JavaFX菜单按钮引发异常

Question

我正在使用fxml中的MenuButton在选项卡之间切换。问题是，菜单按钮会引发异常，但是如果我将其更改为简单按钮，则不会。这是我的fxml代码：

 <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>

在我的控制器类中：

 @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");
    }

和我得到的例外：

"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

编辑：这是我的App类代码，它扩展了Application。App类：

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

您的错误可以通过这个最小的示例重现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();
  }
}

哪个输出：

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)

至少有两种简单的方法可以“修复” NPE：

1. 更改MenuButton的根之前先隐藏Scene。

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

   如果使用的是FXML，则需要将MenuButton注入FXML控制器实例。这是通过在FXML文件中为元素赋予fx:id="foo"属性并将@FXML private MenuItem foo;字段添加到控制器来实现的。然后，您将在更改foo.hide()的根之前调用Scene。

   有关FXML的更多信息，请参见Introduction to FXML。]

2. 更改Scene的Stage，而不更改Scene的根。

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

示例和解决方案均使用JavaFX 14.0.1进行了测试。

---

^{1。请注意，这种示例就是您应该在问题中提供的。}