How can I use a variable from another Controller in JavaFX

Question

I have two scenes Login.fxml and MainView.fxml and two diferent controllers LoginController.java and MainViewControler.java

In LoginController I do the whole process

Answer 1

Use a variation on the solution in Passing Parameters JavaFX FXML.

Setup a LoginManager which has a reference to both the LoginController and the MainViewController.

1. The loginManager creates a login screen using the loginController and passes a reference to itself to the loginController.
2. When login has passed, the loginController notifies the loginManager of the login sessionID.
3. The loginManager can then create a MainViewController, passing the mainViewController the sessionID and replacing the scene contents with the main view.

Here is a link to some sample code to demonstrate this approach.