public ActionForward execute(ActionMapping mapping,
    ActionForm form,
    HttpServletRequest request,
    HttpServletResponse response) throws IOException, ServletException 
{
      String username = ((LogonForm) form).getUsername();
      String password = ((LogonForm) form).getPassword();
      // Save our logged-in user in the session,
      // because we use it again later.
      HttpSession session = request.getSession();
      session.setAttribute(Constants.USER_KEY, form);
      // Log this event
      StringBuffer message = new StringBuffer("LogonAction: User '");
      message.append(username);
      message.append("' logged on in session ");
      message.append(session.getId());
      servlet.log(message.toString());
      // Forward control to the success URI
      // specified in struts-config.xml
      return (mapping.findForward(Constants.CONTINUE));
}

Login the user. The event is logged if the debug level is >= Constants.DEBUG.