发送者其实比较简单，需要做的就是首先确定往哪里发送，其次怎么让消息发送顺畅。我们就看一下具体的代码吧。
首先调用start方法。完成各个类的初始化，启动多个定时任务，其中一个定时任务是updateTopicRouteInfoFromNameServer，这个方法里面和nameService建立长连接，同时维护了topicRouteTable和brokerAddrTable等缓存。topicRouteTable里面维护了这个topic包括有哪些queue和broker。这样producer才可以知道要发往哪里。
启动的流程主要在这个方法中：
MQClientInstance#start
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 server
if (null == this.clientConfig.getNamesrvAddr()) {
this.mQClientAPIImpl.fetchNameServerAddr();
}
// Start request-response channel
this.mQClientAPIImpl.start();
// Start various schedule tasks
this.startScheduledTask();
// Start pull service
this.pullMessageService.start();
// Start rebalance service
this.rebalanceService.start();
// Start push service
this.defaultMQProducer.getDefaultMQProducerImpl().start(false);
log.info("the client factory [{}] start OK", this.clientId);
this.serviceState = ServiceState.RUNNING;
break;
case START_FAILED:
throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null);
default:
break;
}
}
}其中启动了一系列定时任务，包括org.apache.rocketmq.client.impl.factory.MQClientInstance#updateTopicRouteInfoFromNameServer这个方法
public boolean updateTopicRouteInfoFromNameServer(final String topic, boolean isDefault,
DefaultMQProducer defaultMQProducer) {
try {
if (this.lockNamesrv.tryLock(LOCK_TIMEOUT_MILLIS, TimeUnit.MILLISECONDS)) {
try {
TopicRouteData topicRouteData;
if (isDefault && defaultMQProducer != null) {
// 从nameServer获取topciRouteData
topicRouteData = this.mQClientAPIImpl.getDefaultTopicRouteInfoFromNameServer(defaultMQProducer.getCreateTopicKey(),
clientConfig.getMqClientApiTimeout());
if (topicRouteData != null) {
for (QueueData data : topicRouteData.getQueueDatas()) {
int queueNums = Math.min(defaultMQProducer.getDefaultTopicQueueNums(), data.getReadQueueNums());
data.setReadQueueNums(queueNums);
data.setWriteQueueNums(queueNums);
}
}
} else {
topicRouteData = this.mQClientAPIImpl.getTopicRouteInfoFromNameServer(topic, clientConfig.getMqClientApiTimeout());
}
if (topicRouteData != null) {
TopicRouteData old = this.topicRouteTable.get(topic);
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) {
for (BrokerData bd : topicRouteData.getBrokerDatas()) {
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
{
// 生成topicPublishInfo
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 topicPublishInfo
impl.updateTopicPublishInfo(topic, publishInfo);
}
}
}
// Update sub info
if (!consumerTable.isEmpty()) {
Set<MessageQueue> subscribeInfo = topicRouteData2TopicSubscribeInfo(topic, topicRouteData);
for (Entry<String, MQConsumerInner> entry : this.consumerTable.entrySet()) {
MQConsumerInner impl = entry.getValue();
if (impl != null) {
impl.updateTopicSubscribeInfo(topic, subscribeInfo);
}
}
}
TopicRouteData cloneTopicRouteData = new TopicRouteData(topicRouteData);
log.info("topicRouteTable.put. Topic = {}, TopicRouteData[{}]", topic, cloneTopicRouteData);
this.topicRouteTable.put(topic, cloneTopicRouteData);
return true;
}
} else {
log.warn("updateTopicRouteInfoFromNameServer, getTopicRouteInfoFromNameServer return null, Topic: {}. [{}]", topic, this.clientId);
}
} catch (MQClientException e) {
if (!topic.startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX) && !topic.equals(TopicValidator.AUTO_CREATE_TOPIC_KEY_TOPIC)) {
log.warn("updateTopicRouteInfoFromNameServer Exception", e);
}
} catch (RemotingException e) {
log.error("updateTopicRouteInfoFromNameServer Exception", e);
throw new IllegalStateException(e);
} finally {
this.lockNamesrv.unlock();
}
} else {
log.warn("updateTopicRouteInfoFromNameServer tryLock timeout {}ms. [{}]", LOCK_TIMEOUT_MILLIS, this.clientId);
}
} catch (InterruptedException e) {
log.warn("updateTopicRouteInfoFromNameServer Exception", e);
}
return false;
}通过方法名也知道是从nameServer获取这个topic相关的broke数据，拿到TopicRouteData数据。先更新brokerAddrTable，存储borker具体的地址。然后在org.apache.rocketmq.client.impl.factory.MQClientInstance#topicRouteData2TopicPublishInfo里面再进一步生成TopicPublishInfo数据。TopicPublishInfo是对TopicRouteData的一个封装，除了TopicRouteData，还有messageQueue数据，messageQueue是Queue和Borker的交集，会根据配置的queue数量，生成具体的messageQueue，queueId就是0，1，2，3，4他们自己的顺序。
所以有了TopicPublishInfo数据，就知道往哪里发了。
发送消息的过程。
- 先找到TopicPublishInfo。TopicPublishInfo里面有一个MessageQueue的list。
- 从MessageQueueList里面拿到一个messageQueue。 如果没有开启sendLatencyFaultEnable，默认就是采用轮询方法。具体的轮询方式就是，TopicPublishInfo里面维护了一个序号index，每次index自增1，然后通过index去MessageQueueList里面拿一个。
- 拿到了MessageQueue之后，里面有broker的name，根据name去找broker的ip地址，发送数据。这个ip地址就是前面提到的brokerAddrTable变量，在updateTopicRouteInfoFromNameServer方法里面维护的。
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