使用Spring-RocketMQ时，只需要引入rocketmq-spring-boot-starter包，并且定义以下消费者，就可以很简单的实现消息消费

@Component
@RocketMQMessageListener(topic = "first-topic", consumerGroup = "my-producer-group", selectorExpression = "tag1")
public class RocketMQConsumer implements RocketMQListener<String>{

    @Override
    public void onMessage(String message) {
        System.out.println(message);
    }

可以看到只需要添加@RocketMQMessageListener注解，并实现RocketMQListener接口就可以完成消息的接受、处理逻辑

@RocketMQMessageListener实现原理

可以看到在ListenerContainerConfiguration中获取了所有加了RocketMQMessageListener注解的bean

 ListenerContainerConfiguration#afterSingletonsInstantiated
     
     //ListenerContainerConfiguration实现了SmartInitializingSingleton接口，会在bean都实例化完之后，触发afterSingletonsInstantiated方法
	@Override
    public void afterSingletonsInstantiated() {
     //获取所有加了RocketMQMessageListener注解的bean
        Map<String, Object> beans = this.applicationContext.getBeansWithAnnotation(RocketMQMessageListener.class)
            .entrySet().stream().filter(entry -> !ScopedProxyUtils.isScopedTarget(entry.getKey()))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
	    //循环调用registerContainer方法
        beans.forEach(this::registerContainer);
    }

    private void registerContainer(String beanName, Object bean) {
        ......
            
        //拿到RocketMQMessageListener注解
        RocketMQMessageListener annotation = clazz.getAnnotation(RocketMQMessageListener.class);
	   //获取注解上定义的consumerGroup
        String consumerGroup = this.environment.resolvePlaceholders(annotation.consumerGroup());
        //获取注解上定义的topic
        String topic = this.environment.resolvePlaceholders(annotation.topic());

        //定义beanName
        String containerBeanName = String.format("%s_%s", DefaultRocketMQListenerContainer.class.getName(),counter.incrementAndGet());
        GenericApplicationContext genericApplicationContext = (GenericApplicationContext) applicationContext;
        //注册bean  调用createRocketMQListenerContainer初始化一些属性
        genericApplicationContext.registerBean(containerBeanName, DefaultRocketMQListenerContainer.class,
            () -> createRocketMQListenerContainer(containerBeanName, bean, annotation));
        DefaultRocketMQListenerContainer container = genericApplicationContext.getBean(containerBeanName,
            DefaultRocketMQListenerContainer.class);
        if (!container.isRunning()) {
            try {
                //调用start方法
                container.start();
            } catch (Exception e) {
                log.error("Started container failed. {}", container, e);
                throw new RuntimeException(e);
            }
        }

        log.info("Register the listener to container, listenerBeanName:{}, containerBeanName:{}", beanName, containerBeanName);
    }

createRocketMQListenerContainer里面就是初始化了DefaultRocketMQListenerContainer这个对象，并且设置了一些消费相关的属性，比如nameServer、topic、tags、consumerGroup消费者组，rocketMQListener我们定义的消费监听者等

可以看到这里面并没有定义具体的消费者实例

//DefaultRocketMQListenerContainer定义  实现了InitializingBean接口，在bean初始化的时候会调用afterPropertiesSet方法
public class DefaultRocketMQListenerContainer implements InitializingBean,
    RocketMQListenerContainer, SmartLifecycle, ApplicationContextAware {
        
       
     @Override
    public void afterPropertiesSet() throws Exception {
        //通过方法名可以看到是初始化MQ消费者实例
        initRocketMQPushConsumer();

        this.messageType = getMessageType();
        this.methodParameter = getMethodParameter();
        log.debug("RocketMQ messageType: {}", messageType);
    }

    private void initRocketMQPushConsumer() throws MQClientException {
        ......
            
        if (Objects.nonNull(rpcHook)) {
            //初始化DefaultMQPushConsumer对象
            consumer = new DefaultMQPushConsumer(consumerGroup, rpcHook, new AllocateMessageQueueAveragely(),
                enableMsgTrace, this.applicationContext.getEnvironment().
                resolveRequiredPlaceholders(this.rocketMQMessageListener.customizedTraceTopic()));
            consumer.setVipChannelEnabled(false);
            consumer.setInstanceName(RocketMQUtil.getInstanceName(rpcHook, consumerGroup));
        } else {
            log.debug("Access-key or secret-key not configure in " + this + ".");
            consumer = new DefaultMQPushConsumer(consumerGroup, enableMsgTrace,
                this.applicationContext.getEnvironment().
                    resolveRequiredPlaceholders(this.rocketMQMessageListener.customizedTraceTopic()));
        }
	    //消息模式 广播还是集群
        switch (messageModel) {
            case BROADCASTING:
                consumer.setMessageModel(org.apache.rocketmq.common.protocol.heartbeat.MessageModel.BROADCASTING);
                break;
            case CLUSTERING:
                consumer.setMessageModel(org.apache.rocketmq.common.protocol.heartbeat.MessageModel.CLUSTERING);
                break;
            default:
                throw new IllegalArgumentException("Property 'messageModel' was wrong.");
        }

        //筛选方式  TAG和SQL92
        switch (selectorType) {
            case TAG:
                consumer.subscribe(topic, selectorExpression);
                break;
            case SQL92:
                consumer.subscribe(topic, MessageSelector.bySql(selectorExpression));
                break;
            default:
                throw new IllegalArgumentException("Property 'selectorType' was wrong.");
        }
	   //消费模式 顺序和并发
        switch (consumeMode) {
            case ORDERLY:
                consumer.setMessageListener(new DefaultMessageListenerOrderly());
                break;
            case CONCURRENTLY:
                consumer.setMessageListener(new DefaultMessageListenerConcurrently());
                break;
            default:
                throw new IllegalArgumentException("Property 'consumeMode' was wrong.");
        }

    }

回到上面registerContainer，最后拿到DefaultRocketMQListenerContainer的bean，调用start方法

    DefaultRocketMQListenerContainer#start
        
	@Override
    public void start() {
        if (this.isRunning()) {
            throw new IllegalStateException("container already running. " + this.toString());
        }

        try {
            //当前consumer就是上面分析的DefaultMQPushConsumer
            consumer.start();
        } catch (MQClientException e) {
            throw new IllegalStateException("Failed to start RocketMQ push consumer", e);
        }
        this.setRunning(true);

        log.info("running container: {}", this.toString());
    }

    DefaultMQPushConsumer#start
	@Override
    public void start() throws MQClientException {
        setConsumerGroup(NamespaceUtil.wrapNamespace(this.getNamespace(), this.consumerGroup));
        //可以看到调用的是defaultMQPushConsumerImpl.start()方法
        this.defaultMQPushConsumerImpl.start();
        if (null != traceDispatcher) {
            try {
                traceDispatcher.start(this.getNamesrvAddr(), this.getAccessChannel());
            } catch (MQClientException e) {
                log.warn("trace dispatcher start failed ", e);
            }
        }
    }

defaultMQPushConsumerImpl是什么时候初始化的呢

上面说到初始化DefaultMQPushConsumer对象时，点进去构造方法

    public DefaultMQPushConsumer(final String namespace, final String consumerGroup, RPCHook rpcHook,
        AllocateMessageQueueStrategy allocateMessageQueueStrategy, boolean enableMsgTrace, final String customizedTraceTopic) {
        this.consumerGroup = consumerGroup;
        this.namespace = namespace;
        this.allocateMessageQueueStrategy = allocateMessageQueueStrategy;
        //可以看到就在构造方法里面初始化的，通过名字可以猜想就是DefaultMQPushConsumer的实现类，但是并不是通过实现接口的方式，而是组合的方式
        defaultMQPushConsumerImpl = new DefaultMQPushConsumerImpl(this, rpcHook);
        if (enableMsgTrace) {
            try {
                AsyncTraceDispatcher dispatcher = new AsyncTraceDispatcher(customizedTraceTopic, rpcHook);
                dispatcher.setHostConsumer(this.getDefaultMQPushConsumerImpl());
                traceDispatcher = dispatcher;
                this.getDefaultMQPushConsumerImpl().registerConsumeMessageHook(
                    new ConsumeMessageTraceHookImpl(traceDispatcher));
            } catch (Throwable e) {
                log.error("system mqtrace hook init failed ,maybe can't send msg trace data");
            }
        }
    }

接口看DefaultMQPushConsumerImpl的start方法

    DefaultMQPushConsumerImpl#start
        
	public synchronized void start() throws MQClientException {
        switch (this.serviceState) {
            case CREATE_JUST:
			   //顺序消息 ConsumeMessageOrderlyService处理
                if (this.getMessageListenerInner() instanceof MessageListenerOrderly) {
                    this.consumeOrderly = true;
                    this.consumeMessageService =
                        new ConsumeMessageOrderlyService(this, (MessageListenerOrderly) this.getMessageListenerInner());
                //并发消息 ConsumeMessageConcurrentlyService处理
                } else if (this.getMessageListenerInner() instanceof MessageListenerConcurrently) {
                    this.consumeOrderly = false;
                    this.consumeMessageService =
                        new ConsumeMessageConcurrentlyService(this, (MessageListenerConcurrently) this.getMessageListenerInner());
                }

                this.consumeMessageService.start();
                //调用start方法
                mQClientFactory.start();
    }

     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 可以看到这里开启拉取消息
                    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;
            }
        }
    }

    ServiceThread#start
        
	public void start() {
        log.info("Try to start service thread:{} started:{} lastThread:{}", getServiceName(), started.get(), thread);
        if (!started.compareAndSet(false, true)) {
            return;
        }
        stopped = false;
        //new了一个Thread对象，this表示自己就是一个Runnable对象
        this.thread = new Thread(this, getServiceName());
        this.thread.setDaemon(isDaemon);
        //调用start方法
        this.thread.start();
    }

//ServiceThread实现了Runnable接口，并且是抽象的，找实现类
public abstract class ServiceThread implements Runnable {
}

可以看到PullMessageService和我们找的有关，找到它的run方法

    PullMessageService#run
        
	@Override
    public void run() {
        log.info(this.getServiceName() + " service started");

        //通过while循环拉取消息
        while (!this.isStopped()) {
            try {
                //消息存入LinkedBlockingQueue中，通过take方法阻塞获取
                PullRequest pullRequest = this.pullRequestQueue.take();
                //调用pullMessage处理消息
                this.pullMessage(pullRequest);
            } catch (InterruptedException ignored) {
            } catch (Exception e) {
                log.error("Pull Message Service Run Method exception", e);
            }
        }

        log.info(this.getServiceName() + " service end");
    }

    private void pullMessage(final PullRequest pullRequest) {
        //选择一个消费者实例
        final MQConsumerInner consumer = this.mQClientFactory.selectConsumer(pullRequest.getConsumerGroup());
        if (consumer != null) {
            //转换为DefaultMQPushConsumerImpl对象，应该很熟悉吧
            DefaultMQPushConsumerImpl impl = (DefaultMQPushConsumerImpl) consumer;
            //调用pullMessage方法继续处理
            impl.pullMessage(pullRequest);
        } else {
            log.warn("No matched consumer for the PullRequest {}, drop it", pullRequest);
        }
    }

    public void pullMessage(final PullRequest pullRequest) {
        ......
        
        PullCallback pullCallback = new PullCallback() {
            @Override
            public void onSuccess(PullResult pullResult) {
         
                //调用consumeMessageService的submitConsumeRequest方法
                //consumeMessageService上面提到过，包含顺序和并发消费
                DefaultMQPushConsumerImpl.this.consumeMessageService.submitConsumeRequest(
                                    pullResult.getMsgFoundList(),
                                    processQueue,
                                    pullRequest.getMessageQueue(),
                                    dispatchToConsume);
             ......
            }

            @Override
            public void onException(Throwable e) {
                if (!pullRequest.getMessageQueue().getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                    log.warn("execute the pull request exception", e);
                }

                DefaultMQPushConsumerImpl.this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
            }
        };
    }

   ConsumeMessageOrderlyService#submitConsumeRequest
       
	@Override
    public void submitConsumeRequest(
        final List<MessageExt> msgs,
        final ProcessQueue processQueue,
        final MessageQueue messageQueue,
        final boolean dispathToConsume) {
        if (dispathToConsume) {
            //ConsumeRequest是一个Runnable
            ConsumeRequest consumeRequest = new ConsumeRequest(processQueue, messageQueue);
            //提交到线程池中处理
            this.consumeExecutor.submit(consumeRequest);
        }
    }

来看ConsumeRequest的run方法

       ConsumeMessageOrderlyService.ConsumeRequest#run
           
	   @Override
        public void run() {
         ......
         //核心在这里消费消息       
		status = messageListener.consumeMessage(Collections.unmodifiableList(msgs), context);
        }

DefaultRocketMQListenerContainer.DefaultMessageListenerOrderly#consumeMessage        

        @Override
        public ConsumeOrderlyStatus consumeMessage(List<MessageExt> msgs, ConsumeOrderlyContext context) {
            for (MessageExt messageExt : msgs) {
                log.debug("received msg: {}", messageExt);
                try {
                    long now = System.currentTimeMillis();
                    //处理消息
                    handleMessage(messageExt);
                    long costTime = System.currentTimeMillis() - now;
                    log.info("consume {} cost: {} ms", messageExt.getMsgId(), costTime);
                } catch (Exception e) {
                    log.warn("consume message failed. messageExt:{}", messageExt, e);
                    context.setSuspendCurrentQueueTimeMillis(suspendCurrentQueueTimeMillis);
                    return ConsumeOrderlyStatus.SUSPEND_CURRENT_QUEUE_A_MOMENT;
                }
            }

            return ConsumeOrderlyStatus.SUCCESS;
        }

    DefaultRocketMQListenerContainer#handleMessage
        
	private void handleMessage(
        MessageExt messageExt) throws MQClientException, RemotingException, InterruptedException {
        if (rocketMQListener != null) {
            //可以看到最终调用到onMessage方法，也就是开头我们实现的接口中的onMessage方法
            rocketMQListener.onMessage(doConvertMessage(messageExt));
        } else if (rocketMQReplyListener != null) {
            Object replyContent = rocketMQReplyListener.onMessage(doConvertMessage(messageExt));
            Message<?> message = MessageBuilder.withPayload(replyContent).build();

            org.apache.rocketmq.common.message.Message replyMessage = MessageUtil.createReplyMessage(messageExt, convertToBytes(message));
            consumer.getDefaultMQPushConsumerImpl().getmQClientFactory().getDefaultMQProducer().send(replyMessage, new SendCallback() {
                @Override public void onSuccess(SendResult sendResult) {
                    if (sendResult.getSendStatus() != SendStatus.SEND_OK) {
                        log.error("Consumer replies message failed. SendStatus: {}", sendResult.getSendStatus());
                    } else {
                        log.info("Consumer replies message success.");
                    }
                }

                @Override public void onException(Throwable e) {
                    log.error("Consumer replies message failed. error: {}", e.getLocalizedMessage());
                }
            });
        }
    }

至此整个流程也就通了

总结

@RocketMQMessageListener相当于定义一个消费者，topic、consumerGroup、selectorExpression、consumeMode、messageModel定义了消费者的一些属性

实现RocketMQListener接口来处理具体消费逻辑

每个消费者初始化了一个DefaultRocketMQListenerContainer对象，该对象中包含消费实例和消费者的属性

服务启动的时候开启一个线程轮训队列中的消息，如果没有就一直阻塞，拿到消息后，最终会调用自己实现的onMessage方法

推荐阅读

万字长文深度剖析 RocketMQ 设计原理
浅谈如何解决RocketMQ消息堆积的问题
RocketMQ的顺序消息（顺序消费）