javafx call controller function from another thread

The question of the day is: how to call controllers function from another thread. My application looks like this:

public class Server {
//(...)
    public String setMsg(String string) {
       msg.set(string+"\n");
       mainScreenController.updateLog();
    }
//(...)
   while (true){
   doThings();
   }
}

    public class MainScreenController {
//(...)
    public void startServer(){
    new Thread(server::start).start();
    }
     public void updateLog(){
            Platform.runLater(()->{ testAreaLog.setText(testAreaLog.getText() + server.getMsg()); });
        }
//(...)
    }

I want to call updateLog() in the finally block, so every time server updates msg GUI adds this message to log window. My msg is

private volatile AtomicReference<String> msg = new AtomicReference<String>();

it works when i call updateLog(); in startServer(), it displays the first message Starting server as you may guessed, but calling another updateLog(); there returns null so I wanted to call it directly after getMsg() is used.

Solution

It's not really clear why you can't just do

public class MainScreenController {
    //(...)

    public void startServer(){
        new Thread(server::start).start();
    }

    public void updateLog(String message){
        Platform.runLater(()-> testAreaLog.appendText(message + "\n"));
    }

    //(...)
}

and

public class Server {

   public void start() {
       while (true){
           doThings();
           String newMessage = ... ;
           mainScreenController.updateLog(newMessage);
           doMoreThings();
       }
   }

}

This assumes something in your while loop is a blocking (or time-consuming) call, so that you are not flooding the UI thread with too many Platform.runLater() calls.