【RocketMQ】同一个项目中，同一个topic，可以存在多个消费者么？

一、问题答案

　　是不可以的

而且后注册的会替换前注册的，MqConsumer2会替换MqConsumer，并且只结束tag-2的消息

/**
 * @date 2019/05/28
 */
@Component
@Slf4j
public class MqConsumer implements MessageConsumer {


    @Override
    @Transactional(rollbackFor = Throwable.class, propagation = Propagation.REQUIRED)
    public void onMessage(String msg) {
        log.info("接收到的库存MQ消息：{}", msg);
        log.info("接收到的库存MQ消息：{}", msg);
        log.info("接收到的库存MQ消息：{}", msg);
    }

    @Override
    public String getTopic() {
        return "topic-1";
    }

    @Override
    public String getTag() {
        return "tag-1";
    }
}

@Component
@Slf4j
public class MqConsumer2 implements MessageConsumer {


    @Override
    @Transactional(rollbackFor = Throwable.class, propagation = Propagation.REQUIRED)
    public void onMessage(String msg) {
        log.info("接收到的库存MQ消息：{}", msg);
        log.info("接收到的库存MQ消息：{}", msg);
        log.info("接收到的库存MQ消息：{}", msg);
    }

    @Override
    public String getTopic() {
        return "topic-1";
    }

    @Override
    public String getTag() {
        return "tag-2";
    }
}

二、为什么呢？

我们从源码的角度来分析下

1.订阅消息的方法 public void subscribe(String topic, String subExpression, MessageListener listener) ，其中subExpression即为tag

package com.aliyun.openservices.ons.api.impl.rocketmq;
....
@Generated("ons-client")
public class ConsumerImpl extends ONSConsumerAbstract implements Consumer {
    private final ConcurrentHashMap<String, MessageListener> subscribeTable = new ConcurrentHashMap<String, MessageListener>();

    public ConsumerImpl(final Properties properties) {
        super(properties);
        boolean postSubscriptionWhenPull = Boolean.parseBoolean(properties.getProperty(PropertyKeyConst.PostSubscriptionWhenPull, "false"));
        this.defaultMQPushConsumer.setPostSubscriptionWhenPull(postSubscriptionWhenPull);

        String messageModel = properties.getProperty(PropertyKeyConst.MessageModel, PropertyValueConst.CLUSTERING);
        this.defaultMQPushConsumer.setMessageModel(MessageModel.valueOf(messageModel));
    }

    @Override
    public void start() {
        this.defaultMQPushConsumer.registerMessageListener(new MessageListenerImpl());
        super.start();
    }


    @Override
    public void subscribe(String topic, String subExpression, MessageListener listener) {
        if (null == topic) {
            throw new ONSClientException("topic is null");
        }

        if (null == listener) {
            throw new ONSClientException("listener is null");
        }
        this.subscribeTable.put(topic, listener);
        super.subscribe(topic, subExpression);
    }

.....
}

从上面的类中我们可以从this.subscribeTable.put(topic, listener);看到subscribeTable这样的一个Map，该Map与tag无关

2.我们再看super.subscribe(topic, subExpression)方法，属于ONSConsumerAbstract类中

protected void subscribe(String topic, String subExpression) {
        try {
            this.defaultMQPushConsumer.subscribe(topic, subExpression);
        } catch (MQClientException e) {
            throw new ONSClientException("defaultMQPushConsumer subscribe exception", e);
        }
    }

DefaultMQPushConsumer中：

@Override
    public void subscribe(String topic, String subExpression) throws MQClientException {
        this.defaultMQPushConsumerImpl.subscribe(withNamespace(topic), subExpression);
    }

DefaultMQPushConsumerImpl中：

public void subscribe(String topic, String subExpression) throws MQClientException {
        try {
　　　　　　//此处用来构建订阅数据，并且指定了tag
            SubscriptionData subscriptionData = FilterAPI.buildSubscriptionData(this.defaultMQPushConsumer.getConsumerGroup(),
                topic, subExpression);
　　　　　　//此处将topic和该topic的订阅数据存放到subscriptionInner这个Map中
　　　//　protected final ConcurrentMap<String /* topic */, SubscriptionData> subscriptionInner = new ConcurrentHashMap<String, SubscriptionData>();
            this.rebalanceImpl.getSubscriptionInner().put(topic, subscriptionData);
            if (this.mQClientFactory != null) {
                this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
            }
        } catch (Exception e) {
            throw new MQClientException("subscription exception", e);
        }
    }

三、总结

从上面简单的源码可以看到，有用到两个Map，

subscribeTable 和 subscriptionInner ，并且Map的key都为topic。所以我们可以笃定，RocketMQ在同一个项目中，只支持注册一个topic消费者，那么也就只能指定一个tag