How to make a singleton class threadsafe?

Question

I am implementing a singleton class in Java to make sure no more than one instance of the class is created.

Answer 1

The best way to make a singleton? Use an enum.

public enum Singleton {
    INSTANCE;
    public void foo(){ ... }
}

// Usage:
Singleton.INSTANCE.foo();

You get lots of help from the VM not only to avoid double instantiation, but it also helps you avoid deserialization corruption.