Singleton

public class Singleton {private static Singleton singleton;private Singleton(){}public static Singleton getInstance(){//其中两次判断singleton == null的原因：当实例已经创建后，我们不需要再做加锁操作if(singleton == null){//如果不要这一行，每次getInstance时都会加一个同步锁，而加锁是一个非常耗时的操作，应尽量避免。synchronized (Singleton.class){if(singleton == null){singleton = new Singleton();}}}return singleton;}}