java - 从另一个类引用 @FXML TextField，但收到 NullPointerException

Question

我试图从另一个 Controller 类引用 TextField (@FXML)，但出现 NullPointerException。以下代码解释了该项目。我想了解为什么会发生这种情况。感谢您提前做出的努力!

Foo.fxml 文件

//Foo.fxml
...
  <TextField fx:id="textField" prefWidth="54" text="someText" />
...

FooController.java 文件

//FooController.java
public class FooController implements Initializable {

        @FXML
        private TextField textField;

        public TextField gettextField(){
            return textField;
        }
    }

MainController.java 文件

//MainController.java
public class MainController implements Initializable {

    private FooController fooControllerClass;

    @Override
    public void initialize(URL url, ResourceBundle rb) {
      System.out.println(fooControllerClass.gettextField().getText()); // this line throws java.lang.NullPointerException
    }
}

Main.java 文件

//Main.java
public class Main extends Application {
    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("Main.fxml"));
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
        }
    public static void main(String[] args) {launch(args);}
}

堆栈跟踪:

java.lang.NullPointerException
file:/home/vaib/shared/NetBeansProjects/JRAD/dist/run1408613327/Main.jar!/jrad/Main.fxml
  at jrad.MainController.initialize(MainController.java:6)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2185)  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2185)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2061)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2778)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2757)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2743)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2730)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2719)
  at jrad.Main.start(Main.java:34)
  at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2185)
  at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:216)
  at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:179)
  at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:176)
  at java.security.AccessController.doPrivileged(Native Method)
  at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:176)
  at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
  at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
  at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:89)
  at java.lang.Thread.run(Thread.java:744)

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.javafx.main.Main.launchApp(Main.java:698)
    at com.javafx.main.Main.main(Main.java:871)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:403)
    at com.sun.javafx.application.LauncherImpl.access$000(LauncherImpl.java:47)
    at com.sun.javafx.application.LauncherImpl$1.run(LauncherImpl.java:115)
    at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.NullPointerException
    at jrad.JRADController.initialize(JRADController.java:196)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2185)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2061)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2778)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2757)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2743)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2730)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2719)
    at jrad.JRAD.start(JRAD.java:34)
    at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
    at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:216)
    at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:179)
    at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:176)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:176)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
    at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:89)
    ... 1 more
Java Result: 1

Answer 1

所以我认为你已经

Main.fxml

<!-- root element -->< ... fx:controller="MainController">
...
<fx:include source="foo.fxml"/>
...

如果是这样，请将 fx:id 放在 fx:include 上:

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

然后您可以使用 @FXML 注释将 foo.fxml 的 Controller 注入(inject)到 MainController 中:

public class MainController {
  @FXML
  private FooController fooController ;
  // ...
}

这里的规则是，如果包含的 fx:id 是“x”，那么注入(inject) Controller 的变量名称必须是“xController”。请参阅Nested Controller位于 FXML 简介中。