Working of a Servlet

Life Cycle of Servlet –
  • Loading –
    • It is the responsibility of the classloader to load the servlet class.
    • When the web container receives the first request for the servlet, it loads the servlet class.
    • During this step, a Servlet container creates a ServletContext object. An interface that defines the set of methods that a servlet can use to communicate with the servlet container is known as ServletContext.
  • Instantiation –
    • After loading the servlet class, the web container creates the instance of a servlet.
    • The servlet life cycle creates the servlet instance only once.
  • Initialization –
    • The init method initializes the servlet.
    • The init method is responsible to create or load the data that will be used throughout the life of the servlet.
    • Syntax – public void init(ServletConfig config)
  • Service –
    • Each time the web container receives the request for the servlet, it calls the service method.
    • The service() method is the main method to perform the actual task i.e. to handle requests coming from the client and to write the formatted response back to the client.
    • The container calls the service method and this method invokes doGet, doPost, doPut, doDelete, etc. methods as appropriate.
    • Syntax – public void service(ServletRequest request, ServletResponse response) 
  • Destruction –
    • It calls the destroy method only once at the end of the life cycle of a servlet.
    • The web container calls the destroy method before removing the servlet instance from the service to give a chance to –
      • clean up any resource
      • close database connections
      • halt background threads
      • write cookie lists
      • hit counts to disk
      • perform other such cleanup activities
    • Syntax – public void destroy()  
Advantages of Servlet –
  • Better Performance as it works on threads.
  • Portability
  • Robust
  • Secure
  • Platform Independent
Flow of Servlet Execution –
  • The client sends the request to the webserver.
  • Then, the web server receives the request.
  • Then, the request is passed to the corresponding servlet by the webserver.
  • Then, the servlet processes the request and generates a response in the form of output.
  • Then, the servlet sends the response back to the webserver.
  • Then, the response is sent back to the client by the web server and the client browser displays it on the screen.
How Servlet handles the request ?
  • The web container maps the request with the servlet in the web.xml file.
  • So, the web container creates request (HttpServletRequest) and response (HttpServletResponse) objects for this request.
  • So for that request, the web container will create or allocate a thread and call the Servlet’s service() method and passes the request, response objects as arguments.
  • Based on HTTP Request Method(Get, Post etc) sent by the client, the service() method, then decides which servlet method, doGet() or doPost() to call.
  • The Servlet sends back the response to the client using the response object.
  • And, after the service() method is executes, the thread dies. Then, the web container deletes the request and response objects and is ready for garbage collection.
import java.io.*;
import javax.servlet.*;

public class ServletLifeCycle implements Servlet {
	
	@Override
	public void init(ServletConfig config) throws ServletException {
	}
	
	@Override
	public ServletConfig getServletConfig() {
		return null;
	}
	
	@Override
	public void service(ServletRequest request, ServletRespnse response) throws ServletException, IOException {
	}
	
	@Override
	public String getServletInfo() {
		return null;
	}
	
	@Override
	public void destroy() {
	}
}

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