I would suggest using RMI (Remote Method invocation) it is robust and seems fitted for what you are trying to accomplish:
http://en.wikipedia.org/wiki/Java_remote_method_invocation
I would also suggest using LipeRMI, which is a good, easy to follow RMI library.
Here is a good link to give you a quick idea on how it works and what your code would look like: http://lipermi.sourceforge.net/documentation.php
It would allow you to call methods on the server and send/receive serialized objects. Only thing is, if you need to do stuff like file transfer, this wouldnt work it only works with java objects.