HttpSessionListener
Before going into the details of the SessionListener
we should firstly know about the sessions. As we know that Http protocol is
a "stateless" protocol. The term stateless means that it can't
persist the information. It can't remember the previous transactions. Whenever a
client makes a request for any resources to the server, the server receives the
request and processes the request and sends back the response. After sending the
response the server closes the connection and forgets about the previous
requests. Whenever a client sends any request to the server, the server treats
each request as a new request. To remove this we have been provided the facility
of the session. In session tracking whenever a client sends a request to the
server then server creates a unique id for that request and sends back the
unique id to the client along with the response object, now whenever a client
sends a request to the server it also sends a unique id with it so that the
server can know from where the request is coming.
Listeners listen the events. Whenever any event
occurs it is listened by the listener. The listener will be controller by the
web servers.
HttpSessionListener is an interface which
extends java.util.EventListener class. The main purpose of this listener
is to notify whenever there is a change in the list of active sessions in a web
application
This interface has two methods:
- sessionCreated(HttpSessionEvent event): It
will notify when the session is created.
- sessionDestroyed(HttpSessionEvent event): It
will notify when the session gets invalidated.
In the above methods we can see that we have used
HttpSessionEvent as the parameter of both the methods. This class has only one
method getSession() which returns the current session.
The code of the program is given below:
Make the entry of this file in the deployment
descriptor file that is web.xml
import javax.servlet.*;
import javax.servlet.http.*;
public class MySessionListener
implements HttpSessionListener {
public MySessionListener() {
}
public void sessionCreated(HttpSessionEvent sessionEvent) {
// Get the session
HttpSession session = sessionEvent.getSession();
try {
System.out.println("Session created: "+session);
session.setAttribute("foo","bar");
} catch (Exception e) {
System.out.println("Error in setting session attribute: "
}+ e.getMessage());
}
}
public void sessionDestroyed(HttpSessionEvent sessionEvent) {
// Get the session that was invalidated
HttpSession session = sessionEvent.getSession();
// Log a message
System.out.println("Session invalidated: "+session);
System.out.println("The name is: " + session.getAttribute("foo"));
}
} |
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class ServletSessionListener extends HttpServlet{
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
HttpSession session= request.getSession();
String str = (String)session.getAttribute("foo");
pw.println("The name is " + str);
}
} |
The output of the program is given below:
Download this example.
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