code: public String getMessageCode(BaseMessage externalTrxMessage)
      {
          if (externalTrxMessage.getMessageHeader() == null)
              return null;
          return (String)externalTrxMessage.getMessageHeader().get(TrxMessageHeader.MESSAGE_CODE);
      }
docs: From this external message, figure out the message type.
  @externalMessage The external message just received.
  @return The message type for this kind of message (transport specific).