Dubbo序列化的默认方式是Hessian2，对应到源码里面的类便是Hessian2Serialization

public class Hessian2Serialization implements Serialization {

    @Override
    public byte getContentTypeId() {
        return HESSIAN2_SERIALIZATION_ID;
    }

    @Override
    public String getContentType() {
        return "x-application/hessian2";
    }

    @Override
    public ObjectOutput serialize(URL url, OutputStream out) throws IOException {
        return new Hessian2ObjectOutput(out);
    }

    @Override
    public ObjectInput deserialize(URL url, InputStream is) throws IOException {
        return new Hessian2ObjectInput(is);
    }

}

在服务端，是在ExchangeCodec类中通过Hessian2Serialization来序列化或者反序列化数据

    protected void encodeRequest(Channel channel, ChannelBuffer buffer, Request req) throws IOException {
        Serialization serialization = getSerialization(channel);
        // header.
        byte[] header = new byte[HEADER_LENGTH];
        // set magic number.
        Bytes.short2bytes(MAGIC, header);

        // set request and serialization flag.
        header[2] = (byte) (FLAG_REQUEST | serialization.getContentTypeId());

        if (req.isTwoWay()) {
            header[2] |= FLAG_TWOWAY;
        }
        if (req.isEvent()) {
            header[2] |= FLAG_EVENT;
        }

        // set request id.
        Bytes.long2bytes(req.getId(), header, 4);

        // encode request data.
        int savedWriteIndex = buffer.writerIndex();
        buffer.writerIndex(savedWriteIndex + HEADER_LENGTH);
        ChannelBufferOutputStream bos = new ChannelBufferOutputStream(buffer);
        ObjectOutput out = serialization.serialize(channel.getUrl(), bos);
        if (req.isEvent()) {
            encodeEventData(channel, out, req.getData());
        } else {
            encodeRequestData(channel, out, req.getData(), req.getVersion());
        }
        out.flushBuffer();
        if (out instanceof Cleanable) {
            ((Cleanable) out).cleanup();
        }
        bos.flush();
        bos.close();
        int len = bos.writtenBytes();
        checkPayload(channel, len);
        Bytes.int2bytes(len, header, 12);

        // write
        buffer.writerIndex(savedWriteIndex);
        buffer.writeBytes(header); // write header.
        buffer.writerIndex(savedWriteIndex + HEADER_LENGTH + len);
    }

而在消费者端，除了通过ExchangeCodec类来序列化请求，还通过DecodeableRpcResult中的decode方法来反序列化请求结果

    public Object decode(Channel channel, InputStream input) throws IOException {
        if (log.isDebugEnabled()) {
            Thread thread = Thread.currentThread();
            log.debug("Decoding in thread -- [" + thread.getName() + "#" + thread.getId() + "]");
        }

        ObjectInput in = CodecSupport.getSerialization(channel.getUrl(), serializationType)
                .deserialize(channel.getUrl(), input);

        byte flag = in.readByte();
        switch (flag) {
            case DubboCodec.RESPONSE_NULL_VALUE:
                break;
            case DubboCodec.RESPONSE_VALUE:
                handleValue(in);
                break;
            case DubboCodec.RESPONSE_WITH_EXCEPTION:
                handleException(in);
                break;
            case DubboCodec.RESPONSE_NULL_VALUE_WITH_ATTACHMENTS:
                handleAttachment(in);
                break;
            case DubboCodec.RESPONSE_VALUE_WITH_ATTACHMENTS:
                handleValue(in);
                handleAttachment(in);
                break;
            case DubboCodec.RESPONSE_WITH_EXCEPTION_WITH_ATTACHMENTS:
                handleException(in);
                handleAttachment(in);
                break;
            default:
                throw new IOException("Unknown result flag, expect '0' '1' '2' '3' '4' '5', but received: " + flag);
        }
        if (in instanceof Cleanable) {
            ((Cleanable) in).cleanup();
        }
        return this;
    }