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copied!<p>Yes, you should absolutely synchronize (or use a better idiom like the <a href="http://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom" rel="nofollow"><em>Singleton Holder idiom</em></a>). Otherwise you run the risk of multiple threads initializing your object multiple times (and then subsequently using different instances).</p> <p>Consider a sequence of events like this:</p> <ol> <li>Thread A enters <code>getMyObj()</code> and sees that <code>obj == null</code></li> <li>Thread B enters <code>getMyObj()</code> and sees that <code>obj == null</code></li> <li>Thread A constructs a <code>new MyObj()</code> - let's call it <code>objA</code></li> <li>Thread B constructs a <code>new MyObj()</code> - let's call it <code>objB</code></li> <li>Thread A assigns <code>objA</code> to <code>obj</code></li> <li>Thread B assigns <code>objB</code> to <code>obj</code> (which is not <code>null</code> anymore at this point, so the reference to <code>objA</code>, assigned by Thread A, is overwritten)</li> <li>Thread A exits <code>getMyObj()</code> and starts to use <code>objA</code></li> <li>Thread B exits <code>getMyObj()</code> and starts to use <code>objB</code></li> </ol> <p>This scenario can happen with any number of threads. Note that although here, for the sake of simplicity, I assumed a strict ordering of events, in a real multithreaded environment events 1-2, 3-4 and/or 7-8 can partly or fully overlap in time, without changing the end result.</p> <p>An example to the holder idiom:</p> <pre><code>public class Something { private Something() { } private static class LazyHolder { public static final Something INSTANCE = new Something(); } public static Something getInstance() { return LazyHolder.INSTANCE; } } </code></pre> <p>This is guaranteed to be safe, as <code>INSTANCE</code> is <code>final</code>. The Java memory model guarantees that <code>final</code> fields are initialized and made visible correctly to any number of threads, upon loading the containing class. Since <code>LazyHolder</code> is <code>private</code> and is referenced only by <code>getInstance()</code>, it will only get loaded when <code>getInstance()</code> is first called. And at that point, <code>INSTANCE</code> is initialized in the background and published safely by the JVM.</p>

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