单例模式

单例模式很多种，但其他几种都有缺点，只需要记住下面这一种单例模式：

public class Singleton {    private volatile static Singleton uniqueInstance;    private Singleton(){    }    public static Singleton getInstance(){        if (uniqueInstance == null) {            synchronized (Singleton.class) {                if (uniqueInstance == null) {                    uniqueInstance = new Singleton();                }            }        }        return uniqueInstance;    }}