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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
14  <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .  Community Draft review.  To check for further updates, access a
15    preliminary prototype release of main functionality, or join a mailing
16  <p>  list discussing this JSR, 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  .  <p>
20   as-is. It is not a supported product. Use it at your own risk. The  
21   specification, language and implementation are subject to change as a  <em> <b>Disclaimer</b>. The prototype implementation is experimental
22   result of your feedback. Because these features have not yet been  code developed as part of JCP JSR-166 is made available to the
23   approved for addition to the Java language, there is no schedule for  developer community for use as-is. It is not a supported product. Use
24   their inclusion in a product.  it at your own risk. The specification, language and implementation
25  </em>  are subject to change as a result of your feedback. Because these
26    features have not yet been approved for addition to the Java language,
27  <p> Package java.util.concurrent contains utility classes commonly  there is no schedule for their inclusion in a product.  </em>
28  useful in concurrent programming. Like package java.util, it includes  
29  a few small standardized extensible frameworks, as well as some  <p> <em> <b>Disclaimer</b>.  This draft specification was produced
30  classes that provide useful functionality and are otherwise tedious or  using JDK1.4 tools plus some preprocessing. The resulting javadocs do
31  difficult to implement.  JSR166 also includes a few changes and  not yet correctly render other planned JDK1.5 constructs on which
32  additions in packages outside of java.util.concurrent: java.lang, to  JSR-166 relies, most notably the use of generic types. We are
33  address uncaught exceptions, and java.util to better integrate with  releasing this version now (before the availability of JDK1.5-based
34  collections.  Since the target release is JDK1.5, many APIs use  tools) because, even though they are misformatted and sometimes lack
35  generics to parameterize on types.  Here are brief descriptions of the  proper cross-referencing, they otherwise convey the intended
36  main components.  specifications.  </em>
37    
38  <h2>Executors</h2>  <p> JSR-166 introduces package <tt>java.util.concurrent</tt>
39    containing utility classes commonly useful in concurrent
40  {@link java.util.concurrent.Executor} is a simple standardized  programming. Like package <tt>java.util</tt>, it includes a few small
41  interface for defining custom thread-like subsystems, including thread  standardized extensible frameworks, as well as some classes that
42  pools, asynch-IO, and lightweight task frameworks.  Depending on which  provide useful functionality and are otherwise tedious or difficult to
43  concrete Executor class is being used, tasks may execute in a newly  implement.
44  created thread, an existing task-execution thread, or the thread  
45  calling <tt>execute()</tt>, and may execute sequentially or  <p>JSR-166 focusses on breadth, providing critical functionality
46  concurrently.  Executors also standardize ways of calling threads that  useful across a wide range of concurrent programming styles and
47  compute functions returning results, via a {@link  applications, ranging from low-level atomic operations, to
48  java.util.concurrent.Future}. This is supported in part by defining  customizable locks and synchronization aids, to various concurrent
49  interface {@link java.util.concurrent.Callable}, the argument/result  data structures, to high-level execution agents including thread
50  analog of Runnable.  pools. This diversity reflects the range of contexts in which
51    developers of concurrent programs have been found to require or desire
52  <p> {@link java.util.concurrent.ExecutorService} provides a more  support not previously available in J2SE, which also keeping the
53  complete framework for executing Runnables.  An ExecutorService  resulting package small; providing only that minimial support for
54  manages queueing and scheduling of tasks, and allows controlled  which it makes sense to standardize.
55  shutdown.  The two primary implementations of ExecutorService are  
56  {@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and  <p>Descriptions and brief motivations for the main components may be
57  flexible thread pool and {@link  found in the associated package documentation.  JSR-166 also includes
58  java.util.concurrent.ScheduledExecutor}, which adds support for  a few changes and additions in packages outside of
59  delayed and periodic task execution.  These, and other Executors can  java.util.concurrent.  Here are brief descriptions.
 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.  
60    
61  <h2>Queues</h2>  <h2>Queues</h2>
62    
63  A basic (nonblocking) {@link java.util.Queue} interface extending  A basic (nonblocking) {@link java.util.Queue} interface extending
64  java.util.Collection is introduced into java.util. Existing class  {@link java.util.Collection} is introduced into
65  java.util.LinkedList is adapted to support Queue, and a new  <tt>java.util</tt>. Existing class {@link java.util.LinkedList} is
66  non-thread-safe {@link java.util.concurrent.java.util.PriorityQueue}  adapted to support Queue, and a new non-thread-safe {@link
67  is added.  The java.util.concurrent {@link  java.util.PriorityQueue} is added.
68  java.util.concurrent.LinkedQueue} class supplies an efficient  
69  thread-safe non-blocking queue.  <h2>Threads</h2>
70    
71  <p> Five implementations in java.util.concurrent support the extended  Three minor changes are introduced to the {@link java.lang.Thread}
72  {@link java.util.concurrent.BlockingQueue} interface, that defines  class:
73  blocking versions of put and take: {@link  <ul>
74  java.util.concurrent.LinkedBlockingQueue}, {@link    <li> It now allows per-thread installation of handlers for uncaught
75  java.util.concurrent.ArrayBlockingQueue}, {@link    exceptions. Ths optionally disassociates handlers from ThreadGroups,
76  java.util.concurrent.SynchronousQueue}, {@link    which has proven to be too inflexible. (Note that the combination of
77  java.util.concurrent.PriorityBlockingQueue}, and {@link DelayQueue}.    features in JSR-166 make ThreadGroups even less likely to be used in
78      most programs. Perhaps they will eventually be deprecated.)
79    
80  <h2>Locks</h2>    <li> Access checks are no longer required when a Thread interrupts
81      <em>itself</em>.  The <tt>interrupt</tt> method is the only way to
82  The {@link java.util.concurrent.Lock} interface supports locking    re-assert a thread's interruption status (and in the case of
83  disciplines that differ in semantics (reentrant, fair, etc), and that    self-interruption has no other effect than this).  The check here
84  can be used in non-block-structured contexts including hand-over-hand    previously caused unjustifiable and uncontrollable failures when
85  and lock reordering algorithms. This flexibility comes at the price of    restricted code invoked library code that must reassert interruption
86  more awkward syntax.  Implementations include {@link    to correctly propagate status when encountering some
87  java.util.concurrent.ReentrantLock} and {@link    <tt>InterruptedExceptions</tt>.
88  java.util.concurrent.FairReentrantLock}.    <li> The <tt>destroy</tt> method, which has never been implemented,
89      has finally been deprecated. This is just a spec change, reflecting
90  <p> The {@link java.util.concurrent.Locks} class additionally supports    the fact that that the reason it has never been implmented is that
91  some common trylock-designs using builtin locks.    it was undesirable and unworkable.
92    </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.  
93    
94  <h2>Timing</h2>  <h2>Timing</h2>
95    
96  The {@link java.util.concurrent.TimeUnit} class provides multiple  Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It
97  granularities (including nanoseconds) for both accessing time and  provides a high-precision timing facility that is distinct from and
98  performing time-out based operations.  uncoordinated with <tt>System.currentTimeMillis</tt>.
99    
100  <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.)  
101    
102  <p> Additionally, java.lang.ThreadLocal now supports a means to remove  The {@link java.lang.ThreadLocal} class now supports a means to remove
103  a ThreadLocal, which is needed in some thread-pool and worker-thread  a ThreadLocal, which is needed in some thread-pool and worker-thread
104  designs.  designs.
105    
106    
107    
108    <hr>    <hr>
109    <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>    <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
110   </body>   </body>
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