| 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 |
| 19 |
> |
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 |
| 21 |
|
specification, language and implementation are subject to change as a |
| 22 |
|
result of your feedback. Because these features have not yet been |
| 24 |
|
their inclusion in a product. |
| 25 |
|
</em> |
| 26 |
|
|
| 27 |
< |
<p> |
| 28 |
< |
Package java.util.concurrent contains utility classes that are |
| 29 |
< |
commonly useful in concurrent programming. Like package java.util, it |
| 30 |
< |
includes a few small standardized extensible frameworks, as well as |
| 31 |
< |
some classes that provide useful functionality and are otherwise |
| 32 |
< |
tedious or difficult to implement. In this JSR, we have been |
| 33 |
< |
conservative in selecting only those APIs and implementations that are |
| 34 |
< |
useful enough to encourage nearly all concurrent programmers to use |
| 35 |
< |
routinely. JSR 166 also includes a few changes and additions in |
| 36 |
< |
packages outside of java.util.concurrent: java.lang, to address |
| 37 |
< |
uncaught exceptions, and java.util to better integrate queues. |
| 38 |
< |
The API covers: |
| 39 |
< |
|
| 40 |
< |
<ul> |
| 41 |
< |
<li> Queues |
| 42 |
< |
<li> Executors |
| 43 |
< |
<li> Locks |
| 44 |
< |
<li> Condition variables |
| 45 |
< |
<li> Atomic variables |
| 46 |
< |
<li> Timing |
| 47 |
< |
<li> Synchronizers |
| 48 |
< |
<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. |
| 27 |
> |
<p> Package java.util.concurrent contains utility classes commonly |
| 28 |
> |
useful in concurrent programming. Like package java.util, it includes |
| 29 |
> |
a few small standardized extensible frameworks, as well as some |
| 30 |
> |
classes that provide useful functionality and are otherwise tedious or |
| 31 |
> |
difficult to implement. JSR166 also includes a few changes and |
| 32 |
> |
additions in packages outside of java.util.concurrent: java.lang, to |
| 33 |
> |
address uncaught exceptions, and java.util to better integrate with |
| 34 |
> |
collections. Since the target release is JDK1.5, many APIs use |
| 35 |
> |
generics to parameterize on types. Here are brief descriptions of the |
| 36 |
> |
main components. |
| 37 |
|
|
| 38 |
< |
<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> |
| 38 |
> |
<h2>Executors</h2> |
| 39 |
|
|
| 40 |
+ |
{@link java.util.concurrent.Executor} is a simple standardized |
| 41 |
+ |
interface for defining custom thread-like subsystems, including thread |
| 42 |
+ |
pools, asynch-IO, and lightweight task frameworks. Depending on which |
| 43 |
+ |
concrete Executor class is being used, tasks may execute in a newly |
| 44 |
+ |
created thread, an existing task-execution thread, or the thread |
| 45 |
+ |
calling <tt>execute()</tt>, and may execute sequentially or |
| 46 |
+ |
concurrently. Executors also standardize ways of calling threads that |
| 47 |
+ |
compute functions returning results, via a {@link |
| 48 |
+ |
java.util.concurrent.Future}. This is supported in part by defining |
| 49 |
+ |
interface {@link java.util.concurrent.Callable}, the argument/result |
| 50 |
+ |
analog of Runnable. |
| 51 |
+ |
|
| 52 |
+ |
<p> {@link java.util.concurrent.ExecutorService} provides a more |
| 53 |
+ |
complete framework for executing Runnables. An ExecutorService |
| 54 |
+ |
manages queueing and scheduling of tasks, and allows controlled |
| 55 |
+ |
shutdown. The two primary implementations of ExecutorService are |
| 56 |
+ |
{@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and |
| 57 |
+ |
flexible thread pool and {@link |
| 58 |
+ |
java.util.concurrent.ScheduledExecutor}, which adds support for |
| 59 |
+ |
delayed and periodic task execution. These, and other Executors can |
| 60 |
+ |
be used in conjunction with a {@link java.util.concurrent.FutureTask} |
| 61 |
+ |
to asynchronously |
| 62 |
+ |
start a potentially long-running computation and query the FutureTask |
| 63 |
+ |
to determine if its execution has completed, or cancel it. |
| 64 |
+ |
|
| 65 |
+ |
<p> The {@link java.util.concurrent.Executors} class provides factory |
| 66 |
+ |
methods for the most common kinds and styles of Executors, as well as |
| 67 |
+ |
a few utilities methods for using them. |
| 68 |
|
|
| 69 |
|
<h2>Queues</h2> |
| 70 |
|
|
| 71 |
< |
A basic (nonblocking) Queue interface that is compatatible with |
| 72 |
< |
java.util.Collections will be introduced into java.util. Also, |
| 73 |
< |
although it is at the borders of being in scope of JSR-166, |
| 74 |
< |
java.util.LinkedList will be adapted to support Queue, and |
| 75 |
< |
a new non-thread-safe java.util.HeapPriorityQueue will be added. |
| 71 |
> |
A basic (nonblocking) {@link java.util.Queue} interface extending |
| 72 |
> |
java.util.Collection is introduced into java.util. Existing class |
| 73 |
> |
java.util.LinkedList is adapted to support Queue, and a new |
| 74 |
> |
non-thread-safe {@link java.util.PriorityQueue} is added. The |
| 75 |
> |
java.util.concurrent {@link |
| 76 |
> |
java.util.concurrent.ConcurrentLinkedQueue} class supplies an |
| 77 |
> |
efficient sclable thread-safe non-blocking FIFO queue, and {@link |
| 78 |
> |
java.util.concurrent.ConcurrentLinkedStack} provides a similar |
| 79 |
> |
non-blocking LIFO stack. |
| 80 |
|
|
| 81 |
|
<p> Five implementations in java.util.concurrent support the extended |
| 82 |
< |
BlockingQueue interface, that defines blocking versions of put and |
| 83 |
< |
take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, |
| 84 |
< |
PriorityBlockingQueue, and DelayQueue. Additionally, |
| 85 |
< |
java.util.concurrent.LinkedQueue supplies an efficient thread-safe |
| 86 |
< |
non-blocking queue. |
| 87 |
< |
|
| 88 |
< |
<p> Since the target release is JDK1.5, and generics are slated to be |
| 88 |
< |
in 1.5, Queues are parametrized on element type. (Also some others |
| 89 |
< |
below.) |
| 90 |
< |
|
| 82 |
> |
{@link java.util.concurrent.BlockingQueue} interface, that defines |
| 83 |
> |
blocking versions of put and take: {@link |
| 84 |
> |
java.util.concurrent.LinkedBlockingQueue}, {@link |
| 85 |
> |
java.util.concurrent.ArrayBlockingQueue}, {@link |
| 86 |
> |
java.util.concurrent.SynchronousQueue}, {@link |
| 87 |
> |
java.util.concurrent.PriorityBlockingQueue}, and |
| 88 |
> |
{@link java.util.concurrent.DelayQueue}. |
| 89 |
|
|
| 92 |
– |
<h2>Executors</h2> |
| 93 |
– |
|
| 94 |
– |
Executors provide a simple standardized interface for defining custom |
| 95 |
– |
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 |
| 99 |
– |
argument/result analog of Runnable. |
| 100 |
– |
|
| 101 |
– |
<p> While the Executor framework is intended to be extensible the most |
| 102 |
– |
commonly used Executor will be ThreadExecutor, which can be configured |
| 103 |
– |
to act as all sorts of thread pools, background threads, etc. The |
| 104 |
– |
class is designed to be general enough to suffice for the vast |
| 105 |
– |
majority of usages, even sophisticated ones, yet also includes methods |
| 106 |
– |
and functionality that simplify routine usage. |
| 90 |
|
|
| 91 |
|
<h2>Locks</h2> |
| 92 |
|
|
| 93 |
< |
The Lock interface supports locking disciplines that differ in |
| 94 |
< |
semantics (reentrant, semaphore-based, etc), and that can be used in |
| 95 |
< |
non-block-structured contexts including hand-over-hand and lock |
| 96 |
< |
reordering algorithms. This flexibility comes at the price of more |
| 97 |
< |
awkward syntax. Implementations include Semaphore, ReentrantMutex |
| 98 |
< |
FIFOSemaphore, and CountDownLatch. |
| 99 |
< |
|
| 100 |
< |
<p> |
| 101 |
< |
The Locks class additionally supports trylock-designs using builtin |
| 102 |
< |
locks without needing to use Lock classes. This requires adding new |
| 103 |
< |
capabilities to builtin locks inside JVMs. |
| 104 |
< |
|
| 105 |
< |
<p> |
| 106 |
< |
A ReadWriteLock interface similarly defines locks that may be shared |
| 107 |
< |
among readers but are exclusive to writers. For this release, only a |
| 125 |
< |
single implementation, ReentrantReadWriteLock, is planned, since it |
| 93 |
> |
The {@link java.util.concurrent.Lock} interface supports locking |
| 94 |
> |
disciplines that differ in semantics (reentrant, fair, etc), and that |
| 95 |
> |
can be used in non-block-structured contexts including hand-over-hand |
| 96 |
> |
and lock reordering algorithms. This flexibility comes at the price of |
| 97 |
> |
more awkward syntax. Implementations include {@link |
| 98 |
> |
java.util.concurrent.ReentrantLock} and {@link |
| 99 |
> |
java.util.concurrent.FairReentrantLock}. |
| 100 |
> |
|
| 101 |
> |
<p> The {@link java.util.concurrent.Locks} class additionally supports |
| 102 |
> |
some common trylock-designs using builtin locks. |
| 103 |
> |
|
| 104 |
> |
<p> The {@link java.util.concurrent.ReadWriteLock} interface similarly |
| 105 |
> |
defines locks that may be shared among readers but are exclusive to |
| 106 |
> |
writers. Only a single implementation, {@link |
| 107 |
> |
java.util.concurrent.ReentrantReadWriteLock}, is provided, since it |
| 108 |
|
covers all standard usage contexts. But programmers may create their |
| 109 |
|
own implementations to cover nonstandard requirements. |
| 110 |
|
|
| 111 |
|
<h2>Conditions</h2> |
| 112 |
|
|
| 113 |
< |
A Condition class provides the kinds of condition variables associated |
| 114 |
< |
with monitors in other cocurrent languages, as well as pthreads |
| 115 |
< |
condvars. Their support reduces the need for tricky and/or |
| 116 |
< |
inefficient solutions to many classic concurrent problems. Conditions |
| 117 |
< |
also address the annoying problem that Object.wait(msecs) does not |
| 118 |
< |
return an indication of whether the wait timed out. This leads to |
| 119 |
< |
error-prone code. Since this method is in class Object, the problem is |
| 120 |
< |
basically unfixable. |
| 121 |
< |
<p> |
| 122 |
< |
To avoid compatibility problems, the names of Condition methods need |
| 123 |
< |
to be different than Object versions. The downside of this is that |
| 124 |
< |
people can make the mistake of calling cond.notify instead of |
| 143 |
< |
cond.signal. However, they will get IllegalMonitorState exceptions if |
| 144 |
< |
they do, so they can detect the error if they ever run the code. |
| 145 |
< |
<p> |
| 146 |
< |
The implementation requires VM magic to atomically suspend and release |
| 147 |
< |
lock. But it is unlikely to be very challenging for JVM providers, |
| 148 |
< |
since most layer Java monitors on top of posix condvars or similar |
| 149 |
< |
low-level functionality anyway. |
| 150 |
< |
|
| 151 |
< |
<h2>Atomic variables</h2> |
| 152 |
< |
|
| 153 |
< |
Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and |
| 154 |
< |
AtomicReference provide simple scalar variables supporting |
| 155 |
< |
compareAndSwap (CAS) and related atomic operations. These are |
| 156 |
< |
desparately needed by those performing low-level concurrent system |
| 157 |
< |
programming, but much less commonly useful in higher-level frameworks. |
| 158 |
< |
|
| 113 |
> |
The {@link java.util.concurrent.Condition} interface describes the |
| 114 |
> |
kinds of condition variables associated with monitors in other |
| 115 |
> |
concurrent languages, as well as pthreads-style condvars. Their |
| 116 |
> |
support reduces the need for tricky and/or inefficient solutions to |
| 117 |
> |
many classic concurrent problems. To avoid compatibility problems, |
| 118 |
> |
the names of Condition methods are different than Object versions. |
| 119 |
> |
|
| 120 |
> |
<h2>Atomics</h2> |
| 121 |
> |
|
| 122 |
> |
The atomic subpackage includes a small library of classes, including |
| 123 |
> |
AtomicInteger, AtomicLong, and AtomicReference that support |
| 124 |
> |
compareAndSet (CAS) and related atomic operations. |
| 125 |
|
|
| 126 |
|
<h2>Timing</h2> |
| 127 |
|
|
| 128 |
< |
Java has always supported sub-millisecond versions of several native |
| 129 |
< |
time-out-based methods (such as Object.wait), but not methods to |
| 130 |
< |
actually perform timing in finer-grained units. We address this by |
| 165 |
< |
introducing class Clock, which provides multiple granularities for |
| 166 |
< |
both accessing time and performing time-out based operations. |
| 167 |
< |
|
| 128 |
> |
The {@link java.util.concurrent.TimeUnit} class provides multiple |
| 129 |
> |
granularities (including nanoseconds) for both accessing time and |
| 130 |
> |
performing time-out based operations. |
| 131 |
|
|
| 132 |
|
<h2>Synchronizers</h2> |
| 133 |
|
|
| 134 |
|
Five classes aid common special-purpose synchronization idioms. |
| 135 |
< |
Semaphores and FifoSemaphores are classic concurrency tools. Latches |
| 136 |
< |
are very simple yet very common objects useful for blocking until a |
| 137 |
< |
single signal, event, or condition holds. CyclicBarriers are |
| 138 |
< |
resettable multiway synchronization points very common in some styles |
| 139 |
< |
of parallel programming. Exchangers allow two threads to exchange |
| 140 |
< |
objects at a rendezvous point. |
| 141 |
< |
|
| 135 |
> |
{@link java.util.concurrent.Semaphore} and {@link |
| 136 |
> |
java.util.concurrent.FairSemaphore} are classic concurrency tools. |
| 137 |
> |
{@link java.util.concurrent.CountDownLatch} is very simple yet very |
| 138 |
> |
common utility for blocking until a single signal, event, or condition |
| 139 |
> |
holds. A {@link java.util.concurrent.CyclicBarrier} is a resettable multiway |
| 140 |
> |
synchronization point common in some styles of parallel |
| 141 |
> |
programming. An {@link java.util.concurrent.Exchanger} allows two |
| 142 |
> |
threads to exchange objects at a rendezvous point. |
| 143 |
|
|
| 144 |
|
<h2>Concurrent Collections</h2> |
| 145 |
|
|
| 146 |
< |
JSR 166 will supply a few Collection implementations designed for use |
| 147 |
< |
in multithreaded contexts: ConcurrentHashTable, CopyOnWriteArrayList, |
| 148 |
< |
and CopyOnWriteArraySet. |
| 146 |
> |
Besides Queues, this package supplies a few Collection implementations |
| 147 |
> |
designed for use in multithreaded contexts: {@link |
| 148 |
> |
java.util.concurrent.ConcurrentHashMap}, {@link |
| 149 |
> |
java.util.concurrent.CopyOnWriteArrayList}, and {@link |
| 150 |
> |
java.util.concurrent.CopyOnWriteArraySet}. |
| 151 |
> |
|
| 152 |
> |
<p>The "Concurrent" prefix for classes is a shorthand |
| 153 |
> |
indicating several differences from similar "synchronized" |
| 154 |
> |
classes. For example <tt>java.util.Hashtable</tt> and |
| 155 |
> |
<tt>Collections.synchronizedMap(new HashMap())</tt> are |
| 156 |
> |
synchronized. But {@link |
| 157 |
> |
java.util.concurrent.ConcurrentHashMap} is "concurrent". |
| 158 |
> |
A concurrent collection (among other kinds of classes) is |
| 159 |
> |
thread-safe, but not governed by a single exclusion lock. So, in the |
| 160 |
> |
particular case of ConcurrentHashMap, it safely permits any number of |
| 161 |
> |
concurrent reads as well as a tunable number of concurrent writes. |
| 162 |
> |
There may still be a role for "synchronized" classes in some |
| 163 |
> |
multithreaded programs -- they can sometimes be useful when you need |
| 164 |
> |
to prevent ALL access to a collection via a single lock, at the |
| 165 |
> |
expense of much poor scalability. In all other cases, "concurrent" |
| 166 |
> |
versions are normally preferable. |
| 167 |
> |
|
| 168 |
> |
<p> Most concurrent Collection implementations (including most Queues) |
| 169 |
> |
also differ from the usual java.util conventions in that their Iterators |
| 170 |
> |
provide <em>weakly consistent</em> rather than fast-fail traversal. A |
| 171 |
> |
weakly consistent iterator is thread-safe, but does not necessarily |
| 172 |
> |
freeze the collection while iterating, so it may (or may not) reflect |
| 173 |
> |
any updates since the iterator was created. |
| 174 |
|
|
| 175 |
|
<h2>Uncaught Exception Handlers</h2> |
| 176 |
|
|
| 177 |
< |
The java.lang.Thread class will be modified to allow per-thread |
| 177 |
> |
The java.lang.Thread class is modified to allow per-thread |
| 178 |
|
installation of handlers for uncaught exceptions. Ths optionally |
| 179 |
|
disassociates these handlers from ThreadGroups, which has proven to be |
| 180 |
< |
too inflexible in many multithreaded programs. (Note that the combination |
| 181 |
< |
of features in JSR 166 make ThreadGroups even less likely to |
| 182 |
< |
be used in most programs. Perhaps they will eventually be deprecated.) |
| 183 |
< |
<p> |
| 184 |
< |
Additionally, ThreadLocals will now support a means to |
| 185 |
< |
remove a ThreadLocals, which is needed in some thread-pool and |
| 186 |
< |
worker-thread designs. |
| 180 |
> |
too inflexible in many multithreaded programs. (Note that the |
| 181 |
> |
combination of features in JSR166 make ThreadGroups even less likely |
| 182 |
> |
to be used in most programs. Perhaps they will eventually be |
| 183 |
> |
deprecated.) |
| 184 |
> |
|
| 185 |
> |
<p> Additionally, java.lang.ThreadLocal now supports a means to remove |
| 186 |
> |
a ThreadLocal, which is needed in some thread-pool and worker-thread |
| 187 |
> |
designs. |
| 188 |
|
|
| 189 |
|
<hr> |
| 190 |
|
<address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address> |
