依赖项和配置信息参见另一篇博文@KafkaListener的配置使用，这里主要借助源码分析@KafkaListener和KafkaTemplate自动装配原理。

1、KafkaAutoConfiguration 源码分析

KafkaAutoConfiguration类自动装配生成了生产者客户端KafkaTemplate的bean和消费者基础ConsumerFactory的bean，KafkaAutoConfiguration导入KafkaAnnotationDrivenConfiguration，KafkaAnnotationDrivenConfiguration最终生成了ConcurrentKafkaListenerContainerFactory的bean, 该bean是@KafkaListener默认使用的容器工厂，即指定了消费的kafka集群。

package org.springframework.boot.autoconfigure.kafka;import java.io.IOException;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.kafka.core.ConsumerFactory;
import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
import org.springframework.kafka.core.DefaultKafkaProducerFactory;
import org.springframework.kafka.core.KafkaAdmin;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.kafka.core.ProducerFactory;
import org.springframework.kafka.security.jaas.KafkaJaasLoginModuleInitializer;
import org.springframework.kafka.support.LoggingProducerListener;
import org.springframework.kafka.support.ProducerListener;
import org.springframework.kafka.support.converter.RecordMessageConverter;
import org.springframework.kafka.transaction.KafkaTransactionManager;@Configuration(proxyBeanMethods = false
)
/// KafkaTemplate在类路径下存在时，加载该配置类
@ConditionalOnClass({KafkaTemplate.class})  
@EnableConfigurationProperties({KafkaProperties.class})
/// 将KafkaAnnotationDrivenConfiguration、KafkaStreamsAnnotationDrivenConfiguration配置类合并过来
@Import({KafkaAnnotationDrivenConfiguration.class, KafkaStreamsAnnotationDrivenConfiguration.class})  
public class KafkaAutoConfiguration {private final KafkaProperties properties;public KafkaAutoConfiguration(KafkaProperties properties) {this.properties = properties;}@Bean  /// KafkaTemplate的bean未定义时，自动生成该bean@ConditionalOnMissingBean({KafkaTemplate.class})public KafkaTemplate<?, ?> kafkaTemplate(ProducerFactory<Object, Object> kafkaProducerFactory, ProducerListener<Object, Object> kafkaProducerListener, ObjectProvider<RecordMessageConverter> messageConverter) {KafkaTemplate<Object, Object> kafkaTemplate = new KafkaTemplate(kafkaProducerFactory);messageConverter.ifUnique(kafkaTemplate::setMessageConverter);kafkaTemplate.setProducerListener(kafkaProducerListener);kafkaTemplate.setDefaultTopic(this.properties.getTemplate().getDefaultTopic());return kafkaTemplate;}@Bean@ConditionalOnMissingBean({ProducerFactory.class})public ProducerFactory<?, ?> kafkaProducerFactory(ObjectProvider<DefaultKafkaProducerFactoryCustomizer> customizers) {DefaultKafkaProducerFactory<?, ?> factory = new DefaultKafkaProducerFactory(this.properties.buildProducerProperties());String transactionIdPrefix = this.properties.getProducer().getTransactionIdPrefix();if (transactionIdPrefix != null) {factory.setTransactionIdPrefix(transactionIdPrefix);}customizers.orderedStream().forEach((customizer) -> {customizer.customize(factory);});return factory;}@Bean@ConditionalOnMissingBean({ProducerListener.class})public ProducerListener<Object, Object> kafkaProducerListener() {return new LoggingProducerListener();}@Bean/// ConsumerFactory的bean未定义时，自动生成该bean@ConditionalOnMissingBean({ConsumerFactory.class})public ConsumerFactory<?, ?> kafkaConsumerFactory(ObjectProvider<DefaultKafkaConsumerFactoryCustomizer> customizers) {DefaultKafkaConsumerFactory<Object, Object> factory = new DefaultKafkaConsumerFactory(this.properties.buildConsumerProperties());customizers.orderedStream().forEach((customizer) -> {customizer.customize(factory);});return factory;}@Bean@ConditionalOnProperty(name = {"spring.kafka.producer.transaction-id-prefix"})@ConditionalOnMissingBeanpublic KafkaTransactionManager<?, ?> kafkaTransactionManager(ProducerFactory<?, ?> producerFactory) {return new KafkaTransactionManager(producerFactory);}@Bean@ConditionalOnProperty(name = {"spring.kafka.jaas.enabled"})@ConditionalOnMissingBeanpublic KafkaJaasLoginModuleInitializer kafkaJaasInitializer() throws IOException {KafkaJaasLoginModuleInitializer jaas = new KafkaJaasLoginModuleInitializer();KafkaProperties.Jaas jaasProperties = this.properties.getJaas();if (jaasProperties.getControlFlag() != null) {jaas.setControlFlag(jaasProperties.getControlFlag());}if (jaasProperties.getLoginModule() != null) {jaas.setLoginModule(jaasProperties.getLoginModule());}jaas.setOptions(jaasProperties.getOptions());return jaas;}@Bean@ConditionalOnMissingBeanpublic KafkaAdmin kafkaAdmin() {KafkaAdmin kafkaAdmin = new KafkaAdmin(this.properties.buildAdminProperties());kafkaAdmin.setFatalIfBrokerNotAvailable(this.properties.getAdmin().isFailFast());return kafkaAdmin;}
}

2、KafkaAnnotationDrivenConfiguration 源码分析

package org.springframework.boot.autoconfigure.kafka;import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.kafka.KafkaProperties.Listener.Type;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.kafka.annotation.EnableKafka;
import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
import org.springframework.kafka.config.KafkaListenerConfigUtils;
import org.springframework.kafka.core.ConsumerFactory;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.kafka.listener.AfterRollbackProcessor;
import org.springframework.kafka.listener.BatchErrorHandler;
import org.springframework.kafka.listener.ConsumerAwareRebalanceListener;
import org.springframework.kafka.listener.ErrorHandler;
import org.springframework.kafka.listener.RecordInterceptor;
import org.springframework.kafka.support.converter.BatchMessageConverter;
import org.springframework.kafka.support.converter.BatchMessagingMessageConverter;
import org.springframework.kafka.support.converter.MessageConverter;
import org.springframework.kafka.support.converter.RecordMessageConverter;
import org.springframework.kafka.transaction.KafkaAwareTransactionManager;/*** Configuration for Kafka annotation-driven support.** @author Gary Russell* @author Eddú Meléndez*/
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(EnableKafka.class)
class KafkaAnnotationDrivenConfiguration {private final KafkaProperties properties;private final RecordMessageConverter messageConverter;private final BatchMessageConverter batchMessageConverter;private final KafkaTemplate<Object, Object> kafkaTemplate;private final KafkaAwareTransactionManager<Object, Object> transactionManager;private final ConsumerAwareRebalanceListener rebalanceListener;private final ErrorHandler errorHandler;private final BatchErrorHandler batchErrorHandler;private final AfterRollbackProcessor<Object, Object> afterRollbackProcessor;private final RecordInterceptor<Object, Object> recordInterceptor;KafkaAnnotationDrivenConfiguration(KafkaProperties properties,ObjectProvider<RecordMessageConverter> messageConverter,ObjectProvider<BatchMessageConverter> batchMessageConverter,ObjectProvider<KafkaTemplate<Object, Object>> kafkaTemplate,ObjectProvider<KafkaAwareTransactionManager<Object, Object>> kafkaTransactionManager,ObjectProvider<ConsumerAwareRebalanceListener> rebalanceListener, ObjectProvider<ErrorHandler> errorHandler,ObjectProvider<BatchErrorHandler> batchErrorHandler,ObjectProvider<AfterRollbackProcessor<Object, Object>> afterRollbackProcessor,ObjectProvider<RecordInterceptor<Object, Object>> recordInterceptor) {this.properties = properties;this.messageConverter = messageConverter.getIfUnique();this.batchMessageConverter = batchMessageConverter.getIfUnique(() -> new BatchMessagingMessageConverter(this.messageConverter));this.kafkaTemplate = kafkaTemplate.getIfUnique();this.transactionManager = kafkaTransactionManager.getIfUnique();this.rebalanceListener = rebalanceListener.getIfUnique();this.errorHandler = errorHandler.getIfUnique();this.batchErrorHandler = batchErrorHandler.getIfUnique();this.afterRollbackProcessor = afterRollbackProcessor.getIfUnique();this.recordInterceptor = recordInterceptor.getIfUnique();}@Bean@ConditionalOnMissingBeanConcurrentKafkaListenerContainerFactoryConfigurer kafkaListenerContainerFactoryConfigurer() {ConcurrentKafkaListenerContainerFactoryConfigurer configurer = new ConcurrentKafkaListenerContainerFactoryConfigurer();configurer.setKafkaProperties(this.properties);MessageConverter messageConverterToUse = (this.properties.getListener().getType().equals(Type.BATCH))? this.batchMessageConverter : this.messageConverter;configurer.setMessageConverter(messageConverterToUse);configurer.setReplyTemplate(this.kafkaTemplate);configurer.setTransactionManager(this.transactionManager);configurer.setRebalanceListener(this.rebalanceListener);configurer.setErrorHandler(this.errorHandler);configurer.setBatchErrorHandler(this.batchErrorHandler);configurer.setAfterRollbackProcessor(this.afterRollbackProcessor);configurer.setRecordInterceptor(this.recordInterceptor);return configurer;}@Bean@ConditionalOnMissingBean(name = "kafkaListenerContainerFactory")  /// ConcurrentKafkaListenerContainerFactory的bean未定义时，自动生成该bean（这是@KafkaListener默认使用的容器工厂）ConcurrentKafkaListenerContainerFactory<?, ?> kafkaListenerContainerFactory(ConcurrentKafkaListenerContainerFactoryConfigurer configurer,ConsumerFactory<Object, Object> kafkaConsumerFactory) {ConcurrentKafkaListenerContainerFactory<Object, Object> factory = new ConcurrentKafkaListenerContainerFactory<>();configurer.configure(factory, kafkaConsumerFactory);return factory;}@Configuration(proxyBeanMethods = false)@EnableKafka@ConditionalOnMissingBean(name = KafkaListenerConfigUtils.KAFKA_LISTENER_ANNOTATION_PROCESSOR_BEAN_NAME)static class EnableKafkaConfiguration {}}