Starting JavaFX from Main method of class which doesn't extend Application

Question

I'm having problem to start a JavaFX Application from a Main method of a class which doesn't extend javafx.application.Application

In my application there is the MainApp.java which should start the overriden method start() in the MainUIController.java, which extends Applciation

When I start the Main method from the MainUIController.java everything works fine.

MainApp.java

public class MainApp {

    public static void main(String[] args) {
        PersonJDBCTemplate jdbc = connect();
        MainUIController mUIc = new MainUIController(jdbc);
        mUIc.start(new Stage());

    }

    public static PersonJDBCTemplate connect() {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "Beans.xml");
        PersonJDBCTemplate personJDBCTemplate = (PersonJDBCTemplate) context
                .getBean("personJDBCTemplate");
        return personJDBCTemplate;
    }
}

MainUIController.java

public class MainUIController extends Application {

    private Stage stage;
    // private User loggedUser;
    private final double MINIMUM_WINDOW_WIDTH = 800.0;
    private final double MINIMUM_WINDOW_HEIGHT = 570.0;
    private String version = "0.6";
    private PersonJDBCTemplate jdbc;

    public MainUIController(PersonJDBCTemplate jdbc) {
        this.jdbc = jdbc;

    }

    @Override
    public void start(Stage primaryStage) {
        try {
            stage = primaryStage;
            stage.setTitle("Sharp");
            stage.setMinWidth(MINIMUM_WINDOW_WIDTH);
            stage.setMinHeight(MINIMUM_WINDOW_HEIGHT);
            stage.setResizable(false);
            gotoLogin();
            primaryStage.show();
        } catch (Exception ex) {
            Logger.getLogger(MainUIController.class.getName()).log(
                    Level.SEVERE, null, ex);
        }
    }

    public void gotoLogin() {
        try {
            LoginController login = (LoginController) replaceSceneContent("/fxml/Login.fxml");
            login.setApp(this);
        } catch (Exception ex) {
            Logger.getLogger(MainUIController.class.getName()).log(
                    Level.SEVERE, null, ex);
        }
    }
}

After running the MainApp, I get the following Error :

Exception in thread "main" java.lang.ExceptionInInitializerError
at javafx.stage.Window.<init>(Window.java:1110)
at javafx.stage.Stage.<init>(Stage.java:236)
at javafx.stage.Stage.<init>(Stage.java:224)
at ch.kit.sharp.main.MainApp.main(MainApp.java:15)
Caused by: java.lang.IllegalStateException: This operation is permitted on the event thread only; currentThread = main
at com.sun.glass.ui.Application.checkEventThread(Application.java:445)
at com.sun.glass.ui.Screen.setEventHandler(Screen.java:245)
at com.sun.javafx.tk.quantum.QuantumToolkit.setScreenConfigurationListener(QuantumToolkit.java:600)
at javafx.stage.Screen.<clinit>(Screen.java:80)
... 4 more

Answer 1

In addition to what Nejinx said, you must not directly call your start(), always call launch(), because it sets up the JavaFX environment, including creation of stage and calls start() passing the stage as an parameter to it.

The docs has a note specially stating this

NOTE: This method is called on the JavaFX Application Thread

The launch() can be called from any class, taking into consideration if the class is directly not extending javafx.application.Application, then you must pass the class extending it as an argument to the launch method.

For example, consider you have a class JavaFXMain which extends Application

class JavaFXMain extends Application {...}

You can use any other class, to start the JavaFX Application.

class Main {
   ...
   public void someMethod() {
      ...
      JavaFXMain.launch(JavaFXMain.class); // Launch the JavaFX application
      ...
   }
}

In your case, you can try something like this inside the main method of MainApp:

// You may remove args if you don't intend to pass any arguments
MainUIController.launch(MainUIController.class, args)