HashMap ConcurrentHashMap linkedhashmap 数据结构 讲解， volatile 讲解

https://www.hollischuang.com/archives/2091——hashmap

https://blog.csdn.net/u010412719/article/details/52145145——concurrenthashmap

https://juejin.im/entry/59fc786d518825297f3fa968——concurrenthashmap

https://www.cnblogs.com/dolphin0520/p/3920373.html——Java并发编程：volatile关键字解析

https://www.jianshu.com/p/bbc8087bd9ce——ListHashmap