| 24 |
|
their inclusion in a product. |
| 25 |
|
</em> |
| 26 |
|
|
| 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 |
< |
<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. |
| 27 |
> |
<p> JSR166 introduces package <tt>java.util.concurrent</tt> containing utility |
| 28 |
> |
classes commonly useful in concurrent programming. Like package |
| 29 |
> |
java.util, it includes a few small standardized extensible frameworks, |
| 30 |
> |
as well as some classes that provide useful functionality and are |
| 31 |
> |
otherwise tedious or difficult to implement. Descriptions of the main |
| 32 |
> |
components may be found in the associated package documentation. |
| 33 |
> |
|
| 34 |
> |
<p> JSR166 also includes a few changes and additions in packages |
| 35 |
> |
outside of java.util.concurrent: java.lang, to address uncaught |
| 36 |
> |
exceptions, and java.util to better integrate with collections. Here |
| 37 |
> |
are brief descriptions. |
| 38 |
|
|
| 39 |
|
<h2>Queues</h2> |
| 40 |
|
|
| 41 |
|
A basic (nonblocking) {@link java.util.Queue} interface extending |
| 42 |
|
java.util.Collection is introduced into java.util. Existing class |
| 43 |
|
java.util.LinkedList is adapted to support Queue, and a new |
| 44 |
< |
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 |
< |
{@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 |
< |
|
| 90 |
< |
|
| 91 |
< |
<h2>Locks</h2> |
| 92 |
< |
|
| 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 |
< |
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 |
< |
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 |
< |
{@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 |
< |
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. |
| 44 |
> |
non-thread-safe {@link java.util.PriorityQueue} is added. |
| 45 |
|
|
| 46 |
|
<h2>Uncaught Exception Handlers</h2> |
| 47 |
|
|
