从控制器加载FXML文件时出现JavaFX8空指针异常

Question

我将StackPane作为根，并在主应用程序Start（）方法中加载了BorderPane布局。在BorderPane布局中，我有两个按钮：一个用于添加fxml文件（Menu.fxml），另一个用于删除此文件。一旦菜单。fxml已加载，菜单中有一个按钮。fxml，单击时应在先前加载的BorderPane布局的中心加载一个fxml文件（One.fxml）。然而，在这个阶段，我得到了一个空指针异常。你能指出我哪里出了问题吗。非常感谢。

以下是我的代码：

public class Main extends Application {

    private static StackPane rootStack;
    private static BorderPane rootBorder;

    @Override
    public void start(Stage stage) throws Exception {
        rootStack = FXMLLoader.load(getClass().getResource("Base.fxml"));
        rootBorder = FXMLLoader.load(getClass().getResource("Border.fxml"));
        rootStack.getChildren().addAll(rootBorder);

        Scene scene = new Scene(rootStack);
        stage.setScene(scene);
        stage.show();
    } // start

    public static StackPane getRootStack() {
        return rootStack;
    } // end of method getRootStack

    public static BorderPane getRootBorder() {
        return rootBorder;
    } // end of method getRootBorder

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

} // Main

public class BorderController {

    private StackPane rootStack = Main.getRootStack();
    VBox menu;

    public void initialize() throws IOException {
        menu = FXMLLoader.load(getClass().getResource("Menu.fxml"));
    } // initialize    

    @FXML
    private void addMenuPane(ActionEvent event) throws IOException {
        rootStack.getChildren().add(menu);
    } // addMenuPane

    @FXML
    private void removeNode(ActionEvent event) {
        rootStack.getChildren().remove(menu);
    } // removeNode

} // BorderController

public class MenuController {

    private BorderPane rootBorder = Main.getRootBorder();

    @FXML
    private JFXButton oneButton;

    public void initialize() {
        oneButton.addEventHandler(MouseEvent.MOUSE_CLICKED, (event) -> {
            try {
                rootBorder.setCenter(FXMLLoader.load(getClass().getResource("One.fxml")));
            } catch (IOException ex) {
                Logger.getLogger(MenuController.class.getName()).log(Level.SEVERE, null, ex);
            }
        });

    } // initialize    

} // MenuController

堆栈跟踪：

Executing C:\Users\User\Documents\NetBeansProjects\Stack\dist\run2093523570\Stack.jar using platform C:\Program Files\Java\jdk1.8.0_121\jre/bin/java
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at stack.MenuController.lambda$initialize$0(MenuController.java:22)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$ClickGenerator.postProcess(Scene.java:3470)
    at javafx.scene.Scene$ClickGenerator.access$8100(Scene.java:3398)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3766)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:381)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:417)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:416)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
Deleting directory C:\Users\User\Documents\NetBeansProjects\Stack\dist\run2093523570

Answer 1

加载FXML文件时，如果FXML文件的根元素具有fx:controller属性，则实例化该属性指定的类，注入任何@FXML-注释字段，并在控制器类实例（如果有）上调用initialize（）。所有这些都是FXMLLoader的load（）方法的一部分。

如果您考虑＜code＞应用程序中的代码行。start（）方法：

rootBorder = FXMLLoader.load(getClass().getResource("Border.fxml"));

这将加载< code>Border.fxml，创建控制器，调用其< code>initialize()方法，等等，只有当所有这些都完成时，它才会将结果分配给< code>rootBorder。被调用的< code>BorderController中的< code>initialize()方法加载< code > menu . fxml ；进而在< code>Menu.fxml中实例化控制器，该控制器(我假设)是< code>MenuController。< code>MenuController的初始化代码包括

private BorderPane rootBorder = Main.getRootBorder();

请注意，此时，正在加载Border.fxml的load（）方法仍未完成，因此Main中的静态root边境字段尚未分配，因此Main.getRootBorder（）返回null。

因此，稍后当您尝试执行

rootBorder.setCenter(...);

由于为root边境分配了值null，因此您会得到一个空指针异常。

一个快速而肮脏的修复方法就是在您需要的时候检索边框窗格：

public class MenuController {

    @FXML
    private JFXButton oneButton;

    public void initialize() {
        oneButton.addEventHandler(MouseEvent.MOUSE_CLICKED, (event) -> {
            try {
                Main.getRootBorder().setCenter(FXMLLoader.load(getClass().getResource("One.fxml")));
            } catch (IOException ex) {
                Logger.getLogger(MenuController.class.getName()).log(Level.SEVERE, null, ex);
            }
        });

    } // initialize    

} // MenuController

然而，这种在＜code＞应用程序