java - 如果类路径中不存在类，spring boot autoconfigure 如何工作

Question

我正在查看 spring-boot 中的 Kafka 自动配置。

阅读文章 Spring-boot auto-configure

下面是从github上复制的代码Here .

我的问题是，如果包含 KafkaTemplate.class 的 jar 不 出现在 class path 中，如何spring 设法在自动配置时执行以下代码。由于该类不存在，它不应该导致错误吗？还是 spring 在自动配置时根本不会执行此操作？

@Configuration
@ConditionalOnClass(KafkaTemplate.class)
@EnableConfigurationProperties(KafkaProperties.class)
@Import({ KafkaAnnotationDrivenConfiguration.class,
        KafkaStreamsAnnotationDrivenConfiguration.class })
public class KafkaAutoConfiguration {

    private final KafkaProperties properties;

    private final RecordMessageConverter messageConverter;

    public KafkaAutoConfiguration(KafkaProperties properties,
            ObjectProvider<RecordMessageConverter> messageConverter) {
        this.properties = properties;
        this.messageConverter = messageConverter.getIfUnique();
    }

    @Bean
    @ConditionalOnMissingBean(KafkaTemplate.class)
    public KafkaTemplate<?, ?> kafkaTemplate(
            ProducerFactory<Object, Object> kafkaProducerFactory,
            ProducerListener<Object, Object> kafkaProducerListener) {
        KafkaTemplate<Object, Object> kafkaTemplate = new KafkaTemplate<>(
                kafkaProducerFactory);
        if (this.messageConverter != null) {
            kafkaTemplate.setMessageConverter(this.messageConverter);
        }
        kafkaTemplate.setProducerListener(kafkaProducerListener);
        kafkaTemplate.setDefaultTopic(this.properties.getTemplate().getDefaultTopic());
        return kafkaTemplate;
    }

    @Bean
    @ConditionalOnMissingBean(ProducerListener.class)
    public ProducerListener<Object, Object> kafkaProducerListener() {
        return new LoggingProducerListener<>();
    }

    @Bean
    @ConditionalOnMissingBean(ConsumerFactory.class)
    public ConsumerFactory<?, ?> kafkaConsumerFactory() {
        return new DefaultKafkaConsumerFactory<>(
                this.properties.buildConsumerProperties());
    }

    @Bean
    @ConditionalOnMissingBean(ProducerFactory.class)
    public ProducerFactory<?, ?> kafkaProducerFactory() {
        DefaultKafkaProducerFactory<?, ?> factory = new DefaultKafkaProducerFactory<>(
                this.properties.buildProducerProperties());
        String transactionIdPrefix = this.properties.getProducer()
                .getTransactionIdPrefix();
        if (transactionIdPrefix != null) {
            factory.setTransactionIdPrefix(transactionIdPrefix);
        }
        return factory;
    }

    @Bean
    @ConditionalOnProperty(name = "spring.kafka.producer.transaction-id-prefix")
    @ConditionalOnMissingBean
    public KafkaTransactionManager<?, ?> kafkaTransactionManager(
            ProducerFactory<?, ?> producerFactory) {
        return new KafkaTransactionManager<>(producerFactory);
    }

    @Bean
    @ConditionalOnProperty(name = "spring.kafka.jaas.enabled")
    @ConditionalOnMissingBean
    public KafkaJaasLoginModuleInitializer kafkaJaasInitializer() throws IOException {
        KafkaJaasLoginModuleInitializer jaas = new KafkaJaasLoginModuleInitializer();
        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
    @ConditionalOnMissingBean
    public KafkaAdmin kafkaAdmin() {
        KafkaAdmin kafkaAdmin = new KafkaAdmin(this.properties.buildAdminProperties());
        kafkaAdmin.setFatalIfBrokerNotAvailable(this.properties.getAdmin().isFailFast());
        return kafkaAdmin;
    }

}

感谢帮助

Answer 1

引用spring boot documentation :

The @ConditionalOnClass and @ConditionalOnMissingClass annotations let @Configuration classes be included based on the presence or absence of specific classes. Due to the fact that annotation metadata is parsed by using ASM, you can use the value attribute to refer to the real class, even though that class might not actually appear on the running application classpath. You can also use the name attribute if you prefer to specify the class name by using a String value.

This mechanism does not apply the same way to @Bean methods where typically the return type is the target of the condition: before the condition on the method applies, the JVM will have loaded the class and potentially processed method references which will fail if the class is not present

为了回答您的问题，spring 甚至不包含 KafkaAutoConfiguration 类。