I want to find a VBox node in a scene loaded with FXMLoader thanks to Node#lookup() but I get the following exception :
java.lang.Cla
The easiest way to get a reference to the VBox is by calling FXMLLoader#getNamespace(). For example:
VBox myvbox = (VBox)fxmlLoader.getNamespace().get("myvbox");
Note that you'll need to create an instance of FXMLLoader and call the non-static version of load() in order for this to work:
FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("test.fxml"));
AnchorPane page = (AnchorPane) fxmlLoader.load();