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