Java的singleton模式的double-check问题

以下是double-checked locking的java代码:

public class Singleton {
    private Singleton instance = null;
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized(this) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

但double-check在J2SE 1.4或早期版本在多线程或者JVM调优时由于out-of-order writes，是不可用的。

这个问题在J2SE 5.0中已经被修复，可以使用volatile关键字来保证多线程下的单例。

public class Singleton {
    private volatile Singleton instance = null;
    public Singleton getInstance() {
        if (instance == null) {
            synchronized(this) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

推荐方法是Initialization on Demand Holder（IODH）,详见 http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom

public class Singleton {
    static class SingletonHolder {
        static Singleton instance = new Singleton();
    }
    
    public static Singleton getInstance(){
        return SingletonHolder.instance;
    }
}