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HttpSessionEvent and HttpSessionListener

The HttpSessionEvent is notified when session object is changed. The corresponding Listener interface for this event is HttpSessionListener.

We can perform some operations at this event such as counting total and current logged-in users, maintaing a log of user details such as login time, logout time etc.

Methods of HttpSessionListener interface

There are two methods declared in the HttpSessionListener interface which must be implemented by the servlet programmer to perform some action.
  1. public void sessionCreated(HttpSessionEvent e): is invoked when session object is created.
  2. public void sessionDestroyed(ServletContextEvent e): is invoked when session is invalidated.

Example of HttpSessionEvent and HttpSessionListener to count total and current logged-in users

In this example, are counting the total and current logged-in users. For this purpose, we have created three files:
  1. index.html: to get input from the user.
  2. MyListener.java: A listener class that counts total and current logged-in users and stores this information in ServletContext object as an attribute.
  3. First.java: A Servlet class that creates session and prints the total and current logged-in users.
  4. Logout.java: A Servlet class that invalidates session.

index.html


<form action="servlet1">
Name:<input type="text" name="username"><br>
Password:<input type="password" name="userpass"><br>

<input type="submit" value="login"/>
</form>

MyListener.java


import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class CountUserListener implements HttpSessionListener{
	ServletContext ctx=null;
	static int total=0,current=0;
	
	public void sessionCreated(HttpSessionEvent e) {
	total++;
	current++;
	
	ctx=e.getSession().getServletContext();
	ctx.setAttribute("totalusers", total);
	ctx.setAttribute("currentusers", current);
	
	}

	public void sessionDestroyed(HttpSessionEvent e) {
		current--;
		ctx.setAttribute("currentusers",current);
	}

}


First.java


import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class First extends HttpServlet {
public void doGet(HttpServletRequest request,
 HttpServletResponse response)
	throws ServletException, IOException {

		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
	
		String n=request.getParameter("username");
		out.print("Welcome "+n);
		
		HttpSession session=request.getSession();
		session.setAttribute("uname",n);
		
		//retrieving data from ServletContext object
		ServletContext ctx=getServletContext();
		int t=(Integer)ctx.getAttribute("totalusers");
		int c=(Integer)ctx.getAttribute("currentusers");
		out.print("<br>total users= "+t);
		out.print("<br>current users= "+c);
		
		out.print("<br><a href='logout'>logout</a>");
		
		out.close();
	}

}


Logout.java


import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;


public class LogoutServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
 HttpServletResponse response)
		throws ServletException, IOException {

		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
	
		HttpSession session=request.getSession(false);
		session.invalidate();//invalidating session
		
		out.print("You are successfully logged out");
		
		
		out.close();
	}

}





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