一、消息发送入口

  消息发送有三种模式：同步消息、异步消息、单向消息。

同步消息：producer向broker发送消息，等待知道broker反馈结果才结束
异步消息：producer向broker发送消息，同时指定回调方法；执行发送之后立即返回，异步等待broker反馈结果后执行回调方法
当向消息：producer向broker发送消息，执行发送后立即返回，也不等待broker结果

  消息默认以同步的方式进行发送，发送入口为org.apache.rocketmq.client.producer.DefaultMQProducer#send(org.apache.rocketmq.common.message.Message)，代码如下

    /*** Send message in synchronous mode. This method returns only when the sending procedure totally completes. </p>** <strong>Warn:</strong> this method has internal retry-mechanism, that is, internal implementation will retry* {@link #retryTimesWhenSendFailed} times before claiming failure. As a result, multiple messages may be potentially* delivered to broker(s). It's up to the application developers to resolve potential duplication issue.** @param msg Message to send.* @return {@link SendResult} instance to inform senders details of the deliverable, say Message ID of the message,* {@link SendStatus} indicating broker storage/replication status, message queue sent to, etc.* @throws MQClientException if there is any client error.* @throws RemotingException if there is any network-tier error.* @throws MQBrokerException if there is any error with broker.* @throws InterruptedException if the sending thread is interrupted.*/@Overridepublic SendResult send(Message msg) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {msg.setTopic(withNamespace(msg.getTopic()));return this.defaultMQProducerImpl.send(msg);}

  消息发送入口简单，只调用内部默认实现服务（defaultMQProducerImpl）的send方法，最终调用核心实现sendDefaultImpl方法。
  sendDefaultImpl方法入参如下：

• Message msg：发送的消息对象
• CommunicationMode communicationMode：消息的发送模式，默认同步
• SendCallback sendCallback：发送完后的回调接口
• long timeout：超时时间：默认3000ms
  
    通过sendDefaultImpl方法的代码可以看出，消息发送的流程主要包含以下几个步骤：消息验证、查找路由、消息发送。让我们接着往下看。

二、消息发送流程

1、消息验证

  消息发送之前的验证分为2部分：消息主题验证和消息内容验证。

//org.apache.rocketmq.client.Validators#checkMessage消息验证
public static void checkMessage(Message msg, DefaultMQProducer defaultMQProducer) throws MQClientException {if (null == msg) {throw new MQClientException(ResponseCode.MESSAGE_ILLEGAL, "the message is null");}// topicValidators.checkTopic(msg.getTopic());Validators.isNotAllowedSendTopic(msg.getTopic());// bodyif (null == msg.getBody()) {throw new MQClientException(ResponseCode.MESSAGE_ILLEGAL, "the message body is null");}if (0 == msg.getBody().length) {throw new MQClientException(ResponseCode.MESSAGE_ILLEGAL, "the message body length is zero");}if (msg.getBody().length > defaultMQProducer.getMaxMessageSize()) {throw new MQClientException(ResponseCode.MESSAGE_ILLEGAL,"the message body size over max value, MAX: " + defaultMQProducer.getMaxMessageSize());}}

1）消息主题验证

  消息主题验证，规则包含以下几点：

• topic不能为空
• topic长度不能超过最大值127
• topic只能包含大小写字母和数字
• topic不能与部预留的主题相同

rocketmq内部预留的topic有如下几种：

• SCHEDULE_TOPIC_XXXX：延迟消息所使用的topic
• BenchmarkTest：基准测试使用的topic
• RMQ_SYS_TRANS_HALF_TOPIC：事务消息使用的topic
• RMQ_SYS_TRACE_TOPIC：轨迹消息使用的topic
• RMQ_SYS_TRANS_OP_HALF_TOPIC：事务消息使用的topic（二阶段）
• TRANS_CHECK_MAX_TIME_TOPIC：事务检查超时使用的topic
• SELF_TEST_TOPIC：自测使用
• OFFSET_MOVED_EVENT：消息偏移量改变消息使用

    public static void checkTopic(String topic) throws MQClientException {//topic不能为空if (UtilAll.isBlank(topic)) {throw new MQClientException("The specified topic is blank", null);}//topic长度不能超过最大值127if (topic.length() > TOPIC_MAX_LENGTH) {throw new MQClientException(String.format("The specified topic is longer than topic max length %d.", TOPIC_MAX_LENGTH), null);}//topic只能包含大小写字母和数字if (isTopicOrGroupIllegal(topic)) {throw new MQClientException(String.format("The specified topic[%s] contains illegal characters, allowing only %s", topic,"^[%|a-zA-Z0-9_-]+$"), null);}}

2）消息内容验证

  消息内容验证包含以下几点：

• 消息内容对象（msg）不能为null
• 消息内容主体（body）不能为null
• 消息内容主题（body）长度不能为0，或超过消息允许发送的最大长度4M（1024 * 1024 * 4）

2、查找路由

  消息发送需要根据topic找到对应的路由信息即broker。查找路由在DefaultMQProducerImpl#tryToFindTopicPublishInfo方法中完成。

    private TopicPublishInfo tryToFindTopicPublishInfo(final String topic) {TopicPublishInfo topicPublishInfo = this.topicPublishInfoTable.get(topic);if (null == topicPublishInfo || !topicPublishInfo.ok()) {this.topicPublishInfoTable.putIfAbsent(topic, new TopicPublishInfo());this.mQClientFactory.updateTopicRouteInfoFromNameServer(topic);topicPublishInfo = this.topicPublishInfoTable.get(topic);}if (topicPublishInfo.isHaveTopicRouterInfo() || topicPublishInfo.ok()) {return topicPublishInfo;} else {//发送消息，获取topic路由信息，需要获取默认值//是否允许获取默认值，在broker配置文件中配置，配置了默认值的，会注册到nameserverthis.mQClientFactory.updateTopicRouteInfoFromNameServer(topic, true, this.defaultMQProducer);topicPublishInfo = this.topicPublishInfoTable.get(topic);return topicPublishInfo;}}

• 1、先从路由缓存表topicPublishInfoTable中获取
• 2、如果取到的路由为null或路由所持有的队列信息信息为空，则向nameSrv查询topic的路由信息并更新到路由缓存表
• 3、如果已经有topic路由信息或持有队列则返回此路由，否则再次向nameSrv查询更新路由信息到缓存表

3、消息发送

  消息发送根据消息通信模式（同步、异步、单向）决定消息发送次数。如果是同步，则只发送一次，如果是其他（异步、单向）则发送失败会进行重试（3次）

1）选择消息队列

    private SendResult sendDefaultImpl(Message msg,final CommunicationMode communicationMode,final SendCallback sendCallback,final long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {//......//异步失败发3次，同步发1次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();//查找除lastBrokerName外的broker消息队列MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);//......}         

  执行消息发送时，选择所发送消息topic的一个队列，这个队列不能是上一次重试时使用的队列（lastBrokerName）

在重试发送时，记录上一次发送失败的broker（lastBrokerName），这样在下一次重试时能避免再次往失败的broker发送。

2）消息发送-内核实现

  在做完上述消息验证、路由选取动作之后，调用核心方法DefaultMQProducerImpl#sendKernelImpl进行发送消息，下面让我们来看看sendKernelImpl都做了些什么

sendKernelImpl方法参数

 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 {//......
}

参数：

• Message msg：消息实体
• MessageQueue mq：本次发送使用的消息队列
• CommunicationMode communicationMode：消息发送通信模式
• SendCallback sendCallback：回调函数
• TopicPublishInfo topicPublishInfo：路由信息
• long timeout：超时时间

获取brokerAddr

  根据mq队列对象传入的brokerName，从MQClientInstance本地缓存的brokerAddrTable中获取主broker地址。如果查找不到主broker，尝试从nameServer更新topic的路由信息再获取。

        //从MQClientlnIstance的brokerAddrTable中查找broker地址String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());if (null == brokerAddr) {//没找到broker地址，尝试从nameServer更新topic的路由信息再获取tryToFindTopicPublishInfo(mq.getTopic());brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());}

添加消息全局唯一id

                //for MessageBatch,ID has been set in the generating processif (!(msg instanceof MessageBatch)) {MessageClientIDSetter.setUniqID(msg);}

  对于单条消息，为其创建一个全局唯一消息id；批量消息则已经提前设置

设置实例id

  如果topic设置了命名空间（namespace），则为消息添加对应的INSTANCE_ID实例id

                boolean topicWithNamespace = false;if (null != this.mQClientFactory.getClientConfig().getNamespace()) {msg.setInstanceId(this.mQClientFactory.getClientConfig().getNamespace());topicWithNamespace = true;}

设置系统标记

                int sysFlag = 0;boolean msgBodyCompressed = false;if (this.tryToCompressMessage(msg)) {//标记已压缩sysFlag |= MessageSysFlag.COMPRESSED_FLAG;sysFlag |= compressType.getCompressionFlag();msgBodyCompressed = true;}final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);if (Boolean.parseBoolean(tranMsg)) {//标记事务消息sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE;}

  如果消息体超过4K，则进行压缩后添加压缩标记；如果消息是事务消息，添加事务标记

执行消息前置钩子

  前置的消息钩子有两种：检查禁用钩子和消息发送钩子

                //执行禁用钩子函数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);}//执行消息发送钩子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);}

构建发送消息请求体

  SendMessageRequestHeader消息发送请求对象，包含发送消息必要的信息（topic、队列等）

                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);if (requestHeader.getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {String reconsumeTimes = MessageAccessor.getReconsumeTime(msg);if (reconsumeTimes != null) {requestHeader.setReconsumeTimes(Integer.valueOf(reconsumeTimes));MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_RECONSUME_TIME);}String maxReconsumeTimes = MessageAccessor.getMaxReconsumeTimes(msg);if (maxReconsumeTimes != null) {requestHeader.setMaxReconsumeTimes(Integer.valueOf(maxReconsumeTimes));MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_MAX_RECONSUME_TIMES);}}

执行发送消息

  根据消息通信模式，调用MQClientAPIImpl#sendMessage方法进行发送消息。如果是异步消息，发送时会传入回调函数sendCallback以便发送后进行后续操作

                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/66tmpMessage = MessageAccessor.cloneMessage(msg);messageCloned = true;msg.setBody(prevBody);}if (topicWithNamespace) {if (!messageCloned) {tmpMessage = MessageAccessor.cloneMessage(msg);messageCloned = true;}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,mq.getBrokerName(),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,mq.getBrokerName(),msg,requestHeader,timeout - costTimeSync,communicationMode,context,this);break;default:assert false;break;}

执行后置钩子

  消息发送完后，执行发送后置钩子函数

                if (this.hasSendMessageHook()) {//注册了消息发送钩子函数，执行after逻辑context.setSendResult(sendResult);this.executeSendMessageHookAfter(context);}

  同样，在发送过程中发生异常也会执行发送后置钩子函数。这里异常主要包括：RemotingException 、MQBrokerException、InterruptedException

            } catch (RemotingException e) {if (this.hasSendMessageHook()) {context.setException(e);this.executeSendMessageHookAfter(context);}throw e;} catch (MQBrokerException e) {if (this.hasSendMessageHook()) {context.setException(e);this.executeSendMessageHookAfter(context);}throw e;} catch (InterruptedException e) {if (this.hasSendMessageHook()) {context.setException(e);this.executeSendMessageHookAfter(context);}throw e;} finally {