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    <p>Without <code>volatile</code> the code doesn't work correctly with multiple threads.</p> <p>From Wikipedia's <a href="http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java" rel="noreferrer">Double-checked locking</a>:</p> <blockquote> <p>As of J2SE 5.0, this problem has been fixed. The volatile keyword now ensures that multiple threads handle the singleton instance correctly. This new idiom is described in <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html" rel="noreferrer">The "Double-Checked Locking is Broken" Declaration</a>:</p> </blockquote> <pre><code>// Works with acquire/release semantics for volatile // Broken under Java 1.4 and earlier semantics for volatile class Foo { private volatile Helper helper = null; public Helper getHelper() { Helper result = helper; if (result == null) { synchronized(this) { result = helper; if (result == null) { helper = result = new Helper(); } } } return result; } // other functions and members... } </code></pre> <p>In general you should avoid double-check locking if possible, as it is difficult to get right and if you get it wrong it can be difficult to find the error. Try this simpler approach instead:</p> <blockquote> <p>If the helper object is static (one per class loader), an alternative is the <a href="http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom" rel="noreferrer">initialization on demand holder idiom</a></p> </blockquote> <pre><code>// Correct lazy initialization in Java @ThreadSafe class Foo { private static class HelperHolder { public static Helper helper = new Helper(); } public static Helper getHelper() { return HelperHolder.helper; } } </code></pre>
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