code: protected void setLoggedInUser(final HttpServletRequest _request,
                                     final String _userName)
      {
          if (_userName == null) {
              _request.getSession(true).removeAttribute(this.sessionParameterLoginName);
          } else {
              _request.getSession(true).setAttribute(this.sessionParameterLoginName,
                              _userName);
          }
      }
docs: Stores the logged in user name in a session attribute of the http servlet
  request. If the new user name is <code>null</code>, the session attribute
  is removed.
  
  @param _request http servlet request
  @param _userName name of logged in user to set (or null if not defined)