RocketMQ源码阅读-Producer发消息-编程知识

RocketMQ源码阅读-Producer发消息

1. 从单元测试入手
2. 启动过程
3. 同步消息发送过程
4. 异步消息发送过程
5. 小结

Producer是消息的生产者。

Producer和Consummer对Rocket来说都是Client，Server是NameServer。

客户端在源码中是一个单独的Model，目录为rocketmq/client。

类DefaultMQProducer是Producer的默认入口实现类。

继承了类ClientConfig，客户端配置类，存储上下文配置信息。
实现接口MQProducer：

定义了Producer对外提供的接口，也就是所有的发送消息的方法，同时MQProducer接口继承了MQAdmin接口。
如上图中MQAdmin是元数据管理接口，定义了对Message操作的一些方法。

DefaultMQProducer中有一个重要的成员变量：

protected final transient DefaultMQProducerImpl defaultMQProducerImpl;

DefaultMQProducer的所有操作，基本没什么业务逻辑，都是调用DefaultMQProducerImpl类中的方法。

DefaultMQProducerImpl类是Producer操作的具体实现类。

1. 从单元测试入手

看源码的流程都是从单元测试入手

Producer的单元测试在类org.apache.rocketmq.client.producer.DefaultMQProducerTest中。
单元测试的所有方法，就对应着这个类的全部功能。
DefaultMQProducerTest的方法列表如下：

其中 init 和 terminate 是测试开始初始化和测试结束销毁时需要执行的代码。
其他的方法是测试不同功能的测试用例，也就是测试不同的发消息的方式。

2. 启动过程

从单元测试中的init方法入手：

@Before
public void init() throws Exception {String producerGroupTemp = producerGroupPrefix + System.currentTimeMillis();// 创建一个Producer，并赋予名字producer = new DefaultMQProducer(producerGroupTemp);// 设置NameServer的地址producer.setNamesrvAddr("127.0.0.1:9876");// 消息长度大于16开启压缩producer.setCompressMsgBodyOverHowmuch(16);// 创建不同的Messagemessage = new Message(topic, new byte[] {'a'});zeroMsg = new Message(topic, new byte[] {});bigMessage = new Message(topic, "This is a very huge message!".getBytes());// 启动Producerproducer.start();// 设置mQClientFactory和mQClientAPIImpl，后面再讲Field field = DefaultMQProducerImpl.class.getDeclaredField("mQClientFactory");field.setAccessible(true);field.set(producer.getDefaultMQProducerImpl(), mQClientFactory);field = MQClientInstance.class.getDeclaredField("mQClientAPIImpl");field.setAccessible(true);field.set(mQClientFactory, mQClientAPIImpl);// 注册Producerproducer.getDefaultMQProducerImpl().getmQClientFactory().registerProducer(producerGroupTemp, producer.getDefaultMQProducerImpl());when(mQClientAPIImpl.sendMessage(anyString(), anyString(), any(Message.class), any(SendMessageRequestHeader.class), anyLong(), any(CommunicationMode.class),nullable(SendMessageContext.class), any(DefaultMQProducerImpl.class))).thenCallRealMethod();when(mQClientAPIImpl.sendMessage(anyString(), anyString(), any(Message.class), any(SendMessageRequestHeader.class), anyLong(), any(CommunicationMode.class),nullable(SendCallback.class), nullable(TopicPublishInfo.class), nullable(MQClientInstance.class), anyInt(), nullable(SendMessageContext.class), any(DefaultMQProducerImpl.class))).thenReturn(createSendResult(SendStatus.SEND_OK));
}

这段代码就是创建了一个DefaultMQProducer，设置参数，并调用start()方法启动，之后注册到NameServer中。

首先，看下DefaultMQProducer#start()方法：

@Override
public void start() throws MQClientException {this.setProducerGroup(withNamespace(this.producerGroup));// 直接调用defaultMQProducerImpl的start方法this.defaultMQProducerImpl.start();if (null != traceDispatcher) {try {traceDispatcher.start(this.getNamesrvAddr(), this.getAccessChannel());} catch (MQClientException e) {log.warn("trace dispatcher start failed ", e);}}
}

其中直接调用了DefaultMQProducerImpl的start方法：

public void start() throws MQClientException {this.start(true);
}

public void start(final boolean startFactory) throws MQClientException {switch (this.serviceState) {case CREATE_JUST:this.serviceState = ServiceState.START_FAILED;this.checkConfig();if (!this.defaultMQProducer.getProducerGroup().equals(MixAll.CLIENT_INNER_PRODUCER_GROUP)) {this.defaultMQProducer.changeInstanceNameToPID();}// 获取MQClientInstance实例this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQProducer, rpcHook);// 注册boolean registerOK = mQClientFactory.registerProducer(this.defaultMQProducer.getProducerGroup(), this);if (!registerOK) {this.serviceState = ServiceState.CREATE_JUST;throw new MQClientException("The producer group[" + this.defaultMQProducer.getProducerGroup()+ "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL),null);}this.topicPublishInfoTable.put(this.defaultMQProducer.getCreateTopicKey(), new TopicPublishInfo());if (startFactory) {// 启动mQClientFactorymQClientFactory.start();}log.info("the producer [{}] start OK. sendMessageWithVIPChannel={}", this.defaultMQProducer.getProducerGroup(),this.defaultMQProducer.isSendMessageWithVIPChannel());this.serviceState = ServiceState.RUNNING;break;case RUNNING:case START_FAILED:case SHUTDOWN_ALREADY:throw new MQClientException("The producer service state not OK, maybe started once, "+ this.serviceState+ FAQUrl.suggestTodo(FAQUrl.CLIENT_SERVICE_NOT_OK),null);default:break;}// 给所有的broker发心跳this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
}

RocketMQ 使用一个成员变量 serviceState 来记录和管理自身的服务状态，这实际上是状态模式 (State Pattern) 这种设计模式的变种实现。

对于启动过程，状态serviceState为CREATE_JUST。
CREATE_JUST分支中，会获取一个MQClientManager（单例模式），通过方法getAndCreateMQClientInstance()获取一个MQClientInstance实例，赋值给成员变量：

private MQClientInstance mQClientFactory;

然后调用MQClientInstance的registerProducer()方法，将自己注册到MQClientInstance中。
随后，调用MQClientInstance的start()，启动mQClientFactory。
最后，给所有的broker发心跳。

进一步看MQClientInstance的start()方法：

public void start() throws MQClientException {synchronized (this) {switch (this.serviceState) {case CREATE_JUST:this.serviceState = ServiceState.START_FAILED;// If not specified,looking address from name serverif (null == this.clientConfig.getNamesrvAddr()) {this.mQClientAPIImpl.fetchNameServerAddr();}// Start request-response channelthis.mQClientAPIImpl.start();// Start various schedule tasksthis.startScheduledTask();// Start pull servicethis.pullMessageService.start();// Start rebalance servicethis.rebalanceService.start();// Start push servicethis.defaultMQProducer.getDefaultMQProducerImpl().start(false);log.info("the client factory [{}] start OK", this.clientId);this.serviceState = ServiceState.RUNNING;break;case RUNNING:break;case SHUTDOWN_ALREADY:break;case START_FAILED:throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null);default:break;}}
}

这一部分代码的注释比较清楚，流程是这样的：

启动实例 mQClientAPIImpl，其中 mQClientAPIImpl 是类 MQClientAPIImpl 的实例，封装了客户端与 Broker 通信的方法；
启动各种定时任务，包括与 Broker 之间的定时心跳，定时与 NameServer 同步数据等任务；
启动拉取消息服务；
启动 Rebalance 服务；
启动默认的 Producer 服务。

以上就是 Producer 的启动流程。

3. 同步消息发送过程

分析 Producer 发送消息的流程。
接口 MQProducer 中，定义了 19 个不同参数的发消息的方法。
这19个接口可以分为3类：

单向发送（Oneway）：发送消息后立即返回，不处理响应，不关心是否发送成功；
同步发送（Sync）：发送消息后等待响应；
异步发送（Async）：发送消息后立即返回，在提供的回调方法中处理响应。

先看下同步发送消息的方法（异步发送消息，只是将同步发送方法提交给线程池）。
DefaultMQProducer中对同步发送方法的实现为：

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

实际调用的为DefaultMQProducerImpl的send()方法：

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);
}

最终调用的方法为DefaultMQProducerImpl的sendDefaultImpl()方法，源码位置为517行：
（源码偏长，在下方进行解读）

private SendResult sendDefaultImpl(Message msg,final CommunicationMode communicationMode,final SendCallback sendCallback,final long timeout
) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {this.makeSureStateOK();Validators.checkMessage(msg, this.defaultMQProducer);final long invokeID = random.nextLong();long beginTimestampFirst = System.currentTimeMillis();long beginTimestampPrev = beginTimestampFirst;long endTimestamp = beginTimestampFirst;// 获取Topic信息TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic());if (topicPublishInfo != null && topicPublishInfo.ok()) {boolean callTimeout = false;MessageQueue mq = null;Exception exception = null;SendResult sendResult = null;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();MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);if (mqSelected != null) {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;}// 发消息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:if (sendResult.getSendStatus() != SendStatus.SEND_OK) {if (this.defaultMQProducer.isRetryAnotherBrokerWhenNotStoreOK()) {continue;}}return sendResult;default:break;}} catch (RemotingException e) {endTimestamp = System.currentTimeMillis();this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);log.warn(msg.toString());exception = e;continue;} catch (MQClientException e) {endTimestamp = System.currentTimeMillis();this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);log.warn(msg.toString());exception = e;continue;} catch (MQBrokerException e) {endTimestamp = System.currentTimeMillis();this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);log.warn(msg.toString());exception = e;switch (e.getResponseCode()) {case ResponseCode.TOPIC_NOT_EXIST:case ResponseCode.SERVICE_NOT_AVAILABLE:case ResponseCode.SYSTEM_ERROR:case ResponseCode.NO_PERMISSION:case ResponseCode.NO_BUYER_ID:case ResponseCode.NOT_IN_CURRENT_UNIT:continue;default:if (sendResult != null) {return sendResult;}throw e;}} catch (InterruptedException e) {endTimestamp = System.currentTimeMillis();this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false);log.warn(String.format("sendKernelImpl exception, throw exception, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);log.warn(msg.toString());log.warn("sendKernelImpl exception", e);log.warn(msg.toString());throw e;}} else {break;}}if (sendResult != null) {return sendResult;}String info = String.format("Send [%d] times, still failed, cost [%d]ms, Topic: %s, BrokersSent: %s",times,System.currentTimeMillis() - beginTimestampFirst,msg.getTopic(),Arrays.toString(brokersSent));info += FAQUrl.suggestTodo(FAQUrl.SEND_MSG_FAILED);MQClientException mqClientException = new MQClientException(info, exception);if (callTimeout) {throw new RemotingTooMuchRequestException("sendDefaultImpl call timeout");}if (exception instanceof MQBrokerException) {mqClientException.setResponseCode(((MQBrokerException) exception).getResponseCode());} else if (exception instanceof RemotingConnectException) {mqClientException.setResponseCode(ClientErrorCode.CONNECT_BROKER_EXCEPTION);} else if (exception instanceof RemotingTimeoutException) {mqClientException.setResponseCode(ClientErrorCode.ACCESS_BROKER_TIMEOUT);} else if (exception instanceof MQClientException) {mqClientException.setResponseCode(ClientErrorCode.BROKER_NOT_EXIST_EXCEPTION);}throw mqClientException;}List<String> nsList = this.getmQClientFactory().getMQClientAPIImpl().getNameServerAddressList();if (null == nsList || nsList.isEmpty()) {throw new MQClientException("No name server address, please set it." + FAQUrl.suggestTodo(FAQUrl.NAME_SERVER_ADDR_NOT_EXIST_URL), null).setResponseCode(ClientErrorCode.NO_NAME_SERVER_EXCEPTION);}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);
}

首先是获取Topic信息，然后调用sendKernelImpl()方法发送消息，sendKernelImpl()源码如下：

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 brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());if (null == brokerAddr) {tryToFindTopicPublishInfo(mq.getTopic());brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());}SendMessageContext context = null;if (brokerAddr != null) {brokerAddr = MixAll.brokerVIPChannel(this.defaultMQProducer.isSendMessageWithVIPChannel(), brokerAddr);byte[] prevBody = msg.getBody();try {//for MessageBatch,ID has been set in the generating processif (!(msg instanceof MessageBatch)) {MessageClientIDSetter.setUniqID(msg);}boolean topicWithNamespace = false;if (null != this.mQClientFactory.getClientConfig().getNamespace()) {msg.setInstanceId(this.mQClientFactory.getClientConfig().getNamespace());topicWithNamespace = true;}int sysFlag = 0;boolean msgBodyCompressed = false;if (this.tryToCompressMessage(msg)) {sysFlag |= MessageSysFlag.COMPRESSED_FLAG;msgBodyCompressed = true;}final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);if (tranMsg != null && Boolean.parseBoolean(tranMsg)) {sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE;}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);}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);}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);}}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");}// 同步发送消息sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage(brokerAddr,mq.getBrokerName(),msg,requestHeader,timeout - costTimeSync,communicationMode,context,this);break;default:assert false;break;}if (this.hasSendMessageHook()) {context.setSendResult(sendResult);this.executeSendMessageHookAfter(context);}return sendResult;} 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 {msg.setBody(prevBody);msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));}}throw new MQClientException("The broker[" + mq.getBrokerName() + "] not exist", null);
}

也很长，但是逻辑简单，主要功能就是构建发送消息的头 RequestHeader 和上下文 SendMessageContext，然后调用方法 MQClientAPIImpl#sendMessage()（上述代码第150行），将消息发送给队列所在的 Broker。

至此，消息被发送给远程调用的封装类 MQClientAPIImpl，完成后续序列化和网络传输等步骤。

这一篇主要看发送代码，后续序列化和网络传输等步骤的代码后面再去探究。

ps：推荐IEDA插件SequenceDiagram，能一键生成时序图

整体的时序图为：
同步发送消息时序图.png

4. 异步消息发送过程

上一节讲到异步发送消息，只是将同步发送方法提交给线程池。对于MQProducer接口中方法为：

void send(final Message msg, final SendCallback sendCallback, final long timeout)throws MQClientException, RemotingException, InterruptedException;

也就是带有回调方法，DefaultMQProducer中的实现为：

@Override
public void send(Message msg, SendCallback sendCallback, long timeout)
throws MQClientException, RemotingException, InterruptedException {msg.setTopic(withNamespace(msg.getTopic()));this.defaultMQProducerImpl.send(msg, sendCallback, timeout);
}

调用了DefaultMQProducerImpl的send方法：

@Deprecated
public void send(final Message msg, final SendCallback sendCallback, final long timeout)
throws MQClientException, RemotingException, InterruptedException {final long beginStartTime = System.currentTimeMillis();ExecutorService executor = this.getAsyncSenderExecutor();try {executor.submit(new Runnable() {@Overridepublic void run() {long costTime = System.currentTimeMillis() - beginStartTime;if (timeout > costTime) {try {sendDefaultImpl(msg, CommunicationMode.ASYNC, sendCallback, timeout - costTime);} catch (Exception e) {sendCallback.onException(e);}} else {sendCallback.onException(new RemotingTooMuchRequestException("DEFAULT ASYNC send call timeout"));}}});} catch (RejectedExecutionException e) {throw new MQClientException("executor rejected ", e);}}

可以看到，实际上是将DefaultMQProducerImpl的sendDefaultImpl()方法（对应源码517行）提交到线程池执行，后面的流程都和同步发送一致，参看上一节分析。

异步发送消息时序图为：
异步发消息时序图.png
至此，异步消息发送也完成了。

5. 小结

MQProducer定义了19种发送消息的方法，其默认实现为DefaultMQProducer。DefaultMQProducer中的方法没有业务逻辑，最终会调用DefaultMQProducerImpl类中的具体逻辑。

DefaultMQProducerImpl中发消息的方法最终都会调用其sendKernelImpl()方法，其主要功能就是构建发送消息的头 RequestHeader 和上下文 SendMessageContext，然后调用方法 MQClientAPIImpl#sendMessage()，将消息发送给队列所在的 Broker。

最终消息被发送给远程调用的封装类 MQClientAPIImpl，完成后续序列化和网络传输等步骤。

类之间的关系为：
发消息类图.png