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<p> |
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<em> |
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Disclaimer - This prototype is experimental code developed as part of |
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JSR166 and made available to the developer community for use |
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JCP JSR166 and made available to the developer community for use |
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as-is. It is not a supported product. Use it at your own risk. The |
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specification, language and implementation are subject to change as a |
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result of your feedback. Because these features have not yet been |
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their inclusion in a product. |
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</em> |
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|
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<p> |
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Package java.util.concurrent contains utility classes that are |
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commonly useful in concurrent programming. Like package java.util, it |
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includes a few small standardized extensible frameworks, as well as |
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some classes that provide useful functionality and are otherwise |
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tedious or difficult to implement. In this JSR, we have been |
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conservative in selecting only those APIs and implementations that are |
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useful enough to encourage nearly all concurrent programmers to use |
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routinely. JSR 166 also includes a few changes and additions in |
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packages outside of java.util.concurrent: java.lang, to address |
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uncaught exceptions, and java.util to better integrate queues. |
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The API covers: |
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|
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<ul> |
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<li> Queues |
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<li> Executors |
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<li> Locks |
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< |
<li> Condition variables |
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< |
<li> Atomic variables |
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< |
<li> Timing |
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< |
<li> Synchronizers |
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<li> Concurrent Collections |
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<li> Uncaught Exception Handlers |
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</ul> |
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|
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|
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The main rationale for JSR 166 is that threading primitives, such as |
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synchronized blocks, Object.wait and Object.notify, are insufficient |
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for many programming tasks. Currently, developers can use only the |
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concurrency control constructs provided in the Java language |
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itself. These are too low level for some applications, and are |
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incomplete for others. As a result, application programmers are often |
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forced to implement their own concurrency facilities, resulting in |
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enormous duplication of effort creating facilities that are |
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notoriously hard to get right and even harder to optimize. Offering a |
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standard set of concurrency utilities will ease the task of writing a |
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wide variety of multithreaded applications and generally improve the |
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quality of the applications that use them. |
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> |
<p> Package java.util.concurrent contains utility classes commonly |
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> |
useful in concurrent programming. Like package java.util, it includes |
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> |
a few small standardized extensible frameworks, as well as some |
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> |
classes that provide useful functionality and are otherwise tedious or |
31 |
> |
difficult to implement. JSR166 also includes a few changes and |
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> |
additions in packages outside of java.util.concurrent: java.lang, to |
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> |
address uncaught exceptions, and java.util to better integrate with |
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> |
collections. Since the target release is JDK1.5, many APIs use |
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> |
generics to parameterize on types. Here are brief descriptions of the |
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main components. |
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|
|
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<p> |
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Here are brief descriptions and rationales of the main components. |
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For details see the javadocs at <a |
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href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a> |
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<h2>Executors</h2> |
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|
|
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{@link java.util.concurrent.Executor} is a simple standardized |
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interface for defining custom thread-like subsystems, including thread |
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pools, asynch-IO, and lightweight task frameworks. Depending on which |
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+ |
concrete Executor class is being used, tasks may execute in a newly |
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+ |
created thread, an existing task-execution thread, or the thread |
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+ |
calling <tt>execute()</tt>, and may execute sequentially or |
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+ |
concurrently. Executors also standardize ways of calling threads that |
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+ |
compute functions returning results, via a {@link |
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java.util.concurrent.Future}. This is supported in part by defining |
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interface {@link java.util.concurrent.Callable}, the argument/result |
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+ |
analog of Runnable. |
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+ |
|
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<p> {@link java.util.concurrent.ExecutorService} provides a more |
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+ |
complete framework for executing Runnables. An ExecutorService |
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+ |
manages queueing and scheduling of tasks, and allows controlled |
55 |
+ |
shutdown. The two primary implementations of ExecutorService are |
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+ |
{@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and |
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+ |
flexible thread pool and {@link |
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+ |
java.util.concurrent.ScheduledExecutor}, which adds support for |
59 |
+ |
delayed and periodic task execution. These, and other Executors can |
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+ |
be used in conjunction with a {@link FutureTask} to asynchronously |
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+ |
start a potentially long-running computation and query the FutureTask |
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+ |
to determine if its execution has completed, or cancel it. |
63 |
+ |
|
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+ |
<p> The {@link java.util.concurrent.Executors} class provides factory |
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+ |
methods for the most common kinds and styles of Executors, as well as |
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+ |
a few utilities methods for using them. |
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|
|
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|
<h2>Queues</h2> |
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|
|
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A basic (nonblocking) Queue interface that is compatatible with |
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java.util.Collections will be introduced into java.util. Also, |
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although it is at the borders of being in scope of JSR-166, |
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java.util.LinkedList will be adapted to support Queue, and |
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< |
a new non-thread-safe java.util.PriorityQueue will be added. |
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> |
A basic (nonblocking) {@link java.util.Queue} interface extending |
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> |
java.util.Collection is introduced into java.util. Existing class |
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> |
java.util.LinkedList is adapted to support Queue, and a new |
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> |
non-thread-safe {@link java.util.concurrent.java.util.PriorityQueue} |
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> |
is added. The java.util.concurrent {@link |
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> |
java.util.concurrent.LinkedQueue} class supplies an efficient |
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> |
thread-safe non-blocking queue. |
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|
|
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|
<p> Five implementations in java.util.concurrent support the extended |
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< |
BlockingQueue interface, that defines blocking versions of put and |
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< |
take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, |
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< |
PriorityBlockingQueue, and DelayQueue. Additionally, |
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< |
java.util.concurrent.LinkedQueue supplies an efficient thread-safe |
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< |
non-blocking queue. |
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|
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< |
<p> Since the target release is JDK1.5, and generics are slated to be |
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< |
in 1.5, Queues are parametrized on element type. (Also some others |
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< |
below.) |
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< |
|
91 |
< |
|
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< |
<h2>Executors</h2> |
93 |
< |
|
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< |
Executors provide a simple standardized interface for defining custom |
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< |
thread-like subsystems, including thread pools, asynch-IO, and |
96 |
< |
lightweight task frameworks. Executors also standardize ways of |
97 |
< |
calling threads that compute functions returning results, via |
98 |
< |
Futures. This is supported in part by defining interface Callable, the |
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< |
argument/result analog of Runnable. |
100 |
< |
|
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< |
<p> Executors provide a framework for executing Runnables. The |
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< |
Executor manages queueing and scheduling of tasks, and creation and |
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< |
teardown of threads. Depending on which concrete Executor class is |
104 |
< |
being used, tasks may execute in a newly created thread, an existing |
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< |
task-execution thread, or the thread calling execute(), and may |
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< |
execute sequentially or concurrently. |
107 |
< |
|
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< |
<p> Several concrete implementations of Executor are included in |
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< |
java.util.concurrent, including ThreadPoolExecutor, a flexible thread |
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< |
pool and ScheduledExecutor, which adds support for delayed and |
111 |
< |
periodic task execution. Executor can be used in conjunction with |
112 |
< |
FutureTask (which implements Runnable) to asynchronously start a |
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< |
potentially long-running computation and query the FutureTask to |
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< |
determine if its execution has completed. |
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< |
|
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<p> The <tt>Executors</tt> class provides factory methods for all |
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< |
of the types of executors provided in |
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<tt>java.util.concurrent</tt>. |
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> |
{@link java.util.concurrent.BlockingQueue} interface, that defines |
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> |
blocking versions of put and take: {@link |
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> |
java.util.concurrent.LinkedBlockingQueue}, {@link |
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> |
java.util.concurrent.ArrayBlockingQueue}, {@link |
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> |
java.util.concurrent.SynchronousQueue}, {@link |
84 |
> |
java.util.concurrent.PriorityBlockingQueue}, and {@link DelayQueue}. |
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|
|
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|
|
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|
<h2>Locks</h2> |
88 |
|
|
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< |
The Lock interface supports locking disciplines that differ in |
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< |
semantics (reentrant, fair, etc), and that can be used in |
91 |
< |
non-block-structured contexts including hand-over-hand and lock |
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< |
reordering algorithms. This flexibility comes at the price of more |
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< |
awkward syntax. Implementations include ReentrantLock and |
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< |
FairReentrantLock. |
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< |
|
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< |
<p> |
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< |
The Locks class additionally supports trylock-designs using builtin |
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< |
locks without needing to use Lock classes. This requires adding new |
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< |
capabilities to builtin locks inside JVMs. |
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< |
|
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< |
<p> |
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< |
A ReadWriteLock interface similarly defines locks that may be shared |
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< |
among readers but are exclusive to writers. For this release, only a |
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< |
single implementation, ReentrantReadWriteLock, is planned, since it |
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> |
The {@link java.util.concurrent.Lock} interface supports locking |
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> |
disciplines that differ in semantics (reentrant, fair, etc), and that |
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> |
can be used in non-block-structured contexts including hand-over-hand |
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> |
and lock reordering algorithms. This flexibility comes at the price of |
93 |
> |
more awkward syntax. Implementations include {@link |
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> |
java.util.concurrent.ReentrantLock} and {@link |
95 |
> |
java.util.concurrent.FairReentrantLock}. |
96 |
> |
|
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> |
<p> The {@link java.util.concurrent.Locks} class additionally supports |
98 |
> |
some common trylock-designs using builtin locks. |
99 |
> |
|
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> |
<p> The {@link java.util.concurrent.ReadWriteLock} interface similarly |
101 |
> |
defines locks that may be shared among readers but are exclusive to |
102 |
> |
writers. Only a single implementation, {@link |
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> |
java.util.concurrent.ReentrantReadWriteLock}, is provided, since it |
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|
covers all standard usage contexts. But programmers may create their |
105 |
|
own implementations to cover nonstandard requirements. |
106 |
|
|
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|
<h2>Conditions</h2> |
108 |
|
|
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< |
A Condition class provides the kinds of condition variables associated |
110 |
< |
with monitors in other cocurrent languages, as well as pthreads |
111 |
< |
condvars. Their support reduces the need for tricky and/or |
112 |
< |
inefficient solutions to many classic concurrent problems. Conditions |
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< |
also address the annoying problem that Object.wait(msecs) does not |
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< |
return an indication of whether the wait timed out. This leads to |
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< |
error-prone code. Since this method is in class Object, the problem is |
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< |
basically unfixable. |
152 |
< |
<p> |
153 |
< |
To avoid compatibility problems, the names of Condition methods need |
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< |
to be different than Object versions. The downside of this is that |
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< |
people can make the mistake of calling cond.notify instead of |
156 |
< |
cond.signal. However, they will get IllegalMonitorState exceptions if |
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< |
they do, so they can detect the error if they ever run the code. |
158 |
< |
|
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> |
The {@link java.util.concurrent.Condition} interface describes the |
110 |
> |
kinds of condition variables associated with monitors in other |
111 |
> |
concurrent languages, as well as pthreads-style condvars. Their |
112 |
> |
support reduces the need for tricky and/or inefficient solutions to |
113 |
> |
many classic concurrent problems. To avoid compatibility problems, |
114 |
> |
the names of Condition methods are different than Object versions. |
115 |
|
|
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< |
<h2>Atomic variables</h2> |
116 |
> |
<h2>Atomics</h2> |
117 |
|
|
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|
The atomic subpackage includes a small library of classes, including |
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< |
AtomicInteger, AtomicLong, and AtomicReference that support variables |
120 |
< |
performinf compareAndSet (CAS) and related atomic operations. |
119 |
> |
AtomicInteger, AtomicLong, and AtomicReference that support |
120 |
> |
compareAndSet (CAS) and related atomic operations. |
121 |
|
|
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|
<h2>Timing</h2> |
123 |
|
|
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< |
Java has always supported sub-millisecond versions of several native |
125 |
< |
time-out-based methods (such as Object.wait), but not methods to |
126 |
< |
actually perform timing in finer-grained units. We address this by |
171 |
< |
introducing class TimeUnit, which provides multiple granularities for |
172 |
< |
both accessing time and performing time-out based operations. |
173 |
< |
|
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> |
The {@link java.util.concurrent.TimeUnit} class provides multiple |
125 |
> |
granularities (including nanoseconds) for both accessing time and |
126 |
> |
performing time-out based operations. |
127 |
|
|
128 |
|
<h2>Synchronizers</h2> |
129 |
|
|
130 |
|
Five classes aid common special-purpose synchronization idioms. |
131 |
< |
Semaphores and FairSemaphores are classic concurrency tools. |
132 |
< |
CountDownLatches are very simple yet very common objects useful for |
133 |
< |
blocking until a single signal, event, or condition holds. |
134 |
< |
CyclicBarriers are resettable multiway synchronization points very |
135 |
< |
common in some styles of parallel programming. Exchangers allow two |
131 |
> |
{@link java.util.concurrent.Semaphore} and {@link |
132 |
> |
java.util.concurrent.FairSemaphore} are classic concurrency tools. |
133 |
> |
{@link java.util.concurrent.CountDownLatch} is very simple yet very |
134 |
> |
common utility for blocking until a single signal, event, or condition |
135 |
> |
holds. A {@link CyclicBarrier} is a resettable multiway |
136 |
> |
synchronization point common in some styles of parallel |
137 |
> |
programming. An {@link java.util.concurrent.Exchanger} allows two |
138 |
|
threads to exchange objects at a rendezvous point. |
139 |
|
|
185 |
– |
|
140 |
|
<h2>Concurrent Collections</h2> |
141 |
|
|
142 |
< |
JSR 166 will supply a few Collection implementations designed for use |
143 |
< |
in multithreaded contexts: ConcurrentHashMap, CopyOnWriteArrayList, |
144 |
< |
and CopyOnWriteArraySet. |
142 |
> |
This package supplies a few Collection implementations designed for |
143 |
> |
use in multithreaded contexts: {@link |
144 |
> |
java.util.concurrent.ConcurrentHashMap}, {@link |
145 |
> |
java.util.concurrent.CopyOnWriteArrayList}, and {@link |
146 |
> |
java.util.concurrent.CopyOnWriteArraySet}. |
147 |
> |
|
148 |
> |
<p> Most concurrent Collection implementations (including most Queues) |
149 |
> |
differ from the usual java.util conventions in that their Iterators |
150 |
> |
provide <em>weakly consistent</em> rather than fast-fail traversal. A |
151 |
> |
weakly consistent iterator is thread-safe, but does not necessarily |
152 |
> |
freeze the collection while iterating, so it may (or may not) reflect |
153 |
> |
any updates since the iterator was created. |
154 |
|
|
155 |
|
<h2>Uncaught Exception Handlers</h2> |
156 |
|
|
157 |
< |
The java.lang.Thread class will be modified to allow per-thread |
157 |
> |
The java.lang.Thread class is modified to allow per-thread |
158 |
|
installation of handlers for uncaught exceptions. Ths optionally |
159 |
|
disassociates these handlers from ThreadGroups, which has proven to be |
160 |
< |
too inflexible in many multithreaded programs. (Note that the combination |
161 |
< |
of features in JSR 166 make ThreadGroups even less likely to |
162 |
< |
be used in most programs. Perhaps they will eventually be deprecated.) |
160 |
> |
too inflexible in many multithreaded programs. (Note that the |
161 |
> |
combination of features in JSR166 make ThreadGroups even less likely |
162 |
> |
to be used in most programs. Perhaps they will eventually be |
163 |
> |
deprecated.) |
164 |
|
|
165 |
< |
<p> Additionally, ThreadLocals will now support a means to remove a |
166 |
< |
ThreadLocal, which is needed in some thread-pool and worker-thread |
165 |
> |
<p> Additionally, java.lang.ThreadLocal now supports a means to remove |
166 |
> |
a ThreadLocal, which is needed in some thread-pool and worker-thread |
167 |
|
designs. |
168 |
|
|
169 |
|
<hr> |