Spring Boot 启动的过程中 ConfigurationClassPostProcessor 类的

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) ;

processConfigBeanDefinitions(BeanDefinitionRegistry registry) 将会扫描解析的所有的 Bean Definniton。

实际上 Configuration Class 的认定是通过 ConfigurationClassUtils 一下方法来判定的：

    /**
     * Check whether the given bean definition is a candidate for a configuration class
     * (or a nested component class declared within a configuration/component class,
     * to be auto-registered as well), and mark it accordingly.
     * @param beanDef the bean definition to check
     * @param metadataReaderFactory the current factory in use by the caller
     * @return whether the candidate qualifies as (any kind of) configuration class
     */
    public static boolean checkConfigurationClassCandidate(BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {
        String className = beanDef.getBeanClassName();
        if (className == null || beanDef.getFactoryMethodName() != null) {
            return false;
        }

        AnnotationMetadata metadata;
        if (beanDef instanceof AnnotatedBeanDefinition &&
                className.equals(((AnnotatedBeanDefinition) beanDef).getMetadata().getClassName())) {
            // Can reuse the pre-parsed metadata from the given BeanDefinition...
            metadata = ((AnnotatedBeanDefinition) beanDef).getMetadata();
        }
        else if (beanDef instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) beanDef).hasBeanClass()) {
            // Check already loaded Class if present...
            // since we possibly can't even load the class file for this Class.
            Class<?> beanClass = ((AbstractBeanDefinition) beanDef).getBeanClass();
            metadata = new StandardAnnotationMetadata(beanClass, true);
        }
        else {
            try {
                MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(className);
                metadata = metadataReader.getAnnotationMetadata();
            }
            catch (IOException ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Could not find class file for introspecting configuration annotations: " + className, ex);
                }
                return false;
            }
        }

        if (isFullConfigurationCandidate(metadata)) {
            beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);
        }
        else if (isLiteConfigurationCandidate(metadata)) {
            beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
        }
        else {
            return false;
        }

        // It's a full or lite configuration candidate... Let's determine the order value, if any.
        Map<String, Object> orderAttributes = metadata.getAnnotationAttributes(Order.class.getName());
        if (orderAttributes != null) {
            beanDef.setAttribute(ORDER_ATTRIBUTE, orderAttributes.get(AnnotationUtils.VALUE));
        }

        return true;
    }
     /**
     * Check the given metadata for a full configuration class candidate
     * (i.e. a class annotated with {@code @Configuration}).
     * @param metadata the metadata of the annotated class
     * @return {@code true} if the given class is to be processed as a full
     * configuration class, including cross-method call interception
     */
    public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
        return metadata.isAnnotated(Configuration.class.getName());
    }

    /**
     * Check the given metadata for a lite configuration class candidate
     * (e.g. a class annotated with {@code @Component} or just having
     * {@code @Import} declarations or {@code @Bean methods}).
     * @param metadata the metadata of the annotated class
     * @return {@code true} if the given class is to be processed as a lite
     * configuration class, just registering it and scanning it for {@code @Bean} methods
     */
    public static boolean isLiteConfigurationCandidate(AnnotationMetadata metadata) {
        // Do not consider an interface or an annotation...
        if (metadata.isInterface()) {
            return false;
        }

        // Any of the typical annotations found?
        for (String indicator : candidateIndicators) {
            if (metadata.isAnnotated(indicator)) {
                return true;
            }
        }

        // Finally, let's look for @Bean methods...
        try {
            return metadata.hasAnnotatedMethods(Bean.class.getName());
        }
        catch (Throwable ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to introspect @Bean methods on class [" + metadata.getClassName() + "]: " + ex);
            }
            return false;
        }
    }

checkConfigurationClassCandidate(BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) 优先判断 Class 是否被 @Configuration 注解声明，其次在判断有没有被 @Component、@Import、@ImportResource、@ComponentScan 等注解声明，否则判断类的方法是否被 @Bean 注解声明。