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 getI         


        

Answer 1

The easiest way is to use a static inner holder class :

public class Singleton {

    private Singleton() {
    }

    public static Singleton getInstance() {
        return Holder.INSTANCE;
    }

    private static class Holder {
        private static final Singleton INSTANCE = new Singleton();
    }
}