Protective copy & stateless

         This means of avoiding sharing by creating replica objects is called 【 Protective copy （defensive copy）】

         stay web Stage learning , Design Servlet In order to ensure its thread safety , There will be such suggestions , Don't do it Servlet Set member variables , This class without any member variables is thread safe .

   Because the data saved by member variables can also be called state information , Therefore, if there is no member variable, it is called 【 No state 】