不使用Lock 实现单例

Singleton.java

public class Singleton {
    private static final AtomicReference<Singleton> INSTANCE = new AtomicReference<Singleton>(); 

    private Singleton() {}

    public static Singleton getInstance() {
        for (;;) {
            Singleton singleton = INSTANCE.get();
            if (null != singleton) {
                return singleton;
            }

            singleton = new Singleton();
            if (INSTANCE.compareAndSet(null, singleton)) {
                return singleton;
            }
        }
    }
}

from http://www.hollischuang.com/archives/1866