fxmlLoader.getController()导致空指针异常？

Question

我试图访问加载fxml的窗口的控制器类实例。它加载时没有错误，但是当我尝试使用

System.out.println("from NAW: "+ NAC.newAccModel.users.getNumAccs());

它给出了下面的空指针异常。(第36行是println)

java.lang.NullPointerException
at muselogin.newAccountWindow.<init>(newAccountWindow.java:36)
at muselogin.MuseLoginController.initialize(MuseLoginController.java:84)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2441)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3214)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3175)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3148)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3124)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3104)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3097)
at muselogin.MuseLogin.start(MuseLogin.java:22)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)

这里是我试图调用getController()的地方

public class newAccountWindow extends Application {



    Stage stage = new Stage();
    newAccountController NAC = new newAccountController();

public newAccountWindow(){
    Parent root=null;
    try{
    FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("newAccountWindow.fxml"));
    root = fxmlLoader.load(getClass().getResource("newAccountWindow.fxml"));
   // fxmlLoader.setBuilderFactory(new JavaFXBuilderFactory());
    NAC = (newAccountController) fxmlLoader.getController();
    System.out.println("from NAW: "+NAC.newAccModel.users.getNumAccs());
    }catch(Exception e){
        e.printStackTrace();
    }
    Scene scene = new Scene(root);
    //scene.getStylesheets().add(MuseLogin.class.getResource("newAccCSS.css").toExternalForm());        
    stage.setScene(scene);
}

这是控制器，如果有关系的话

public class newAccountController implements Initializable {

    newAccountModel newAccModel;

    @FXML
    private TextField usernameField;
    @FXML
    private PasswordField passwordField;
    @FXML
    private Button createAccount;
    @FXML
    private PasswordField confirmField;
    @FXML
    private Label usernameBlankMessage;
    @FXML
    private Label usernameTakenMessage;
    @FXML
    private Label passwordMessage;


    //counter to check if all 3 conditions are met
    int makeAcc = 0;

    //action event for make account button clicked
    @FXML
    private void createAccountClicked(ActionEvent event) {

        //does account creation stuff

    }

    public int getNumAccs(){
        return newAccModel.users.getNumAccs();
    }

    //required initialize function, initializes model
    @Override
    public void initialize(URL url, ResourceBundle rb) {

        newAccModel = new newAccountModel();

    }

}

Answer 1

通过将一个URL传递给load方法，您可以使用FXMLLoader的static load方法之一，当然它不能将控制器存储在FXMLLoader实例中，因为没有有关该实例的信息。

由于您已经在URL构造函数中指定了FXMLLoader，所以只需使用无参数的load方法：

root = fxmlLoader.load();

Answer 2

 NAC = (newAccountController) fxmlLoader.getController();
 System.out.println("from NAW: "+NAC.newAccModel.users.getNumAccs());

不能在sysout之后执行getController。您要做的是在控制器newAccModel方法中创建或初始化initilize。一段时间后才接到电话。当加载完成时，它将调用。但

 NAC = (newAccountController) fxmlLoader.getController();

在这条线之后，负载还没有准备好。所以它只是给NPE。请确保在控制器中执行初始化方法后访问newAccModel。