I\'m trying to implement a middle-ware for group communication in a distributed system using Java RMI.
In there, I need to send an object to the server and modify it
RMI isn't magic. It is Remote Method Invocation. You have to call a remote method to get something to happen in a remote place. Just changing a variable and having it magically propagate across a network isn't within the scope of RMI.
Whatever you want to happen remotely has to be defined by methods in your remote interface(s).