src/main/java/com/alipay/remoting/codec/ProtocolCodeBasedDecoder.java
protected ProtocolCode decodeProtocolCode(ByteBuf in) {
        if (in.readableBytes() >= protocolCodeLength) {
            byte[] protocolCodeBytes = new byte[protocolCodeLength];
            in.readBytes(protocolCodeBytes);
            return ProtocolCode.fromBytes(protocolCodeBytes);
        }
        return null;
    }
protected ProtocolCode decodeProtocolCode(ByteBuf in) {
        if (in.readableBytes() >= protocolCodeLength) {
            byte[] protocolCodeBytes = new byte[protocolCodeLength];
            in.readBytes(protocolCodeBytes);
            return ProtocolCode.fromBytes(protocolCodeBytes);
        }
        return null;
    }
decode the protocol code

@param in input byte buf
@return an instance of ProtocolCode