JavaFX and FXML. When are the references to components loaded into the controller class?

I have a very simple fxml file, with a checkbox:

...
<AnchorPane id="AnchorPane" xmlns:fx="http://javafx.com/fxml" fx:controller="jfx01.T01">
...
<CheckBox fx:id="checkBox1" text="CheckBox" />
...

the very simple controller class is the following:

public class T01 extends Application {

    @FXML protected CheckBox checkBox1;

    @Override
    public void start(Stage primaryStage) throws IOException {
         Parent root = FXMLLoader.load(getClass().getResource("t01.fxml"));   
         primaryStage.setScene(new Scene(root));
         primaryStage.show();

         //here, here is the problem!
         System.out.println("checkBox1==null? "+ (checkBox1==null?"yes":"no"));

    }
}

The output of this basic app is:

checkBox1==null? yes

Inside the "start" method the components are obviously created, but not yet assigned to @FXML fields of the controller class. As a test that there aren't other errors, i've added a button and an event. There the checkBox1 IS assigned!

    @FXML protected void handleButton1Action(ActionEvent event) {
        System.out.println("button pressed");
        checkBox1.setSelected(!checkBox1.isSelected());
    }

Question

If I can't use start method because init is not yet complete, what is the first method available where the init is complete and so the components are available?