| 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 |
