关于单利模式：双因素检查法以及枚举方式实现单利模式

URL:

https://blog.csdn.net/goodlixueyong/article/details/51935526

https://www.cnblogs.com/cielosun/p/6596475.html

https://www.cnblogs.com/yangzhilong/p/6148639.html