code: public static void addActionErrorExpression( HttpServletRequest request, String propertyName, String expression,
                                                   Object[] messageArgs )
      {
          ExpressionMessage msg = new ExpressionMessage( expression, messageArgs );
          InternalUtils.addActionError( propertyName, msg, request );
      }
docs: Add a property-related message as an expression that will be evaluated and shown with the Errors and Error tags.
  
  @param request the current ServletRequest.
  @param propertyName the name of the property with which to associate this error.
  @param expression the JSP 2.0-style expression (e.g., <code>${pageFlow.myProperty}</code>) or literal string
  that will be used as the message.
  @param messageArgs zero or more arguments to the message.