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Consumer源码分析

消费者消费消息有两种模式：推模式和拉模式。

所谓拉模式，就是由consumer主动从broker拉取消息进行消费。

推模式，就是由broker将一批消息推送给consumer消费。

rocketmq中这两种模式都可以使用，我们通常使用较多的是push模式，因为处理起来比较简单。如果使用pull模式需要我们再consume代码中自己处理消息的拉取。

但实际上，严格来说，RocketMq并没有实现push模式。RocketMQ消息推模式的实现基于拉模式，在拉模式上包装一层，在消费端开启一个线程 PullMessageService 循环向 Broke r拉取消息，一次拉取任务结束后马上又发起另一次拉取操作，实现准实时自动拉取。给我们的感觉好像是broker主动推过来的。

从start方法开始

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@Override
public void start() throws MQClientException {
	// 配置消费者组名
    setConsumerGroup(NamespaceUtil.wrapNamespace(this.getNamespace(), this.consumerGroup));
    // start方法。入口
    this.defaultMQPushConsumerImpl.start();
    if (null != traceDispatcher) {
        try {
        	// 消息轨迹追踪
            traceDispatcher.start(this.getNamesrvAddr(), this.getAccessChannel());
        } catch (MQClientException e) {
            log.warn("trace dispatcher start failed ", e);
        }
    }
}

我们现在重点看defaultMQPushConsumerImpl.start()

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public synchronized void start() throws MQClientException {
    switch (this.serviceState) {
        case CREATE_JUST:
            log.info("the consumer [{}] start beginning. messageModel={}, isUnitMode={}", this.defaultMQPushConsumer.getConsumerGroup(),
                this.defaultMQPushConsumer.getMessageModel(), this.defaultMQPushConsumer.isUnitMode());
            this.serviceState = ServiceState.START_FAILED;
			// 检查消费者配置
            this.checkConfig();
			// 
            this.copySubscription();

            if (this.defaultMQPushConsumer.getMessageModel() == MessageModel.CLUSTERING) {
                this.defaultMQPushConsumer.changeInstanceNameToPID();
            }

            this.mQClientFactory = MQClientManager.getInstance().getOrCreateMQClientInstance(this.defaultMQPushConsumer, this.rpcHook);

            this.rebalanceImpl.setConsumerGroup(this.defaultMQPushConsumer.getConsumerGroup());
            this.rebalanceImpl.setMessageModel(this.defaultMQPushConsumer.getMessageModel());
            this.rebalanceImpl.setAllocateMessageQueueStrategy(this.defaultMQPushConsumer.getAllocateMessageQueueStrategy());
            this.rebalanceImpl.setmQClientFactory(this.mQClientFactory);

            this.pullAPIWrapper = new PullAPIWrapper(
                mQClientFactory,
                this.defaultMQPushConsumer.getConsumerGroup(), isUnitMode());
            this.pullAPIWrapper.registerFilterMessageHook(filterMessageHookList);

            if (this.defaultMQPushConsumer.getOffsetStore() != null) {
                this.offsetStore = this.defaultMQPushConsumer.getOffsetStore();
            } else {
                switch (this.defaultMQPushConsumer.getMessageModel()) {
                    case BROADCASTING:
                        this.offsetStore = new LocalFileOffsetStore(this.mQClientFactory, this.defaultMQPushConsumer.getConsumerGroup());
                        break;
                    case CLUSTERING:
                        this.offsetStore = new RemoteBrokerOffsetStore(this.mQClientFactory, this.defaultMQPushConsumer.getConsumerGroup());
                        break;
                    default:
                        break;
                }
                this.defaultMQPushConsumer.setOffsetStore(this.offsetStore);
            }
            this.offsetStore.load();

            if (this.getMessageListenerInner() instanceof MessageListenerOrderly) {
                this.consumeOrderly = true;
                this.consumeMessageService =
                    new ConsumeMessageOrderlyService(this, (MessageListenerOrderly) this.getMessageListenerInner());
            } else if (this.getMessageListenerInner() instanceof MessageListenerConcurrently) {
                this.consumeOrderly = false;
                this.consumeMessageService =
                    new ConsumeMessageConcurrentlyService(this, (MessageListenerConcurrently) this.getMessageListenerInner());
            }

            this.consumeMessageService.start();

            boolean registerOK = mQClientFactory.registerConsumer(this.defaultMQPushConsumer.getConsumerGroup(), this);
            if (!registerOK) {
                this.serviceState = ServiceState.CREATE_JUST;
                this.consumeMessageService.shutdown(defaultMQPushConsumer.getAwaitTerminationMillisWhenShutdown());
                throw new MQClientException("The consumer group[" + this.defaultMQPushConsumer.getConsumerGroup()
                    + "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL),
                    null);
            }

            mQClientFactory.start();
            log.info("the consumer [{}] start OK.", this.defaultMQPushConsumer.getConsumerGroup());
            this.serviceState = ServiceState.RUNNING;
            break;
        case RUNNING:
        case START_FAILED:
        case SHUTDOWN_ALREADY:
            throw new MQClientException("The PushConsumer service state not OK, maybe started once, "
                + this.serviceState
                + FAQUrl.suggestTodo(FAQUrl.CLIENT_SERVICE_NOT_OK),
                null);
        default:
            break;
    }

    this.updateTopicSubscribeInfoWhenSubscriptionChanged();
    this.mQClientFactory.checkClientInBroker();
    this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
    this.mQClientFactory.rebalanceImmediately();
}