Following Scenario:

I define 2 FXML + Controllers:

A.fxml
A.java

B.fxml
B.java

Now I use fx:include to import B into A. Now I embed B into the hierarchy graph of A.fxml.
In A.java I use @FXML to get the reference to the Node which is represented by the included B.fxml. Works like a charm.

My Problem is now:

Inside of A.java I do need a reference to the instance of the controller (B.java) which belongs to the @FXML-injected node of the B.fxml. How to achieve this?