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} |
75 |
< |
is added. The java.util.concurrent {@link |
76 |
< |
java.util.concurrent.LinkedQueue} class supplies an efficient |
77 |
< |
thread-safe non-blocking queue. |
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 |
|
{@link java.util.concurrent.BlockingQueue} interface, that defines |
143 |
|
|
144 |
|
<h2>Concurrent Collections</h2> |
145 |
|
|
146 |
< |
This package supplies a few Collection implementations designed for |
147 |
< |
use in multithreaded contexts: {@link |
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 |
< |
differ from the usual java.util conventions in that their Iterators |
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 |