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 |
|
|