code: @Handler
      public void onOutput(Output<ByteBuffer> event, Channel channel) {
          Writer writer = outputWriters.get(channel);
          if (writer != null) {
              writer.write(event.buffer());
          }
      }
docs: Handle {@link Output} events by writing them to the file, if
  a channel exists.
  
  @param event the event
  @param channel the channel