JavaFX FXML 场景切换不起作用 [NullPointerException]

Question

当我按下按钮时，我正在尝试切换布局。我尝试了很多东西，也在这里在 stackoverflow 上进行了搜索，但给出的答案似乎不适合我。

我的 FrontendView.class：

/* FrontendView Class by @Aebian */
package org.aebian.umFrontend.view;

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.image.Image;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

import java.io.IOException;


public class FrontendView extends Application {

    @FXML
    protected static Button btnLogin, btnSave, btnDiscard, btnRefresh, btnEdit;
    @FXML
    private Stage primaryStage;
    @FXML
    private Scene scene;
    @FXML
    private VBox vDefault;
    @FXML
    protected BorderPane rootLayout;

    private double xOffset = 0;
    private double yOffset = 0;

    @Override
    public void start(Stage primaryStage) {

        GridPane root = new GridPane();
        primaryStage.setMinWidth(800);
        primaryStage.initStyle(StageStyle.UNDECORATED);

        primaryStage.setMinHeight(600);
        primaryStage.setResizable(false);
        primaryStage.getIcons().add(new Image("file:res/images/uMgmt.png"));

        primaryStage.setTitle("User Management");
        Scene scene = new Scene(root, 800, 600);

        primaryStage.setScene(scene);
        this.primaryStage = primaryStage;
        root.setAlignment(Pos.CENTER);

        showRoot();
        showLogin();
    }

    public void showRoot() { // Load root layout
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(FrontendView.class.getResource("UI/umFrontendRoot.fxml"));
            rootLayout = loader.load();
            rootLayout.setId("umFrontend");

            //Let the root Layout be able to moved around.
            rootLayout.getTop().setOnMousePressed(new EventHandler<MouseEvent>() {
                @Override
                public void handle(MouseEvent event) {
                    xOffset = event.getSceneX();
                    yOffset = event.getSceneY();
                }
            });
            rootLayout.getTop().setOnMouseDragged(new EventHandler<MouseEvent>() {
                @Override
                public void handle(MouseEvent event) {
                    primaryStage.setX(event.getScreenX() - xOffset);
                    primaryStage.setY(event.getScreenY() - yOffset);
                }
            });

            // Show the scene containing the root layout.
            Scene scene = new Scene(rootLayout);
            primaryStage.setScene(scene);
            String css = FrontendView.class.getResource("UI/umFrontendStyles.css").toExternalForm();
            scene.getStylesheets().clear();
            scene.getStylesheets().add(css);

            primaryStage.show();

        } catch (IOException e) {
        }
    }

    public void showLogin() { // Load the login page.
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(FrontendView.class.getResource("UI/umFrontendLogin.fxml"));
            vDefault = loader.load();
            // Set login to center of root layout.
            rootLayout.setCenter(vDefault);
        } catch (IOException e) {
        }
    }

    public void showAbout() {  // Load the about page.
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(FrontendView.class.getResource("UI/umFrontendAbout.fxml"));
            vDefault = loader.load();

            // Set about page to center of root layout.
            this.rootLayout.setCenter(vDefault);
        } catch (IOException e) {
        }
    }

    public void showAdminOverview() {  // Load the admin overview / edit page.
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(FrontendView.class.getResource("UI/umFrontendAdminOverview.fxml"));
            VBox showAdminOverview = loader.load();
            // Set admin overview page to center of root layout.
            rootLayout.setCenter(showAdminOverview);
        } catch (IOException e) {
        }
    }

    public void showUserOverview() {  // Load the user overview / edit page.
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(FrontendView.class.getResource("UI/umFrontendUserOverview.fxml"));
            VBox showUserOverview = loader.load();
            // Set admin overview page to center of root layout.
            rootLayout.setCenter(showUserOverview);
        } catch (IOException e) {
        }
    }

    /**
     * Getter and setter.
     *
     * @return
     */
    public BorderPane getBorderPane() {
        return rootLayout;
    }

}

在第 62 行我打电话给showLogin()，它工作正常。如果我尝试通过按钮运行它，它将失败。同样适用于showAbout()。如果我用它替换第 62 行上的showLogin()，它将起作用。在按钮上它不会。

我的 FrontendViewController.class：

package org.aebian.umFrontend.view;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.Parent;
import javafx.scene.control.Alert;
import javafx.scene.control.Button;
import javafx.scene.control.ButtonType;
import javafx.scene.control.MenuItem;

import java.io.IOException;
import java.util.Optional;

public class FrontendViewController {

    FrontendView FView = new FrontendView();
    Boolean admin = false;

    @FXML
    MenuItem appClose;
    @FXML
    MenuItem appAbout;
    @FXML
    Button buttonLogin;
    @FXML
    Parent root;

    /* Button Events  */

    @FXML
    private void handleButtonAction(ActionEvent e) throws IOException {

        if (e.getSource() == appClose) { // Exit application when user clicks on Edit > Close
            System.exit(0);
        }

        else if (e.getSource() == appAbout) { // Show About page when user clicks on Help > About
            FView.showAbout();
        }

        else if (e.getSource() == buttonLogin) { // Function that gets triggered when the user presses the Login Button
            if (admin == true) {
                FView.showAdminOverview();
            } else {
                FView.showUserOverview();
            }
        }

    }

     /* Frontend Dialogs  */

    public void delUserConfirmDialog() {

        Alert alert = new Alert(Alert.AlertType.CONFIRMATION);
        alert.setTitle("Delete User Dialog");
        alert.setHeaderText("Delete User $ss");
        alert.setContentText("Are you sure you want to delete the selected action?");

        Optional<ButtonType> result = alert.showAndWait();
        if (result.get() == ButtonType.OK) {
            FView.showAdminOverview();
        } else {
            // ... user chose CANCEL or closed the dialog
        }

    }
}

我也尝试动态设置控制器，但这并没有改变任何东西。 我从编译器得到的错误是这个：

"C:\Program Files\Java\jdk1.8.0_121\bin\java" -agentlib:jdwp=transport=dt_socket,address=127.0.0.1:6660,suspend=y,server=n -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_121\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_121\jre\lib\rt.jar;C:\Users\agoebbel\Tempo Box\Development & PS\Eclipse-Workspaces\userMan\out\production\userMan;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.3.4\lib\idea_rt.jar" org.aebian.umFrontend.Frontend
Connected to the target VM, address: '127.0.0.1:6660', transport: 'socket'
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    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.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    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:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
    at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1405)
    at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$343(ContextMenuContent.java:1358)
    at com.sun.javafx.event.CompositeEventHandler$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.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$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$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$148(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
    ... 43 more
Caused by: java.lang.NullPointerException
    at org.aebian.umFrontend.view.FrontendView.showAbout(FrontendView.java:115)
    at org.aebian.umFrontend.view.FrontendViewController.handleButtonAction(FrontendViewController.java:39)
    ... 53 more

所以我可以直接运行 showLogin()、showAbout() 等方法，这不是输入流的错误。 如果您需要我的 FXML 文件之一：

umFrontendAbout.fxml：

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

<?import javafx.scene.paint.*?>
<?import javafx.scene.text.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.paint.Color?>
<?import javafx.scene.text.Font?>

<VBox alignment="CENTER" prefHeight="600.0" prefWidth="900.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="org.aebian.umFrontend.view.FrontendView">
    <children>
        <SplitPane focusTraversable="true" orientation="VERTICAL" prefHeight="-1.0" prefWidth="-1.0" VBox.vgrow="ALWAYS">
            <items>
                <AnchorPane prefHeight="576.0" prefWidth="328.0">
               <children>
                  <Label layoutX="272.0" layoutY="121.0" text="Java User Managment">
                     <font>
                        <Font size="24.0" />
                     </font>
                  </Label>
                  <Label layoutX="228.0" layoutY="164.0" text="A java written user management solution with mysql support" />
                  <Label layoutX="251.0" layoutY="181.0" text="© 2017 by Adrian (Simmarith) &amp; Alexander (Aebian)" />
                  <Label alignment="CENTER" layoutX="38.0" layoutY="14.0" minWidth="60.0" prefWidth="-1.0" style="&#10;" text="User Management \ About" textAlignment="CENTER" wrapText="false">
                     <font>
                        <Font size="18.0" fx:id="x1" />
                     </font>
                     <textFill>
                        <Color blue="0.624" green="0.624" red="0.624" fx:id="x2" />
                     </textFill>
                  </Label>
                  <Label layoutX="338.0" layoutY="220.0" text="aebian@aebian.org" />
               </children>
                </AnchorPane>
            </items>
        </SplitPane>
    </children>
</VBox>

我该如何解决这个问题？

我尝试过但对我不起作用的事情：

• 通过
动态设置控制器

( (控制器) loader.getController() ).setPrimaryStage(primaryStage);

• 在控制器中创建了一个 setPrimaryStage 方法

Answer 1

我在调试和尝试后自己解决了这个问题。 问题是控制器没有获得初始化值。只有在方法启动运行时才存在。因此，我要么需要在我的控制器中有“应用程序”，要么因为 JavaFX 已经基于 MVC，所以我可以将我的控制器与 View 类本身合并。

我做了第二个选项，它现在可以工作了。 感谢您的帮助。

Aebian 出来了。

Answer 2

你在调用handleButtonAction()方法之前初始化rootLayout了吗？

据我了解，您在 showRoot() 上加载了 rootLayout，但是当应用程序到达方法 handleButtonAction() 时，rootLayout 没有被初始化。

尝试保存您在 showRoot() 上初始化的 rootLayout，以便您可以通过 handleButtonAction() 在 showAbout 上使用它；