Java – singleton using atomicreference

Is the deferred initialization singleton correctly implemented using atomicreference? If not – what are the possible problems?

import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.concurrent.atomic.AtomicReference;

public class Singleton implements Serializable {

    private static final Singleton _instance = new Singleton();

    private static AtomicReference<Singleton> instance = new AtomicReference<Singleton>();

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance.compareAndSet(null,_instance)) {
            synchronized (_instance) {
                _instance.init();
                instance.set(_instance);
            }
        }
        return instance.get();
    }

    private void init() {
        // do initialization
    }

    private Object readResolve() throws ObjectStreamException {
        return getInstance();
    }

}

Solution

No, it's bad:

public static Singleton getInstance() {
    // new "singleton" for every method call
    Singleton s = new Singleton();
                   ^^^^^^^^^^^^^^
    if (instance.compareAndSet(null,s)) {
        synchronized (s) {
            s.init();
        }
    }
    return instance.get();
}

public final class Singleton{
    private Singleton(){}
    private static final Singleton INSTANCE = new Singleton();
    public Singleton getInstance(){return INSTANCE;}
}

public final class Singleton{
    private Singleton(){}
    private static class Holder{
        private static final Singleton INSTANCE = new Singleton();
    }
    public Singleton getInstance(){return Holder.INSTANCE;}
}

public enum Singleton{
    INSTANCE;
}