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本文只要研究一下KafkaListener的实现机制

KafkaListener

org/springframework/kafka/annotation/KafkaListener.java

@Target({ ElementType.TYPE, ElementType.METHOD, ElementType.ANNOTATION_TYPE })
@Retention(RetentionPolicy.RUNTIME)
@MessageMapping
@Documented
@Repeatable(KafkaListeners.class)
public @interface KafkaListener {String id() default "";String containerFactory() default "";String[] topics() default {};String topicPattern() default "";TopicPartition[] topicPartitions() default {};String containerGroup() default "";String errorHandler() default "";String groupId() default "";boolean idIsGroup() default true;String clientIdPrefix() default "";String beanRef() default "__listener";String concurrency() default "";String autoStartup() default "";String[] properties() default {};
}

KafkaListener注解定义了id、topics、groupId等属性

KafkaListenerAnnotationBeanPostProcessor

org/springframework/kafka/annotation/KafkaListenerAnnotationBeanPostProcessor.java

public class KafkaListenerAnnotationBeanPostProcessor<K, V>implements BeanPostProcessor, Ordered, BeanFactoryAware, SmartInitializingSingleton {private final KafkaListenerEndpointRegistrar registrar = new KafkaListenerEndpointRegistrar();	@Overridepublic int getOrder() {return LOWEST_PRECEDENCE;}@Overridepublic void setBeanFactory(BeanFactory beanFactory) {this.beanFactory = beanFactory;if (beanFactory instanceof ConfigurableListableBeanFactory) {this.resolver = ((ConfigurableListableBeanFactory) beanFactory).getBeanExpressionResolver();this.expressionContext = new BeanExpressionContext((ConfigurableListableBeanFactory) beanFactory,this.listenerScope);}}@Overridepublic void afterSingletonsInstantiated() {this.registrar.setBeanFactory(this.beanFactory);if (this.beanFactory instanceof ListableBeanFactory) {Map<String, KafkaListenerConfigurer> instances =((ListableBeanFactory) this.beanFactory).getBeansOfType(KafkaListenerConfigurer.class);for (KafkaListenerConfigurer configurer : instances.values()) {configurer.configureKafkaListeners(this.registrar);}}if (this.registrar.getEndpointRegistry() == null) {if (this.endpointRegistry == null) {Assert.state(this.beanFactory != null,"BeanFactory must be set to find endpoint registry by bean name");this.endpointRegistry = this.beanFactory.getBean(KafkaListenerConfigUtils.KAFKA_LISTENER_ENDPOINT_REGISTRY_BEAN_NAME,KafkaListenerEndpointRegistry.class);}this.registrar.setEndpointRegistry(this.endpointRegistry);}if (this.defaultContainerFactoryBeanName != null) {this.registrar.setContainerFactoryBeanName(this.defaultContainerFactoryBeanName);}// Set the custom handler method factory once resolved by the configurerMessageHandlerMethodFactory handlerMethodFactory = this.registrar.getMessageHandlerMethodFactory();if (handlerMethodFactory != null) {this.messageHandlerMethodFactory.setMessageHandlerMethodFactory(handlerMethodFactory);}else {addFormatters(this.messageHandlerMethodFactory.defaultFormattingConversionService);}// Actually register all listenersthis.registrar.afterPropertiesSet();}@Overridepublic Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {if (!this.nonAnnotatedClasses.contains(bean.getClass())) {Class<?> targetClass = AopUtils.getTargetClass(bean);Collection<KafkaListener> classLevelListeners = findListenerAnnotations(targetClass);final boolean hasClassLevelListeners = classLevelListeners.size() > 0;final List<Method> multiMethods = new ArrayList<>();Map<Method, Set<KafkaListener>> annotatedMethods = MethodIntrospector.selectMethods(targetClass,(MethodIntrospector.MetadataLookup<Set<KafkaListener>>) method -> {Set<KafkaListener> listenerMethods = findListenerAnnotations(method);return (!listenerMethods.isEmpty() ? listenerMethods : null);});if (hasClassLevelListeners) {Set<Method> methodsWithHandler = MethodIntrospector.selectMethods(targetClass,(ReflectionUtils.MethodFilter) method ->AnnotationUtils.findAnnotation(method, KafkaHandler.class) != null);multiMethods.addAll(methodsWithHandler);}if (annotatedMethods.isEmpty()) {this.nonAnnotatedClasses.add(bean.getClass());if (this.logger.isTraceEnabled()) {this.logger.trace("No @KafkaListener annotations found on bean type: " + bean.getClass());}}else {// Non-empty set of methodsfor (Map.Entry<Method, Set<KafkaListener>> entry : annotatedMethods.entrySet()) {Method method = entry.getKey();for (KafkaListener listener : entry.getValue()) {processKafkaListener(listener, method, bean, beanName);}}if (this.logger.isDebugEnabled()) {this.logger.debug(annotatedMethods.size() + " @KafkaListener methods processed on bean '"+ beanName + "': " + annotatedMethods);}}if (hasClassLevelListeners) {processMultiMethodListeners(classLevelListeners, multiMethods, bean, beanName);}}return bean;}		
}		

KafkaListenerAnnotationBeanPostProcessor实现了BeanPostProcessor, Ordered, BeanFactoryAware, SmartInitializingSingleton接口，其getOrder返回LOWEST_PRECEDENCE
其afterSingletonsInstantiated方法(SmartInitializingSingleton接口)首先获取KafkaListenerConfigurer，然后设置configureKafkaListeners为registrar，最后是执行registrar.afterPropertiesSet()
其postProcessAfterInitialization方法(BeanPostProcessor接口)会收集标注KafkaListener的bean的方法，然后针对每个方法执行processKafkaListener

processKafkaListener

	protected void processKafkaListener(KafkaListener kafkaListener, Method method, Object bean, String beanName) {Method methodToUse = checkProxy(method, bean);MethodKafkaListenerEndpoint<K, V> endpoint = new MethodKafkaListenerEndpoint<>();endpoint.setMethod(methodToUse);processListener(endpoint, kafkaListener, bean, methodToUse, beanName);}protected void processListener(MethodKafkaListenerEndpoint<?, ?> endpoint, KafkaListener kafkaListener,Object bean, Object adminTarget, String beanName) {String beanRef = kafkaListener.beanRef();if (StringUtils.hasText(beanRef)) {this.listenerScope.addListener(beanRef, bean);}endpoint.setBean(bean);endpoint.setMessageHandlerMethodFactory(this.messageHandlerMethodFactory);endpoint.setId(getEndpointId(kafkaListener));endpoint.setGroupId(getEndpointGroupId(kafkaListener, endpoint.getId()));endpoint.setTopicPartitions(resolveTopicPartitions(kafkaListener));endpoint.setTopics(resolveTopics(kafkaListener));endpoint.setTopicPattern(resolvePattern(kafkaListener));endpoint.setClientIdPrefix(resolveExpressionAsString(kafkaListener.clientIdPrefix(), "clientIdPrefix"));String group = kafkaListener.containerGroup();if (StringUtils.hasText(group)) {Object resolvedGroup = resolveExpression(group);if (resolvedGroup instanceof String) {endpoint.setGroup((String) resolvedGroup);}}String concurrency = kafkaListener.concurrency();if (StringUtils.hasText(concurrency)) {endpoint.setConcurrency(resolveExpressionAsInteger(concurrency, "concurrency"));}String autoStartup = kafkaListener.autoStartup();if (StringUtils.hasText(autoStartup)) {endpoint.setAutoStartup(resolveExpressionAsBoolean(autoStartup, "autoStartup"));}resolveKafkaProperties(endpoint, kafkaListener.properties());KafkaListenerContainerFactory<?> factory = null;String containerFactoryBeanName = resolve(kafkaListener.containerFactory());if (StringUtils.hasText(containerFactoryBeanName)) {Assert.state(this.beanFactory != null, "BeanFactory must be set to obtain container factory by bean name");try {factory = this.beanFactory.getBean(containerFactoryBeanName, KafkaListenerContainerFactory.class);}catch (NoSuchBeanDefinitionException ex) {throw new BeanInitializationException("Could not register Kafka listener endpoint on [" + adminTarget+ "] for bean " + beanName + ", no " + KafkaListenerContainerFactory.class.getSimpleName()+ " with id '" + containerFactoryBeanName + "' was found in the application context", ex);}}endpoint.setBeanFactory(this.beanFactory);String errorHandlerBeanName = resolveExpressionAsString(kafkaListener.errorHandler(), "errorHandler");if (StringUtils.hasText(errorHandlerBeanName)) {endpoint.setErrorHandler(this.beanFactory.getBean(errorHandlerBeanName, KafkaListenerErrorHandler.class));}this.registrar.registerEndpoint(endpoint, factory);if (StringUtils.hasText(beanRef)) {this.listenerScope.removeListener(beanRef);}}	

processKafkaListener方法将method转换为MethodKafkaListenerEndpoint，然后执行processListener方法，它主要是将KafkaListener注解的信息填充到MethodKafkaListenerEndpoint上，确定KafkaListenerContainerFactory，最后执行registrar.registerEndpoint(endpoint, factory)

registrar.registerEndpoint

org/springframework/kafka/config/KafkaListenerEndpointRegistrar.java

	/*** Register a new {@link KafkaListenerEndpoint} alongside the* {@link KafkaListenerContainerFactory} to use to create the underlying container.* <p>The {@code factory} may be {@code null} if the default factory has to be* used for that endpoint.* @param endpoint the {@link KafkaListenerEndpoint} instance to register.* @param factory the {@link KafkaListenerContainerFactory} to use.*/public void registerEndpoint(KafkaListenerEndpoint endpoint, KafkaListenerContainerFactory<?> factory) {Assert.notNull(endpoint, "Endpoint must be set");Assert.hasText(endpoint.getId(), "Endpoint id must be set");// Factory may be null, we defer the resolution right before actually creating the containerKafkaListenerEndpointDescriptor descriptor = new KafkaListenerEndpointDescriptor(endpoint, factory);synchronized (this.endpointDescriptors) {if (this.startImmediately) { // Register and start immediatelythis.endpointRegistry.registerListenerContainer(descriptor.endpoint,resolveContainerFactory(descriptor), true);}else {this.endpointDescriptors.add(descriptor);}}}

KafkaListenerEndpointRegistrar的registerEndpoint会创建KafkaListenerEndpointDescriptor，然后执行endpointRegistry.registerListenerContainer

endpointRegistry.registerListenerContainer

org/springframework/kafka/config/KafkaListenerEndpointRegistry.java

	public void registerListenerContainer(KafkaListenerEndpoint endpoint, KafkaListenerContainerFactory<?> factory,boolean startImmediately) {Assert.notNull(endpoint, "Endpoint must not be null");Assert.notNull(factory, "Factory must not be null");String id = endpoint.getId();Assert.hasText(id, "Endpoint id must not be empty");synchronized (this.listenerContainers) {Assert.state(!this.listenerContainers.containsKey(id),"Another endpoint is already registered with id '" + id + "'");MessageListenerContainer container = createListenerContainer(endpoint, factory);this.listenerContainers.put(id, container);if (StringUtils.hasText(endpoint.getGroup()) && this.applicationContext != null) {List<MessageListenerContainer> containerGroup;if (this.applicationContext.containsBean(endpoint.getGroup())) {containerGroup = this.applicationContext.getBean(endpoint.getGroup(), List.class);}else {containerGroup = new ArrayList<MessageListenerContainer>();this.applicationContext.getBeanFactory().registerSingleton(endpoint.getGroup(), containerGroup);}containerGroup.add(container);}if (startImmediately) {startIfNecessary(container);}}}/*** Start the specified {@link MessageListenerContainer} if it should be started* on startup.* @param listenerContainer the listener container to start.* @see MessageListenerContainer#isAutoStartup()*/private void startIfNecessary(MessageListenerContainer listenerContainer) {if (this.contextRefreshed || listenerContainer.isAutoStartup()) {listenerContainer.start();}}	

KafkaListenerEndpointRegistry的registerListenerContainer方法会根据endpoint和factory来创建MessageListenerContainer，然后放入到listenerContainers中，对于startImmediately的会执行startIfNecessary，它主要是执行listenerContainer.start()

MessageListenerContainer

org/springframework/kafka/listener/MessageListenerContainer.java

public interface MessageListenerContainer extends SmartLifecycle {void setupMessageListener(Object messageListener);Map<String, Map<MetricName, ? extends Metric>> metrics();default ContainerProperties getContainerProperties() {throw new UnsupportedOperationException("This container doesn't support retrieving its properties");}default Collection<TopicPartition> getAssignedPartitions() {throw new UnsupportedOperationException("This container doesn't support retrieving its assigned partitions");}default void pause() {throw new UnsupportedOperationException("This container doesn't support pause");}default void resume() {throw new UnsupportedOperationException("This container doesn't support resume");}default boolean isPauseRequested() {throw new UnsupportedOperationException("This container doesn't support pause/resume");}default boolean isContainerPaused() {throw new UnsupportedOperationException("This container doesn't support pause/resume");}default void setAutoStartup(boolean autoStartup) {// empty}default String getGroupId() {throw new UnsupportedOperationException("This container does not support retrieving the group id");}@Nullabledefault String getListenerId() {throw new UnsupportedOperationException("This container does not support retrieving the listener id");}
}

MessageListenerContainer继承了SmartLifecycle接口，它有一个泛型接口为GenericMessageListenerContainer，后者有一个抽象类为AbstractMessageListenerContainer，然后它有两个子类，分别是KafkaMessageListenerContainer与ConcurrentMessageListenerContainer

AbstractMessageListenerContainer

public abstract class AbstractMessageListenerContainer<K, V>implements GenericMessageListenerContainer<K, V>, BeanNameAware, ApplicationEventPublisherAware {@Overridepublic final void start() {checkGroupId();synchronized (this.lifecycleMonitor) {if (!isRunning()) {Assert.isTrue(this.containerProperties.getMessageListener() instanceof GenericMessageListener,() -> "A " + GenericMessageListener.class.getName() + " implementation must be provided");doStart();}}}@Overridepublic final void stop() {synchronized (this.lifecycleMonitor) {if (isRunning()) {final CountDownLatch latch = new CountDownLatch(1);doStop(latch::countDown);try {latch.await(this.containerProperties.getShutdownTimeout(), TimeUnit.MILLISECONDS); // NOSONARpublishContainerStoppedEvent();}catch (InterruptedException e) {Thread.currentThread().interrupt();}}}}//......
}		

AbstractMessageListenerContainer的start方法会回调子类的doStart方法，其stop方法会回调子类的doStop方法

KafkaMessageListenerContainer

org/springframework/kafka/listener/KafkaMessageListenerContainer.java

public class KafkaMessageListenerContainer<K, V> // NOSONAR comment densityextends AbstractMessageListenerContainer<K, V> {@Overrideprotected void doStart() {if (isRunning()) {return;}if (this.clientIdSuffix == null) { // stand-alone containercheckTopics();}ContainerProperties containerProperties = getContainerProperties();checkAckMode(containerProperties);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");GenericMessageListener<?> listener = (GenericMessageListener<?>) messageListener;ListenerType listenerType = deteremineListenerType(listener);this.listenerConsumer = new ListenerConsumer(listener, listenerType);setRunning(true);this.listenerConsumerFuture = containerProperties.getConsumerTaskExecutor().submitListenable(this.listenerConsumer);}//......
}		

KafkaMessageListenerContainer的doStart方法会获取到messageListener，然后创建ListenerConsumer，最后提交到线程池中执行

ConcurrentMessageListenerContainer

org/springframework/kafka/listener/ConcurrentMessageListenerContainer.java

public class ConcurrentMessageListenerContainer<K, V> extends AbstractMessageListenerContainer<K, V> {@Overrideprotected void doStart() {if (!isRunning()) {checkTopics();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);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.setGenericErrorHandler(getGenericErrorHandler());container.setAfterRollbackProcessor(getAfterRollbackProcessor());container.setRecordInterceptor(getRecordInterceptor());container.setEmergencyStop(() -> {stop(() -> {// NOSONAR});publishContainerStoppedEvent();});if (isPaused()) {container.pause();}container.start();this.containers.add(container);}}}//......	
}

ConcurrentMessageListenerContainer的doStart会根据concurrency值来创建对应的KafkaMessageListenerContainer，然后执行其start方法

ListenerConsumer

org/springframework/kafka/listener/KafkaMessageListenerContainer.java

private final class ListenerConsumer implements SchedulingAwareRunnable, ConsumerSeekCallback {@Overridepublic 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();initAsignedPartitions();while (isRunning()) {try {pollAndInvoke();}catch (@SuppressWarnings(UNUSED) WakeupException e) {// Ignore, we're stopping or applying immediate foreign acks}catch (NoOffsetForPartitionException nofpe) {this.fatalError = true;ListenerConsumer.this.logger.error("No offset and no reset policy", nofpe);break;}catch (Exception e) {handleConsumerException(e);}catch (Error e) { // NOSONAR - rethrownRunnable runnable = KafkaMessageListenerContainer.this.emergencyStop;if (runnable != null) {runnable.run();}this.logger.error("Stopping container due to an Error", e);wrapUp();throw e;}}wrapUp();}protected void pollAndInvoke() {if (!this.autoCommit && !this.isRecordAck) {processCommits();}processSeeks();checkPaused();ConsumerRecords<K, V> records = this.consumer.poll(this.pollTimeout);this.lastPoll = System.currentTimeMillis();checkResumed();debugRecords(records);if (records != null && records.count() > 0) {if (this.containerProperties.getIdleEventInterval() != null) {this.lastReceive = System.currentTimeMillis();}invokeListener(records);}else {checkIdle();}}private void invokeListener(final ConsumerRecords<K, V> records) {if (this.isBatchListener) {invokeBatchListener(records);}else {invokeRecordListener(records);}}private void doInvokeBatchOnMessage(final ConsumerRecords<K, V> records,List<ConsumerRecord<K, V>> recordList) {switch (this.listenerType) {case ACKNOWLEDGING_CONSUMER_AWARE:this.batchListener.onMessage(recordList,this.isAnyManualAck? new ConsumerBatchAcknowledgment(records): null, this.consumer);break;case ACKNOWLEDGING:this.batchListener.onMessage(recordList,this.isAnyManualAck? new ConsumerBatchAcknowledgment(records): null);break;case CONSUMER_AWARE:this.batchListener.onMessage(recordList, this.consumer);break;case SIMPLE:this.batchListener.onMessage(recordList);break;}}private void doInvokeOnMessage(final ConsumerRecord<K, V> recordArg) {ConsumerRecord<K, V> record = recordArg;if (this.recordInterceptor != null) {record = this.recordInterceptor.intercept(record);}if (record == null) {if (this.logger.isDebugEnabled()) {this.logger.debug("RecordInterceptor returned null, skipping: " + recordArg);}}else {switch (this.listenerType) {case ACKNOWLEDGING_CONSUMER_AWARE:this.listener.onMessage(record,this.isAnyManualAck? new ConsumerAcknowledgment(record): null, this.consumer);break;case CONSUMER_AWARE:this.listener.onMessage(record, this.consumer);break;case ACKNOWLEDGING:this.listener.onMessage(record,this.isAnyManualAck? new ConsumerAcknowledgment(record): null);break;case SIMPLE:this.listener.onMessage(record);break;}}}									//......	
}

ListenerConsumer实现了org.springframework.scheduling.SchedulingAwareRunnable接口(它继承了Runnable接口)以及org.springframework.kafka.listener.ConsumerSeekCallback接口
其run方法主要是执行initAsignedPartitions，然后循环执行pollAndInvoke，对于NoOffsetForPartitionException则跳出异常，对于其他Exception则执行handleConsumerException，对于Error执行emergencyStop与wrapUp方法
pollAndInvoke方法主要是执行consumer.poll()，然后通过invokeListener(records)回调，最后是通过doInvokeBatchOnMessage、doInvokeOnMessage去回调listener.onMessage方法

小结

KafkaListenerAnnotationBeanPostProcessor主要是收集标注KafkaListener的bean的方法，然后针对每个方法执行processKafkaListener，processKafkaListener方法将method转换为MethodKafkaListenerEndpoint，执行registrar.registerEndpoint(endpoint, factory)
KafkaListenerEndpointRegistry的registerListenerContainer方法会根据endpoint和factory来创建MessageListenerContainer，然后放入到listenerContainers中，对于startImmediately的会执行startIfNecessary，它主要是执行listenerContainer.start()
MessageListenerContainer有两个主要的实现类分别是KafkaMessageListenerContainer与ConcurrentMessageListenerContainer，后者的start方法主要是根据concurrency创建对应数量的KafkaMessageListenerContainer，最后都是执行KafkaMessageListenerContainer的start方法，它会创建ListenerConsumer，最后提交到线程池中执行；ListenerConsumer主要是执行pollAndInvoke，拉取消息，然后回到listener的onMessage方法
整体的链路就是KafkaListenerAnnotationBeanPostProcessor --> KafkaListenerEndpointRegistry --> MessageListenerContainer --> GenericMessageListener.onMessage