How Servlet Works?

Now learn in this page how to servlet works, we are going to get the internal detail about the first servlet program.

The server checks if the servlet is requested for the first time.

If yes, web container does the following steps-

  • loads the servlet class.
  • instantiates the servlet class.
  • calls the init method passing the ServletConfig object

otherwise calls the service method passing request and response objects

How to handles web container the servlet request?

The web container is responsible to handle the request.

  • The request with the servlet in the web.xml file.
  • creates request and response objects for this request
  • calls the service method.
  • The public service method internally calls the protected service method
  • The protected service method calls the doGet method depending on the type of request.
  • The doGet method generates the response and it is passed to the client.
  • After sending the response, the web container deletes the request and response objects

What is written inside the public service method?


public void service(ServletRequest req, ServletResponse res)  
       throws ServletException, IOException  
   {  
     HttpServletRequest request;  
     HttpServletResponse response;  
      try  
      {  
        request = (HttpServletRequest)req;  
        response = (HttpServletResponse)res;  
       }  
      catch(ClassCastException e)  
      {  
      throw new ServletException("non-HTTP request or response");  
      }  
   service(request, response);  
   }  

What is written inside the protected service method?

protected void service(HttpServletRequest req, HttpServletResponse resp)  
       throws ServletException, IOException  
   {  
       String method = req.getMethod();  
       if(method.equals("GET"))  
       {  
          long lastModified = getLastModified(req);  
         if(lastModified == -1L)  
           {  
               doGet(req, resp);  
           }   
  ....  
   //rest of the code  
       }