Yes the field name the controller is injected to is always constructed by concatenating the fx:id
of the <fx:include>
tag with "Controller"
.
It’s “hidden” in the documentation of the FXMLLoader.CONTROLLER_SUFFIX
field.
A suffix for controllers of included fxml files. The full key is stored in namespace map.
(The namespace map contains all the objects by the field name they are injected to, if such a field exists.)
You can verify that it’s value is "Controller"
here: https://docs.oracle.com/javase/8/javafx/api/constant-values.html#javafx.fxml.FXMLLoader.CONTROLLER_SUFFIX