SimpleDateFormat是线程不安全的

该类源码上的解释

 * Date formats are not synchronized.
 * It is recommended to create separate format instances for each thread.
 * If multiple threads access a format concurrently, it must be synchronized externally.

*日期格式不同步。

*建议为每个线程创建单独的格式实例。

*如果多个线程同时访问一种格式，则必须在外部同步该格式。

 

https://blog.youkuaiyun.com/kangbin825/article/details/72564531

https://blog.youkuaiyun.com/piaoslowly/article/details/81476059