Session class to get user input and keep main method as small as possible

I am trying to keep my main method as small as possible. An application should merely be bootstrapped by the main method. Therefore I am trying to put the user input session into separate classes/methods. My initial idea was to use AtomicBoolean running. Unfortunately, I am only allowed to use classes from java.util. That's why I tried it this way:

I created a class Session.java:

public class Session {
    protected boolean running = true;

    /**
     * After starting the session this method remains in a loop until the 
     * terminate method is called.
     */
    void run() {
        RailRegister register = new Register();

        while (running) {
            String input = Terminal.readLine();
            try {
                UserInterface userInterface = new UserInterface(register);
                userInterface.execute(input);
            } catch (UnknownCommandException | BadSyntaxException e) {
                Terminal.printError(e.getMessage());
            }
        }
    }

    // Terminates the session
    public void terminate() {
        running = false;
    }
}

and here is my main class with the main method:

    public class Main {
        public static void main(String[] args) {
            Session session = new Session();
            session.run();
        }
    }

The quit command should terminate the session. However, I am not sure how to access the current session.

    public class QuitCommand extends AbstractCommand {
        public QuitCommand(RailRegister register) {
            super(register);
        }

        @Override
        public void execute(List<String> arguments) throws BadSyntaxException {
            // TODO: terminate session
        }
    }

Can someone show me how this can be implemented?