HttpSessionEvent and HttpSessionListener
| The HttpSessionEvent is notified when session object is changed. The corresponding Listener interface for this event is HttpSessionListener.
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| 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.
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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.
- public void sessionCreated(HttpSessionEvent e): is invoked when session object is created.
- public void sessionDestroyed(ServletContextEvent e): is invoked when session is invalidated.
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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:
- index.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.
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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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