JavaFX FXML und SceneBuilder in IntelliJ

${eclipse-project-dir}
  + src
    + my
      + demo
        + view
		  + Base.fxml
		  + ContentArea.fxml
		  + MenuBarArea.fxml
		  + MyDemoApp.fxml
		+ view_ctlr
		  + AccessToAll.java
		  + BaseController.java
		  + ContentAreaController.java
		  + MenuBarAreaController.java
		+ MyDemoApp.java
        + FxmlControllerIface.java
		+ my-demo-app.css

package my.demo;

import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;
import my.demo.view_ctlr.AccessToAll;
import my.demo.view_ctlr.BaseController;

public class FxmlApp extends Application {

    public FxmlApp() {

    }

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        //Schlecht: (kein Zugriff auf Controller):
        //          Parent root = loader.load(getClass().getResource("MyApp.fxml"));
        // besser - ermöglicht Zugriff auf Controller und damit UI-Elemente:
        // FXMLLoader loader = new FXMLLoader(getClass().getResource(FxmlApp.BASE_FXML));
        // Parent root = loader.load();
        // MUSS NACH loader.load() erfolgen - sonst wird 'null' !!!!! zurückgeliefert:
        // BaseController baseController = loader.<BaseController>getController();

        // Am besten: Alle Controller implementieren Interface FxmlControllerIface:
        // Damit ist es möglich, eine Methode 'createFromFxml(...)' zu implementieren, die 
        // das Laden und Konfigurieren einheitlich erledigt. 
        // Die Generic-Parameter (hier <BorderPane, BaseController>) werden nur für den 
        // Rückgabetyp benötigt und können weggelassen werden, da der Compiler den erforderlichen
        // Rückgabetyp bei der Zuweisung ohnehin prüfen kann/muss.
        BaseController baseCtlr = AccessToAll.singleton()
                .<BorderPane, BaseController>createFromFxml(AccessToAll.BASE_FXML);
        Parent root = baseCtlr.getFxmlParent();
        Scene scene = new Scene(root);
        AccessToAll.singleton().setScene(scene);
        AccessToAll.singleton().setBaseController(baseCtlr);

        scene.getStylesheets().add(getClass().getResource("my-demo-fxmlapp.css").toExternalForm());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

}

package my.demo.view_ctlr;

import java.io.IOException;

import javafx.event.Event;
import javafx.fxml.FXMLLoader;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Control;
import javafx.scene.control.MenuItem;
import javafx.scene.control.TextField;
import javafx.scene.layout.Pane;
import my.demo.FxmlControllerIface;

public class AccessToAll { // könnte mann auch 'AppContext' o.ä. nennen
    public static final String BASE_FXML = "/my/demo/view/Base.fxml";
    public static final String MENUBAR_AREA_FXML = "/my/demo/view/MenuBarArea.fxml";
    public static final String CONTENT_AREA_FXML = "/my/demo/view/ContentArea.fxml";

    private static AccessToAll instance;
    private Scene scene;
    private BaseController baseController;
    private MenuBarAreaController menuBarAreaController;
    private ContentAreaController contentAreaController;

    public static AccessToAll singleton() {  // immer SELBES Objekt zurückliefern
        if (instance == null) {
            instance = new AccessToAll();  // kann nur ein einziges Mal instanziert werden
        }
        return instance;
    }

    private AccessToAll() { // Erzeugung Singleton: Verhindern, dass mit 'new AssignToAll()'
                            // neue Objekte erstellt werden können -> privater Konstruktor und
    } // Instanzieren nur mit Methode 'singleton(), wenn noch KEINE Instanz da'

    public Scene getScene() {
        return scene;
    }

    public void setScene(Scene scene) {
        this.scene = scene;
    }

    public BaseController getBaseController() {
        return baseController;
    }

    public void setBaseController(BaseController baseController) {
        assert baseController == null || baseController.getFxmlParent() != null
                : noFxmlParentMsg(BASE_FXML);
        this.baseController = baseController;
    }

    public MenuBarAreaController getMenuBarAreaController() {
        return menuBarAreaController;
    }

    public void setMenuBarAreaController(MenuBarAreaController menuBarAreaController) {
        assert menuBarAreaController == null || menuBarAreaController.getFxmlParent() != null
                : noFxmlParentMsg(MENUBAR_AREA_FXML);
        this.menuBarAreaController = menuBarAreaController;
    }

    public ContentAreaController getContentAreaController() {
        return contentAreaController;
    }

    public void setContentAreaController(ContentAreaController contentAreaController) {
        assert contentAreaController == null || contentAreaController.getFxmlParent() != null
                : noFxmlParentMsg(CONTENT_AREA_FXML);
        this.contentAreaController = contentAreaController;
    }

    private static String noFxmlParentMsg(String fxmlPath) {
        return "FXML-File '" + fxmlPath + "': fx:id des Wurzelelem. MUSS wegen "
                + "Controller-Basisklasse 'MyFxmlController' zwingend 'fxmlParent' sein";
    }

    /**
     * 
     * @param <P>      Typ der Wurzel-Pane der gegebenen FXML-Datei (BorderPane, HBox, VBox, ...)
     * @param <TT>     Typ des Controllers (BaseController, MenuBarAreaController, ...)
     * @param fxmlPath relativer Pfad der FXML-Datei (auch unter Windows '/' als Verzeichnis-Trenner
     *                 - Werte in AccessToAll als Konstante vorhanden (BASE_FXML, MENUBAR_AREA_FXML,
     *                 CONTENT_AREA_FXML)
     * 
     * @return C Generic Type für den zurückzuliefernden Controller
     */
    public <P extends Pane, TT extends FxmlControllerIface<P>> TT createFromFxml(String fxmlPath) {
        FXMLLoader loader = new FXMLLoader(getClass().getResource(fxmlPath));
        try {     // loader.load MUSS ZUERST erfolgen, damit 'loader' alles weiß!
            loader.load();  // Parent parent = ... Rückgabewert nicht genutzt
        } catch (IOException ioe) {
            throw new IllegalArgumentException("Lade-Problem bei FXML-Datei", ioe);
        }
        return loader.getController();
    }

    // Methode, die eleganter in anderer Klasse enthalten wäre - Kompromiss wg. Kürze:
    public static String eventInfo(Event event) {
        String txt = "Event Typ " + event.getEventType() + ", ausgelöst von ";
        Object evtSrc = event.getSource();
        if (evtSrc instanceof MenuItem mi) {
            txt = "MenuItem '" + mi.getText() + "'";
        } else if (evtSrc instanceof Button bt) {
            txt = "Button '" + bt.getText() + "'";
        } else if (evtSrc instanceof TextField tf) {
            txt = "TextField '" + tf.getText() + "'";
        } else if (evtSrc instanceof Pane pn) {
            txt = "Pane - " + pn.toString();
        } else if (evtSrc instanceof Control cl) {
            txt = "Control - " + cl.toString();
        } else if (evtSrc instanceof Node nd) {
            txt = "Node - " + nd.toString();
        } else {
            txt = "Other Source Type - " + evtSrc.toString();
        }
        return txt;
    }
}

package my.demo;

import javafx.fxml.FXML;
import javafx.scene.layout.Pane;

public interface FxmlControllerIface<P extends Pane> {

    P getFxmlParent();

    @FXML
    void initialize();  // called by the FXMLLoader when initialization is complete
}

/**
 * Sample Skeleton for 'MyDemoAppBase.fxml' Controller Class
 */

package my.demo.view_ctlr;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.Parent;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import my.demo.FxmlControllerIface;

public class BaseController implements FxmlControllerIface<BorderPane> {

    @FXML // ResourceBundle that was given to the FXMLLoader
    private ResourceBundle resources;

    @FXML // URL location of the FXML file that was given to the FXMLLoader
    private URL location;

    @FXML
    private BorderPane fxmlParent;

    @FXML // fx:id="btnDrueckenFuerMenue"
    private Button btnDrueckenFuerMenue; // Value injected by FXMLLoader

    @FXML
    private Label lblInfoToDo;

    @FXML
    void menueZeigenAction(ActionEvent event) throws IOException {
        System.out.println("Menü wird instanziert und aktiviert - b");

        // MenuBarAreaController menuBarAreaCtlr = AccessToAll.singleton()
        // .<HBox, MenuBarAreaController>createFromFxml(AccessToAll.MENUBAR_AREA_FXML);
        // Explizite Rückgabe-Typ-Parameter korrekt, aber unnötig (Compiler kann's selbst):
        MenuBarAreaController menuBarAreaCtlr = AccessToAll.singleton()
                .createFromFxml(AccessToAll.MENUBAR_AREA_FXML);
        Parent menuBarArea = menuBarAreaCtlr.getFxmlParent();
        AccessToAll.singleton().setMenuBarAreaController(menuBarAreaCtlr);

        fxmlParent.setTop(menuBarArea);
        btnDrueckenFuerMenue.setDisable(true);
        lblInfoToDo.setDisable(false);
    }

    @Override
    @FXML // This method is called by the FXMLLoader when initialization is complete
    public void initialize() {
        assert fxmlParent != null : "fx:id=\"fxmlParent\" was not injected:"
                + " check your FXML file 'ContentArea.fxml'.";
        assert btnDrueckenFuerMenue != null : "fx:id=\"btnDrueckenFuerMenue\" was not injected:"
                + " check your FXML file 'MyDemoAppBase.fxml'.";
        assert lblInfoToDo != null : "fx:id=\"lblInfoToDo\" was not injected:"
                + " check your FXML file 'Base.fxml'.";
        // hier könnten weitere dynamische Setup-Aktivitäten erfolgen ...
    }

    @Override
    public BorderPane getFxmlParent() {
        return fxmlParent;
    }

}

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Menu?>
<?import javafx.scene.control.MenuBar?>
<?import javafx.scene.control.MenuItem?>
<?import javafx.scene.layout.HBox?>

<HBox xmlns="http://javafx.com/javafx/16" xmlns:fx="http://javafx.com/fxml/1" fx:controller="my.demo.view_ctlr.MenuBarAreaController">
   <children>
      <MenuBar styleClass="menu-element">
        <menus>
          <Menu mnemonicParsing="false" styleClass="menu-element" text="File">
            <items>
              <MenuItem mnemonicParsing="false" onAction="#infoNotImplemented" styleClass="menu-element" text="Close" />
            </items>
          </Menu>
          <Menu mnemonicParsing="false" styleClass="menu-element" text="Edit">
            <items>
              <MenuItem mnemonicParsing="false" onAction="#infoNotImplemented" styleClass="menu-element" text="Delete" />
            </items>
          </Menu>
            <Menu mnemonicParsing="false" styleClass="menu-element" text="Settings">
              <items>
                <MenuItem mnemonicParsing="false" onAction="#setupContentArea" styleClass="menu-element" text="Inhaltsbereich aktivieren" />
                  <MenuItem mnemonicParsing="false" onAction="#tabAdd" text="Tab hinzufügen" />
              </items>
            </Menu>
          <Menu mnemonicParsing="false" styleClass="menu-element" text="Help">
            <items>
              <MenuItem mnemonicParsing="false" onAction="#infoNotImplemented" styleClass="menu-element" text="About" />
            </items>
          </Menu>
        </menus>
      </MenuBar>
   </children>
</HBox>

package my.demo.view_ctlr;

import java.io.IOException;
import java.net.URL;
import java.util.List;
import java.util.ResourceBundle;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.Parent;
import javafx.scene.control.Label;
import javafx.scene.control.MenuItem;
import javafx.scene.control.Tab;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import my.demo.FxmlControllerIface;

public class MenuBarAreaController implements FxmlControllerIface<HBox> {

    @FXML // ResourceBundle that was given to the FXMLLoader
    private ResourceBundle resources;

    @FXML // URL location of the FXML file that was given to the FXMLLoader
    private URL location;

    @FXML
    private HBox fxmlParent;

    @FXML
    private MenuItem mniTabAdd;

    @FXML
    public void infoNotImplemented(ActionEvent event) {
        System.out.println("Noch nicht implementiert - " + AccessToAll.eventInfo(event));
    }

    @FXML
    public void setupContentArea(ActionEvent event) throws IOException {
        System.out.println(AccessToAll.eventInfo(event));

        ContentAreaController contentAreaCtlr = AccessToAll.singleton()
                .<VBox, ContentAreaController>createFromFxml(AccessToAll.CONTENT_AREA_FXML);
        AccessToAll.singleton().setContentAreaController(contentAreaCtlr);
        AccessToAll.singleton().getBaseController().getFxmlParent()
                .setCenter(contentAreaCtlr.getFxmlParent());
        mniTabAdd.setDisable(false);
    }

    @FXML
    public void showAbout(ActionEvent event) {
        String msg = "Die beste JavaFX-App - definitiv!";
        ButtonType btEnough = new ButtonType("Genug Info");
        ButtonType btShowMore = new ButtonType("Zeige mehr Info");
        ButtonType btShowAll = new ButtonType("Zeige komplette Info");
        Alert alertConfirm = new Alert(AlertType.CONFIRMATION, msg, //
                btEnough, btShowMore, btShowAll);
        alertConfirm.setHeaderText("RX-HeaderText");
        alertConfirm.setTitle("RX-Title");
        Optional<ButtonType> result = alertConfirm.showAndWait();
        if (result.get() == btEnough) {
            System.out.println("genug Info ... Ende!");
        } else if (result.get() == btShowMore) {
            System.out.println("mehr Info ... OK");
            Alert alertInfoMore = new Alert(AlertType.INFORMATION, "Hier mehr Info");
            alertInfoMore.show();
        } else {
            System.out.println("alle Info ... wird gemacht");
            Alert alertInfoAll = new Alert(AlertType.WARNING, "mehr gibt's nicht!");
            alertInfoAll.show();
        }
    }

    @FXML
    public void tabAdd(ActionEvent event) {
        System.out.println(AccessToAll.eventInfo(event));

        List<Tab> tabs = AccessToAll.singleton().getContentAreaController().getTbpContentTabPane()
                .getTabs();
        int nr = tabs.size() + 1;
        VBox contentVBox = new VBox();
        Tab newTab = new Tab("Tab " + nr, contentVBox);
        contentVBox.getChildren().add(new Label("Inhalt von Tab " + nr));
        tabs.add(newTab);
    }

    @FXML
    public void resetApp(ActionEvent event) throws IOException {
        System.out.println(AccessToAll.eventInfo(event));

        BaseController baseCtlr = AccessToAll.singleton()
                .<BorderPane, BaseController>createFromFxml(AccessToAll.BASE_FXML);
        Parent root = baseCtlr.getFxmlParent();
        AccessToAll.singleton().getScene().setRoot(root); // Scene erhält neue Rootcontainer-Instanz

        AccessToAll.singleton().setBaseController(baseCtlr); // neue BaseController-Instanz
        AccessToAll.singleton().setMenuBarAreaController(null); // alte Instanz nun ungültig
        AccessToAll.singleton().setContentAreaController(null); // detto
    }

    @Override
    @FXML // This method is called by the FXMLLoader when initialization is complete
    public void initialize() {
        assert fxmlParent != null : "fx:id=\"fxmlParent\" was not injected:"
                + " check your FXML file 'MenuBarArea.fxml'.";
        assert mniTabAdd != null : "fx:id=\"mniTabAdd\" was not injected:"
                + " check your FXML file 'MenuBarArea.fxml'.";
    }

    @Override
    public HBox getFxmlParent() {
        return fxmlParent;
    }

}

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Label?>
<?import javafx.scene.control.Tab?>
<?import javafx.scene.control.TabPane?>
<?import javafx.scene.layout.VBox?>


<VBox xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/16" fx:controller="my.demo.view_ctlr.ContentAreaController">
   <children>
      <TabPane fx:id="tbpContentTabPane" prefHeight="200.0" prefWidth="349.0" tabClosingPolicy="UNAVAILABLE">
        <tabs>
          <Tab closable="false" text="Tab 1">
               <content>
                  <VBox prefHeight="169.0" prefWidth="271.0">
                     <children>
                        <Label text="Erster Tab, auf nicht schließbar gesetzt" />
                     </children>
                  </VBox>
               </content>
            </Tab>
        </tabs>
      </TabPane>
   </children>
</VBox>

/**
 * Sample Skeleton for 'ContentArea.fxml' Controller Class
 */

package my.demo.view_ctlr;

import java.net.URL;
import java.util.ResourceBundle;

import javafx.fxml.FXML;
import javafx.scene.control.TabPane;
import javafx.scene.layout.VBox;
import my.demo.FxmlControllerIface;

public class ContentAreaController implements FxmlControllerIface<VBox> {

    @FXML // ResourceBundle that was given to the FXMLLoader
    private ResourceBundle resources;

    @FXML // URL location of the FXML file that was given to the FXMLLoader
    private URL location;

    @FXML
    private VBox fxmlParent;

    @FXML // fx:id="tbpContentTabPane"
    private TabPane tbpContentTabPane; // Value injected by FXMLLoader

    @Override
    @FXML // This method is called by the FXMLLoader when initialization is complete
    public void initialize() {
        //super.initialize();
        assert fxmlParent != null : "fx:id=\"fxmlParent\" was not injected:"
                + " check your FXML file 'ContentArea.fxml'.";
        assert tbpContentTabPane != null : "fx:id=\"tbpContentTabPane\" was not injected:"
                + " check your FXML file 'ContentArea.fxml'.";
    }

    public TabPane getTbpContentTabPane() {
        return tbpContentTabPane;
    }

    @Override
    public VBox getFxmlParent() {
        return fxmlParent;
    }

}

/* JavaFX CSS - Leave this comment until you have at least create one rule which uses -fx-Property */

.root {  /*Styling-Basisklasse*/
    -fx-font-size: 14pt;
    /*-fx-font-family: "Courier New";*/
    -fx-text-fill: blue;
    -fx-background-color: rgb(255, 255, 223);
}

.menu-element {
    -fx-border-width: 2px;
    -fx-border-style: solid;
    -fx-border-color: red;
    -fx-border-radius: 0.9em;
}

#base-borderpane {
    -fx-border-width: 2px;
    -fx-border-style: solid;
    -fx-border-color: darkviolet;
    -fx-border-radius: 1.0em;
}

Menü wird instanziert und aktiviert - b
MenuItem 'Inhaltsbereich aktivieren'
MenuItem 'Tab hinzufügen'
MenuItem 'Reset App'
Menü wird instanziert und aktiviert - b
MenuItem 'Inhaltsbereich aktivieren'
MenuItem 'Tab hinzufügen'