• In life cycle of Java Servlet page (JSP) , there are 3 important methods:

  • init()

  • service()

  • destroy()

• When a user enters the url in the the web browser and makes a request, the browser generates the HTTP request and sends it to the web server. Web Server maps this request to the corresponding servlet.

  

                      fig. Servlet Life Cycle
  

The init( ) method:

• It request for a servlet received by the servlet engine, checks to see if the servlet is already loaded.

• If not, uses a class loader to get the required servlet class and instantiates it by calling the constructor method.

• After the servlet is loaded, but before it services any requests, the init ( ) method is called.

• Inside init( ), the resources used by the servlet are initialize like establishing database connection.

• This method is called only once just before the servlet is placed into service.

• The init( ) method takes a ServletConfig object as a parameter. Its signature is:

• public void init(ServletConfig config) throws 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.

• Only a single instance of the servlet is created and the servlet engine dispatches each request in a single thread.

• The service( ) method is used only when GenericServlet class is extended.

• Since servlets are designed to operate in the HTTP environment, the HttpServlet class is extended.

• The service(HttpServletRequest, HttpServletResponse) method examines the request and calls the appropriate doGet() or doPost() method.

• A typical Http servlet includes overrides to one or more of these subsidiary methods rather than an override to service().

The destroy( ) method:

• This method signifies the end of a servlet’s life. This method can be invoked by Servlet engine only.

• The resources allocated during init( ) are released.

• It saves persistent information that will be used the next time the servlet isloaded.

• The servlet engine unloads the servlet.