JavaFX 菜单按钮抛出异常

Question

I'm using MenuButton in fxml to switch between tabs. The problem is, the menubuttons throw exceptions, but if I change it to a simple button, it doesn't. Here's my fxml code:

 <MenuButton mnemonicParsing="false" prefHeight="27.0" prefWidth="101.0" style="-fx-background-color: #666666;" text="Select menu" textFill="WHITE" GridPane.columnIndex="4">
<items>
<MenuItem mnemonicParsing="false" onAction="#switchToMoney" text="1" />
<MenuItem mnemonicParsing="false" onAction="#switchToWeight" text="2" />
<MenuItem mnemonicParsing="false" onAction="#switchToTemperature" text="3" />
</items>
</MenuButton>

In my controller class:

 @FXML
    private void switchToMoney() throws IOException {
        App.setRoot("money");
    }

    @FXML
    private void switchToWeight() throws IOException {
        App.setRoot("weight");
    }

    @FXML
    private void switchToTemperature() throws IOException {
        App.setRoot("temperature");
    }

and the exception I get:

"C:\Program Files\Java\jdk-14\bin\java.exe" --add-modules javafx.base,javafx.graphics --add-reads javafx.base=ALL-UNNAMED --add-reads javafx.graphics=ALL-UNNAMED "-javaagent:D:\IntelliJ IDEA 2019.3.4\lib\idea_rt.jar=64566:D:\IntelliJ IDEA 2019.3.4\bin" -Dfile.encoding=UTF-8 -p "C:\Users\tarbe\.m2\repository\org\openjfx\javafx-base\14\javafx-base-14-win.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-graphics\14\javafx-graphics-14-win.jar;D:\IntelliJ IDEA 2019.3.4\projects\proba\target\classes;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-controls\14\javafx-controls-14.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-controls\14\javafx-controls-14-win.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-graphics\14\javafx-graphics-14.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-base\14\javafx-base-14.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-fxml\14\javafx-fxml-14.jar;C:\Users\tarbe\.m2\repository\org\openjfx\javafx-fxml\14\javafx-fxml-14-win.jar" -m org.alkfejl/org.alkfejl.App
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:222)
    at javafx.controls/javafx.scene.control.skin.MenuButtonSkinBase.lambda$new$7(MenuButtonSkinBase.java:198)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:832)

Process finished with exit code 0

EDIT: Here's my code of the App class, which extends the Application. the App class:

public class App extends Application {

    private static Scene scene;

    @Override
    public void start(Stage stage) throws IOException {
        scene = new Scene(loadFXML("general"));
        stage.setTitle("Calculator");
        stage.setScene(scene);
        stage.show();
    }

    static void setRoot(String fxml) throws IOException {
        scene.setRoot(loadFXML(fxml));
    }

    private static Parent loadFXML(String fxml) throws IOException {
        FXMLLoader fxmlLoader = new FXMLLoader(App.class.getResource(fxml + ".fxml"));
        return fxmlLoader.load();
    }

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

}

Answer 1

Your error can be reproduced with this minimal example ¹ :

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.MenuButton;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class Main extends Application {

  @Override
  public void start(Stage primaryStage) {
    MenuButton btn = new MenuButton("Switch...");

    MenuItem item = new MenuItem("To 'Hello, World!'");
    item.setOnAction(
        e -> {
          StackPane root = new StackPane(new Label("Hello, World!"));
          primaryStage.getScene().setRoot(root);
        });
    btn.getItems().add(item);

    primaryStage.setScene(new Scene(new StackPane(btn), 500, 300));
    primaryStage.show();
  }
}

Which outputs:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:222)
    at javafx.controls/javafx.scene.control.skin.MenuButtonSkinBase.lambda$new$7(MenuButtonSkinBase.java:198)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:832)

There are at least two simple ways to "fix" the NPE:

1. Hide the MenuButton before changing the root of the Scene .

    StackPane root = new StackPane(new Label("Hello, World;")). btn;hide(). primaryStage.getScene();setRoot(root);

   If you're using FXML then you'll need to inject the MenuButton into the FXML controller instance. This is accomplished by giving the element an fx:id="foo" attribute in the FXML file and adding an @FXML private MenuButton foo; field to the controller. Then you would invoke foo.hide() before changing the root of the Scene .

   See Introduction to FXML for more information about FXML.

2. Change the Scene of the Stage instead of changing the root of the Scene .

    StackPane root = new StackPane(new Label("Hello, World;")). primaryStage,setScene(new Scene(root, 500; 300));

Both the example and solutions were tested using JavaFX 14.0.1.

---

^{1. Note this sort of example is what should be provided by you, in the question.}