Java APIs

Hi all.

I've developed a quite comprehensive client-server system, and is now confronting the task of getting the network communication right.
What I've got is:

1. A server application, that communicates with a local database.
2. Client applications, that communicate with the server.

I've written a TCP server for the server, that sets up a session for each connecting client.

My question now is, how should the client and server communicate?
The communication will be vast, both requesting for custom class information and sending custom classes. The clients should be asked to login as well.

I've had a look at RMI, but I don't feel its appropriate for session based communication with multiple clients. Perhaps I'm wrong?

Thanks in advance.
Jonas