| 148 |
|
writes of shared variables. The results of a write by one thread are |
| 149 |
|
guaranteed to be visible to a read by another thread only if the write |
| 150 |
|
operation <i>happens-before</i> the read operation. The |
| 151 |
< |
<tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the |
| 152 |
< |
<tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form |
| 151 |
> |
{@code synchronized} and {@code volatile} constructs, as well as the |
| 152 |
> |
{@code Thread.start()} and {@code Thread.join()} methods, can form |
| 153 |
|
<i>happens-before</i> relationships. In particular: |
| 154 |
|
|
| 155 |
|
<ul> |
| 156 |
|
<li>Each action in a thread <i>happens-before</i> every action in that |
| 157 |
|
thread that comes later in the program's order. |
| 158 |
|
|
| 159 |
< |
<li>An unlock (<tt>synchronized</tt> block or method exit) of a |
| 160 |
< |
monitor <i>happens-before</i> every subsequent lock (<tt>synchronized</tt> |
| 159 |
> |
<li>An unlock ({@code synchronized} block or method exit) of a |
| 160 |
> |
monitor <i>happens-before</i> every subsequent lock ({@code synchronized} |
| 161 |
|
block or method entry) of that same monitor. And because |
| 162 |
|
the <i>happens-before</i> relation is transitive, all actions |
| 163 |
|
of a thread prior to unlocking <i>happen-before</i> all actions |
| 164 |
|
subsequent to any thread locking that monitor. |
| 165 |
|
|
| 166 |
< |
<li>A write to a <tt>volatile</tt> field <i>happens-before</i> every |
| 166 |
> |
<li>A write to a {@code volatile} field <i>happens-before</i> every |
| 167 |
|
subsequent read of that same field. Writes and reads of |
| 168 |
< |
<tt>volatile</tt> fields have similar memory consistency effects |
| 168 |
> |
{@code volatile} fields have similar memory consistency effects |
| 169 |
|
as entering and exiting monitors, but do <em>not</em> entail |
| 170 |
|
mutual exclusion locking. |
| 171 |
|
|
| 172 |
< |
<li>A call to <tt>start</tt> on a thread <i>happens-before</i> any action in the |
| 172 |
> |
<li>A call to {@code start} on a thread <i>happens-before</i> any action in the |
| 173 |
|
started thread. |
| 174 |
|
|
| 175 |
|
<li>All actions in a thread <i>happen-before</i> any other thread |
| 176 |
< |
successfully returns from a <tt>join</tt> on that thread. |
| 176 |
> |
successfully returns from a {@code join} on that thread. |
| 177 |
|
|
| 178 |
|
</ul> |
| 179 |
|
|
| 180 |
|
|
| 181 |
< |
The methods of all classes in <tt>java.util.concurrent</tt> and its |
| 181 |
> |
The methods of all classes in {@code java.util.concurrent} and its |
| 182 |
|
subpackages extend these guarantees to higher-level |
| 183 |
|
synchronization. In particular: |
| 184 |
|
|
| 185 |
|
<ul> |
| 186 |
|
|
| 187 |
< |
<li>State changes to any object made prior to placement into any |
| 188 |
< |
concurrent collection <i>happen-before</i> this element is accessed via or |
| 189 |
< |
removed from that collection. |
| 190 |
< |
|
| 191 |
< |
<li>State changes to a <tt>Runnable</tt> object made by the |
| 192 |
< |
submitting thread prior to submission to an <tt>Executor</tt> |
| 193 |
< |
<i>happen-before</i> its execution. Similarly, state changes to a |
| 194 |
< |
<tt>Callable</tt> object made by the submitting thread before being |
| 195 |
< |
submitted to an <tt>ExecutorService</tt> <i>happen-before</i> its |
| 196 |
< |
execution. |
| 197 |
< |
|
| 198 |
< |
<li>State changes to a <tt>Future</tt> that |
| 199 |
< |
<i>happen-before</i> it is made available |
| 200 |
< |
<i>happen-before</i> access via <tt>Future.get()</tt>. |
| 187 |
> |
<li>Actions in a thread prior to placing an object into any concurrent |
| 188 |
> |
collection <i>happen-before</i> actions subsequent to the access or |
| 189 |
> |
removal of that element from the collection in another thread. |
| 190 |
> |
|
| 191 |
> |
<li>Actions in a thread prior to the submission of a {@code Runnable} |
| 192 |
> |
to an {@code Executor} <i>happen-before</i> its execution begins. |
| 193 |
> |
Similarly for {@code Callables} submitted to an {@code ExecutorService}. |
| 194 |
> |
|
| 195 |
> |
<li>Actions taken by the asynchronous computation represented by a |
| 196 |
> |
{@code Future} <i>happen-before</i> actions subsequent to the |
| 197 |
> |
retrieval of the result via {@code Future.get()} in another thread. |
| 198 |
|
|
| 199 |
|
<li>Actions prior to "releasing" synchronizer methods such as |
| 200 |
< |
<tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>, and |
| 201 |
< |
<tt>CountDownLatch.countDown</tt> |
| 202 |
< |
<i>happen-before</i> actions subsequent to a successful "acquiring" method such as |
| 203 |
< |
<tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, <tt>Condition.await</tt>, and |
| 204 |
< |
<tt>CountDownLatch.await</tt> on the same synchronizer object. |
| 200 |
> |
{@code Lock.unlock}, {@code Semaphore.release}, and |
| 201 |
> |
{@code CountDownLatch.countDown} <i>happen-before</i> actions |
| 202 |
> |
subsequent to a successful "acquiring" method such as |
| 203 |
> |
{@code Lock.lock}, {@code Semaphore.acquire}, |
| 204 |
> |
{@code Condition.await}, and {@code CountDownLatch.await} on the |
| 205 |
> |
same synchronizer object in another thread. |
| 206 |
|
|
| 207 |
|
<li>For each pair of threads that successfully exchange objects via |
| 208 |
< |
an <tt>Exchanger</tt>, actions prior to the <tt>exchange()</tt> |
| 208 |
> |
an {@code Exchanger}, actions prior to the {@code exchange()} |
| 209 |
|
in each thread <i>happen-before</i> those subsequent to the |
| 210 |
< |
corresponding <tt>exchange()</tt> in the other thread. |
| 210 |
> |
corresponding {@code exchange()} in another thread. |
| 211 |
|
|
| 212 |
< |
<li>Actions prior to calling <tt>CyclicBarrier.await</tt> |
| 212 |
> |
<li>Actions prior to calling {@code CyclicBarrier.await} |
| 213 |
|
<i>happen-before</i> actions performed by the barrier action, and |
| 214 |
|
actions performed by the barrier action <i>happen-before</i> actions |
| 215 |
< |
subsequent to a successful return from the corresponding <tt>await</tt> |
| 215 |
> |
subsequent to a successful return from the corresponding {@code await} |
| 216 |
|
in other threads. |
| 217 |
|
|
| 218 |
|
</ul> |
