I'd like to show all logged in users in my application in a window for an administrator. I think I have to use a Java action, but I'm not sure which core functions to call. (M2EE can do the same, but this is not an option in this case.)
You could also enable PersistentSessions. The active sessions will be logged in the database. By using the System.Session entity you could display a datagrid with all active sessions. This way you don't need Java. Please be aware that persistent sessions has influence on your application performance.