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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
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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.PriorityQueue 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> Executors provide a framework for executing Runnables.  The  
 Executor manages queueing and scheduling of tasks, and creation and  
 teardown of threads.  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 execute(), and may  
 execute sequentially or concurrently.  
   
 <p> Several concrete implementations of Executor are included in  
 java.util.concurrent, including ThreadPoolExecutor, a flexible thread  
 pool and ScheduledExecutor, which adds support for delayed and  
 periodic task execution.  Executor can be used in conjunction with  
 FutureTask (which implements Runnable) to asynchronously start a  
 potentially long-running computation and query the FutureTask to  
 determine if its execution has completed.  
   
 <p> The <tt>Executors</tt> class provides factory methods for all  
 of the types of executors provided in  
 <tt>java.util.concurrent</tt>.  
   
   
 <h2>Locks</h2>  
   
 The Lock interface supports locking disciplines that differ in  
 semantics (reentrant, fair, 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 ReentrantLock and  
 FairReentrantLock.  
   
 <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.  
   
   
 <h2>Atomic variables</h2>  
   
 The atomic subpackage includes a small library of classes, including  
 AtomicInteger, AtomicLong, and AtomicReference that support variables  
 performinf compareAndSet (CAS) and related atomic operations.  
   
 <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 TimeUnit, 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 FairSemaphores are classic concurrency tools.  
 CountDownLatches 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: ConcurrentHashMap, 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    deprecated.)
58    
59    <h2>High precision timing</h2>
60    
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  <p> Additionally, ThreadLocals will now support a means to remove a  The java.lang.ThreadLocal class now supports a means to remove a
68  ThreadLocal, which is needed in some thread-pool and worker-thread  ThreadLocal, which is needed in some thread-pool and worker-thread
69  designs.  designs.
70    
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