单例模式

参考：http://my.oschina.net/suyewanwan/blog/102525

1.懒汉式：

public class Singleton {
    private static Singleton singleton;
    private Singleton() {}  //此类不能被实例化
    public static synchronized Singleton getInstance() {
        if (singleton == null) {
            singleton = new Singleton();
        }
        return singleton;
    }
}


2.饿汉式:

public class Singleton {
    private static final Singleton SINGLETON = new Singleton();
    private Singleton() {}  //此类不能被实例化
    public static Singleton getInstance() {
        return SINGLETON;
    }
}


3.内部类单例模式：

public class Singleton {
    private Singleton() {} //构造方法是私有的，从而避免外界利用构造方法直接创建任意多实例。
    public static Singleton getInstance() {
        return Holder.SINGLETON;
    }
    private static class Holder {
       private static final Singleton SINGLETON = new Singleton();
    }
}