http://ifeve.com/easy-happens-before/
https://en.wikipedia.org/wiki/Happened-before
https://blog.csdn.net/ns_code/article/details/17348313
https://www.zhihu.com/question/57682334
https://www.jianshu.com/p/1508eedba54d
原文：https://www.cnblogs.com/gotodsp/p/8838205.html