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Message
message = 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Producer
producer.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Producer
producer.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mQClientFactory
mQClientFactory.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 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 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 process
if (!(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/66
tmpMessage = 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() {
@Override
public 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：
