转载源自 Gentryhuang 大佬，感谢大佬的帮助！！！万分感谢！！！ 概述HashMap 不是线程安全的，在高并发环境下可能会出现系列问题，如链表成环（JDK1.7)、数据覆盖（JDK1.8)。想要避免 HashMap 的线程安全问题

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