RocketMQ源码15- consumer启动流程

本系列RocketMQ4.8注释github地址，希望对大家有所帮助，要是觉得可以的话麻烦给点一下Star哈

org.apache.rocketmq.example.quickstart.Consumer:

public class Consumer {

    public static void main(String[] args) throws InterruptedException, MQClientException {

        // todo
        DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("please_rename_unique_group_name_4");

        consumer.setNamesrvAddr("127.0.0.1:9876");
       
        consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET);

        consumer.subscribe("TopicTest", "*");

        // 注册监听器，监听消息
        consumer.registerMessageListener(new MessageListenerConcurrently() {

            @Override
            public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> msgs,
                ConsumeConcurrentlyContext context) {
                // 这里获得了消息
                System.out.printf("%s Receive New Messages: %s %n", Thread.currentThread().getName(), msgs);
                return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
            }
        });
        // todo 启动
        consumer.start();

        System.out.printf("Consumer Started.%n");
    }
}
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consumer使用起来还是挺简单的，先是创建了一个DefaultMQPushConsumer对象，然后配置了一些属性，比较关键的就是注册消息监听器（在这个监听器里会获取消息），之后就调用start()方法启动consumer.

接下来我们就来分析这块的消费过程。

1. 构造方法：DefaultMQPushConsumer

consumer的处理类为DefaultMQPushConsumer，我们先来看看DefaultMQPushConsumer的属性值和构造方法：

public class DefaultMQPushConsumer extends ClientConfig implements MQPushConsumer {

    private final InternalLogger log = ClientLogger.getLog();

    protected final transient DefaultMQPushConsumerImpl defaultMQPushConsumerImpl;

    // 消费者所属组
    private String consumerGroup;
    // 消息消费模式，分为集群模式、广播模式，默认为集群模式
    private MessageModel messageModel = MessageModel.CLUSTERING;
    // 第一次消费时指定消费策略
    private ConsumeFromWhere consumeFromWhere = ConsumeFromWhere.CONSUME_FROM_LAST_OFFSET;

    private String consumeTimestamp = UtilAll.timeMillisToHumanString3(System.currentTimeMillis() - (1000 * 60 * 30));

    // 集群模式下消息队列的负载策略
    private AllocateMessageQueueStrategy allocateMessageQueueStrategy;

    /**
     * Subscription relationship 订阅信息
     */
    private Map<String /* topic */, String /* sub expression */> subscription = new HashMap<String, String>();

    /**
     * Message listener 消息业务监听器
     */
    private MessageListener messageListener;

    /**
     * Offset Storage 消息消费进度存储器
     */
    private OffsetStore offsetStore;

    /**
     * Minimum consumer thread number 消费者最小线程数
     */
    private int consumeThreadMin = 20;

    /**
     * Max consumer thread number  消费者最大线程数，因为消费
     * 者线程池使用无界队列，所以此参数不生效
     */
    private int consumeThreadMax = 20;

    /**
     * Threshold for dynamic adjustment of the number of thread pool
     */
    private long adjustThreadPoolNumsThreshold = 100000;

    /**
     * Concurrently max span offset.it has no effect on sequential consumption
     * 并发消息消费时处理队列最大跨度，默认2000，表示如果消息处理队列中偏移量最大的消息
     * 与偏移量最小的消息的跨度超过2000，则延迟50ms后再拉取消息
     */
    private int consumeConcurrentlyMaxSpan = 2000;

    /**
     * Flow control threshold on queue level, each message queue will cache at most 1000 messages by default,
     * Consider the {@code pullBatchSize}, the instantaneous value may exceed the limit
     * 默认1000，表示每1000
     * 次流控后打印流控日志
     */
    private int pullThresholdForQueue = 1000;
    private int pullThresholdSizeForTopic = -1;

    /**
     * Message pull Interval
     * 推模式下拉取任务的间隔时间，默
     * 认一次拉取任务完成后继续拉取
     */
    private long pullInterval = 0;

    /**
     * Batch consumption size 消息并发消费时一次
     * 消费消息的条数，通俗点说，就是每次传入
     * MessageListener#consumeMessage中的消息条数
     */
    private int consumeMessageBatchMaxSize = 1;

    /**
     * Batch pull size 每次消息拉取的条数，默认32条
     */
    private int pullBatchSize = 32;

    /**
     * Whether update subscription relationship when every pull
     * 是否每次拉取消息都更
     * 新订阅信息，默认为false
     */
    private boolean postSubscriptionWhenPull = false;

    private boolean unitMode = false;
    /**
     *
     * 最大消费重试次数。如果消息消费次数超过maxReconsumeTimes还未成功，则将该消息转移到一个失败
     * 队列，等待被删除
     */
    private int maxReconsumeTimes = -1;

    /**
     * 延迟将该队列的消
     * 息提交到消费者线程的等待时间，默认延迟1s
     */
    private long suspendCurrentQueueTimeMillis = 1000;

    /**
     * Maximum amount of time in minutes a message may block the consuming thread.
     * 消息消费超时时间，默认为15，
     * 单位为分钟
     */
    private long consumeTimeout = 15;
    ...
    
    public DefaultMQPushConsumer(final String consumerGroup) {
        // 指定了队列分配策略 AllocateMessageQueueAveragely
        this(null, consumerGroup, null,