| 1 |
|
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> |
| 2 |
|
<html> |
| 3 |
|
<head> |
| 4 |
< |
<title>JSR 166 Snapshot Introduction.</title> |
| 4 |
> |
<title>JSR 166 Introduction.</title> |
| 5 |
|
</head> |
| 6 |
|
|
| 7 |
|
<body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000"> |
| 8 |
< |
<h1>JSR 166 Snapshot Introduction.</h1> |
| 8 |
> |
<h1>JSR 166 Introduction.</h1> |
| 9 |
|
|
| 10 |
|
by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a> |
| 11 |
|
<p> |
| 12 |
|
|
| 13 |
< |
To join a mailing list discussing this JSR, go to: |
| 14 |
< |
<A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> . |
| 15 |
< |
|
| 16 |
< |
<p> |
| 17 |
< |
<em> |
| 18 |
< |
Disclaimer - This prototype is experimental code developed as part of |
| 19 |
< |
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 |
| 21 |
< |
specification, language and implementation are subject to change as a |
| 22 |
< |
result of your feedback. Because these features have not yet been |
| 23 |
< |
approved for addition to the Java language, there is no schedule for |
| 24 |
< |
their inclusion in a product. |
| 13 |
> |
This is maintenance repository of JSR166 specifications. For further |
| 14 |
> |
information, go to: <A |
| 15 |
> |
HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> |
| 16 |
> |
http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A>. |
| 17 |
> |
|
| 18 |
> |
<p><em>Note: The javadocs here do <em>not</em> include pre-existing |
| 19 |
> |
java classes (for example <tt>java.lang.Thread</tt>) that were changed |
| 20 |
> |
as part of the JSR166 spec. On the other hand, the javadocs here do |
| 21 |
> |
include some existing java.util Collection interfaces and classes that |
| 22 |
> |
are not part of the spec, but are included because some new methods |
| 23 |
> |
implement or inherit from their specifications. |
| 24 |
|
</em> |
| 25 |
|
|
| 26 |
< |
<p> |
| 27 |
< |
Package java.util.concurrent contains utility classes that are |
| 28 |
< |
commonly useful in concurrent programming. Like package java.util, it |
| 29 |
< |
includes a few small standardized extensible frameworks, as well as |
| 30 |
< |
some classes that provide useful functionality and are otherwise |
| 31 |
< |
tedious or difficult to implement. In this JSR, we have been |
| 32 |
< |
conservative in selecting only those APIs and implementations that are |
| 33 |
< |
useful enough to encourage nearly all concurrent programmers to use |
| 34 |
< |
routinely. JSR 166 also includes a few changes and additions in |
| 35 |
< |
packages outside of java.util.concurrent: java.lang, to address |
| 36 |
< |
uncaught exceptions, and java.util to better integrate queues. |
| 37 |
< |
The API covers: |
| 38 |
< |
|
| 39 |
< |
<ul> |
| 40 |
< |
<li> Queues |
| 41 |
< |
<li> Executors |
| 42 |
< |
<li> Locks |
| 43 |
< |
<li> Condition variables |
| 44 |
< |
<li> Atomic variables |
| 45 |
< |
<li> Timing |
| 46 |
< |
<li> Synchronizers |
| 47 |
< |
<li> Concurrent Collections |
| 49 |
< |
<li> Uncaught Exception Handlers |
| 50 |
< |
</ul> |
| 51 |
< |
|
| 52 |
< |
|
| 53 |
< |
The main rationale for JSR 166 is that threading primitives, such as |
| 54 |
< |
synchronized blocks, Object.wait and Object.notify, are insufficient |
| 55 |
< |
for many programming tasks. Currently, developers can use only the |
| 56 |
< |
concurrency control constructs provided in the Java language |
| 57 |
< |
itself. These are too low level for some applications, and are |
| 58 |
< |
incomplete for others. As a result, application programmers are often |
| 59 |
< |
forced to implement their own concurrency facilities, resulting in |
| 60 |
< |
enormous duplication of effort creating facilities that are |
| 61 |
< |
notoriously hard to get right and even harder to optimize. Offering a |
| 62 |
< |
standard set of concurrency utilities will ease the task of writing a |
| 63 |
< |
wide variety of multithreaded applications and generally improve the |
| 64 |
< |
quality of the applications that use them. |
| 65 |
< |
|
| 66 |
< |
<p> |
| 67 |
< |
Here are brief descriptions and rationales of the main components. |
| 68 |
< |
For details see the javadocs at <a |
| 69 |
< |
href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a> |
| 70 |
< |
|
| 26 |
> |
<p>JSR-166 introduces package <tt>java.util.concurrent</tt> |
| 27 |
> |
containing utility classes commonly useful in concurrent |
| 28 |
> |
programming. Like package <tt>java.util</tt>, it includes a few small |
| 29 |
> |
standardized extensible frameworks, as well as other classes that |
| 30 |
> |
provide useful functionality and are otherwise tedious or difficult to |
| 31 |
> |
implement. |
| 32 |
> |
|
| 33 |
> |
<p>JSR-166 focuses on breadth, providing critical functionality |
| 34 |
> |
useful across a wide range of concurrent programming styles and |
| 35 |
> |
applications, ranging from low-level atomic operations, to |
| 36 |
> |
customizable locks and synchronization aids, to various concurrent |
| 37 |
> |
data structures, to high-level execution agents including thread |
| 38 |
> |
pools. This diversity reflects the range of contexts in which |
| 39 |
> |
developers of concurrent programs have been found to require or desire |
| 40 |
> |
support not previously available in J2SE, which also keeping the |
| 41 |
> |
resulting package small; providing only functionality that has been |
| 42 |
> |
found to be worthwhile to standardize. |
| 43 |
> |
|
| 44 |
> |
<p>Descriptions and brief motivations for the main components may be |
| 45 |
> |
found in the associated package documentation. JSR-166 also includes |
| 46 |
> |
a few changes and additions in packages outside of |
| 47 |
> |
java.util.concurrent. Here are brief descriptions. |
| 48 |
|
|
| 49 |
|
<h2>Queues</h2> |
| 50 |
|
|
| 51 |
< |
A basic (nonblocking) Queue interface that is compatatible with |
| 52 |
< |
java.util.Collections will be introduced into java.util. Also, |
| 53 |
< |
although it is at the borders of being in scope of JSR-166, |
| 54 |
< |
java.util.LinkedList will be adapted to support Queue, and |
| 55 |
< |
a new non-thread-safe java.util.PriorityQueue will be added. |
| 56 |
< |
|
| 57 |
< |
<p> Five implementations in java.util.concurrent support the extended |
| 58 |
< |
BlockingQueue interface, that defines blocking versions of put and |
| 59 |
< |
take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, |
| 60 |
< |
PriorityBlockingQueue, and DelayQueue. Additionally, |
| 61 |
< |
java.util.concurrent.LinkedQueue supplies an efficient thread-safe |
| 62 |
< |
non-blocking queue. |
| 63 |
< |
|
| 64 |
< |
<p> Since the target release is JDK1.5, and generics are slated to be |
| 65 |
< |
in 1.5, Queues are parametrized on element type. (Also some others |
| 66 |
< |
below.) |
| 67 |
< |
|
| 68 |
< |
|
| 69 |
< |
<h2>Executors</h2> |
| 70 |
< |
|
| 71 |
< |
Executors provide a simple standardized interface for defining custom |
| 72 |
< |
thread-like subsystems, including thread pools, asynch-IO, and |
| 73 |
< |
lightweight task frameworks. Executors also standardize ways of |
| 74 |
< |
calling threads that compute functions returning results, via |
| 75 |
< |
Futures. This is supported in part by defining interface Callable, the |
| 76 |
< |
argument/result analog of Runnable. |
| 77 |
< |
|
| 78 |
< |
<p> Executors provide a framework for executing Runnables. The |
| 79 |
< |
Executor manages queueing and scheduling of tasks, and creation and |
| 80 |
< |
teardown of threads. Depending on which concrete Executor class is |
| 104 |
< |
being used, tasks may execute in a newly created thread, an existing |
| 105 |
< |
task-execution thread, or the thread calling execute(), and may |
| 106 |
< |
execute sequentially or concurrently. |
| 107 |
< |
|
| 108 |
< |
<p> Several concrete implementations of Executor are included in |
| 109 |
< |
java.util.concurrent, including ThreadPoolExecutor, a flexible thread |
| 110 |
< |
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 |
| 113 |
< |
potentially long-running computation and query the FutureTask to |
| 114 |
< |
determine if its execution has completed. |
| 115 |
< |
|
| 116 |
< |
<p> The <tt>Executors</tt> class provides factory methods for all |
| 117 |
< |
of the types of executors provided in |
| 118 |
< |
<tt>java.util.concurrent</tt>. |
| 119 |
< |
|
| 120 |
< |
|
| 121 |
< |
<h2>Locks</h2> |
| 122 |
< |
|
| 123 |
< |
The Lock interface supports locking disciplines that differ in |
| 124 |
< |
semantics (reentrant, fair, etc), and that can be used in |
| 125 |
< |
non-block-structured contexts including hand-over-hand and lock |
| 126 |
< |
reordering algorithms. This flexibility comes at the price of more |
| 127 |
< |
awkward syntax. Implementations include ReentrantLock and |
| 128 |
< |
FairReentrantLock. |
| 129 |
< |
|
| 130 |
< |
<p> |
| 131 |
< |
The Locks class additionally supports trylock-designs using builtin |
| 132 |
< |
locks without needing to use Lock classes. This requires adding new |
| 133 |
< |
capabilities to builtin locks inside JVMs. |
| 134 |
< |
|
| 135 |
< |
<p> |
| 136 |
< |
A ReadWriteLock interface similarly defines locks that may be shared |
| 137 |
< |
among readers but are exclusive to writers. For this release, only a |
| 138 |
< |
single implementation, ReentrantReadWriteLock, is planned, since it |
| 139 |
< |
covers all standard usage contexts. But programmers may create their |
| 140 |
< |
own implementations to cover nonstandard requirements. |
| 141 |
< |
|
| 142 |
< |
<h2>Conditions</h2> |
| 143 |
< |
|
| 144 |
< |
A Condition class provides the kinds of condition variables associated |
| 145 |
< |
with monitors in other cocurrent languages, as well as pthreads |
| 146 |
< |
condvars. Their support reduces the need for tricky and/or |
| 147 |
< |
inefficient solutions to many classic concurrent problems. Conditions |
| 148 |
< |
also address the annoying problem that Object.wait(msecs) does not |
| 149 |
< |
return an indication of whether the wait timed out. This leads to |
| 150 |
< |
error-prone code. Since this method is in class Object, the problem is |
| 151 |
< |
basically unfixable. |
| 152 |
< |
<p> |
| 153 |
< |
To avoid compatibility problems, the names of Condition methods need |
| 154 |
< |
to be different than Object versions. The downside of this is that |
| 155 |
< |
people can make the mistake of calling cond.notify instead of |
| 156 |
< |
cond.signal. However, they will get IllegalMonitorState exceptions if |
| 157 |
< |
they do, so they can detect the error if they ever run the code. |
| 158 |
< |
|
| 159 |
< |
|
| 160 |
< |
<h2>Atomic variables</h2> |
| 161 |
< |
|
| 162 |
< |
The atomic subpackage includes a small library of classes, including |
| 163 |
< |
AtomicInteger, AtomicLong, and AtomicReference that support variables |
| 164 |
< |
performinf compareAndSet (CAS) and related atomic operations. |
| 51 |
> |
A basic (nonblocking) {@link java.util.Queue} interface extending |
| 52 |
> |
{@link java.util.Collection} is introduced into |
| 53 |
> |
<tt>java.util</tt>. Existing class {@link java.util.LinkedList} is |
| 54 |
> |
adapted to support Queue, and a new non-thread-safe {@link |
| 55 |
> |
java.util.PriorityQueue} is added. |
| 56 |
> |
|
| 57 |
> |
<h2>Threads</h2> |
| 58 |
> |
|
| 59 |
> |
Three minor changes are introduced to the {@link java.lang.Thread} |
| 60 |
> |
class: |
| 61 |
> |
<ul> |
| 62 |
> |
<li> It now allows per-thread installation of handlers for uncaught |
| 63 |
> |
exceptions. This optionally disassociates handlers from ThreadGroups, |
| 64 |
> |
which has proven to be too inflexible. (Note that the combination of |
| 65 |
> |
features in JSR-166 make ThreadGroups even less likely to be used in |
| 66 |
> |
most programs. Perhaps they will eventually be deprecated.) |
| 67 |
> |
|
| 68 |
> |
<li> Access checks are no longer required when a Thread interrupts |
| 69 |
> |
<em>itself</em>. The <tt>interrupt</tt> method is the only way to |
| 70 |
> |
re-assert a thread's interruption status (and in the case of |
| 71 |
> |
self-interruption has no other effect than this). The check here |
| 72 |
> |
previously caused unjustifiable and uncontrollable failures when |
| 73 |
> |
restricted code invoked library code that must reassert interruption |
| 74 |
> |
to correctly propagate status when encountering some |
| 75 |
> |
<tt>InterruptedExceptions</tt>. |
| 76 |
> |
<li> The <tt>destroy</tt> method, which has never been implemented, |
| 77 |
> |
has finally been deprecated. This is just a spec change, reflecting |
| 78 |
> |
the fact that that the reason it has never been implemented is that |
| 79 |
> |
it was undesirable and unworkable. |
| 80 |
> |
</ul> |
| 81 |
|
|
| 82 |
|
<h2>Timing</h2> |
| 83 |
|
|
| 84 |
< |
Java has always supported sub-millisecond versions of several native |
| 85 |
< |
time-out-based methods (such as Object.wait), but not methods to |
| 86 |
< |
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 |
< |
|
| 174 |
< |
|
| 175 |
< |
<h2>Synchronizers</h2> |
| 176 |
< |
|
| 177 |
< |
Five classes aid common special-purpose synchronization idioms. |
| 178 |
< |
Semaphores and FairSemaphores are classic concurrency tools. |
| 179 |
< |
CountDownLatches are very simple yet very common objects useful for |
| 180 |
< |
blocking until a single signal, event, or condition holds. |
| 181 |
< |
CyclicBarriers are resettable multiway synchronization points very |
| 182 |
< |
common in some styles of parallel programming. Exchangers allow two |
| 183 |
< |
threads to exchange objects at a rendezvous point. |
| 84 |
> |
Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It |
| 85 |
> |
provides a high-precision timing facility that is distinct from and |
| 86 |
> |
uncoordinated with <tt>System.currentTimeMillis</tt>. |
| 87 |
|
|
| 88 |
+ |
<h2>Removing ThreadLocals</h2> |
| 89 |
|
|
| 90 |
< |
<h2>Concurrent Collections</h2> |
| 91 |
< |
|
| 92 |
< |
JSR 166 will supply a few Collection implementations designed for use |
| 189 |
< |
in multithreaded contexts: ConcurrentHashMap, CopyOnWriteArrayList, |
| 190 |
< |
and CopyOnWriteArraySet. |
| 191 |
< |
|
| 192 |
< |
<h2>Uncaught Exception Handlers</h2> |
| 90 |
> |
The {@link java.lang.ThreadLocal} class now supports a means to remove |
| 91 |
> |
a ThreadLocal, which is needed in some thread-pool and worker-thread |
| 92 |
> |
designs. |
| 93 |
|
|
| 194 |
– |
The java.lang.Thread class will be modified to allow per-thread |
| 195 |
– |
installation of handlers for uncaught exceptions. Ths optionally |
| 196 |
– |
disassociates these handlers from ThreadGroups, which has proven to be |
| 197 |
– |
too inflexible in many multithreaded programs. (Note that the combination |
| 198 |
– |
of features in JSR 166 make ThreadGroups even less likely to |
| 199 |
– |
be used in most programs. Perhaps they will eventually be deprecated.) |
| 94 |
|
|
| 201 |
– |
<p> Additionally, ThreadLocals will now support a means to remove a |
| 202 |
– |
ThreadLocal, which is needed in some thread-pool and worker-thread |
| 203 |
– |
designs. |
| 95 |
|
|
| 96 |
|
<hr> |
| 206 |
– |
<address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address> |
| 97 |
|
</body> |
| 98 |
|
</html> |
