In Java, HttpSessionEvent is a class, which is representing event notifications for changes or updates to sessions within a web application. Similarly, the interface for this event is HttpSessionListener, which is for receiving notification events about HttpSession lifecycle changes. As a means to, accept these notification events, the implementation of the class must follow one of the following:
- It should be declared in the deployment descriptor of the web application, annotated with WebListener
- It should be registered via one of the addListener methods defined on ServletContext
There are two methods of the HttpSessionListner interface:
- sessionCreated: It receives the notification that a session has been created.
- sessionDestroyed: It receives the notification that a session is almost invalidated.
// Parameter: se – the HttpSessionEvent containing the session
void sessionCreated(HttpSessionEvent se)
// Parameter: se – the HttpSessionEvent containing the session
void sessionDestroyed(ServletContextEvent se)
Example
Count the total and the active sessions, using HttpSessionEvent and HttpSessionListener. Now, we have to create these files:
- index.html
- CountUserListner.java
- LoginServlet.java
- LogoutServlet.java
index.html: Login Credentials
HTML
<!DOCTYPE html><html> <body> <h2>Login Credentials</h2> <form action="servlet"> Username:<input type="text" name="username"><br> Password:<input type="password" name="userpass"><br> <input type="Submit" value="Sign-in"/> </form> </body></html> |
CountUserListner.java: This listener class counts the total and active sessions and stores this information as an attribute in the ServletContext object.
Java
import javax.servlet.ServletContext;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener; public class CountUserListener implements HttpSessionListener{ ServletContext scx =null; static int all =0, active =0; // It receives the notification that // a session has been created. public void sessionCreated(HttpSessionEvent se) { all++; active++; scx =se.getSession().getServletContext(); scx.setAttribute("All Users", all); scx.setAttribute("Active Users", active); } // It receives the notification that // a session is almost invalidated public void sessionDestroyed(HttpSessionEvent se) { active--; scx.setAttribute("Active Users", active); } } |
LoginServlet.java: This Servlet class creates a session and prints the total and current active users.
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 LoginServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html; charset=UTF-8"); PrintWriter out = response.getWriter(); String str = request.getParameter("Username"); out.print("Welcome "+str); HttpSession session = request.getSession(); session.setAttribute("Uname",str); // this retrieve data from ServletContext object ServletContext scx = getServletContext(); int au = (Integer)scx.getAttribute("All Users"); int acu = (Integer)scx.getAttribute("Active Users"); out.print("<br>All Users = "+au); out.print("<br>Active Users = "+acu); out.print("<br><a href='logout'>Logout</a>"); out.close(); } } |
LogoutServlet.java: This Servlet class invalidates session.
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); // This invalidates the session session.invalidate(); out.print("You are successfully logged out"); out.close(); } } |
