code: @Override
      public void declareOutputFields(OutputFieldsDeclarer declarer)
      {
          List<String> fields = Lists.newArrayList(FieldName.MESSAGE_KEY, FieldName.MESSAGE_VALUE);
  
          // Declare default stream
          declarer.declare(new Fields(fields));
  
          for (String stream : getOutputStreams())
          {
              declarer.declareStream(stream, new Fields(fields));
          }
      }
docs: Declare output fields and streams.<br>
  Declare fields are following.<br>
  <ol>
  <li>messageKey   : Groupingkey if exists.</li>
  <li>messageValue : Message value.</li>
  </ol>
  Streams are "default" and user setting streams.
  
  @param declarer declarer object