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copied!<p>This is sort of a side note explanation on some of the details of volatile. Writing this here because it is too much for an comment. I want to give some examples which show how volatile affects visibility, and how that changed in jdk 1.5.</p> <p>Given the following example code:</p> <pre><code>public class MyClass { private int _n; private volatile int _volN; public void setN(int i) { _n = i; } public void setVolN(int i) { _volN = i; } public int getN() { return _n; } public int getVolN() { return _volN; } public static void main() { final MyClass mc = new MyClass(); Thread t1 = new Thread() { public void run() { mc.setN(5); mc.setVolN(5); } }; Thread t2 = new Thread() { public void run() { int volN = mc.getVolN(); int n = mc.getN(); System.out.println("Read: " + volN + ", " + n); } }; t1.start(); t2.start(); } } </code></pre> <p>The behavior of this test code is well defined in jdk1.5+, but is <strong>not</strong> well defined pre-jdk1.5.</p> <p>In the pre-jdk1.5 world, there was no defined relationship between volatile accesses and non-volatile accesses. therefore, the output of this program could be:</p> <ol> <li>Read: 0, 0</li> <li>Read: 0, 5</li> <li>Read: 5, 0</li> <li>Read: 5, 5</li> </ol> <p>In the jdk1.5+ world, the semantics of volatile were changed so that volatile accesses affect non-volatile accesses in exactly the same way as synchronization. therefore, only certain outputs are possible in the jdk1.5+ world:</p> <ol> <li>Read: 0, 0</li> <li>Read: 0, 5</li> <li>Read: 5, 0 <strong><- not possible</strong></li> <li>Read: 5, 5</li> </ol> <p>Output 3. is not possible because the reading of "5" from the volatile _volN establishes a synchronization point between the 2 threads, which means all actions from t1 taken before the assignment to _volN <em>must</em> be visible to t2.</p> <p>Further reading:</p> <ul> <li><a href="http://www.ibm.com/developerworks/library/j-jtp02244.html" rel="noreferrer">Fixing the java memory model, part 1</a></li> <li><a href="http://www.ibm.com/developerworks/library/j-jtp03304/" rel="noreferrer">Fixing the java memory model, part 2</a></li> </ul>

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