Messaging in Android using Servlet

Hi there !
I'm working on a android app that can detect friends's location. Indeed it's a system with Android, tomcat and MySQL.
I trying to build something so that user can exchange messages when they're both online using servlet, because my system only uses servlet.
And with servlet, can user exchange instant messages or offline messages or both or impossible ???
Did anyone solve similar problem ?

Best regards !

My not use a web service instead of a servlet? You might have an easer time of having the Android app communicate with a web service. That is not to say that using a servlet would not work - after all you have control over the communication protocol.

thanks for reply ! It's very useful.