servlet生命周期

The life cycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.

1. If an instance of the servlet does not exist, the Web container
   1. Loads the servlet class.
   2. Creates an instance of the servlet class.
   3. Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet.
2. Invokes the service method, passing a request and response object. Service methods are discussed in Writing Service Methods.

If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's destroy method. Finalization is discussed in Finalizing a Servlet.