,
56      <li> Locks    which has proven to be too inflexible. (Note that the combination of
57      <li> Condition variables    features in JSR-166 make ThreadGroups even less likely to be used in
58      <li> Atomic variables    most programs. Perhaps they will eventually be deprecated.)
59      <li> Timing  
60      <li> Synchronizers    <li> Access checks are no longer required when a Thread interrupts
61      <li> Concurrent Collections    <em>itself</em>.  The <code>interrupt</code> method is the only way to
62      <li> Uncaught Exception Handlers    re-assert a thread's interruption status (and in the case of
63      self-interruption has no other effect than this).  The check here
64      previously caused unjustifiable and uncontrollable failures when
65      restricted code invoked library code that must reassert interruption
66      to correctly propagate status when encountering some
67      <code>InterruptedExceptions</code>.
68      <li> The <code>destroy</code> method, which has never been implemented,
69      has finally been deprecated. This is just a spec change, reflecting
70      the fact that the reason it has never been implemented is that
71      it was undesirable and unworkable.
72    </ul>    </ul>
74    <h2>Timing</h2>
76  The main rationale for JSR 166 is that threading primitives, such as  Method <code>nanoTime</code> is added to {@link java.lang.System}. It
77  synchronized blocks, Object.wait and Object.notify, are insufficient  provides a high-precision timing facility that is distinct from and
78  for many programming tasks.  Currently, developers can use only the  uncoordinated with <code>System.currentTimeMillis</code>.
79  concurrency control constructs provided in the Java language  
80  itself. These are too low level for some applications, and are  <h2>Removing ThreadLocals</h2>
81  incomplete for others.  As a result, application programmers are often  
82  forced to implement their own concurrency facilities, resulting in  The {@link java.lang.ThreadLocal} class now supports a means to remove
83  enormous duplication of effort creating facilities that are  a ThreadLocal, which is needed in some thread-pool and worker-thread
84  notoriously hard to get right and even harder to optimize.  Offering a  designs.
 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  
 A basic (nonblocking) Queue interface that is compatatible with  
 java.util.Collections will be introduced into java.util. Also,  
 although it is at the borders of being in scope of JSR-166,  
 java.util.LinkedList will be adapted to support Queue, and  
 a new non-thread-safe java.util.HeapPriorityQueue will be added.  
 <p> Five implementations in java.util.concurrent support the extended  
 BlockingQueue interface, that defines blocking versions of put and  
 take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue,  
 PriorityBlockingQueue, and DelayQueue. Additionally,  
 java.util.concurrent.LinkedQueue supplies an efficient thread-safe  
 non-blocking queue.  
 <p> Since the target release is JDK1.5, and generics are slated to be  
 in 1.5, Queues are parametrized on element type. (Also some others  
 Executors provide a simple standardized interface for defining custom  
 thread-like subsystems, including thread pools, asynch-IO, and  
 lightweight task frameworks.  Executors also standardize ways of  
 calling threads that compute functions returning results, via  
 Futures. This is supported in part by defining interface Callable, the  
 argument/result analog of Runnable.  
 <p> While the Executor framework is intended to be extensible the most  
 commonly used Executor will be ThreadExecutor, which can be configured  
 to act as all sorts of thread pools, background threads, etc. The  
 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.  
 Java has always supported sub-millisecond versions of several native  
 time-out-based methods (such as Object.wait), but not methods to  
 actually perform timing in finer-grained units. We address this by  
 introducing class Clock, which provides multiple granularities for  
 both accessing time and performing time-out based operations.  
 Five classes aid common special-purpose synchronization idioms.  
 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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