Servlet Basics - Developing Java Servlets - page 20

The Life Cycle of a Servlet
The life cycle of a Java servlet is a very simple object-oriented design. A servlet is constructed
and initialized. It then services zero or more requests until the service that it extends shuts
down. At this point the servlet is destroyed and garbage is collected. This design explains why
servlets are such a good replacement for CGI: The servlet is loaded only once and it stays resi-
dent in memory while servicing requests.
The javax.servlet.Servlet interface declares this framework. The Servlet interface defines
the life cycle methods. These methods are the init() , the service() , and the destroy()
methods.
The init() Method
The init() method is where the servlet's life begins. It is called by the server immediately
after the servlet is instantiated. It is called only once. In the init() method the servlet creates
and initializes the resources that it will be using while handling requests. The init() method's
signature is defined as follows:
public void init(ServletConfig config) throws ServletException;
The init() method takes a ServletConfig object as a parameter. You should save this object
so that it can be referenced later. The most common way of doing this is to have the init()
method call super.init() passing it the ServletConfig object.
You will also notice that the init() method can throw a ServletException . If, for some rea-
son, the servlet cannot initialize the resources necessary to handle requests, the init() method
should throw a ServletException .
The service() Method
The service() method handles all requests sent by a client. It cannot start servicing requests
until the init() method has been executed. You will not usually implement this method
directly, unless you extend the GenericServlet abstract class.
The most common implementation of the service() method is in the HttpServlet class.
The HttpServlet class implements the Servlet interface by extending GenericServlet . Its
service() method supports standard HTTP/1.1 requests by determining the request type and
calling the appropriate method. The signature of the service() method is shown below.
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;