SpringBoot 处理 @KafkaListener 消息

消息监听容器

1、KafkaMessageListenerContainer

由spring提供用于监听以及拉取消息，并将这些消息按指定格式转换后交给由@KafkaListener注解的方法处理，相当于一个消费者；

看看其整体代码结构：

可以发现其入口方法为doStart(), 往上追溯到实现了SmartLifecycle接口，很明显，由spring管理其start和stop操作；

ListenerConsumer, 内部真正拉取消息消费的是这个结构，其实现了Runable接口，简言之，它就是一个后台线程轮训拉取并处理消息（while true死循环拉取消息）。

在doStart方法中会创建ListenerConsumer并交给线程池处理

以上步骤就开启了消息监听过程。

KafkaMessageListenerContainer#doStart

protected void doStart() {
		if (isRunning()) {
			return;
		}
		ContainerProperties containerProperties = getContainerProperties();
		if (!this.consumerFactory.isAutoCommit()) {
			AckMode ackMode = containerProperties.getAckMode();
			if (ackMode.equals(AckMode.COUNT) || ackMode.equals(AckMode.COUNT_TIME)) {
				Assert.state(containerProperties.getAckCount() > 0, "'ackCount' must be > 0");
			}
			if ((ackMode.equals(AckMode.TIME) || ackMode.equals(AckMode.COUNT_TIME))
					&& containerProperties.getAckTime() == 0) {
				containerProperties.setAckTime(5000);
			}
		}

		Object messageListener = containerProperties.getMessageListener();
		Assert.state(messageListener != null, "A MessageListener is required");
		if (containerProperties.getConsumerTaskExecutor() == null) {
			SimpleAsyncTaskExecutor consumerExecutor = new SimpleAsyncTaskExecutor(
					(getBeanName() == null ? "" : getBeanName()) + "-C-");
			containerProperties.setConsumerTaskExecutor(consumerExecutor);
		}
		Assert.state(messageListener instanceof GenericMessageListener, "Listener must be a GenericListener");
		this.listener = (GenericMessageListener<?>) messageListener;
		ListenerType listenerType = ListenerUtils.determineListenerType(this.listener);
		if (this.listener instanceof DelegatingMessageListener) {
			Object delegating = this.listener;
			while (delegating instanceof DelegatingMessageListener) {
				delegating = ((DelegatingMessageListener<?>) delegating).getDelegate();
			}
			listenerType = ListenerUtils.determineListenerType(delegating);
		}
        // 这里创建了监听消费者对象
		this.listenerConsumer = new ListenerConsumer(this.listener, listenerType);
		setRunning(true);
        // 将消费者对象放入到线程池中执行
		this.listenerConsumerFuture = containerProperties
				.getConsumerTaskExecutor()
				.submitListenable(this.listenerConsumer);
	}

KafkaMessageListenerContainer.ListenerConsumer#run

public void run() {
			this.consumerThread = Thread.currentThread();
			if (this.genericListener instanceof ConsumerSeekAware) {
				((ConsumerSeekAware) this.genericListener).registerSeekCallback(this);
			}
			if (this.transactionManager != null) {
				ProducerFactoryUtils.setConsumerGroupId(this.consumerGroupId);
			}
			this.count = 0;
			this.last = System.currentTimeMillis();
			if (isRunning() && this.definedPartitions != null) {
				try {
					initPartitionsIfNeeded();
				}
				catch (Exception e) {
					this.logger.error("Failed to set initial offsets", e);
				}
			}
			long lastReceive = System.currentTimeMillis();
			long lastAlertAt = lastReceive;
			while (isRunning()) {
				try {
					if (!this.autoCommit && !this.isRecordAck) {
						processCommits();
					}
					processSeeks();
					if (!this.consumerPaused && isPaused()) {
						this.consumer.pause(this.consumer.assignment());
						this.consumerPaused = true;
						if (this.logger.isDebugEnabled()) {
							this.logger.debug("Paused consumption from: " + this.consumer.paused());
						}
						publishConsumerPausedEvent(this.consumer.assignment());
					}
                    // 拉取信息
					ConsumerRecords<K, V> records = this.consumer.poll(this.containerProperties.getPollTimeout());
					this.lastPoll = System.currentTimeMillis();
					if (this.consumerPaused && !isPaused()) {
						if (this.logger.isDebugEnabled()) {
							this.logger.debug("Resuming consumption from: " + this.consumer.paused());
						}
						Set<TopicPartition> paused = this.consumer.paused();
						this.consumer.resume(paused);
						this.consumerPaused = false;
						publishConsumerResumedEvent(paused);
					}
					if (records != null && this.logger.isDebugEnabled()) {
						this.logger.debug("Received: " + records.count() + " records");
						if (records.count() > 0 && this.logger.isTraceEnabled()) {
							this.logger.trace(records.partitions().stream()
								.flatMap(p -> records.records(p).stream())
								// map to same format as send metadata toString()
								.map(r -> r.topic() + "-" + r.partition() + "@" + r.offset())
								.collect(Collectors.toList()));
						}
					}
					if (records != null && records.count() > 0) {
						if (this.containerProperties.getIdleEventInterval() != null) {
							lastReceive = System.currentTimeMillis();
						}
						invokeListener(records);
					}
					else {
						if (this.containerProperties.getIdleEventInterval() != null) {
							long now = System.currentTimeMillis();
							if (now > lastReceive + this.containerProperties.getIdleEventInterval()
									&& now > lastAlertAt + this.containerProperties.getIdleEventInterval()) {
								publishIdleContainerEvent(now - lastReceive, this.isConsumerAwareListener
										? this.consumer : null, this.consumerPaused);
								lastAlertAt = now;
								if (this.genericListener instanceof ConsumerSeekAware) {
									seekPartitions(getAssignedPartitions(), true);
								}
							}
						}
					}
				}
				catch (WakeupException e) {
					// Ignore, we're stopping
				}
				catch (NoOffsetForPartitionException nofpe) {
					this.fatalError = true;
					ListenerConsumer.this.logger.error("No offset and no reset policy", nofpe);
					break;
				}
				catch (Exception e) {
					handleConsumerException(e);
				}
			}
			ProducerFactoryUtils.clearConsumerGroupId();
			if (!this.fatalError) {
				if (this.kafkaTxManager == null) {
					commitPendingAcks();
					try {
						this.consumer.unsubscribe();
					}
					catch (WakeupException e) {
						// No-op. Continue process
					}
				}
			}
			else {
				ListenerConsumer.this.logger.error("No offset and no reset policy; stopping container");
				KafkaMessageListenerContainer.this.stop();
			}
			this.monitorTask.cancel(true);
			if (!this.taskSchedulerExplicitlySet) {
				((ThreadPoolTaskScheduler) this.taskScheduler).destroy();
			}
			this.consumer.close();
			this.logger.info("Consumer stopped");
		}

2、ConcurrentMessageListenerContainer

并发消息监听，相当于创建消费者；其底层逻辑仍然是通过KafkaMessageListenerContainer实现处理；从实现上看就是在KafkaMessageListenerContainer上做了层包装，有多少的concurrency就创建多个KafkaMessageListenerContainer，也就是concurrency个消费者。

	protected void doStart() {
		if (!isRunning()) {
			ContainerProperties containerProperties = getContainerProperties();
			TopicPartitionInitialOffset[] topicPartitions = containerProperties.getTopicPartitions();
			if (topicPartitions != null
					&& this.concurrency > topicPartitions.length) {
				this.logger.warn("When specific partitions are provided, the concurrency must be less than or "
						+ "equal to the number of partitions; reduced from " + this.concurrency + " to "
						+ topicPartitions.length);
				this.concurrency = topicPartitions.length;
			}
			setRunning(true);

            // 创建多个消费者
			for (int i = 0; i < this.concurrency; i++) {
				KafkaMessageListenerContainer<K, V> container;
				if (topicPartitions == null) {
					container = new KafkaMessageListenerContainer<>(this, this.consumerFactory,
							containerProperties);
				}
				else {
					container = new KafkaMessageListenerContainer<>(this, this.consumerFactory,
							containerProperties, partitionSubset(containerProperties, i));
				}
				String beanName = getBeanName();
				container.setBeanName((beanName != null ? beanName : "consumer") + "-" + i);
				if (getApplicationEventPublisher() != null) {
					container.setApplicationEventPublisher(getApplicationEventPublisher());
				}
				container.setClientIdSuffix("-" + i);
				container.setAfterRollbackProcessor(getAfterRollbackProcessor());
				container.start();
				this.containers.add(container);
			}
		}
	}

3、@KafkaListener底层监听原理

上面已经介绍了KafkaMessageListenerContainer的作用是拉取并处理消息，但还缺少关键的一步，即如何将我们的业务逻辑与KafkaMessageListenerContainer的处理逻辑联系起来？

那么这个桥梁就是@KafkaListener注解

KafkaListenerAnnotationBeanPostProcessor, 从后缀BeanPostProcessor就可以知道这是Spring IOC初始化bean相关的操作，当然这里也是；此类会扫描带@KafkaListener注解的类或者方法，通过 KafkaListenerContainerFactory工厂创建对应的KafkaMessageListenerContainer，并调用start方法启动监听，也就是这样打通了这条路…

4、Spring Boot 自动加载kafka相关配置

1、KafkaAutoConfiguration
自动生成kafka相关配置，比如当缺少这些bean的时候KafkaTemplate、ProducerListener、ConsumerFactory、ProducerFactory等，默认创建bean实例

2、KafkaAnnotationDrivenConfiguration
主要是针对于spring-kafka提供的注解背后的相关操作，比如 @KafkaListener;

在开启了@EnableKafka注解后，spring会扫描到此配置并创建缺少的bean实例，比如当配置的工厂beanName不是kafkaListenerContainerFactory的时候，就会默认创建一个beanName为kafkaListenerContainerFactory的实例，这也是为什么在springboot中不用定义consumer的相关配置也可以通过@KafkaListener正常的处理消息

5、消息处理

1、单条消息处理

@Configuration
public class KafkaConsumerConfiguration {

  

    @Bean
    KafkaListenerContainerFactory<ConcurrentMessageListenerContainer<Integer, String>> kafkaCustomizeContainerFactory() {
        ConcurrentKafkaListenerContainerFactory<Integer, String> factory = new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(consumerFactory());
        factory.setConcurrency(2);
        factory.getContainerProperties().setPollTimeout(3000);
        return factory;
    }

    private ConsumerFactory<Integer, String> consumerFactory() {
        return new DefaultKafkaConsumerFactory<>(consumerConfigs());
    }

    private Map<String, Object> consumerConfigs() {
        Map<String, Object> props = new HashMap<>();
        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bizConfig.getReconciliationInstanceKafkaServers());
        props.put(ConsumerConfig.GROUP_ID_CONFIG, bizConfig.getReconciliationInstanceKafkaConsumerGroupId());
        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");
        props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, true);
        props.put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, 100);
        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
        props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 100);
        props.put(ConsumerConfig.FETCH_MIN_BYTES_CONFIG, 300);
        // poll 一次拉取的阻塞的最大时长，单位：毫秒。这里指的是阻塞拉取需要满足至少 fetch-min-size 大小的消息
        props.put(ConsumerConfig.FETCH_MAX_BYTES_CONFIG, 10000);
        return props;
    }

}

这种方式的@KafkaLisener中的参数是单条的。

2、批量处理

@Configuration
@EnableKafka
public class KafkaConfig {
 
    @Bean
public KafkaListenerContainerFactory<?, ?> batchFactory() {
    ConcurrentKafkaListenerContainerFactory<Integer, String> factory =
            new ConcurrentKafkaListenerContainerFactory<>();
    factory.setConsumerFactory(consumerFactory());
    // 增加开启批量处理
    factory.setBatchListener(true);  // <<<<<<<<<<<<<<<<<<<<<<<<<
    return factory;
}
 
    @Bean
    public ConsumerFactory<Integer, String> consumerFactory() {
        return new DefaultKafkaConsumerFactory<>(consumerConfigs());
    }
    
    @Bean
    public Map<String, Object> consumerConfigs() {
        Map<String, Object> props = new HashMap<>();
        props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, embeddedKafka.getBrokersAsString());
        ...
        return props;
    }
}

// 注意：这里接受的是集合类型
@KafkaListener(id = "list", topics = "myTopic", containerFactory = "batchFactory")
public void listen(List<String> list) {
    ...
}

这种方式的@KafkaLisener中的参数是多条的。

6、线程池相关

如果没有额外给Kafka指定线程池，底层默认用的是SimpleAsyncTaskExecutor类，它不使用线程池，而是为每个任务创建新线程。相当于一个消费者用一个独立的线程来跑。

总结

spring为了将kafka融入其生态，方便在spring大环境下使用kafka，开发了spring-kafa这一模块，本质上是为了帮助开发者更好的以spring的方式使用kafka

@KafkaListener就是这么一个工具，在同一个项目中既可以有单条的消息处理，也可以配置多条的消息处理，稍微改变下配置即可实现，很是方便

当然，@KafkaListener单条或者多条消息处理仍然是spring自行封装处理，与kafka-client客户端的拉取机制无关；比如一次性拉取50条消息，对于单条处理来说就是循环50次处理，而多条消息处理则可以一次性处理50条；本质上来说这套逻辑都是spring处理的，并不是说单条消费就是通过kafka-client一次只拉取一条消息

在使用过程中需要注意spring自动的创建的一些bean实例，当然也可以覆盖其自动创建的实例以满足特定的需求场景。

原文链接：https://blog.csdn.net/yuechuzhixing/article/details/124725713