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

Question

我在春季启动时看看Kafka Auto配置。

阅读艺术Spring-boot auto-configure

以下是从github Here复制的代码。

我的问题是，如果包含KafkaTemplate.class的jar不存在于类路径中，那么在自动配置时弹簧如何设置执行以下代码。由于该类不存在不应该导致错误？或者在自动配置时弹簧是否会执行此操作？

@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

引用春季靴子documentation：

@ConditionalOnClass和@ConditionalOnMissingClass注释允许根据特定类的存在与否来包含@Configuration类。由于使用ASM解析注释元数据这一事实，您可以使用value属性来引用真实类，即使该类实际上可能不会出现在正在运行的应用程序类路径中。如果您希望使用String值指定类名，也可以使用name属性。

此机制不会以相同的方式应用于@Bean方法，其中返回类型通常是条件的目标：在方法的条件适用之前，JVM将加载类和可能处理的方法引用，如果类的话，它将失败不在场

要回答你的问题，spring甚至不包括KafkaAutoConfiguration类。