code: private static Node checkUserIdentifier(final HttpServletRequest request,
  			final ReadResponse response) {
  		final String userId = request
  				.getParameter(ProtocolConstants.Parameters.Read.USER_IDENTIFIER);
  		if (userId != null) {
  			final Node user = NeoUtils.getUserByIdentifier(userId);
  			if (user != null) {
  				return user;
  			}
  			response.userIdentifierInvalid(userId);
  		} else {
  			response.userIdentifierMissing();
  		}
  
  		return null;
  	}
docs: check if the request contains a valid user identifier
  
  @param request
  Tomcat servlet request
  @param response
  response object
  @return user node with the identifier passed<br>
  <b>null</b> if the identifier is invalid