Apache RocketMQ只用了7个类就实现了Nameserver，你知道是如何做到的吗？

public RemotingCommand processRequest(ChannelHandlerContext ctx,        RemotingCommand request) throws RemotingCommandException {
        if (ctx != null) {            log.debug("receive request, {} {} {}",                request.getCode(),                RemotingHelper.parseChannelRemoteAddr(ctx.channel()),                request);        }

        switch (request.getCode()) {            ……            //Broker注册请求处理，可以看到对于不对版本的Broker调用了不同的方法进行处理，测试中版本大于3.0.11，所以我们直接看registerBrokerWithFilterServer方法调用            case RequestCode.REGISTER_BROKER:                Version brokerVersion = MQVersion.value2Version(request.getVersion());                if (brokerVersion.ordinal() >= MQVersion.Version.V3_0_11.ordinal()) {                    return this.registerBrokerWithFilterServer(ctx, request);                } else {                    return this.registerBroker(ctx, request);                }            ……            case RequestCode.GET_ROUTEINFO_BY_TOPIC:                return this.getRouteInfoByTopic(ctx, request);            ……            default:                break;        }        return null;    }

public RemotingCommand registerBrokerWithFilterServer(ChannelHandlerContext ctx, RemotingCommand request)        throws RemotingCommandException {        //构建了请求返回信息        final RemotingCommand response = RemotingCommand.createResponseCommand(RegisterBrokerResponseHeader.class);        final RegisterBrokerResponseHeader responseHeader = (RegisterBrokerResponseHeader) response.readCustomHeader();        //解码Broker注册请求头信息，头信息里包含了Broker地址、名称、brokerId等信息        final RegisterBrokerRequestHeader requestHeader =            (RegisterBrokerRequestHeader) request.decodeCommandCustomHeader(RegisterBrokerRequestHeader.class);
        if (!checksum(ctx, request, requestHeader)) {            response.setCode(ResponseCode.SYSTEM_ERROR);            response.setRemark("crc32 not match");            return response;        }
        RegisterBrokerBody registerBrokerBody = new RegisterBrokerBody();
        if (request.getBody() != null) {            try {                //解码Broker注册请求体信息，请求体中包含了所有的Topic信息                registerBrokerBody = RegisterBrokerBody.decode(request.getBody(), requestHeader.isCompressed());            } catch (Exception e) {                throw new RemotingCommandException("Failed to decode RegisterBrokerBody", e);            }        } else {            registerBrokerBody.getTopicConfigSerializeWrapper().getDataVersion().setCounter(new AtomicLong(0));            registerBrokerBody.getTopicConfigSerializeWrapper().getDataVersion().setTimestamp(0);        }        //调用了RouteInfoManager#registerBroker方法，存储Broker相关信息到路由管理类中        RegisterBrokerResult result = this.namesrvController.getRouteInfoManager().registerBroker(            requestHeader.getClusterName(),            requestHeader.getBrokerAddr(),            requestHeader.getBrokerName(),            requestHeader.getBrokerId(),            requestHeader.getHaServerAddr(),            registerBrokerBody.getTopicConfigSerializeWrapper(),            registerBrokerBody.getFilterServerList(),            ctx.channel());        //构建返回请求        responseHeader.setHaServerAddr(result.getHaServerAddr());        responseHeader.setMasterAddr(result.getMasterAddr());
        byte[] jsonValue = this.namesrvController.getKvConfigManager().getKVListByNamespace(NamesrvUtil.NAMESPACE_ORDER_TOPIC_CONFIG);        response.setBody(jsonValue);
        response.setCode(ResponseCode.SUCCESS);        response.setRemark(null);        return response;    }