Request login info from external servlet

I have a basic login screen (user/pwd/login button) that is displayed when the application starts. [code]
public class ClientUI extends UI
{

@Override
protected void init(VaadinRequest request) 
{
    final HorizontalLayout layout = new HorizontalLayout();
    layout.setSizeFull();
    layout.setMargin(true);
    setContent(layout);

    final LoginDialog loginDialog = new LoginDialog();
    layout.addComponent(loginDialog);
    
    Button loginButton = loginDialog.getLoginButton();
    loginButton.addClickListener(new Button.ClickListener() {
        public void buttonClick(Button.ClickEvent event) 
        {
            String user = loginDialog.getUsernameTextField().getValue();
            String pwd = loginDialog.getPasswordField().getValue();
            
            if (user.matches("^\\w*$"))
                loginDialog.getUsernameTextField().setComponentError(
                        new UserError("Username must be letters and numbers."));
            else
                loginDialog.getUsernameTextField().setComponentError(null);
            
            // need to authenticate using another servlet here
        }
    });
}

}
[/code]
When the user clicks the
Login
button, I would like to authenticate using a pre-existing servlet that validates the user’s credentials against a databse and returns a JSON login ticket.

How can i go about doing this? I haven’t seen explicit references to this type of activity in the book of Vaadin or in my forum searches. Any help would be greatly appreciated.