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POjvm reordering/visibility effect test
 

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    <p>While writing some java article I'm trying to reproduce <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#reordering" rel="nofollow">re-ordering</a> in case of unsynchronized object costruction in multithreaded environment. The case when a heavy object is constructed w/o synchonization/volatiles/finals and other threads get access to it right after constructor call. Here is the code I try:</p> <pre><code>public class ReorderingTest { static SomeObject&lt;JPanel&gt;[] sharedArray = new SomeObject[100]; public static void main(String[] args) { for (int i = 0; i &lt; 100; i++) { String name = "watcher" + i; new Thread(new Watcher(name)).start(); System.out.printf("watcher %s started!%n", name); } } static class Watcher implements Runnable { private String name; Watcher(String name) { this.name = name; } public void run() { while (true) { int randomIndex = (int) (Math.random() * sharedArray.length); SomeObject&lt;JPanel&gt; item = sharedArray[randomIndex]; if (item == null) { //System.out.printf("sharedArray[%s]=null%n", randomIndex); double r = 1 + Math.random() * 1000; sharedArray[randomIndex] = new SomeObject&lt;JPanel&gt;( new JPanel(), UUID.randomUUID().toString(), r, (float)r * 33, (long)r); } else { //System.out.printf("sharedArray[%s]=&lt;obj&gt;!%n", randomIndex); if (item.value == null || (item.stringField == null) || (item.doubleField == 0) || (item.floatField == 0) || (item.longField == 0) ) { System.err.printf("watcher %s sees default values: %s!%n", name, item); } else { // fully initialized! run new construction process double r = 1 + Math.random() * 1000; sharedArray[randomIndex] = new SomeObject&lt;JPanel&gt;( new JPanel(), UUID.randomUUID().toString(), r, (float)r * 37, (long)r); } } /*try { TimeUnit.NANOSECONDS.sleep(randomIndex); } catch (InterruptedException e) { throw new RuntimeException(e); }*/ } } } static class SomeObject&lt;V&gt; { V value; String stringField; double doubleField; float floatField; long longField; SomeObject(V value, String stringField, double doubleField, float floatField, long longField) { this.value = value; this.stringField = stringField; this.doubleField = doubleField; this.floatField = floatField; this.longField = longField; } @Override public String toString() { return "SomeObject{" + "value=" + value == null ? "null" : "&lt;obj&gt;" + ", stringField='" + stringField + '\'' + ", doubleField=" + doubleField + ", floatField=" + floatField + ", longField=" + longField + '}'; } } } </code></pre> <p>-But no effect so far, I've tried on different 2,4 and 8 core Intel/AMD PCs with windows, ran test for a few hours - no reordering effect - System.err.printf("watcher %s sees ...") - is not called, static sharedArray[randomIndex] reference always contains fully constructed values.</p> <p>What's wrong? How to reproduce this? </p>
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    1. COit's _very_ hard to reproduce deliberately. you generally need your code to be running on multiple cpus and to be considered "hot" by hoptspot so that it is aggressively optimized by the jit. and even then, it's still hard to reproduce in testing.
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    2. COaccoring to [jmm](http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#reordering): "There are a number of potential sources of reordering, such as the compiler, the JIT, and the cache" - there are 2 other possibilities of this effect: the compiler and the cache (hardware)... but seems they are also very rare in home conditions.
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