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.

Constructor of HttpSessionEvent class

1. HttpSessionEvent(HttpSession source): Construct a session event from the given source.

Method of HttpSessionEvent class

1. public HttpSession getSession(): Return the session that changed

Methods of HttpSessionListener interface

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. 

In this example, are counting the total and current logged-in users. For this purpose, we have created three files:

    Login.html: to get input from the user.
    MyListener.java: A listener class that counts total and current logged-in users and stores this    
    information in ServletContext object as an attribute.
    First.java: A Servlet class that creates session and prints the total and current logged-in users.
    Logout.java: A Servlet class that invalidates session.

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
 
  <listener>
  <listener-class>CountUserListener</listener-class>
  </listener>
 
  <servlet>
     <servlet-name>First</servlet-name>
    <servlet-class>First</servlet-class>
  </servlet>
  <servlet>
      <servlet-name>LogoutServlet</servlet-name>
    <servlet-class>LogoutServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>First</servlet-name>
    <url-pattern>/servlet1</url-pattern>
  </servlet-mapping>

  <servlet-mapping>
    <servlet-name>LogoutServlet</servlet-name>
    <url-pattern>/logout</url-pattern>
  </servlet-mapping>
 
</web-app>

CountUserListener.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);
    }

}
Login.html

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

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

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);
      
        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();
    }

}

LogoutServlet .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();
      
        out.print("You are successfully logged out");
        out.close();
    }

}