Confused about Life cycle of Servlets

Hi All,

I have following Servlet.

import java.io.*;

import javax.servlet.*;
import javax.servlet.http.*;

/**
 * @author p.chopra
 *
 */
public class HelloWorld extends HttpServlet{ 
  public void doGet(HttpServletRequest request, HttpServletResponse response)
                                   throws ServletException,IOException{
	  System.out.println("doGet");
    response.setContentType("text/html");
    PrintWriter pw = response.getWriter();
    pw.println("<html>");
    pw.println("<head><title>Hello World</title></title>");
    pw.println("<body>");
    pw.println("<h1>Hello World</h1>");
    pw.println("</body></html>");
  }

public HelloWorld() {
			super();
			System.out.println("i am in constructor");
			// TODO Auto-generated constructor stub
		}

/* (non-Javadoc)
 * @see javax.servlet.http.HttpServlet#doPost(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
 */
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
		throws ServletException, IOException {
	// TODO Auto-generated method stub
	System.out.println("doPost");
}

/* (non-Javadoc)
 * @see javax.servlet.http.HttpServlet#service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
 */
@Override
protected void service(HttpServletRequest arg0, HttpServletResponse arg1)
		throws ServletException, IOException {
	// TODO Auto-generated method stub
	super.service(arg0, arg1);
	System.out.println("service HTTP");
}

/* (non-Javadoc)
 * @see javax.servlet.http.HttpServlet#service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
 */
@Override
public void service(ServletRequest arg0, ServletResponse arg1)
		throws ServletException, IOException {
	// TODO Auto-generated method stub
	super.service(arg0, arg1);
	System.out.println("service generic");
	
}

/* (non-Javadoc)
 * @see javax.servlet.GenericServlet#init()
 */
@Override
public void init() throws ServletException {
	// TODO Auto-generated method stub
	System.out.println("init");
}
}

When the very first request comes to this servlet. I got following results:

i am in constructor
init
doPost
service HTTP
service generic

I would like to know the reason behind last 2 outputs. I am confused how these two methods are getting executed.

Pawan,

executes the last 2 lines. Remember doPost() is called by HTTP service method.

If this code is just for experiment then its fine, otherwise in general refrain from overriding the service method. Override the doPost and/or doGet methods instead...

The service(HttpServletRequest, HttpServletResponse) is the HTTP-specific version of the Servlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse) method. Thus, when you make an HTTP request to your server, you get the HTTP service method invoked before the generic service method.

The servlet life cycle is as follows.

1. Creates an Instance of the servlet class (Constructor)
2. Initializes the servlet (init method) and wait for requests.
3. if any request comes in, service method is called and this determines which type of request it is and calls the appropriate method(like doGet, doPost etc...)
4. The destroy method is called before removing the servlet from the container

I think you can trace what is happening by putting as the first line of the method definition and at the end of the method definition.

If this code is just for experiment then its fine, otherwise in general refrain from overriding the service method. Override the doPost and/or doGet methods instead...

+1. The service method doesn't need to be overwritten unless you're writing a servlet to handle HTTP extensions (which would be an exceedingly rare occasion).