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revision 1.6, Thu Jun 26 10:46:55 2003 UTC revision 1.7, Tue Jul 8 00:46:24 2003 UTC
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24   their inclusion in a product.   their inclusion in a product.
25  </em>  </em>
26    
27  <p> Package java.util.concurrent contains utility classes commonly  <p> JSR166 introduces package <tt>java.util.concurrent</tt> containing utility
28  useful in concurrent programming. Like package java.util, it includes  classes commonly useful in concurrent programming. Like package
29  a few small standardized extensible frameworks, as well as some  java.util, it includes a few small standardized extensible frameworks,
30  classes that provide useful functionality and are otherwise tedious or  as well as some classes that provide useful functionality and are
31  difficult to implement.  JSR166 also includes a few changes and  otherwise tedious or difficult to implement.  Descriptions of the main
32  additions in packages outside of java.util.concurrent: java.lang, to  components may be found in the associated package documentation.
33  address uncaught exceptions, and java.util to better integrate with  
34  collections.  Since the target release is JDK1.5, many APIs use  <p> JSR166 also includes a few changes and additions in packages
35  generics to parameterize on types.  Here are brief descriptions of the  outside of java.util.concurrent: java.lang, to address uncaught
36  main components.  exceptions, and java.util to better integrate with collections.  Here
37    are brief descriptions.
 <h2>Executors</h2>  
   
 {@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.  
38    
39  <h2>Queues</h2>  <h2>Queues</h2>
40    
41  A basic (nonblocking) {@link java.util.Queue} interface extending  A basic (nonblocking) {@link java.util.Queue} interface extending
42  java.util.Collection is introduced into java.util. Existing class  java.util.Collection is introduced into java.util. Existing class
43  java.util.LinkedList is adapted to support Queue, and a new  java.util.LinkedList is adapted to support Queue, and a new
44  non-thread-safe {@link java.util.PriorityQueue} is added.  The  non-thread-safe {@link java.util.PriorityQueue} is added.
 java.util.concurrent {@link  
 java.util.concurrent.ConcurrentLinkedQueue} class supplies an  
 efficient sclable thread-safe non-blocking FIFO queue, and {@link  
 java.util.concurrent.ConcurrentLinkedStack} provides a similar  
 non-blocking LIFO stack.  
   
 <p> Five implementations in java.util.concurrent support the extended  
 {@link java.util.concurrent.BlockingQueue} interface, that defines  
 blocking versions of put and take: {@link  
 java.util.concurrent.LinkedBlockingQueue}, {@link  
 java.util.concurrent.ArrayBlockingQueue}, {@link  
 java.util.concurrent.SynchronousQueue}, {@link  
 java.util.concurrent.PriorityBlockingQueue}, and  
 {@link java.util.concurrent.DelayQueue}.  
   
   
 <h2>Locks</h2>  
   
 The {@link java.util.concurrent.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 {@link  
 java.util.concurrent.ReentrantLock} and {@link  
 java.util.concurrent.FairReentrantLock}.  
   
 <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.  
   
 <h2>Timing</h2>  
   
 The {@link java.util.concurrent.TimeUnit} class provides multiple  
 granularities (including nanoseconds) for both accessing time and  
 performing time-out based operations.  
   
 <h2>Synchronizers</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.  
45    
46  <h2>Uncaught Exception Handlers</h2>  <h2>Uncaught Exception Handlers</h2>
47    

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