java - How to implement thread-safe lazy initialization?

Question

What are some recommended approaches to achieving thread-safe lazy initialization? For instance,

// Not thread-safe
public Foo getInstance(){
    if(INSTANCE == null){
        INSTANCE = new Foo();
    }

    return INSTANCE;
}

Answer 1

For singletons there is an elegant solution by delegating the task to the JVM code for static initialization.

public class Something {
    private Something() {
    }

    private static class LazyHolder {
            public static final Something INSTANCE = new Something();
    }

    public static Something getInstance() {
            return LazyHolder.INSTANCE;
    }
}

see

http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom

and this blog post of Crazy Bob Lee

http://blog.crazybob.org/2007/01/lazy-loading-singletons.html