JAVA Observer multi-IHM

My teacher has a JAVA application composed with a controller that was notifying all the different HMI and I want to reproduce it.

But the HMI and the controller are really independent: he had to launch the controller (with a runnable jar) and then with another runnable Jar he could launch as many HMI that he wants.

One solution could be to use socket programming, but I was wondering if he could use another solution without using distance call. An Observer can only notify an object included in its project, isn’t it?

I want to work “locally”, it’s why I was looking for another solution than the socket programming.