I need to do clustering in tomcat but i got some questions :
1- do i need to change anything ( in programming or in logic ) in my application , my application is J2ee ?
2- Is it easy to configure this in the tomcat ? and any examples for this ?
The most important thing in programming is to make sure that your session variables are all serialization-friendly. If the cluster needs to transfer a session to a different server, only the data that can be serialized will make the trip. You don't want holes in you sessions!
It's not too hard to set up a cluster, but I'm going to have to refer you to the documentation.
An IDE is no substitute for an Intelligent Developer.
Also, just a heads up that Amazon EC2 does not support Multicast. So you may need to decide if you want to support/create/use a database or file system for storing sessions. The following page has some more examples (other than the official docs):