I would suggest you go for this library
It is very easy to use. You just need to follow couple of steps
1) Add the Library into your Project.
2) Add the below mentioned entry in web.xml
3) Define below in your applicationContext.xml for your RPC classes
4) Mark your Remove Service with Below mentioned Annotation
5) Make sure that your RPC implementation is marked with below mentioned annotation. Also they should purely implement RemoteServiceInterface. Nothing else.
Let me know if this helps.