//饿汉式，在多线程下是安全的
public class Single {
    private static final Single s=new Single();
    private Single(){};
    public static Single getInstance(){
        return s;
    }
}

//懒汉式，非线程安全，必须使用同步
public class Single {
    private static  Single s=null;
    private Single(){};
    public static Single getInstance(){
        if(s==null)
            synchronized(Single.class){//静态同步，锁是方法所在类的字节码文件对象
                if(s==null)
                s=new Single();
            }
        return s;
    }
}