| 139 |
|
freeze the collection while iterating, so it may (or may not) reflect |
| 140 |
|
any updates since the iterator was created. |
| 141 |
|
|
| 142 |
+ |
<h2> Memory Consistency Properties </h2> |
| 143 |
+ |
|
| 144 |
+ |
<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html"> |
| 145 |
+ |
Chapter 17 of the Java Language Specification</a> defines the |
| 146 |
+ |
<i>happens-before</i> relation on memory operations such as reads and |
| 147 |
+ |
writes of shared variables. The results of a write by one thread are |
| 148 |
+ |
guaranteed to be visible to a read by another thread only if the write |
| 149 |
+ |
operation <i>happens-before</i> the read operation. The |
| 150 |
+ |
<tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the |
| 151 |
+ |
<tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form |
| 152 |
+ |
<i>happens-before</i> relationships. In particular: |
| 153 |
+ |
|
| 154 |
+ |
<ul> |
| 155 |
+ |
<li>Each action in a thread happens before every action in that |
| 156 |
+ |
thread that comes later in the program's order. |
| 157 |
+ |
|
| 158 |
+ |
<li>An unlock (<tt>synchronized</tt> block or method exit) on a |
| 159 |
+ |
monitor happens before every subsequent lock (<tt>synchronized</tt> |
| 160 |
+ |
block or method entry) that same monitor. |
| 161 |
+ |
|
| 162 |
+ |
<li>A write to a <tt>volatile</tt> field happens before every |
| 163 |
+ |
subsequent read of that same field. |
| 164 |
+ |
|
| 165 |
+ |
<li>A call to <tt>start</tt> on a thread happens before any actions in the |
| 166 |
+ |
started thread. |
| 167 |
+ |
|
| 168 |
+ |
<li>All actions in a thread happen before any other thread |
| 169 |
+ |
successfully returns from a <tt>join</tt> on that thread. |
| 170 |
+ |
|
| 171 |
+ |
</ul> |
| 172 |
+ |
|
| 173 |
+ |
The methods of all classes in <tt>java.util.concurrent</tt> and its |
| 174 |
+ |
subpackages extend these guarantees to higher-level |
| 175 |
+ |
synchronization. In particular: |
| 176 |
+ |
|
| 177 |
+ |
<ul> |
| 178 |
+ |
|
| 179 |
+ |
<li>State changes to any object made prior to placement into any |
| 180 |
+ |
concurrent collection happen before this element is accessed via or |
| 181 |
+ |
removed from that collection. |
| 182 |
+ |
|
| 183 |
+ |
<li>State changes to a <tt>Runnable</tt> object made prior to |
| 184 |
+ |
submission to an <tt>Executor</tt> happen before its execution. And |
| 185 |
+ |
similarly for a <tt>Callable</tt> object submitted to an |
| 186 |
+ |
<tt>ExecutorService</tt>. |
| 187 |
+ |
|
| 188 |
+ |
<li>State changes to a <tt>Future</tt> made prior to it becoming |
| 189 |
+ |
available happen before access via <tt>Future.get()</tt>. |
| 190 |
+ |
|
| 191 |
+ |
<li>Actions prior to "releasing" synchronizer methods such as |
| 192 |
+ |
<tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>, |
| 193 |
+ |
<tt>CountDownLatch.countDown<tt> and <tt>Condition.signal</tt> |
| 194 |
+ |
happen before actions subsequent to "acquiring" methods such as |
| 195 |
+ |
<tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, and |
| 196 |
+ |
<tt>CountDownLatch.await</tt> on the same synchronizer object. |
| 197 |
+ |
|
| 198 |
+ |
<li>Actions prior to symmetric synchronizer methods such as |
| 199 |
+ |
<tt>CyclicBarrier.await</tt> and <tt>Exchanger.exchange</tt> happen |
| 200 |
+ |
before those subsequent to the matching actions in other threads. |
| 201 |
+ |
|
| 202 |
+ |
</ul> |
| 203 |
+ |
|
| 204 |
|
@since 1.5 |
| 205 |
|
|
| 206 |
|
</body> </html> |
