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先看有哪些send方法

首先说红圈的

有3个红圈。归类成3种发送方式。假设前提条件，发送的topic，有3个broker，每个broker总共4个write队列，总共有12个队列。

• 普通发送。负载均衡12个队列。指定超时时间
• 指定MessageQueue,发送，指定超时时间
• 指定selector器，指定特定参数，指定超时时间。一般用于局部有序，比如相同userId的，到同一个队列

默认超时时间时3秒

再说蓝圈

• sendDefaultImpl 负载均衡的方式，选择队列。然后调sendKernelImpl
• sendSelectImpl 指定队列selector和arg的方式，选择队列。然后调sendKernelImpl
• sendKernelImpl 最核心的方式。这里已经明确队列，做真实的消息发送

很明显，只需要简单解读sendDefaultImpl和sendSelectImpl如何选择队列。然后重点在于查看sendKernelImpl方法实现

sendDefaultImpl选择队列分析

先看源码

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;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;}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;}

org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendDefaultImpl

第一步，通过topic查找路由信息tryToFindTopicPublishInfo
先从内存中获取。内存是DefaultMQProducerImpl#topicPublishInfoTable
如果内存没有，则从nameserver获取
org.apache.rocketmq.client.impl.factory.MQClientInstance#updateTopicRouteInfoFromNameServer(java.lang.String)

内存是什么时候添加的呢？是有定时器任务更新的。详情看我写的文章rocketmq-push模式-消费侧重平衡-类流程图分析

第二步、设定默认重试3次（包含首次）,选择topic的其中一个队列
org.apache.rocketmq.client.latency.MQFaultStrategy#selectOneMessageQueue

public MessageQueue selectOneMessageQueue(final String lastBrokerName) {if (lastBrokerName == null) {return selectOneMessageQueue();} else {for (int i = 0; i < this.messageQueueList.size(); i++) {int index = this.sendWhichQueue.incrementAndGet();int pos = Math.abs(index) % this.messageQueueList.size();if (pos < 0)pos = 0;MessageQueue mq = this.messageQueueList.get(pos);if (!mq.getBrokerName().equals(lastBrokerName)) {return mq;}}return selectOneMessageQueue();}
}

可以发现，topic对应的TopicPublishInfo，维护者一个ThreadLocalIndex对象。
每个线程先会获取一个index，然后对index取模，得到某一个队列。
这意味着，sendDefaultImpl中，队列的负载均衡是线程独立的。每个线程维护着自己的index，每发送一次，index+1。

public int incrementAndGet() {Integer index = this.threadLocalIndex.get();if (null == index) {index = Math.abs(random.nextInt());this.threadLocalIndex.set(index);}this.threadLocalIndex.set(++index);return Math.abs(index & POSITIVE_MASK);}

第三步、选择完MessageQueue后，调用sendKernelImpl发送消息

sendSelectImpl选择队列分析

先看源码

private SendResult sendSelectImpl(Message msg,MessageQueueSelector selector,Object arg,final CommunicationMode communicationMode,final SendCallback sendCallback, final long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {long beginStartTime = System.currentTimeMillis();this.makeSureStateOK();Validators.checkMessage(msg, this.defaultMQProducer);TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic());if (topicPublishInfo != null && topicPublishInfo.ok()) {MessageQueue mq = null;try {List<MessageQueue> messageQueueList =mQClientFactory.getMQAdminImpl().parsePublishMessageQueues(topicPublishInfo.getMessageQueueList());Message userMessage = MessageAccessor.cloneMessage(msg);String userTopic = NamespaceUtil.withoutNamespace(userMessage.getTopic(), mQClientFactory.getClientConfig().getNamespace());userMessage.setTopic(userTopic);mq = mQClientFactory.getClientConfig().queueWithNamespace(selector.select(messageQueueList, userMessage, arg));} catch (Throwable e) {throw new MQClientException("select message queue threw exception.", e);}long costTime = System.currentTimeMillis() - beginStartTime;if (timeout < costTime) {throw new RemotingTooMuchRequestException("sendSelectImpl call timeout");}if (mq != null) {return this.sendKernelImpl(msg, mq, communicationMode, sendCallback, null, timeout - costTime);} else {throw new MQClientException("select message queue return null.", null);}}validateNameServerSetting();throw new MQClientException("No route info for this topic, " + msg.getTopic(), null);}

org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendSelectImpl

第一步，通过topic查找路由信息tryToFindTopicPublishInfo。分析同上
第二步，通过MessageQueueSelector，找出发送的MessageQueue
MessageQueueSelector的实现方式，可以自定义。提供了2种
SelectMessageQueueByRandom 随机一个
SelectMessageQueueByHash 根据arg的hashcode取模一个。适合局部有序

public class SelectMessageQueueByHash implements MessageQueueSelector {@Overridepublic MessageQueue select(List<MessageQueue> mqs, Message msg, Object arg) {int value = arg.hashCode() % mqs.size();if (value < 0) {value = Math.abs(value);}return mqs.get(value);}
}

第三步、选择完MessageQueue后，调用sendKernelImpl发送消息

sendKernelImpl发送分析

org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendKernelImpl
第一步、通过MessageQueue，获取对应的master节点地址
第二步、设置消息的唯一id。详情看以下实现。明显是客户端生成的，（由于不是分布式唯一ID的创建方式，有点怀疑会重复。后续查看）
org.apache.rocketmq.common.message.MessageClientIDSetter#createUniqID
第三步、对消息body做消息压缩
第四步、判断该消息是否是事务消息。给sysFlag位标志变量加标志
第五步、发送前可做一些自定义的检查CheckForbiddenHook、SendMessageHook
第六步、构建SendMessageRequestHeader requestHeader，将msg的一些内容设置到header上
第七部、根据发送模式communicationMode，调用不同的sendMessage方法
org.apache.rocketmq.client.impl.MQClientAPIImpl#sendMessage

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;
}

第八步、最终会调用NettyRemotingClient的发送方法
SYNC：
org.apache.rocketmq.remoting.netty.NettyRemotingClient#invokeSync
ONEWAY:
org.apache.rocketmq.remoting.netty.NettyRemotingClient#invokeOneway
ASYNC:
org.apache.rocketmq.remoting.netty.NettyRemotingClient#invokeAsync

总结

product的发送有几种API模式，其实目的都是为了选择MessageQueue

• 默认的发送，是根据topic的队列，做负载均衡的方式，topicPublishInfo内部维护着ThreadLocalIndex对象，做线程级别的负载均衡。而且默认都3次重试机会，意味可以选择不同队列做发送；
• 指定messageQueue，是调用方明确知道发送的MessageQueue，这种失败不会做重试；
• 指定MessageQueueSelector等，这种是通过传入的参数，计算出对应的MessageQueue，这种失败不会做重试，适合作为局部有序的发送方式

选择好队列后，就会调用org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendKernelImpl方法，主要是构建SendMessageRequestHeader，执行自定义的发送before和after的处理。
sendKernelImpl最终会调用NettyRemotingClient提供的接口，分别处理SYNC、ONEWAY、ASYNC的三种模式