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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 maintenance repository of JSR166 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><em>Note: The javadocs here do <em>not</em> include pre-existing
19   JCP JSR166 and made available to the developer community for use  java classes (for example <tt>java.lang.Thread</tt>) that were changed
20   as-is. It is not a supported product. Use it at your own risk. The  as part of the JSR166 spec.  On the other hand, the javadocs here do
21   specification, language and implementation are subject to change as a  include some existing java.util Collection interfaces and classes that
22   result of your feedback. Because these features have not yet been  are not part of the spec, but are included because some new methods
23   approved for addition to the Java language, there is no schedule for  implement or inherit from their specifications.
  their inclusion in a product.  
24  </em>  </em>
25    
26  <p> Package java.util.concurrent contains utility classes commonly  <p>JSR-166 introduces package <tt>java.util.concurrent</tt>
27  useful in concurrent programming. Like package java.util, it includes  containing utility classes commonly useful in concurrent
28  a few small standardized extensible frameworks, as well as some  programming. Like package <tt>java.util</tt>, it includes a few small
29  classes that provide useful functionality and are otherwise tedious or  standardized extensible frameworks, as well as other classes that
30  difficult to implement.  JSR166 also includes a few changes and  provide useful functionality and are otherwise tedious or difficult to
31  additions in packages outside of java.util.concurrent: java.lang, to  implement.
32  address uncaught exceptions, and java.util to better integrate with  
33  collections.  Since the target release is JDK1.5, many APIs use  <p>JSR-166 focuses on breadth, providing critical functionality
34  generics to parameterize on types.  Here are brief descriptions of the  useful across a wide range of concurrent programming styles and
35  main components.  applications, ranging from low-level atomic operations, to
36    customizable locks and synchronization aids, to various concurrent
37  <h2>Executors</h2>  data structures, to high-level execution agents including thread
38    pools. This diversity reflects the range of contexts in which
39  {@link java.util.concurrent.Executor} is a simple standardized  developers of concurrent programs have been found to require or desire
40  interface for defining custom thread-like subsystems, including thread  support not previously available in J2SE, which also keeping the
41  pools, asynch-IO, and lightweight task frameworks.  Depending on which  resulting package small; providing only functionality that has been
42  concrete Executor class is being used, tasks may execute in a newly  found to be worthwhile to standardize.
43  created thread, an existing task-execution thread, or the thread  
44  calling <tt>execute()</tt>, and may execute sequentially or  <p>Descriptions and brief motivations for the main components may be
45  concurrently.  Executors also standardize ways of calling threads that  found in the associated package documentation.  JSR-166 also includes
46  compute functions returning results, via a {@link  a few changes and additions in packages outside of
47  java.util.concurrent.Future}. This is supported in part by defining  java.util.concurrent.  Here are brief descriptions.
 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.  
48    
49  <h2>Queues</h2>  <h2>Queues</h2>
50    
51  A basic (nonblocking) {@link java.util.Queue} interface extending  A basic (nonblocking) {@link java.util.Queue} interface extending
52  java.util.Collection is introduced into java.util. Existing class  {@link java.util.Collection} is introduced into
53  java.util.LinkedList is adapted to support Queue, and a new  <tt>java.util</tt>. Existing class {@link java.util.LinkedList} is
54  non-thread-safe {@link java.util.concurrent.java.util.PriorityQueue}  adapted to support Queue, and a new non-thread-safe {@link
55  is added.  The java.util.concurrent {@link  java.util.PriorityQueue} is added.
56  java.util.concurrent.LinkedQueue} class supplies an efficient  
57  thread-safe non-blocking queue.  <h2>Threads</h2>
58    
59  <p> Five implementations in java.util.concurrent support the extended  Three minor changes are introduced to the {@link java.lang.Thread}
60  {@link java.util.concurrent.BlockingQueue} interface, that defines  class:
61  blocking versions of put and take: {@link  <ul>
62  java.util.concurrent.LinkedBlockingQueue}, {@link    <li> It now allows per-thread installation of handlers for uncaught
63  java.util.concurrent.ArrayBlockingQueue}, {@link    exceptions. This optionally disassociates handlers from ThreadGroups,
64  java.util.concurrent.SynchronousQueue}, {@link    which has proven to be too inflexible. (Note that the combination of
65  java.util.concurrent.PriorityBlockingQueue}, and {@link DelayQueue}.    features in JSR-166 make ThreadGroups even less likely to be used in
66      most programs. Perhaps they will eventually be deprecated.)
67    
68  <h2>Locks</h2>    <li> Access checks are no longer required when a Thread interrupts
69      <em>itself</em>.  The <tt>interrupt</tt> method is the only way to
70  The {@link java.util.concurrent.Lock} interface supports locking    re-assert a thread's interruption status (and in the case of
71  disciplines that differ in semantics (reentrant, fair, etc), and that    self-interruption has no other effect than this).  The check here
72  can be used in non-block-structured contexts including hand-over-hand    previously caused unjustifiable and uncontrollable failures when
73  and lock reordering algorithms. This flexibility comes at the price of    restricted code invoked library code that must reassert interruption
74  more awkward syntax.  Implementations include {@link    to correctly propagate status when encountering some
75  java.util.concurrent.ReentrantLock} and {@link    <tt>InterruptedExceptions</tt>.
76  java.util.concurrent.FairReentrantLock}.    <li> The <tt>destroy</tt> method, which has never been implemented,
77      has finally been deprecated. This is just a spec change, reflecting
78  <p> The {@link java.util.concurrent.Locks} class additionally supports    the fact that that the reason it has never been implemented is that
79  some common trylock-designs using builtin locks.    it was undesirable and unworkable.
80    </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.  
81    
82  <h2>Timing</h2>  <h2>Timing</h2>
83    
84  The {@link java.util.concurrent.TimeUnit} class provides multiple  Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It
85  granularities (including nanoseconds) for both accessing time and  provides a high-precision timing facility that is distinct from and
86  performing time-out based operations.  uncoordinated with <tt>System.currentTimeMillis</tt>.
87    
88  <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.)  
89    
90  <p> Additionally, java.lang.ThreadLocal now supports a means to remove  The {@link java.lang.ThreadLocal} class now supports a means to remove
91  a ThreadLocal, which is needed in some thread-pool and worker-thread  a ThreadLocal, which is needed in some thread-pool and worker-thread
92  designs.  designs.
93    
94    
95    
96    <hr>    <hr>
   <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>  
97   </body>   </body>
98  </html>  </html>
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