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16  <p>  <p>
17  <em>  <em>
18   Disclaimer - This prototype is experimental code developed as part of   Disclaimer - This prototype is experimental code developed as part of
19   JSR166 and made available to the developer community for use   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   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   specification, language and implementation are subject to change as a
22   result of your feedback. Because these features have not yet been   result of your feedback. Because these features have not yet been
# Line 24  Line 24 
24   their inclusion in a product.   their inclusion in a product.
25  </em>  </em>
26    
27  <p>  <p> JSR166 introduces package <tt>java.util.concurrent</tt> containing
28  Package java.util.concurrent contains utility classes that are  utility classes commonly useful in concurrent programming. Like
29  commonly useful in concurrent programming. Like package java.util, it  package java.util, it includes a few small standardized extensible
30  includes a few small standardized extensible frameworks, as well as  frameworks, as well as some classes that provide useful functionality
31  some classes that provide useful functionality and are otherwise  and are otherwise tedious or difficult to implement. JSR-166 focusses
32  tedious or difficult to implement.  In this JSR, we have been  on breadth, prviding critical functionality useful across a wide range
33  conservative in selecting only those APIs and implementations that are  of concurrent programming styles and applications, ranging from
34  useful enough to encourage nearly all concurrent programmers to use  low-level atomic operations, to customizable locks and synchronization
35  routinely.  JSR 166 also includes a few changes and additions in  aids, to various concurrent data structures, to thread pools.
36  packages outside of java.util.concurrent: java.lang, to address  Descriptions of the main components may be found in the associated
37  uncaught exceptions, and java.util to better integrate queues.  package documentation.
 The API covers:  
   
   <ul>  
     <li> Queues  
     <li> Executors  
     <li> Locks  
     <li> Condition variables  
     <li> Atomic variables  
     <li> Timing  
     <li> Synchronizers  
     <li> Concurrent Collections  
     <li> Uncaught Exception Handlers  
   </ul>  
   
   
 The main rationale for JSR 166 is that threading primitives, such as  
 synchronized blocks, Object.wait and Object.notify, are insufficient  
 for many programming tasks.  Currently, developers can use only the  
 concurrency control constructs provided in the Java language  
 itself. These are too low level for some applications, and are  
 incomplete for others.  As a result, application programmers are often  
 forced to implement their own concurrency facilities, resulting in  
 enormous duplication of effort creating facilities that are  
 notoriously hard to get right and even harder to optimize.  Offering a  
 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.  
   
 <p>  
 Here are brief descriptions and rationales of the main components.  
 For details see the javadocs at <a  
 href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a>  
38    
39    <p> JSR166 also includes a few changes and additions in packages
40    outside of java.util.concurrent.  Here are brief descriptions.
41    
42  <h2>Queues</h2>  <h2>Queues</h2>
43    
44  A basic (nonblocking) Queue interface that is compatatible with  A basic (nonblocking) {@link java.util.Queue} interface extending
45  java.util.Collections will be introduced into java.util. Also,  java.util.Collection is introduced into java.util. Existing class
46  although it is at the borders of being in scope of JSR-166,  java.util.LinkedList is adapted to support Queue, and a new
47  java.util.LinkedList will be adapted to support Queue, and  non-thread-safe {@link java.util.PriorityQueue} is added.
 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  
 below.)  
   
   
 <h2>Executors</h2>  
   
 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.  
   
 <h2>Locks</h2>  
   
 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.  
   
 <p>  
 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.  
   
 <p>  
 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.  
   
 <h2>Conditions</h2>  
   
 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.  
 <p>  
 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.  
 <p>  
 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.  
   
   
 <h2>Timing</h2>  
   
 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.  
   
   
 <h2>Synchronizers</h2>  
   
 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.  
48    
49  <h2>Uncaught Exception Handlers</h2>  <h2>Uncaught Exception Handlers</h2>
50    
51  The java.lang.Thread class will be modified to allow per-thread  The java.lang.Thread class is modified to allow per-thread
52  installation of handlers for uncaught exceptions. Ths optionally  installation of handlers for uncaught exceptions. Ths optionally
53  disassociates these handlers from ThreadGroups, which has proven to be  disassociates these handlers from ThreadGroups, which has proven to be
54  too inflexible in many multithreaded programs. (Note that the combination  too inflexible in many multithreaded programs. (Note that the
55  of features in JSR 166 make ThreadGroups even less likely to  combination of features in JSR166 make ThreadGroups even less likely
56  be used in most programs. Perhaps they will eventually be deprecated.)  to be used in most programs. Perhaps they will eventually be
57  <p>  deprecated.)
58  Additionally,  ThreadLocals will now support a means to  
59  remove a ThreadLocals, which is needed in some thread-pool and  <h2>High precision timing</h2>
60  worker-thread designs.  
61    Method <tt>nanoTime</tt> is added to <tt>java.lang.System</tt>. It
62    provides a high-precision timing facility that is distinct from
63    and uncoordinated with <tt>System.currentTimeMillis</tt>.
64    
65    <h2>Removing ThreadLocals</h2>
66    
67    The java.lang.ThreadLocal class now supports a means to remove a
68    ThreadLocal, which is needed in some thread-pool and worker-thread
69    designs.
70    
71    <hr>    <hr>
72    <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>

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