Spring Kafka中@KafkaListener源码解析详解？

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目录+前言+一、总体流程+二、源码解读

1.postProcessAfterInitialization

1.1 processKafkaListener 1.2 processListener 1.3 registerEndpoint 1.4 startIfNecessary

2.afterSingletonsInstantiated

2.1 afterPropertiesSet

目录

• 前言
• 一、总体流程
• 二、源码解读
• 1、postProcessAfterInitialization
• 1.1、processKafkaListener
• 1.2、processListener
• 1.3、registerEndpoint
• 1.4、startIfNecessary
• 2、afterSingletonsInstantiated
• 2.1、afterPropertiesSet
• 2.2、registerAllEndpoints
• 总结

前言

本文主要通过深入了解源码，梳理从spring启动到真正监听kafka消息的这套流程

一、总体流程

从spring启动开始处理@KafkaListener，到start消息监听整体流程图

二、源码解读

1、postProcessAfterInitialization

KafkaListenerAnnotationBeanPostProcessor#postProcessAfterInitialization

public Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException { if (!this.nonAnnotatedClasses.contains(bean.getClass())) { Class<?> targetClass = AopUtils.getTargetClass(bean); // 扫描@KafkaListener注解 Collection<KafkaListener> classLevelListeners = findListenerAnnotations(targetClass); ...... if (annotatedMethods.isEmpty()) { this.nonAnnotatedClasses.add(bean.getClass()); 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(); // 遍历扫描到的所有@KafkaListener注解并开始处理 for (KafkaListener listener : entry.getValue()) { processKafkaListener(listener, method, bean, beanName); } } this.logger.debug(() -> annotatedMethods.size() + " @KafkaListener methods processed on bean '" + beanName + "': " + annotatedMethods); } // 处理在类上的@KafkaListener注解 if (hasClassLevelListeners) { processMultiMethodListeners(classLevelListeners, multiMethods, bean, beanName); } } return bean; }

1.1、processKafkaListener

KafkaListenerAnnotationBeanPostProcessor#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); }

1.2、processListener

KafkaListenerAnnotationBeanPostProcessor#processListener

将每个kafkaListener转变成MethodKafkaListenerEndpoint并注册到KafkaListenerEndpointRegistrar容器，方便后续统一启动监听

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(); ...... // 注册已经封装好的消费端-endpoint this.registrar.registerEndpoint(endpoint, factory); if (StringUtils.hasText(beanRef)) { this.listenerScope.removeListener(beanRef); } }

1.3、registerEndpoint

KafkaListenerEndpointRegistrar#registerEndpoint

public void registerEndpoint(KafkaListenerEndpoint endpoint, KafkaListenerContainerFactory<?> factory) { ...... KafkaListenerEndpointDescriptor descriptor = new KafkaListenerEndpointDescriptor(endpoint, factory); synchronized (this.endpointDescriptors) { // 如果到了需要立即启动监听的阶段就直接注册并监听(也就是创建消息监听容器并启动) if (this.startImmediately) { // Register and start immediately this.endpointRegistry.registerListenerContainer(descriptor.endpoint, resolveContainerFactory(descriptor), true); } else { // 一般情况都先走这一步，添加至此列表，待bean后续的生命周期 统一注册并启动 this.endpointDescriptors.add(descriptor); } } } public void registerListenerContainer(KafkaListenerEndpoint endpoint, KafkaListenerContainerFactory<?> factory, boolean startImmediately) { ...... synchronized (this.listenerContainers) { ...... // 1.创建消息监听容器 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); } // 2.是否立即启动消息监听 if (startImmediately) { startIfNecessary(container); } } }

1.4、startIfNecessary

KafkaListenerEndpointRegistry#startIfNecessary
启动消息监听

private void startIfNecessary(MessageListenerContainer listenerContainer) { if (this.contextRefreshed || listenerContainer.isAutoStartup()) { // 启动消息监听 // 到这一步之后，消息监听以及处理都是KafkaMessageListenerContainer的逻辑 // 到此也就打通了@KafkaListener到MessageListenerContainer消息监听容器的逻辑 listenerContainer.start(); } }

2、afterSingletonsInstantiated

这一步是实例化(此处的实例化是已经创建对象并完成了初始化操作)之后，紧接着的操作

KafkaListenerAnnotationBeanPostProcessor#afterSingletonsInstantiated

public void afterSingletonsInstantiated() { this.registrar.setBeanFactory(this.beanFactory); // 对"注册员"信息的完善 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); } ...... // Actually register all listeners // 整个方法这里才是关键 // 创建MessageListenerContainer并注册 this.registrar.afterPropertiesSet(); }

2.1、afterPropertiesSet

KafkaListenerEndpointRegistrar#afterPropertiesSet

public void afterPropertiesSet() { registerAllEndpoints(); }

2.2、registerAllEndpoints

KafkaListenerEndpointRegistrar#registerAllEndpoints

protected void registerAllEndpoints() { synchronized (this.endpointDescriptors) { for (KafkaListenerEndpointDescriptor descriptor : this.endpointDescriptors) { // 这里是真正的创建ListenerContainer监听对象并注册 this.endpointRegistry.registerListenerContainer( descriptor.endpoint, resolveContainerFactory(descriptor)); } // 启动时所有消息监听对象都注册之后，便将参数置为true this.startImmediately = true; // trigger immediate startup } }

总结

以上便是整个流程，总体感觉就是将kafka消息监听融入到spring生命周期中，并完美契合

调试及相关源码版本：

org.springframework.boot:：2.3.3.RELEASE spring-kafka:2.5.4.RELEASE

相关参考：

spring-kafka官方文档
spring容器之refresh方法

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