users are accessing the application at the particular time

Hi,
I have a web application in java written in jsp,spring running on tomcat 7.

I don't want to hit the database for the achieving the below.
How can i come to know how many users are accessing the application at the particular time through JSP?
RockingAsked:
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rrzCommented:
You could use a Listener.
 import javax.servlet.http.HttpSessionListener;
 import javax.servlet.http.HttpSessionEvent;
 public class SessionCounter implements HttpSessionListener {
    public void sessionCreated(HttpSessionEvent event) {
        // here is where you can add user to a data structure
    }
    public void sessionDestroyed(HttpSessionEvent event) {
       // here is where you can remove a user 
    }
 }

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RockingAuthor Commented:
It means we can create a hashmap at global level and store all users which are accessing our application.
based on the size of the map we can find out the current users at a given time.
Am i correct?
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rrzCommented:
Yes, you could create a application-scoped variable that would be accessible in your JSPs.
There is an example at  
http://www.experts-exchange.com/Programming/Languages/Java/J2EE/JSP/Q_20514811.html   
Look at my accepted solution. But, your case will simpler. You don't need the Filter.
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RockingAuthor Commented:
Is there any difference between using listener and filter?
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rrzCommented:
Yes. A Filter doesn't listen to session events.
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