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.

If an instance of the servlet does not exist, the Web container
Loads the servlet class.
Creates an instance of the servlet class.
Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet.
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.

Servlet被服务器实例化后，容器运行其init方法，请求到达时运行其service方法，service方法自动派遣运行与请求对应的doXXX方法（doGet，doPost）等，当服务器决定将实例销毁的时候调用其destroy方法。
与cgi的区别在于servlet处于服务器进程中，它通过多线程方式运行其service方法，一个实例可以服务于多个请求，并且其实例一般不会销毁，而CGI对每个请求都产生新的进程，服务完成后就销毁，所以效率上低于servlet