You can revert the workflow of the FXML node creation and inject FXML content into your own object. This is especially useful if you want to create custom controls with content managed by FXML.

Firstly, the fx:root attribute should be used instead of the direct class name for the root node:

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<fx:root type="HBox" xmlns:fx="http://javafx.com/fxml">
   <children>
      <Label fx:id="label" />
      <TextField fx:id="textField" />
   </children>
</fx:root>

Also, note that you may not set fx:controller here and select the desired type of the root.

Now, we can create the class that will be used as a root for such FXML:

// chapter4/fxroot/MyControl.java
public class MyControl extends HBox {
    
    @FXML
    private TextField textField;

    @FXML
    private Label label;

    public MyControl(String text) throws IOException {
        // here we initialize our HBox
        setAlignment(Pos.CENTER);
        setSpacing(5);
        
        // loading FXML and using current object as it's root and controller
        FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("MyControl.fxml"));
        fxmlLoader.setRoot(this);
        fxmlLoader.setController(this);
        fxmlLoader.load();
        
        // now we already can use @FXML initialized controls
        textField.setText(text);
        label.setText("Message: ");
    }
}

Here, we are using the same class for the Controller and Root, which allows us to contain all logic in one place. So, we can create our new control like a regular JavaFX class:

// chapter4/fxroot/FxRootDemo.java
public class FxRootDemo extends Application {
    
    @Override
    public void start(Stage stage) throws Exception {
        StackPane stackPane = new StackPane();
        stackPane.getChildren().add(
                new MyControl("Hello, World"));
        Scene scene = new Scene(stackPane, 300, 80);
        
        stage.setScene(scene);
        stage.show();
    }
}

And, we will get the following application: