How do I add injection of custom fields to the default ControllerFactory of FXMLLoader?

I want to set some non-UI fields in the controller before the initialize method of the controller gets called automatically upon creation. As I understand it, the way to do it is to provide custom ControllerFactory, since initialize() gets called after ControllerFactory returns the created object. I wanted to use the following code as per this answer:

FXMLLoader loader = new FXMLLoader(mainFXML); // some .fxml file to load
loader.setControllerFactory(param -> {
    Object controller = null;
    try {
        controller = ReflectUtil.newInstance(param); // this is default behaviour
    } catch (InstantiationException | IllegalAccessException e) {
        e.printStackTrace();
    }
    if (controller instanceof Swappable) {
        ((Swappable) controller).setSwapper(swapper); // this is what I want to add
    }
    return controller;
});

However, the ReflectUtil class (which is used in default setControllerFactory method) is part of com.sun.reflect.misc package, which I am not able to use, since compiling fails with error: package sun.reflect.misc does not exist.

As I understand it, I can't use sun packages, since this is not public API. So the question is: what do I do? I can't find any other examples of this, only the ones with ReflectUtil and, well, I want my ControllerFactory to comply with default workflow of JavaFX with @FXML annotations and all that. Is there some other way to inject a field maybe? (other than to synchronize on it and wait in initialize() until the setter method gets called from other thread).