Question

I have a web based application that uses userName and password for login.

now how can i check on certain time which all users are logged in at that very time.
i am using session management and no DB is used in application everything is on filesystem

Edit: 1 more silly doubt.. how to define a variable with application scope.. is this something of this sort?

<env-entry>
<env-entry-name>test/MyEnv2</env-entry-name>
<env-entry-type>java.lang.Boolean</env-entry-type>
<env-entry-value>true</env-entry-value>
</env-entry>

Answer 1

Just collect all logged in users in a Set in the application scope. If your application is well designed, you should have a javabean User which represents the logged-in user. Let it implement HttpSessionBindingListener and add/remove the user from the Set when it’s about to be bound/unbound in the session.

Kickoff example:

public class User implements HttpSessionBindingListener {

    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        Set<User> logins = (Set<User>) event.getSession().getServletContext().getAttribute("logins");
        logins.add(this);
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        Set<User> logins = (Set<User>) event.getSession().getServletContext().getAttribute("logins");
        logins.remove(this);
    }

    // @Override equals() and hashCode() as well!

}

Note that you need to prepare the Set in the application scope so that it doesn’t return null in above methods. You could do that in the same methods by a nullcheck, or with help of ServletContextListener#contextInitialized().

Then, anywhere in your application where you’ve access to the ServletContext, like in a servlet, you can just access the logged-in users as follows:

Set<User> logins = (Set<User>) getServletContext().getAttribute("logins");