JavaFX 访问控制器给出空指针异常

Question

我想要实现的是从 main 方法获取控制器实例，这样我就可以从另一个 class 调用控制器的方法并更新 fxml。无论如何，这是我的代码：

主要class:

public class Main extends Application {

Controller controller;
@Override
public void start(Stage primaryStage) throws Exception{

    FXMLLoader fxmlLoader = new FXMLLoader();

    Parent root = fxmlLoader.load(getClass().getResource("sample.fxml"));
    controller = fxmlLoader.getController();

    primaryStage.setTitle("uPick Smart Service");
    primaryStage.setScene(new Scene(root, 1600, 600));

    primaryStage.show();

    ConnectionHandling connectionHandling = new ConnectionHandling();
    Thread X = new Thread (connectionHandling);
    X.start();

}

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

}

public Controller getController(){
    return controller;
}


}

我的控制器class:

public class Controller {

public HBox billbox;

public int childnr;


public void createBill() {
    System.out.println("Creating");
    TableView<Item> bill = new TableView<>();

    DropShadow dropShadow = new DropShadow();
    dropShadow.setRadius(5.0);
    dropShadow.setOffsetX(3.0);
    dropShadow.setOffsetY(3.0);
    dropShadow.setColor(Color.color(0.4, 0.5, 0.5));

    VBox fullbill = new VBox();
    fullbill.setPadding(new Insets(1, 1, 1, 1));
    fullbill.getStyleClass().add("fullbill");

    TableColumn<Item, String> nameColumn = new TableColumn<>("Emri");
    nameColumn.setMinWidth(200);
    nameColumn.setCellValueFactory(new PropertyValueFactory<>("name"));

    TableColumn<Item, String> quantsColumn = new TableColumn<>("Sasia");
    quantsColumn.setMinWidth(50);
    quantsColumn.setCellValueFactory(new PropertyValueFactory<>("quants"));

    double tablewidth = nameColumn.getWidth() + quantsColumn.getWidth();

    Label tablenrlabel = new Label("Table 5");
    tablenrlabel.getStyleClass().add("tablenr-label");
    tablenrlabel.setMinWidth(tablewidth);

    Button closebutton = new Button("Mbyll");
    closebutton.setMinWidth(tablewidth);
    closebutton.getStyleClass().add("red-tint");

    bill.setItems(getItem());
    bill.setMinWidth(256);
    bill.setColumnResizePolicy(TableView.CONSTRAINED_RESIZE_POLICY);
    bill.getColumns().addAll(nameColumn, quantsColumn);

    fullbill.setEffect(dropShadow);
    fullbill.getChildren().addAll(tablenrlabel, bill, closebutton);

    billbox.getChildren().addAll(fullbill);

    childnr += 1;

    //Loops over every button in every vbox and gives it a seperate listener (the index of the button is hardcoded so it can cause problems if you add more items)
    for (int i = 0; i < childnr; i++) {
        VBox box = (VBox) billbox.getChildren().get(i);
        Button btn = (Button) box.getChildren().get(2); //if sudden issues change this
        btn.setId(Integer.valueOf(i).toString());
        btn.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                int index = billbox.getChildren().indexOf(btn.getParent());
                billbox.getChildren().remove(index);
                childnr -= 1;
                System.out.println(btn.getId());
            }
        });
    }
    System.out.println("done");
}

}

并且 class 试图调用控制器方法：

public class TakeOrder implements Runnable {
Socket SOCK;
Controller controller;

//OrderIndexes: "Order","Waiter","Payment"

private int NonConnectedDatas = 2;

public TakeOrder(Socket X){
    this.SOCK = X;
}

public void CheckConnection() throws IOException{
    System.out.println("Checking connection");
    if(!SOCK.isConnected()){
        System.out.println("Dissconectiong");
        for(int i = 0; i < ConnectionHandling.ConnectionArray.size(); i++){
            if(ConnectionHandling.ConnectionArray.get(i) == SOCK){
                ConnectionHandling.ConnectionArray.remove(i);
            }
        }
    }
}

public void run(){
    try{
        try{
            CheckConnection();


            ObjectInputStream ob = new ObjectInputStream(SOCK.getInputStream());

            String[] structuredArray = (String[])ob.readObject();

            String tablenr = structuredArray[0];
            String index = structuredArray[1];

            ArrayList<String> names = new ArrayList<>();
            ArrayList<String> quants = new ArrayList<>();

            int a = 0;
            int b = 0;
            switch (index) {
                case "Order":
                    for (int i = NonConnectedDatas; i < structuredArray.length; i++) {
                        if (i % 2 == 0) {
                            names.add(a, structuredArray[i]);
                            System.out.println(names.get(a));
                            a++;
                        } else {
                            quants.add(b, structuredArray[i]);
                            System.out.println(quants.get(b));
                            b++;
                        }
                    }
                    break;
            }

            Platform.runLater(new Runnable() {
                @Override
                public void run() {

                }
            });
        }finally{
            SOCK.close();
        }
    }catch(Exception X){
        System.out.print(X);
    }
}



}

这是我的错误信息：

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at sample.TakeOrder.run(TakeOrder.java:80)
at com.sun.javafx.application.PlatformImpl.lambda$null3(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater4(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null8(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)

ConnectionHandling class:

public class ConnectionHandling implements Runnable{

public static ArrayList<Socket> ConnectionArray = new ArrayList<Socket>();


public void run(){
    System.out.println("Starting");
    try{
        final int PORT = 60123;
        ServerSocket SERVER = new ServerSocket(PORT);
        System.out.println("Waiting for clients");
        while(true){
            Socket SOCK = SERVER.accept();
            ConnectionArray.add(SOCK);

            TakeOrder ORDER = new TakeOrder(SOCK);
            Thread X = new Thread(ORDER);
            X.setDaemon(true);
            X.start();
        }
    }catch(Exception x){
        System.out.print(x);
    }

}

Answer 1

首先，您使用的是staticFXMLLoader.load(URL)方法。因为它是 static，您创建的 FXMLLoader 实例的控制器属性未通过调用此 load 方法进行初始化。所以 Main 中的 controller 将是 null.

相反，设置位置并使用实例方法load():

@Override
public void start(Stage primaryStage) throws Exception{

    FXMLLoader fxmlLoader = new FXMLLoader();
    fxmlLoader.setLocation(getClass().getResource("sample.fxml"));

    Parent root = fxmlLoader.load();
    controller = fxmlLoader.getController();

    primaryStage.setTitle("uPick Smart Service");
    primaryStage.setScene(new Scene(root, 1600, 600));

    primaryStage.show();



}

不过，这并不能完全解决您的问题。当您启动该应用程序时，JavaFX 将创建一个 Main 实例并在该实例上调用 start()。通过上述更改，该实例 的 controller 字段将被正确初始化。

但是，在 TakeOrder.run() 中，您创建了 Main 的另一个实例：

Main main = new Main();

该实例的 controller 字段不会被初始化（即使您初始化了它，它也与您想要的实例不同）。所以你真的需要安排 TakeOrder 来访问在 start 方法中创建的 controller 实例 。

下面是对您的代码进行最直接的修复以使其正常工作：

public class ConnectionHandling implements Runnable{

    private final Controller controller ;

    public ConnectionHandling(Controller controller) {
        this.controller = controller ;
    }

    // ...

    public void run(){

        // existing code ...

        TakeOrder ORDER = new TakeOrder(SOCK, connection);

        // ...
    }
}

和

public class TakeOrder implements Runnable {

    Socket SOCK;
    Controller controller;

    //OrderIndexes: "Order","Waiter","Payment"

    private int NonConnectedDatas = 2;

    public TakeOrder(Socket X, Controller controller){
        this.SOCK = X;
        this.controller = controller ;
    }

    // ...

    public void run() {

        // ...

        Platform.runLater(controller::createBill);

        // ...
    }

}

最后

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{

        FXMLLoader fxmlLoader = new FXMLLoader();

        Parent root = fxmlLoader.load(getClass().getResource("sample.fxml"));
        Controller controller = fxmlLoader.getController();

        primaryStage.setTitle("uPick Smart Service");
        primaryStage.setScene(new Scene(root, 1600, 600));

        primaryStage.show();

        ConnectionHandling connectionHandling = new ConnectionHandling(controller);
        Thread X = new Thread (connectionHandling);
        X.start();

    }

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

    }

    public Controller getController(){
        return controller;
    }


}

一般来说，对于这样的应用程序，您可能需要考虑使用 MVC 或 MVP 方法（即您需要一个模型 class，这将适用于服务，例如您的 ConnectionHandler).

您可能还对 this article 在 JavaFX 中集成服务感兴趣。

Answer 2

如果您手动实例化 Application（在本例中为 Main class），则不会调用 start() 函数。尝试使用

启动您的应用程序

Application.launch

而不是

Main main = new Main()

此外，launch 方法在应用程序退出之前不会 return。因此，您可能希望对 getController()

的调用重新排序

还有另一种管理控制器实例的方法，那就是使用 FXMLLoader 中的 setController(Object controller)。

文档：

https://docs.oracle.com/javase/8/javafx/api/javafx/application/Application.html#launch-java.lang.Class-java.lang.String...-

https://docs.oracle.com/javase/8/javafx/api/javafx/fxml/FXMLLoader.html#setController-java.lang.Object-

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