聊聊KafkaListener的实现机制

序

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();    

    @Override
    public int getOrder() {
        return LOWEST_PRECEDENCE;
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) {
        this.beanFactory = beanFactory;
        if (beanFactory instanceof ConfigurableListableBeanFactory) {
            this.resolver = ((ConfigurableListableBeanFactory) beanFactory).getBeanExpressionResolver();
            this.expressionContext = new BeanExpressionContext((ConfigurableListableBeanFactory) beanFactory,
                    this.listenerScope);
        }
    }

    @Override
    public 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 configurer
        MessageHandlerMethodFactory handlerMethodFactory = this.registrar.getMessageHandlerMethodFactory();
        if (handlerMethodFactory != null) {
            this.messageHandlerMethodFactory.setMessageHandlerMethodFactory(handlerMethodFactory);
        }
        else {
            addFormatters(this.messageHandlerMethodFactory.defaultFormattingConversionService);
        }

        // Actually register all listeners
        this.registrar.afterPropertiesSet();
    }

    @Override
    public 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 methods
                for (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 container
        KafkaListenerEndpointDescriptor descriptor = new KafkaListenerEndpointDescriptor(endpoint, factory);
        synchronized (this.endpointDescriptors) {
            if (this.startImmediately) { // Register and start immediately
                this.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");
    }
    @Nullable
    default 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 {

    @Override
    public 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();
            }
        }
    }

    @Override
    public 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); // NOSONAR
                    publishContainerStoppedEvent();
                }
                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 density
        extends AbstractMessageListenerContainer<K, V> {

    @Override
    protected void doStart() {
        if (isRunning()) {
            return;
        }
        if (this.clientIdSuffix == null) { // stand-alone container
            checkTopics();
        }
        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> {

    @Override
    protected 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 {

        @Override
        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();
            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 - rethrown
                    Runnable 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