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1  <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">  <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
2  <html>  <html>
3   <head>   <head>
4     <title>JSR 166 Snapshot Introduction.</title>     <title>JSR 166 Introduction.</title>
5    </head>    </head>
6    
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>
9    
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>
12    
13  To join a mailing list discussing this JSR, go to:  This is an updated version of the specification submitted for JCP Public
14  <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .  Review.  To check for further updates, access a preliminary prototype
15    release of main functionality, or join a mailing list discussing
16  <p>  JSR-166, go to: <A
17  <em>  HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest">
18   Disclaimer - This prototype is experimental code developed as part of  http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A>.
19   JCP JSR166 and made available to the developer community for use  
20   as-is. It is not a supported product. Use it at your own risk. The  <p><em>Note: The javadocs here includes some existing java.util
21   specification, language and implementation are subject to change as a  Collection interfaces and classes that are not part of the JSR-166
22   result of your feedback. Because these features have not yet been  spec, but are included because JSR-166 methods implement or inherit
23   approved for addition to the Java language, there is no schedule for  from their specifications.</em>
24   their inclusion in a product.  
25  </em>  <p> JSR-166 introduces package <tt>java.util.concurrent</tt>
26    containing utility classes commonly useful in concurrent
27  <p> Package java.util.concurrent contains utility classes commonly  programming. Like package <tt>java.util</tt>, it includes a few small
28  useful in concurrent programming. Like package java.util, it includes  standardized extensible frameworks, as well as some classes that
29  a few small standardized extensible frameworks, as well as some  provide useful functionality and are otherwise tedious or difficult to
30  classes that provide useful functionality and are otherwise tedious or  implement.
31  difficult to implement.  JSR166 also includes a few changes and  
32  additions in packages outside of java.util.concurrent: java.lang, to  <p>JSR-166 focusses on breadth, providing critical functionality
33  address uncaught exceptions, and java.util to better integrate with  useful across a wide range of concurrent programming styles and
34  collections.  Since the target release is JDK1.5, many APIs use  applications, ranging from low-level atomic operations, to
35  generics to parameterize on types.  Here are brief descriptions of the  customizable locks and synchronization aids, to various concurrent
36  main components.  data structures, to high-level execution agents including thread
37    pools. This diversity reflects the range of contexts in which
38  <h2>Executors</h2>  developers of concurrent programs have been found to require or desire
39    support not previously available in J2SE, which also keeping the
40  {@link java.util.concurrent.Executor} is a simple standardized  resulting package small; providing only functionality that it makes
41  interface for defining custom thread-like subsystems, including thread  sense to standardize.
42  pools, asynch-IO, and lightweight task frameworks.  Depending on which  
43  concrete Executor class is being used, tasks may execute in a newly  <p>Descriptions and brief motivations for the main components may be
44  created thread, an existing task-execution thread, or the thread  found in the associated package documentation.  JSR-166 also includes
45  calling <tt>execute()</tt>, and may execute sequentially or  a few changes and additions in packages outside of
46  concurrently.  Executors also standardize ways of calling threads that  java.util.concurrent.  Here are brief descriptions.
 compute functions returning results, via a {@link  
 java.util.concurrent.Future}. This is supported in part by defining  
 interface {@link java.util.concurrent.Callable}, the argument/result  
 analog of Runnable.  
   
 <p> {@link java.util.concurrent.ExecutorService} provides a more  
 complete framework for executing Runnables.  An ExecutorService  
 manages queueing and scheduling of tasks, and allows controlled  
 shutdown.  The two primary implementations of ExecutorService are  
 {@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and  
 flexible thread pool and {@link  
 java.util.concurrent.ScheduledExecutor}, which adds support for  
 delayed and periodic task execution.  These, and other Executors can  
 be used in conjunction with a {@link FutureTask} to asynchronously  
 start a potentially long-running computation and query the FutureTask  
 to determine if its execution has completed, or cancel it.  
   
 <p> The {@link java.util.concurrent.Executors} class provides factory  
 methods for the most common kinds and styles of Executors, as well as  
 a few utilities methods for using them.  
47    
48  <h2>Queues</h2>  <h2>Queues</h2>
49    
50  A basic (nonblocking) {@link java.util.Queue} interface extending  A basic (nonblocking) {@link java.util.Queue} interface extending
51  java.util.Collection is introduced into java.util. Existing class  {@link java.util.Collection} is introduced into
52  java.util.LinkedList is adapted to support Queue, and a new  <tt>java.util</tt>. Existing class {@link java.util.LinkedList} is
53  non-thread-safe {@link java.util.concurrent.java.util.PriorityQueue}  adapted to support Queue, and a new non-thread-safe {@link
54  is added.  The java.util.concurrent {@link  java.util.PriorityQueue} is added.
55  java.util.concurrent.LinkedQueue} class supplies an efficient  
56  thread-safe non-blocking queue.  <h2>Threads</h2>
57    
58  <p> Five implementations in java.util.concurrent support the extended  Three minor changes are introduced to the {@link java.lang.Thread}
59  {@link java.util.concurrent.BlockingQueue} interface, that defines  class:
60  blocking versions of put and take: {@link  <ul>
61  java.util.concurrent.LinkedBlockingQueue}, {@link    <li> It now allows per-thread installation of handlers for uncaught
62  java.util.concurrent.ArrayBlockingQueue}, {@link    exceptions. Ths optionally disassociates handlers from ThreadGroups,
63  java.util.concurrent.SynchronousQueue}, {@link    which has proven to be too inflexible. (Note that the combination of
64  java.util.concurrent.PriorityBlockingQueue}, and {@link DelayQueue}.    features in JSR-166 make ThreadGroups even less likely to be used in
65      most programs. Perhaps they will eventually be deprecated.)
66    
67  <h2>Locks</h2>    <li> Access checks are no longer required when a Thread interrupts
68      <em>itself</em>.  The <tt>interrupt</tt> method is the only way to
69  The {@link java.util.concurrent.Lock} interface supports locking    re-assert a thread's interruption status (and in the case of
70  disciplines that differ in semantics (reentrant, fair, etc), and that    self-interruption has no other effect than this).  The check here
71  can be used in non-block-structured contexts including hand-over-hand    previously caused unjustifiable and uncontrollable failures when
72  and lock reordering algorithms. This flexibility comes at the price of    restricted code invoked library code that must reassert interruption
73  more awkward syntax.  Implementations include {@link    to correctly propagate status when encountering some
74  java.util.concurrent.ReentrantLock} and {@link    <tt>InterruptedExceptions</tt>.
75  java.util.concurrent.FairReentrantLock}.    <li> The <tt>destroy</tt> method, which has never been implemented,
76      has finally been deprecated. This is just a spec change, reflecting
77  <p> The {@link java.util.concurrent.Locks} class additionally supports    the fact that that the reason it has never been implmented is that
78  some common trylock-designs using builtin locks.    it was undesirable and unworkable.
79    </ul>
 <p> The {@link java.util.concurrent.ReadWriteLock} interface similarly  
 defines locks that may be shared among readers but are exclusive to  
 writers.  Only a single implementation, {@link  
 java.util.concurrent.ReentrantReadWriteLock}, is provided, since it  
 covers all standard usage contexts. But programmers may create their  
 own implementations to cover nonstandard requirements.  
   
 <h2>Conditions</h2>  
   
 The {@link java.util.concurrent.Condition} interface describes the  
 kinds of condition variables associated with monitors in other  
 concurrent languages, as well as pthreads-style condvars.  Their  
 support reduces the need for tricky and/or inefficient solutions to  
 many classic concurrent problems.  To avoid compatibility problems,  
 the names of Condition methods are different than Object versions.  
   
 <h2>Atomics</h2>  
   
 The atomic subpackage includes a small library of classes, including  
 AtomicInteger, AtomicLong, and AtomicReference that support  
 compareAndSet (CAS) and related atomic operations.  
80    
81  <h2>Timing</h2>  <h2>Timing</h2>
82    
83  The {@link java.util.concurrent.TimeUnit} class provides multiple  Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It
84  granularities (including nanoseconds) for both accessing time and  provides a high-precision timing facility that is distinct from and
85  performing time-out based operations.  uncoordinated with <tt>System.currentTimeMillis</tt>.
86    
87  <h2>Synchronizers</h2>  <h2>Removing ThreadLocals</h2>
   
 Five classes aid common special-purpose synchronization idioms.  
 {@link java.util.concurrent.Semaphore} and {@link  
 java.util.concurrent.FairSemaphore} are classic concurrency tools.  
 {@link java.util.concurrent.CountDownLatch} is very simple yet very  
 common utility for blocking until a single signal, event, or condition  
 holds.  A {link CyclicBarrier} is a resettable multiway  
 synchronization point common in some styles of parallel  
 programming. An {@link java.util.concurrent.Exchanger} allows two  
 threads to exchange objects at a rendezvous point.  
   
 <h2>Concurrent Collections</h2>  
   
 This package supplies a few Collection implementations designed for  
 use in multithreaded contexts: {@link  
 java.util.concurrent.ConcurrentHashMap}, {@link  
 java.util.concurrent.CopyOnWriteArrayList}, and {@link  
 java.util.concurrent.CopyOnWriteArraySet}.  
   
 <p> Most concurrent Collection implementations (including most Queues)  
 differ from the usual java.util conventions in that their Iterators  
 provide <em>weakly consistent</em> rather than fast-fail traversal. A  
 weakly consistent iterator is thread-safe, but does not necessarily  
 freeze the collection while iterating, so it may (or may not) reflect  
 any updates since the iterator was created.  
   
 <h2>Uncaught Exception Handlers</h2>  
   
 The java.lang.Thread class is 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 JSR166 make ThreadGroups even less likely  
 to be used in most programs. Perhaps they will eventually be  
 deprecated.)  
88    
89  <p> Additionally, java.lang.ThreadLocal now supports a means to remove  The {@link java.lang.ThreadLocal} class now supports a means to remove
90  a ThreadLocal, which is needed in some thread-pool and worker-thread  a ThreadLocal, which is needed in some thread-pool and worker-thread
91  designs.  designs.
92    
93    
94    
95    <hr>    <hr>
   <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>  
96   </body>   </body>
97  </html>  </html>

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