Program to demonstrate how to apply listener to Session attributes in Java EE
package com.hubberspot.javaee.listener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
// @WebListener annotation informs container that
// this class is a web listener which will listen to
// various events happening during lifecycle of application
// Here this class listens to changes made to HttpSession
// attributes.
// We make class implements HttpSessionAttributeListener
// which has three methods : attributeRemoved(), attributeAdded()
// and attributeReplaced() , which are called by the container
// whenever attributes are added, removed, or replaced
// within the HTTP session.
@WebListener
public class MySessionAttributeListener implements HttpSessionAttributeListener {
public void attributeRemoved(HttpSessionBindingEvent event) {
System.out.println("Method called when HttpSession attribute removed :");
HttpSession session = event.getSession();
System.out.println("Session ID : " + session.getId());
System.out.println("Session Name : " + event.getName());
System.out.println("Session Value : " + event.getValue());
}
public void attributeAdded(HttpSessionBindingEvent event) {
System.out.println("Method called when HttpSession attribute added :");
HttpSession session = event.getSession();
System.out.println("Session ID : " + session.getId());
System.out.println("Session Name : " + event.getName());
System.out.println("Session Value : " + event.getValue());
}
public void attributeReplaced(HttpSessionBindingEvent event) {
System.out.println("Method called when HttpSession attribute replaced :");
HttpSession session = event.getSession();
System.out.println("Session ID : " + session.getId());
System.out.println("Session Name : " + event.getName());
System.out.println("Session Value : " + event.getValue());
}
}
Run HelloWorldServlet.java and methods mentioned in above listener will be called by the container.package com.hubberspot.javaee;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
// @WebServlet annotation has a initParams field which takes
// in initialization parameters for a servlet.
// @WebInitParam annotation takes in a name and value for the
// initialization parameters for the current Servlet.
@WebServlet(name = "HelloWorldServlet" , urlPatterns = { "/HelloWorldServlet" }
, initParams = { @WebInitParam(name = "user" , value = "Jonty") })
public class HelloWorldServlet extends HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response
) throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
HttpSession session = request.getSession();
// attributeAdded method gets executed
session.setAttribute("user", "Jonty");
// attributeReplaced method gets executed
session.setAttribute("user", "Dinesh");
// attributeRemoved method gets executed
session.removeAttribute("user");
}
}
Output of the program : 
