This chapter mainly elaborated the life cycle of servlet:
1. Inituration (init): He is only initialized once when Servlet is started, only when the server is restarted, handle some unnecessary multiple processing and more complex one-time operation
2. Service: For a user, a thread calling service, who calls the corresponding doxxx () based on the corresponding HTTP request.
3. Call Doxxxx :: If you use servlce equivalent to handle DOGET and DOPOST, there are 2 questions: 1. You can't add requests other than 2 methods. The default service called getlastModify method is overwritten
4. Destory: Destruction ends
