Implementing custom protocol logic in Java?

When implementing a client/server solutions, one of th开发者_Python百科e questions you always need to answer is about protocol.

In simple cases, it's possible that packets are always of the same type, so the protocol could even have no logic at all: client connects to the server and sever just says some fact, the client disconnects and that's it.

In more complex cases, some packets are can only be sent in some specific cases. For instance, imagine an abstract server that requires authorization: clients have to authorized before sending or getting any useful data. In this case, the concept of session appears.

Session is an concept that describes the state of client/server dialog: both client and sever expect something from eachother, while there are also things that both of them don't expect.

Then, going even deeper, pretend that protocol is quite complicated and it's implementation should be easily extendable. I believe, that the theoretically right solution here is using a finite state machine. Are there any Java frameworks/libraries that allow this state machine to be easily implemented? Or probably, any more protocol-specific solutions?

What I'm expecting is a framework that allows me to define states and transitions between them.

Update: the question is not about easiest client/server solution implementation, the question is about implementing custom protocol. So, please, don't recommend using web services.

I remember using Unimod FSM for finite state machines a few years ago, although for serious work I always preferred to implement the finite state machines directly.