code: private ProtocolDecoderOutput getDecoderOut(IoSession session,
              NextFilter nextFilter) {
          ProtocolDecoderOutput out = (ProtocolDecoderOutput) session.getAttribute(DECODER_OUT);
          
          if (out == null) {
              // Create a new instance, and stores it into the session
              out = new ProtocolDecoderOutputImpl();
              session.setAttribute(DECODER_OUT, out);
          }
          
          return out;
      }
docs: Return a reference to the decoder callback. If it's not already created
  and stored into the session, we create a new instance.