一，Consumer实例初始化

DefaultMQPushConsumer#start()

DefaultMQPushConsumerImpl#start()

 二，拉取消息

RocketMQ中推送与拉取消息底层皆为拉取

PullMessageService#run()

DefaultMQPushConsumerImpl#pullMessage()

 三，消费消息组装

PullMessageProcessor#processRequest()

DefaultMessageStore#getMessage()

PullMessageProcessor#processRequest()