I have never seen anything that tracks the number of active session, but there is data about the current number of active requests - the currentThreadsBusy attribute of the jboss.web:name=http-127.0.0.1-8080,type=ThreadPool mbean (your mbean might be named differently based on the IP address your app server is bound to).
Most monitoring tools use JMX, and the MBeans provided by JBossAS, to provide their data. The jmx-console is an MBean browser for JBossAS. To access it, use the url: http://hostname:8080/jmx-console. (If this doesn't work, make sure that the app server has the directory server/xxx/deploy/jmx-console.war - some people remove this directory for production.) Once you know the MBean name and attribute it is usually simple to configure the monitoring tool to access the data.
Thanks for that info. I tried browsing to http://localhost:8180/jmx-console/ on the server and I got a huge listing of stuff (I don't even know what the stuff is!). What I do know is the application I host (Saba) has a single ear file which I deploy (D:\SabaWeb\JBoss_Tomcat\server\default\deploy\saba.ear), and in the ear file is a saba.war file and a saba.jar file, which as I understand it is pretty much standard for J2EE applications. What I am interested in is how many people have an active session in that application.
I found what I think is the mbean you mentioned:
And when I follow the link it shows a value for currentThreadsBusy. I did a test by logging in as several different people, and then refreshed the mbean view, but nothing changed.
I find it odd that there is no easy way to find out how many users have an active session in a particular application - I would have thought that was a metric which most system administrators would like to monitor from time to time.