原文//www.greatytc.com/p/6cc25cd42185

public class SendMessageProcessor extends AbstractSendMessageProcessor implements NettyRequestProcessor {

    private RemotingCommand consumerSendMsgBack(final ChannelHandlerContext ctx, final RemotingCommand request)
        throws RemotingCommandException {

        final RemotingCommand response = RemotingCommand.createResponseCommand(null);
        final ConsumerSendMsgBackRequestHeader requestHeader =
            (ConsumerSendMsgBackRequestHeader)request.decodeCommandCustomHeader(ConsumerSendMsgBackRequestHeader.class);

        String namespace = NamespaceUtil.getNamespaceFromResource(requestHeader.getGroup());
        
        // 重试队列的Topic，RETRY_GROUP_TOPIC_PREFIX + consumerGroup;
        String newTopic = MixAll.getRetryTopic(requestHeader.getGroup());
        int queueIdInt = Math.abs(this.random.nextInt() % 99999999) % subscriptionGroupConfig.getRetryQueueNums();

        int topicSysFlag = 0;
        if (requestHeader.isUnitMode()) {
            topicSysFlag = TopicSysFlag.buildSysFlag(false, true);
        }

        TopicConfig topicConfig = this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(
            newTopic,
            subscriptionGroupConfig.getRetryQueueNums(),
            PermName.PERM_WRITE | PermName.PERM_READ, topicSysFlag);

        // 省略相关代码

        // 首先判断最大的重新消费次数
        int delayLevel = requestHeader.getDelayLevel();
        int maxReconsumeTimes = subscriptionGroupConfig.getRetryMaxTimes();
        if (request.getVersion() >= MQVersion.Version.V3_4_9.ordinal()) {
            // 当前版本由consumer端的MaxReconsumeTimes指定
            maxReconsumeTimes = requestHeader.getMaxReconsumeTimes();
        }
        // 当超过最大的重新消费次数后选择死信队列DLQTopic
        if (msgExt.getReconsumeTimes() >= maxReconsumeTimes
            || delayLevel < 0) {

            // DLQ_GROUP_TOPIC_PREFIX + consumerGroup;
            newTopic = MixAll.getDLQTopic(requestHeader.getGroup());
            queueIdInt = Math.abs(this.random.nextInt() % 99999999) % DLQ_NUMS_PER_GROUP;
            // 创建消费分组对应的死信队列的Topic
            topicConfig = this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(newTopic,
                DLQ_NUMS_PER_GROUP,
                PermName.PERM_WRITE,// 死信队列只有写权限
                0
            );

        } else {
            if (0 == delayLevel) {
                delayLevel = 3 + msgExt.getReconsumeTimes();
            }

            msgExt.setDelayTimeLevel(delayLevel);
        }

        // 重新生成消息体写入到新的topic当中，如果死信队列就重新写入commitLog和consumeQueue
        MessageExtBrokerInner msgInner = new MessageExtBrokerInner();
        msgInner.setTopic(newTopic);
        msgInner.setBody(msgExt.getBody());
        msgInner.setFlag(msgExt.getFlag());
        MessageAccessor.setProperties(msgInner, msgExt.getProperties());
        msgInner.setPropertiesString(MessageDecoder.messageProperties2String(msgExt.getProperties()));
        msgInner.setTagsCode(MessageExtBrokerInner.tagsString2tagsCode(null, msgExt.getTags()));

        msgInner.setQueueId(queueIdInt);
        msgInner.setSysFlag(msgExt.getSysFlag());
        msgInner.setBornTimestamp(msgExt.getBornTimestamp());
        msgInner.setBornHost(msgExt.getBornHost());
        msgInner.setStoreHost(this.getStoreHost());
        msgInner.setReconsumeTimes(msgExt.getReconsumeTimes() + 1);

        String originMsgId = MessageAccessor.getOriginMessageId(msgExt);
        MessageAccessor.setOriginMessageId(msgInner, UtilAll.isBlank(originMsgId) ? msgExt.getMsgId() : originMsgId);

        PutMessageResult putMessageResult = this.brokerController.getMessageStore().putMessage(msgInner);
        if (putMessageResult != null) {
            switch (putMessageResult.getPutMessageStatus()) {
                case PUT_OK:
                    String backTopic = msgExt.getTopic();
                    String correctTopic = msgExt.getProperty(MessageConst.PROPERTY_RETRY_TOPIC);
                    if (correctTopic != null) {
                        backTopic = correctTopic;
                    }

                    this.brokerController.getBrokerStatsManager().incSendBackNums(requestHeader.getGroup(), backTopic);

                    response.setCode(ResponseCode.SUCCESS);
                    response.setRemark(null);

                    return response;
                default:
                    break;
            }

            response.setCode(ResponseCode.SYSTEM_ERROR);
            response.setRemark(putMessageResult.getPutMessageStatus().name());
            return response;
        }

        response.setCode(ResponseCode.SYSTEM_ERROR);
        response.setRemark("putMessageResult is null");
        return response;
    }
}