RocketMQ 5.1.0 源码详解 | Producer 发送流程

初始化DefaultMQProducer实例

详细内容见文章 RocketMQ 5.1.0 源码详解 | Producer 启动流程 第一部分

发送流程

DefaultMQProducer#send

只需要执行以下代码即可开始消息的发送流程

try {
    Message msg = new Message(TOPIC, TAG, "OrderID188", "Hello world".getBytes(StandardCharsets.UTF_8));
    SendResult sendResult = producer.send(msg);
    System.out.printf("%s%n", sendResult);
} catch (Exception e) {
    e.printStackTrace();
}

RocketMQ 发送普通消息有同步（Sync）发送、异步（Async）发送和单向（Oneway）发送三种方式，send() 方法中只传入 message 则默认为 SYNC 模式

producer 的 send 方法内容如下

public SendResult send(
    Message msg) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    msg.setTopic(withNamespace(msg.getTopic()));
    return this.defaultMQProducerImpl.send(msg);
}

可以看到在发送消息时 DefaultMQProducer 也只是一个门面类，具体的实现都是由 DefaultMQProducerImpl 去做的

DefaultMQProducerImpl#send

DefaultMQProducerImpl 的 send 方法内容如下

public SendResult send(
    Message msg) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    return send(msg, this.defaultMQProducer.getSendMsgTimeout());
}

public SendResult send(Message msg,
    long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    return this.sendDefaultImpl(msg, CommunicationMode.SYNC, null, timeout);
}

可以看到，基本就是继续调用了几个函数以补齐缺失的参数如超时时间、发送消息的类型和回调函数（由于是同步发送因此回调函数为 null），发送消息的逻辑则主要是在 sendDefaultImpl 方法中实现的

由于此方法内容太多，因此先看看整体的流程

private SendResult sendDefaultImpl(
    Message msg,
    final CommunicationMode communicationMode,
    final SendCallback sendCallback,
    final long timeout
) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    // 确认生产者处于RUNNING状态
    this.makeSureStateOK();
    // 检查消息是否合法
    Validators.checkMessage(msg, this.defaultMQProducer);
    
    final long invokeID = random.nextLong();
    long beginTimestampFirst = System.currentTimeMillis();
    long beginTimestampPrev = beginTimestampFirst;
    long endTimestamp = beginTimestampFirst;
    
    // 获取topic的路由信息
    TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic());
    
    // topicPublishInfo不为空且可用
    if (topicPublishInfo != null && topicPublishInfo.ok()) {...}
    
    // 校验 NameServer 配置是否正确
    validateNameServerSetting();

    throw new MQClientException("No route info of this topic: " + msg.getTopic() + FAQUrl.suggestTodo(FAQUrl.NO_TOPIC_ROUTE_INFO),
        null).setResponseCode(ClientErrorCode.NOT_FOUND_TOPIC_EXCEPTION);
}

首先检查生产者是否处于 RUNNING 状态，接着检查要发送的消息是否合法，然后会调用 tryToFindTopicPublishInfo 获取路由信息，如果获取成功则进入分支语句中的逻辑，否则校验 NameServer 配置是否正确。如果 NameServer 配置为空则抛出 No name server address 异常，否则抛出 No route info of this topic 异常

由于其他的逻辑相对容易，我们接下来先直接分析 tryToFindTopicPublishInfo 方法的内容

private TopicPublishInfo tryToFindTopicPublishInfo(final String topic) {
    // 从本地缓存(ConcurrentMap< String/* topic */,  TopicPublishInfo>)中尝试获取，第一次肯定为空
    TopicPublishInfo topicPublishInfo = this.topicPublishInfoTable.get(topic);
    if (null == topicPublishInfo || !topicPublishInfo.ok()) {
        this.topicPublishInfoTable.putIfAbsent(topic, new TopicPublishInfo());
        // 1.尝试从NameServer获取特定topic路由信息并更新本地缓存配置
        this.mQClientFactory.updateTopicRouteInfoFromNameServer(topic);
        topicPublishInfo = this.topicPublishInfoTable.get(topic);
    }

    // 如果找到可用的路由信息并返回
    if (topicPublishInfo.isHaveTopicRouterInfo() || topicPublishInfo.ok()) {
        return topicPublishInfo;
    } else { // 2.如果未找到路由信息，则再次尝试使用默认的topic获取路由信息
        this.mQClientFactory.updateTopicRouteInfoFromNameServer(topic, true, this.defaultMQProducer);
        topicPublishInfo = this.topicPublishInfoTable.get(topic);
        return topicPublishInfo;
    }
}

可以看到此方法首先会从本地的 topicPublishInfoTable 中寻找 topicPublishInfo，由于之前没有向 topic 发送过消息，因此第一次必然不会从本地找到

此时会首先向 topicPublishInfoTable 中添加空白 topicPublishInfo，然后再调用 mQClientFactory 对象的 updateTopicRouteInfoFromNameServer 方法来更新 topicPublishInfoTable 中 topicPublishInfo 的数据

又因为是向一个还不存在的 topic 发送消息，因此第一次尝试从 NameServer 获取配置信息并更新本地缓存配置失败，会进行尝试使用默认的 topic 去找路由配置信息

MQClientInstance#updateTopicRouteInfoFromNameServer

由上述章节可知此方法被调用了两次，第一次尝试从 NameServer 获取特定 topic 路由信息并更新本地缓存配置失败，第二次尝试使用默认的 topic 获取路由信息

使用特定 topic 获取路由信息

第一次尝试使用特定 topic 获取路由信息，调用方法为 updateTopicRouteInfoFromNameServer(topic)

public boolean updateTopicRouteInfoFromNameServer(final String topic) {
    return updateTopicRouteInfoFromNameServer(topic, false, null);
}

此方法又会调用其重载方法，即updateTopicRouteInfoFromNameServer(final String topic, boolean isDefault, DefaultMQProducer defaultMQProducer)，其中 isDefault 传入的值为 false

由于方法的内容太多，因此我们只看代码走过的部分

TopicRouteData topicRouteData;
if (isDefault && defaultMQProducer != null) {
   // ... 
} else {
    // 获取指定topic的配置信息
    topicRouteData = this.mQClientAPIImpl.getTopicRouteInfoFromNameServer(topic, clientConfig.getMqClientApiTimeout());
}

isDefault 的值为 false，因此进入 else 分支，尝试从 NameServer 中获取特定 topic 的路由信息，其中 getTopicRouteInfoFromNameServer 方法通过 Netty 使用 RPC 调用获取 Topic 路由信息，方法内容如下

public TopicRouteData getTopicRouteInfoFromNameServer(final String topic, final long timeoutMillis)
    throws RemotingException, MQClientException, InterruptedException {
    return getTopicRouteInfoFromNameServer(topic, timeoutMillis, true);
}

public TopicRouteData getTopicRouteInfoFromNameServer(final String topic, final long timeoutMillis,
    boolean allowTopicNotExist) throws MQClientException, InterruptedException, RemotingTimeoutException, RemotingSendRequestException, RemotingConnectException {
    GetRouteInfoRequestHeader requestHeader = new GetRouteInfoRequestHeader();
    requestHeader.setTopic(topic);
    RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.GET_ROUTEINFO_BY_TOPIC, requestHeader);

    RemotingCommand response = this.remotingClient.invokeSync(null, request, timeoutMillis);
    assert response != null;
    switch (response.getCode()) {
        case ResponseCode.TOPIC_NOT_EXIST: {
            if (allowTopicNotExist) {
                log.warn("get Topic [{}] RouteInfoFromNameServer is not exist value", topic);
            }

            break;
        }
        //...
    }

    throw new MQClientException(response.getCode(), response.getRemark());
}

但是我们向一个不存在的 topic 发送消息，因此进入 case ResponseCode.TOPIC_NOT_EXIST 分支。又因为 allowTopicNotExist 传入的值为 true，所以打印警告并抛出异常，方法结束

使用默认 topic 获取路由信息

第二次获取时调用了 updateTopicRouteInfoFromNameServer(final String topic, boolean isDefault, DefaultMQProducer defaultMQProducer) ，其中 isDefault 传入的值为 true

TopicRouteData topicRouteData;
if (isDefault && defaultMQProducer != null) {
    // 从NameServer中获取默认的topic路由信息
    topicRouteData = this.mQClientAPIImpl.getDefaultTopicRouteInfoFromNameServer(defaultMQProducer.getCreateTopicKey(),
        clientConfig.getMqClientApiTimeout());
    if (topicRouteData != null) {
        // 修正topic路由信息中的读写队列数，使其最大不超过默认的topic队列数
        for (QueueData data : topicRouteData.getQueueDatas()) {
            int queueNums = Math.min(defaultMQProducer.getDefaultTopicQueueNums(), data.getReadQueueNums());
            data.setReadQueueNums(queueNums);
            data.setWriteQueueNums(queueNums);
        }
    }
}

上述代码分为两个步骤：

1. 从 NameServer 中获取默认 topic 即 TBW102 的路由信息
2. 修正获取到的默认 topic 路由信息

此时我们的 topicRouteData 不为空，且其 QueueData 属性也经过了修正，具体内容如下

TopicRouteData [
	orderTopicConf=null, 
	queueDatas=[
		QueueData [
			brokerName=broker-a, 
			readQueueNums=4, 
			writeQueueNums=4, 
			perm=6, 
			topicSysFlag=0
		]
	], 
	brokerDatas=[
		BrokerData [
			brokerName=broker-a, 
			brokerAddrs={0=192.168.142.1:10911}, 
			enableActingMaster=false
		]
	], 
	filterServerTable={}, 
	topicQueueMappingInfoTable=null
]

接着执行下面的代码

if (topicRouteData != null) {
    TopicRouteData old = this.topicRouteTable.get(topic);
    // 与本地缓存中的 topic 发布信息进行比较，如果有变化，则需要同步更新发送者、消费者关于该 topic 的缓存
    boolean changed = topicRouteData.topicRouteDataChanged(old);
    if (!changed) {
        changed = this.isNeedUpdateTopicRouteInfo(topic);
    } else {
        log.info("the topic[{}] route info changed, old[{}] ,new[{}]", topic, old, topicRouteData);
    }

    if (changed) { // 如果有变化，则需要同步更新发送者、消费者关于该 topic 的缓存
        for (BrokerData bd : topicRouteData.getBrokerDatas()) {
            // 更新broker地址
            this.brokerAddrTable.put(bd.getBrokerName(), bd.getBrokerAddrs());
        }

        // Update endpoint map
        {
            ConcurrentMap<MessageQueue, String> mqEndPoints = topicRouteData2EndpointsForStaticTopic(topic, topicRouteData);
            if (!mqEndPoints.isEmpty()) {
                topicEndPointsTable.put(topic, mqEndPoints);
            }
        }

        // Update Pub info
        {
            // 根据topic路由信息组装TopicPublishInfo对象
            TopicPublishInfo publishInfo = topicRouteData2TopicPublishInfo(topic, topicRouteData);
            publishInfo.setHaveTopicRouterInfo(true);
            for (Entry<String, MQProducerInner> entry : this.producerTable.entrySet()) {
                MQProducerInner impl = entry.getValue();
                if (impl != null) {
                    // 更新DefaultMQProducerImpl的topicPublishInfoTable表
                    impl.updateTopicPublishInfo(topic, publishInfo);
                }
            }
        }

        // Update sub info 生产者实例的consumerTable为空
        if (!consumerTable.isEmpty()) {
            //...
        }
        TopicRouteData cloneTopicRouteData = new TopicRouteData(topicRouteData);
        log.info("topicRouteTable.put. Topic = {}, TopicRouteData[{}]", topic, cloneTopicRouteData);
        this.topicRouteTable.put(topic, cloneTopicRouteData);
        return true;
    }
}

很明显新获取到的和本地缓存中的 topic 路由信息相比有变化，因此 changed 为 true

接着会根据 topicRouteData 组装TopicPublishInfo 对象，并将其保存到 DefaultMQProducerImpl 的 topicPublishInfoTable 中，key 为 topic 名称，value 为 TopicPublishInfo 对象

最后将 topicRouteData 保存在 topicRouteTable 中，方法结束

DefaultMQProducerImpl#sendDefaultImpl

现在我们已经获取到了要发送的 topic 的发布路由 topicPublishInfo，之后就开始发送了

boolean callTimeout = false;
MessageQueue mq = null;
Exception exception = null;
SendResult sendResult = null;
// 发送失败后重试最多的次数
int timesTotal = communicationMode == CommunicationMode.SYNC ? 1 + this.defaultMQProducer.getRetryTimesWhenSendFailed() : 1;
int times = 0;
String[] brokersSent = new String[timesTotal];
for (; times < timesTotal; times++) {
    String lastBrokerName = null == mq ? null : mq.getBrokerName();
    // 选择一个MessageQueue发送消息
    MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);
    if (mqSelected != null) {
    	//发送消息...
    } else {
        break;
    }
}

其中 selectOneMessageQueue 方法就是选择一个可用的 MessageQueue 发送消息

如上图所示，MessageQueue 有一个三元组标识唯一一个队列，即 (topic, brokerName, queueId)，最上方的 MessageQueue 的三元组可能是 (TopicTest, broker-a, 0)

当我们得到了要发送的 MessageQueue 后就开始执行发送消息的步骤

mq = mqSelected;
brokersSent[times] = mq.getBrokerName();
try {
    beginTimestampPrev = System.currentTimeMillis();
    if (times > 0) {
        //Reset topic with namespace during resend.
        msg.setTopic(this.defaultMQProducer.withNamespace(msg.getTopic()));
    }
    long costTime = beginTimestampPrev - beginTimestampFirst;
    if (timeout < costTime) {
        callTimeout = true;
        break;
    }

    // 向 MessageQueue 发送消息
    sendResult = this.sendKernelImpl(msg, mq, communicationMode, sendCallback, topicPublishInfo, timeout - costTime);
    
    endTimestamp = System.currentTimeMillis();
    this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false);
    switch (communicationMode) {
        case ASYNC:
            return null;
        case ONEWAY:
            return null;
        case SYNC:
            // 同步调用方式(SYNC)下如果发送失败则执行失败重试策略，默认重试两次，即最多发送三次
            if (sendResult.getSendStatus() != SendStatus.SEND_OK) {
                if (this.defaultMQProducer.isRetryAnotherBrokerWhenNotStoreOK()) {
                    continue;
                }
            }

            return sendResult;
        default:
            break;
    }
}

通过代码可以看出又调用了 sendKernelImpl 方法发送消息

private SendResult sendKernelImpl(final Message msg,
    final MessageQueue mq,
    final CommunicationMode communicationMode,
    final SendCallback sendCallback,
    final TopicPublishInfo topicPublishInfo,
    final long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    long beginStartTime = System.currentTimeMillis();
    String brokerName = this.mQClientFactory.getBrokerNameFromMessageQueue(mq);
    String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(brokerName);
    if (null == brokerAddr) {
        tryToFindTopicPublishInfo(mq.getTopic());
        brokerName = this.mQClientFactory.getBrokerNameFromMessageQueue(mq);
        brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(brokerName);
    }

    SendMessageContext context = null;
    if (brokerAddr != null) {
        // 根据配置判断是否使用VIP通道
        brokerAddr = MixAll.brokerVIPChannel(this.defaultMQProducer.isSendMessageWithVIPChannel(), brokerAddr);

        byte[] prevBody = msg.getBody();
        try {
            //for MessageBatch,ID has been set in the generating process
            // 检查消息是否为 MessageBatch 类型
            if (!(msg instanceof MessageBatch)) {
                MessageClientIDSetter.setUniqID(msg);
            }
 
            boolean topicWithNamespace = false;
            // 检查客户端配置中是否设置了命名空间
            if (null != this.mQClientFactory.getClientConfig().getNamespace()) {
                msg.setInstanceId(this.mQClientFactory.getClientConfig().getNamespace());
                topicWithNamespace = true;
            }

            // sysFlag是消息的系统标志位，包含压缩标志位、事务标志位、批量标志位、多队列标志位等
            int sysFlag = 0;
            boolean msgBodyCompressed = false;
            // 尝试压缩消息体
            if (this.tryToCompressMessage(msg)) {
                sysFlag |= MessageSysFlag.COMPRESSED_FLAG;
                sysFlag |= compressType.getCompressionFlag();
                msgBodyCompressed = true;
            }

            final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);
            // 检查消息是否为事务消息
            if (Boolean.parseBoolean(tranMsg)) {
                sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE;
            }

            // 发送消息的校验钩子
            if (hasCheckForbiddenHook()) {
                CheckForbiddenContext checkForbiddenContext = new CheckForbiddenContext();
                checkForbiddenContext.setNameSrvAddr(this.defaultMQProducer.getNamesrvAddr());
                checkForbiddenContext.setGroup(this.defaultMQProducer.getProducerGroup());
                checkForbiddenContext.setCommunicationMode(communicationMode);
                checkForbiddenContext.setBrokerAddr(brokerAddr);
                checkForbiddenContext.setMessage(msg);
                checkForbiddenContext.setMq(mq);
                checkForbiddenContext.setUnitMode(this.isUnitMode());
                this.executeCheckForbiddenHook(checkForbiddenContext);
            }

            // 发送消息前的钩子
            if (this.hasSendMessageHook()) {
                context = new SendMessageContext();
                context.setProducer(this);
                context.setProducerGroup(this.defaultMQProducer.getProducerGroup());
                context.setCommunicationMode(communicationMode);
                context.setBornHost(this.defaultMQProducer.getClientIP());
                context.setBrokerAddr(brokerAddr);
                context.setMessage(msg);
                context.setMq(mq);
                context.setNamespace(this.defaultMQProducer.getNamespace());
                String isTrans = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);
                if (isTrans != null && isTrans.equals("true")) {
                    context.setMsgType(MessageType.Trans_Msg_Half);
                }

                if (msg.getProperty("__STARTDELIVERTIME") != null || msg.getProperty(MessageConst.PROPERTY_DELAY_TIME_LEVEL) != null) {
                    context.setMsgType(MessageType.Delay_Msg);
                }
                this.executeSendMessageHookBefore(context);
            }

            // 设置发送消息的请求头
            SendMessageRequestHeader requestHeader = new SendMessageRequestHeader();
            requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
            requestHeader.setTopic(msg.getTopic());
            requestHeader.setDefaultTopic(this.defaultMQProducer.getCreateTopicKey());
            requestHeader.setDefaultTopicQueueNums(this.defaultMQProducer.getDefaultTopicQueueNums());
            requestHeader.setQueueId(mq.getQueueId());
            requestHeader.setSysFlag(sysFlag);
            requestHeader.setBornTimestamp(System.currentTimeMillis());
            requestHeader.setFlag(msg.getFlag());
            requestHeader.setProperties(MessageDecoder.messageProperties2String(msg.getProperties()));
            requestHeader.setReconsumeTimes(0);
            requestHeader.setUnitMode(this.isUnitMode());
            requestHeader.setBatch(msg instanceof MessageBatch);
            requestHeader.setBname(brokerName);
            // 如果是重发消息，则设置重发消息的次数
            if (requestHeader.getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                // 重发消息的次数
                String reconsumeTimes = MessageAccessor.getReconsumeTime(msg);
                if (reconsumeTimes != null) {
                    // 设置重发消息的次数
                    requestHeader.setReconsumeTimes(Integer.valueOf(reconsumeTimes));
                    // 清除消息的重发次数属性，因为消息的重发次数属性是在消息重发时设置的
                    MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_RECONSUME_TIME);
                }

                // 消息的最大重发次数
                String maxReconsumeTimes = MessageAccessor.getMaxReconsumeTimes(msg);
                if (maxReconsumeTimes != null) {
                    // 设置消息的最大重发次数
                    requestHeader.setMaxReconsumeTimes(Integer.valueOf(maxReconsumeTimes));
                    // 清除消息的最大重发次数属性，因为消息的最大重发次数属性是在消息重发时设置的
                    MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_MAX_RECONSUME_TIMES);
                }
            }

            SendResult sendResult = null;
            switch (communicationMode) {
                case ASYNC:
                    Message tmpMessage = msg;
                    boolean messageCloned = false;
                    if (msgBodyCompressed) {
                        //If msg body was compressed, msgbody should be reset using prevBody.
                        //Clone new message using commpressed message body and recover origin massage.
                        //Fix bug:https://github.com/apache/rocketmq-externals/issues/66
                        tmpMessage = MessageAccessor.cloneMessage(msg);
                        messageCloned = true;
                        // 防止压缩后的消息体重发时被再次压缩
                        msg.setBody(prevBody);
                    }

                    if (topicWithNamespace) {
                        if (!messageCloned) {
                            tmpMessage = MessageAccessor.cloneMessage(msg);
                            messageCloned = true;
                        }
                        // 防止设置了命名空间的topic重发时被再次设置命名空间
                        msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));
                    }

                    long costTimeAsync = System.currentTimeMillis() - beginStartTime;
                    if (timeout < costTimeAsync) {
                        throw new RemotingTooMuchRequestException("sendKernelImpl call timeout");
                    }
                    sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage(
                        brokerAddr,
                        brokerName,
                        tmpMessage,
                        requestHeader,
                        timeout - costTimeAsync,
                        communicationMode,
                        sendCallback,
                        topicPublishInfo,
                        this.mQClientFactory,
                        this.defaultMQProducer.getRetryTimesWhenSendAsyncFailed(),
                        context,
                        this);
                    break;
                case ONEWAY:
                case SYNC:
                    long costTimeSync = System.currentTimeMillis() - beginStartTime;
                    if (timeout < costTimeSync) {
                        throw new RemotingTooMuchRequestException("sendKernelImpl call timeout");
                    }
                    sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage(
                        brokerAddr,
                        brokerName,
                        msg,
                        requestHeader,
                        timeout - costTimeSync,
                        communicationMode,
                        context,
                        this);
                    break;
                default:
                    assert false;
                    break;
            }

            // 发送消息后的钩子
            if (this.hasSendMessageHook()) {
                context.setSendResult(sendResult);
                this.executeSendMessageHookAfter(context);
            }

            return sendResult;
        } catch (RemotingException | InterruptedException | MQBrokerException e) {
            if (this.hasSendMessageHook()) {
                context.setException(e);
                this.executeSendMessageHookAfter(context);
            }
            throw e;
        } finally {
            msg.setBody(prevBody);
            msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));
        }
    }

    throw new MQClientException("The broker[" + brokerName + "] not exist", null);
}

这段代码虽然比较长，但是结合注释还是挺容易理解的。不过其中在异步 (ASYNC) 发送消息时有下面一段代码可能会让人疑惑

Message tmpMessage = msg;
boolean messageCloned = false;
if (msgBodyCompressed) {
    //If msg body was compressed, msgbody should be reset using prevBody.
    //Clone new message using commpressed message body and recover origin massage.
    //Fix bug:https://github.com/apache/rocketmq-externals/issues/66
    tmpMessage = MessageAccessor.cloneMessage(msg);
    messageCloned = true;
    // 防止压缩后的消息体重发时被再次压缩
    msg.setBody(prevBody);
}

if (topicWithNamespace) {
    if (!messageCloned) {
        tmpMessage = MessageAccessor.cloneMessage(msg);
        messageCloned = true;
    }
    // 防止设置了命名空间的topic重发时被再次设置命名空间
    msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));
}

这段代码主要是克隆了一个和 msg 内容一样的 tmpMessage 并发送，而 msg 本身的 body 被设置成了压缩之前的 body，topic 也被设置成了添加命名空间之前的 topic

发送流程总结

1. 检查消息是否合法
2. 获取 topic 路由信息
   1. 先尝试从本地获取路由信息，没有则向 NameServer 获取
      1. 向 NameServer 获取路由信息并更新本地缓存，没有则抛出异常并返回
      2. 从本地获取路由信息
   2. 如果本地扔获取不到路由信息则获取默认路由信息
      1. 向 NameServer 获取默认路由信息，如果获取不到则抛出异常并返回
      2. 修改获取到的默认路由信息为新的 topic 的路由信息
      3. 更新本地路由信息缓存
3. 获取路由信息成功；失败则跳转到第4步
   1. 选择一个 MessageQueue
   2. 向 MessageQueue 发送消息
      1. 根据配置判断是否使用 VIP 通道
      2. 检查消息是否为 MessageBatch 类型
      3. 检查客户端配置中是否设置了命名空间
      4. 设置消息的标志位 sysFlag
         1. 尝试压缩消息体并更新 sysFlag
         2. 检查消息是否为事务消息并更新 sysFlag
      5. 调用钩子函数
      6. 设置消息请求头
      7. 根据发送消息的方式发送消息
4. 获取路由信息失败
   1. 校验 NameServer 配置是否正确
   2. 抛出异常结束