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Singleton class + Lazy loading

 
Ranch Hand
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How can i load Singleton class using lazy loading?
 
Sheriff
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There are several ways.

The easiest way is to make the entire getInstance() method synchronized. A bit harder is double-checked locking:
Then there is the holder class:
This nested class will only be initialized when needed; that is, when Singleton.getInstance() is called. Only then will it create the Singleton object. Since class loading is atomic, this approach is thread safe as well.
 
Java Cowboy
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Note that double-checked locking is tricky, and it will not work correctly if you use an older Java version than Java 5: The "Double-Checked Locking is Broken" Declaration.

As far as I know, the memory model was changed in Java 5 so that it's no longer broken (I don't remember all the details right now), but it's still tricky.
 
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