JavaFx 节点集.....() -> NullPointer 异常

Question

首先感谢您指出我在创建此帖子时所犯的错误。

我尽可能地缩小了我的代码范围，希望现在更清楚了。

所以发生的问题是NullPointer Exception，例如，当我从场景（根）切换到另一个场景后尝试更改标签的可见性时。这里是headlineLeaderboard.setVisible(false)。在调用方法之前打印 headlineLeaderboard 会返回 null。

即使切换进度相同，我的应用程序中的每个场景也不会发生这种情况。

现在是代码：

在主类中创建舞台、主要场景等。还有切换场景根的方法。

public class Main extends Application {

private Stage primaryStage;
private Scene scene;

// 
@Override
public void start(Stage primaryStage)  {
    this.primaryStage = primaryStage;
    primaryStage.setTitle("ReaktionsTest");
    try {
        FXMLLoader loaderMain = new FXMLLoader(Main.class.getResource("MainWindow.fxml"));
        AnchorPane paneMain = (AnchorPane) loaderMain.load();
        MainWindowController mainWindowController = loaderMain.getController();
        mainWindowController.setMain(this);
        scene = new Scene(paneMain);
        primaryStage.setScene(scene);
        primaryStage.show();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
}

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

public void setSceneLeaderboard() {
    try {
        Parent root = FXMLLoader.load(Main.class.getResource("LeaderboardWindow.fxml"));
        scene.setRoot(root);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

在应用程序中MainWindowController 是所有场景的控制器。 handleLeaderboardAufrufen() 首先改变场景。在下一步中会出现 NullPointer。

public class MainWindowController {

private Main main;

@FXML private Label label1; 
@FXML private Button leaderboard;  
@FXML private TextField fieldName; 
@FXML private Label headlineLeaderboard;

public void setMain(Main main)   {
    this.main = main;   
}

@FXML
public void handleLeaderboardAufrufen() {
    main.setSceneLeaderboard();
    headlineLeaderboard.setVisible(false);
}
}   

LeaderboardWindow 的 fxml 是这样的：（在MainWindow.fxml 中使用相同的控制器：fx:controller="application.MainWindowController"）

<AnchorPane maxHeight="500.0" maxWidth="750.0" minHeight="500.0" minWidth="750.0" prefHeight="500.0" prefWidth="750.0" xmlns="http://javafx.com/javafx/8.0.171" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.MainWindowController">
   <children>
      <TableView fx:id="leaderboardTable" layoutX="10.0" layoutY="92.0" prefHeight="331.0" prefWidth="729.0">
      </TableView>
      <Label fx:id="headlineLeaderboard" layoutX="268.0" layoutY="23.0" text="Leaderboard" textAlignment="CENTER" underline="true">
         <font>
            <Font name="System Bold" size="36.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

错误代码：

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.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:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Node.fireEvent(Node.java:8411)
at javafx.scene.control.Button.fire(Button.java:185)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
at com.sun.javafx.event.CompositeEventHandler$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$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$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$MouseEventNotification.run(GlassViewEventHandler.java:394)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$353(GlassViewEventHandler.java:432)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sun.reflect.misc.Trampoline.invoke(Unknown Source)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1771)
... 48 more
Caused by: java.lang.NullPointerException
at application.MainWindowController.handleLeaderboardAufrufen(MainWindowController.java:25)
... 58 more

Answer 1

我同意@Sedrick 的评论，您应该按照他的建议和指南中的说明更新您的帖子。

但是，我会指出我认为可能导致此问题的一件事，如果我错了，任何人都可以随时纠正我。

您看起来是，从连接到您的 FXML A 的控制器 A，试图从 FXML B (ResultView) 访问 Text 变量。

main.setSceneResult(); 
System.out.println("resultText  " +resultText );
resultText.setText("hello");

正如您在上面的代码中所示，您将根添加到场景 (main.setSceneResult)，尽管当前控制器（我假设您在其中）不知道新父级及其成员.您可以做的是为 ResultView 创建一个新控制器，我建议如下：

FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource(resultView.fxml));
ResultViewController resultViewController = new ResultViewController();
loader.setController(resultViewController);
Parent root = loader.load();

然后您可以从控制器A 调用控制器B 上的方法来更新Text 变量。

您可以尝试类似下面的代码来验证它是否有效。

ResultViewController resultViewController = main.setSceneResult(); /*This method returns the controller which has access to the ResultText variable (if correctly setup which it looked like you had done)*/
resultViewController.setResultText("hello");

我希望这可以帮助你，如果没有，那么我建议你用一个可运行的例子来更新你的帖子:)

编辑： 我又看了一遍代码，因为我写回复时没有那么多时间。如果您有多个控制器或只有 1 个控制器 (MainWindowController)，则很难弄清楚发生了什么。我上面的例子可能会有所帮助，所以请尝试一下，但在再次查看代码后我真的不确定。 另外，我不确定您是否有理由在 fxml 中指定控制器，而不是像我在建议中所做的那样创建它并将其设置为加载器。我也不确定您是否使用“MainWindowController”作为 ResultView 和当前视图的控制器。如果是这种情况，那么我真的建议您创建一个处理 ResultView 的控制器，然后让主控制器处理其余部分。