
### Environment

* Dubbo version: 2.7.0-SNAPSHOT
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

DubboCodec extends from ExchangeCodec, which already implements codec2, but DubboCodec still re-implements codec2, and it makes me confused. Is this designed specifically？

try {
                Object data;
                ObjectInput in = CodecSupport.deserialize(channel.getUrl(), is, proto);
                if (req.isHeartbeat()) {
                    data = decodeHeartbeatData(channel, in);
                } else if (req.isEvent()) {
                    data = decodeEventData(channel, in);
                } else {
                    DecodeableRpcInvocation inv;
                    if (channel.getUrl().getParameter(
                            Constants.DECODE_IN_IO_THREAD_KEY,
                            Constants.DEFAULT_DECODE_IN_IO_THREAD)) {
                        inv = new DecodeableRpcInvocation(channel, req, is, proto);
                        inv.decode();
                    } else {
                        inv = new DecodeableRpcInvocation(channel, req,
                                new UnsafeByteArrayInputStream(readMessageData(is)), proto);
                    }
                    data = inv;
                }
                req.setData(data);
            } catch (Throwable t) {
                if (log.isWarnEnabled()) {
                    log.warn("Decode request failed: " + t.getMessage(), t);
                }
                
                req.setBroken(true);
                req.setData(t);
            }
            return req;