@@ FXML的自定义组件注入不会在Controller中实例化变量，并导致NullPointerException

Question

我创建了一个名为PlayerView的自定义组件，该组件具有关联的PlayerView.fxml FXML布局和PlayerView.java类以实例化该组件。然后，我将该组件的多个实例包含在另一个名为Board.fxml的FXML布局中，并尝试通过使用@FXML批注创建注入来在Board的控制器Board.java中引用这些实例。但是，此注入无法按预期方式工作，因为当我尝试在控制器中引用NullPointerException实例时得到了PlayerView。

PlayerView.fxml

<fx:root type="javafx.scene.layout.VBox" xmlns:fx="http://javafx.com/fxml">
    ...
</fx:root>

PlayerView.java

public class PlayerView extends VBox {
    public PlayerView() {
        FXMLLoader loader = new FXMLLoader(getClass().getResource("PlayerView.fxml"));
        loader.setRoot(this);
        loader.setController(this);
        try {
            loader.load();
        } catch (IOException e) { 
            e.printStackTrace();
        }
    }
    public void init(String name) {
        ...
    }
}

Board.fxml

<VBox xmlns="http://javafx.com/javafx" xmlns:fx="http://javafx.com/fxml"
      fx:controller="pablo.Board" prefHeight="400.0" prefWidth="600.0" alignment="CENTER">
    <BorderPane>
        <top>
            <PlayerView BorderPane.alignment="TOP_CENTER" fx:id="playerView2"/>
        </top>
        <bottom>
            <PlayerView BorderPane.alignment="BOTTOM_CENTER" fx:id="playerView1"/>
        </bottom>
        <left>
            <PlayerView BorderPane.alignment="CENTER_LEFT" fx:id="playerView3"/>
        </left>
        <right>
            <PlayerView BorderPane.alignment="CENTER_RIGHT" fx:id="playerView4"/>
        </right>
        ...
    </BorderPane>
    ...
</VBox>

Board.java

public class Board extends Application {
    ...
    @FXML PlayerView playerView1, playerView2, playerView3, playerView4;
    @Override
    public void start(Stage primaryStage) throws IOException, ClassNotFoundException {
        Parent root = FXMLLoader.load(getClass().getResource("Board.fxml"));
        Scene scene = new Scene(root);
        primaryStage.setTitle("Pablo!");
        primaryStage.setScene(scene);
        primaryStage.show();

        playerViews = new PlayerView[]{ playerView1, playerView2, playerView3, playerView4 };
        playerView1.init(playerName); // NullPointerException occurs at this line
        ...
    }
}

堆栈跟踪

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$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$1(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
    at pablo.Board.start(Board.java:39)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$8(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$7(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$5(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$6(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)

我使用Board.java中的@FXML注释引用Board.fxml中的PlayerView实例的方式有什么问题吗？我应该如何代替他们呢？

Answer 1

定义PlayerView：

PlayerView.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<fx:root type="javafx.scene.layout.VBox" xmlns:fx="http://javafx.com/fxml"> 
    <Label fx:id="nameLbl" alignment="CENTER" contentDisplay="CENTER" prefHeight="66.0" prefWidth="87.0" text="--" />
</fx:root>

PlayerView.java也可以用作控制器：

public class PlayerView extends VBox {

    @FXML Label nameLbl;

    public PlayerView() {

        FXMLLoader loader = new FXMLLoader(getClass().getResource("PlayerView.fxml"));
        loader.setRoot(this);
        loader.setController(this);

        try {
            loader.load();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void init(String name) {
        nameLbl.setText(name);
    }
}

Board.fxml及其控制器：

<?xml version="1.0" encoding="UTF-8"?>

<?import fx_tests.a_test.PlayerView?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.VBox?>

<VBox xmlns="http://javafx.com/javafx" xmlns:fx="http://javafx.com/fxml" prefHeight="400.0" 
prefWidth="600.0" alignment="CENTER" fx:controller="foo.bar.BoardController">
    <BorderPane>
        <top>
            <PlayerView  fx:id="playerView2"/>
        </top>
        <bottom>
            <PlayerView  fx:id="playerView1"/>
        </bottom>
        <left>
            <PlayerView  fx:id="playerView3"/>
        </left>
        <right>
            <PlayerView fx:id="playerView4"/>
        </right>
    </BorderPane>
</VBox>

让控制器根据需要更改Boaed状态：

public class BoardController{

    @FXML PlayerView playerView1, playerView2, playerView3, playerView4;

    public PlayerView getBottom(){
        return playerView1;
    }

    public PlayerView getTop(){
        return playerView2;
    }

    public PlayerView getLeft(){
        return playerView3;
    }

    public PlayerView getRight(){
        return playerView4;
    }
}

在Board中获取控制器并使用它：

public class Board extends Application {

    @FXML PlayerView playerView1, playerView2, playerView3, playerView4;
    @Override
    public void start(Stage primaryStage) throws IOException, ClassNotFoundException {

        FXMLLoader loader = new FXMLLoader(getClass().getResource("Board.fxml"));
        VBox root = loader.load();
        BoardController controller = loader.getController();

        Button btn = new Button("Add Names");
        btn.setOnAction(e->{
            controller.getTop().init("Top");
            controller.getBottom().init("Bottom");
            controller.getLeft().init("Left");
            controller.getRight().init("Right");
        });
        root.getChildren().add(btn);
        Scene scene = new Scene(root);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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