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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 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   JCP 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> Package java.util.concurrent contains utility classes commonly  applications, ranging from low-level atomic operations, to
28  useful in concurrent programming. Like package java.util, it includes  customizable locks and synchronization aids, to various concurrent
29  a few small standardized extensible frameworks, as well as some  data structures, to high-level execution agents including thread
30  classes that provide useful functionality and are otherwise tedious or  pools. This diversity reflects the range of contexts in which
31  difficult to implement.  JSR166 also includes a few changes and  developers of concurrent programs have been found to require or desire
32  additions in packages outside of java.util.concurrent: java.lang, to  support not previously available in J2SE, while also keeping the
33  address uncaught exceptions, and java.util to better integrate with  resulting package small; providing only functionality that has been
34  collections.  Since the target release is JDK1.5, many APIs use  found to be worthwhile to standardize.
35  generics to parameterize on types.  Here are brief descriptions of the  
36  main components.  <p>Descriptions and brief motivations for the main components may be
37    found in the associated package documentation.  JSR-166 also includes
38  <h2>Executors</h2>  a few changes and additions in packages outside of
39    java.util.concurrent.  Here are brief descriptions.
 {@link java.util.concurrent.Executor} is a simple standardized  
 interface for defining custom thread-like subsystems, including thread  
 pools, asynch-IO, and lightweight task frameworks.  Depending on which  
 concrete Executor class is being used, tasks may execute in a newly  
 created thread, an existing task-execution thread, or the thread  
 calling <tt>execute()</tt>, and may execute sequentially or  
 concurrently.  Executors also standardize ways of calling threads that  
 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 java.util.concurrent.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.  
40    
41  <h2>Queues</h2>  <h2>Queues</h2>
42    
43  A basic (nonblocking) {@link java.util.Queue} interface extending  A basic (nonblocking) {@link java.util.Queue} interface extending
44  java.util.Collection is introduced into java.util. Existing class  {@link java.util.Collection} is introduced into
45  java.util.LinkedList is adapted to support Queue, and a new  <code>java.util</code>. Existing class {@link java.util.LinkedList} is
46  non-thread-safe {@link java.util.PriorityQueue} is added.  The  adapted to support Queue, and a new non-thread-safe {@link
47  java.util.concurrent {@link  java.util.PriorityQueue} is added.
48  java.util.concurrent.ConcurrentLinkedQueue} class supplies an  
49  efficient sclable thread-safe non-blocking FIFO queue, and {@link  <h2>Threads</h2>
50  java.util.concurrent.ConcurrentLinkedStack} provides a similar  
51  non-blocking LIFO stack.  Three minor changes are introduced to the {@link java.lang.Thread}
52    class:
53  <p> Five implementations in java.util.concurrent support the extended  <ul>
54  {@link java.util.concurrent.BlockingQueue} interface, that defines    <li> It now allows per-thread installation of handlers for uncaught
55  blocking versions of put and take: {@link    exceptions. This optionally disassociates handlers from ThreadGroups,
56  java.util.concurrent.LinkedBlockingQueue}, {@link    which has proven to be too inflexible. (Note that the combination of
57  java.util.concurrent.ArrayBlockingQueue}, {@link    features in JSR-166 make ThreadGroups even less likely to be used in
58  java.util.concurrent.SynchronousQueue}, {@link    most programs. Perhaps they will eventually be deprecated.)
59  java.util.concurrent.PriorityBlockingQueue}, and  
60  {@link java.util.concurrent.DelayQueue}.    <li> Access checks are no longer required when a Thread interrupts
61      <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  <h2>Locks</h2>    self-interruption has no other effect than this).  The check here
64      previously caused unjustifiable and uncontrollable failures when
65  The {@link java.util.concurrent.Lock} interface supports locking    restricted code invoked library code that must reassert interruption
66  disciplines that differ in semantics (reentrant, fair, etc), and that    to correctly propagate status when encountering some
67  can be used in non-block-structured contexts including hand-over-hand    <code>InterruptedExceptions</code>.
68  and lock reordering algorithms. This flexibility comes at the price of    <li> The <code>destroy</code> method, which has never been implemented,
69  more awkward syntax.  Implementations include {@link    has finally been deprecated. This is just a spec change, reflecting
70  java.util.concurrent.ReentrantLock} and {@link    the fact that the reason it has never been implemented is that
71  java.util.concurrent.FairReentrantLock}.    it was undesirable and unworkable.
72    </ul>
 <p> The {@link java.util.concurrent.Locks} class additionally supports  
 some common trylock-designs using builtin locks.  
   
 <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.  
73    
74  <h2>Timing</h2>  <h2>Timing</h2>
75    
76  The {@link java.util.concurrent.TimeUnit} class provides multiple  Method <code>nanoTime</code> is added to {@link java.lang.System}. It
77  granularities (including nanoseconds) for both accessing time and  provides a high-precision timing facility that is distinct from and
78  performing time-out based operations.  uncoordinated with <code>System.currentTimeMillis</code>.
79    
80  <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 java.util.concurrent.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>  
   
 Besides Queues, 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>The "Concurrent" prefix for classes is a shorthand  
 indicating several differences from similar "synchronized"  
 classes. For example <tt>java.util.Hashtable</tt> and  
 <tt>Collections.synchronizedMap(new HashMap())</tt> are  
 synchronized. But {@link  
 java.util.concurrent.ConcurrentHashMap} is "concurrent".  
 A concurrent collection (among other kinds of classes) is  
 thread-safe, but not governed by a single exclusion lock. So, in the  
 particular case of ConcurrentHashMap, it safely permits any number of  
 concurrent reads as well as a tunable number of concurrent writes.  
 There may still be a role for "synchronized" classes in some  
 multithreaded programs -- they can sometimes be useful when you need  
 to prevent ALL access to a collection via a single lock, at the  
 expense of much poor scalability. In all other cases, "concurrent"  
 versions are normally preferable.  
   
 <p> Most concurrent Collection implementations (including most Queues)  
 also 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.)  
81    
82  <p> Additionally, java.lang.ThreadLocal now supports a means to remove  The {@link java.lang.ThreadLocal} class now supports a means to remove
83  a ThreadLocal, which is needed in some thread-pool and worker-thread  a ThreadLocal, which is needed in some thread-pool and worker-thread
84  designs.  designs.
85    
86    
87    
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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