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：
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：
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：
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