根不能为 JavaFX 中多个 FXML/Controller 的空异常

Question

我正在开发一个管理锻炼的 JavaFX 项目。虽然它有一个大的 .fxml 文件和一个大的 MainController class，但它工作正常：

WorkoutsMain.fxml:

<AnchorPane id="AnchorPane" fx:id="anchorPane" prefHeight="900.0" prefWidth="1600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="workouts.WorkoutsMainController">
    <children>
        <SplitPane fx:id="splitPane" dividerPositions="0.15" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
            <items>
                <AnchorPane>
                    <children>
                        <StackPane fx:id="menuPane" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0"/>
                    </children>
                </AnchorPane>
                <AnchorPane>
                    <children>
                        <Pane fx:id="statisticsPane" visible="false" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="130.0"/>
                        <Pane fx:id="workoutsPane" visible="false" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="130.0">
                        //... other FX elements
                        </Pane>
                        <Pane fx:id="calendarPane" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="130.0"/>
                        <StackPane fx:id="logoPane" onMouseClicked="#setLogo" AnchorPane.bottomAnchor="778.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0"/>
                    </children>
                </AnchorPane>
            </items>
        </SplitPane>
    </children>
</AnchorPane>

WorkoutsMainController.java:

public class WorkoutsMainController implements Initializable, SplitPaneDividerController {
    private final DataBase dataBase = new DataBase();
    private boolean first = true;
    private String logoName;
    @FXML
    private StackPane logoPane, menuPane;
    private final List<Pane> panes = new ArrayList<>();
    @FXML
    private Pane calendarPane, workoutsPane, statisticsPane;
    @FXML
    private SplitPane splitPane;
    @FXML
    private AnchorPane anchorPane;
    // methods and functions
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        setLogo();
        disableSplitPaneDivider(splitPane, 0.1525);
        setUpMainMenu();
    }
}

MainClass.java:

public class MainClass extends Application {
    private Parent anchorPane;
    private final String fxml = "WorkoutsMain.fxml";
    @Override
    public void start(Stage stage) {       
        try {
            anchorPane = FXMLLoader.load(getClass().getResource(fxml));
        } catch (IOException ex) {
            System.out.println("Error when trying to load " + fxml);
          }
        Scene scene = new Scene(anchorPane);
        stage.setScene(scene);
        stage.getIcons().add(new Image(getClass().getResourceAsStream("/calendarIconWhite.png")));
        stage.setTitle("THE WORKOUT CALENDAR 1.7.0 by hazazs®");
        stage.setResizable(false);
        stage.show();
    }
    public static void main(String[] args) {
        launch(args);
    }
}

然后我想从WorkoutsMain.fxml上剪一块。我已经像这样更改了原始大 .fxml 文件中的一个段：
来自

<Pane fx:id="workoutsPane" visible="false" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="130.0">
    //... other FX elements
</Pane>

到

<fx:include source="Workouts.fxml" fx:id="workoutsPane"/>

然后我创建了一个新的 fxml 文件，它有自己的控制器 class:
Workouts.fxml:

<Pane fx:id="workoutsPane" visible="false" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="130.0" xmlns:fx="http://javafx.com/fxml/1" fx:controller="workouts.WorkoutsController">
// other FX elements
</Pane>

我已经创建了一个合适的 WorkoutsController class 并且我已经像这样更改了我的 WorkoutsMainController：

public class WorkoutsMainController implements Initializable, SplitPaneDividerController {
    protected static final DataBase dataBase = new DataBase();
    private final WorkoutsController workoutsController = new WorkoutsController();
    private boolean first = true;
    private String logoName;
    @FXML
    private StackPane logoPane, menuPane;
    private final List<Pane> panes = new ArrayList<>();
    @FXML
    private Pane calendarPane, workoutsPane, statisticsPane;
    @FXML
    protected static SplitPane splitPane;
    @FXML
    protected static AnchorPane anchorPane;
    // and so on..

但是从现在开始，当我尝试启动我的应用程序时出现异常：

Exception in Application start method
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 com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    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.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication9(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException: Root cannot be null
    at javafx.scene.Scene.<init>(Scene.java:336)
    at javafx.scene.Scene.<init>(Scene.java:194)
    at workouts.MainClass.start(MainClass.java:23)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication16(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait9(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null7(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater8(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null2(WinApplication.java:177)
    ... 1 more

大家有什么建议吗，我应该怎么做才能让这项工作再次成功？使用一个 .fxml 和一个 Controller，它工作得很好。
谢谢

Answer 1

您正在 WorkoutsMainController 中创建一个新的 WorkoutsController 实例，而不是使用 FXMLLoader 为您创建的实例。您创建的实例中的 @FXML-annotated 字段将全部为空，因此您可能在某处遇到空指针异常。（您应该会在控制台中看到“尝试加载 WorkoutsMain.fxml 时出错”消息。）因此，您的 start() 方法中的 anchorPane 永远不会初始化，并且当您通过对 Scene 构造函数的空 anchorPane 引用。

另请注意，您（出于某种未知原因）制作了 splitPane 和 anchorPane static：这样做没有任何意义，FXMLLoader 也不会初始化静态字段。所以这些字段在控制器中也是空的。

如果您不压缩通过加载 FXML 抛出的异常，您将能够看到底层异常的实际堆栈跟踪。

修复是：

不要制作 @FXML-注释字段 static。参见 javafx 8 compatibility issues - FXML static fields
要为 FXMLLoader 创建的包含的 FXML 注入实际控制器实例，请参阅文档中的 Nested Controllers。简而言之，既然你的 fx:include 有 fx:id="workoutsPane"，你应该替换
```
 private final WorkoutsController workoutsController = new WorkoutsController();
```
与
```
 @FXML private WorkoutsController workoutsPaneController ;
```
（并在 WorkoutsMainController 的其余部分中将所有出现的 workoutsController 替换为 workoutsPaneController。

根不能为 JavaFX 中多个 FXML/Controller 的空异常

Root cannot be null exception with multiple FXML/Controller in JavaFX

java

exception

javafx

fxml