SpringBoot 第九篇 之 @Configuration, @Bean, @Component 的区别

@Configuration , @Component  都可作为配置类。

但有区别：

    @Configuration 中 所有 带有 @Bean的注解都会被动态代理（CGLIB），

          因此对bean的调用返回的都是同一实例。

    @Component 没有被动态代理，每次调用返回的都是不同实例。

 

原因是ConfigurationClassPostProcessor类中，调用enhanceConfigurationClasses方法，

被注解@Configuration的类进行CGLIB代理。

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
        Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<String, AbstractBeanDefinition>();
        for (String beanName : beanFactory.getBeanDefinitionNames()) {
            BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef)) {//判断是否被@Configuration标注
                if (!(beanDef instanceof AbstractBeanDefinition)) {
                    throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
                            beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
                }
                else if (logger.isWarnEnabled() && beanFactory.containsSingleton(beanName)) {
                    logger.warn("Cannot enhance @Configuration bean definition '" + beanName +
                            "' since its singleton instance has been created too early. The typical cause " +
                            "is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
                            "return type: Consider declaring such methods as 'static'.");
                }
                configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
            }
        }
        if (configBeanDefs.isEmpty()) {
            // nothing to enhance -> return immediately
            return;
        }
        ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
        for (Map.Entry<String, AbstractBeanDefinition> entry : configBeanDefs.entrySet()) {
            AbstractBeanDefinition beanDef = entry.getValue();
            // If a @Configuration class gets proxied, always proxy the target class
            beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);
            try {
                // Set enhanced subclass of the user-specified bean class
                Class<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);
                Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);//生成代理的class
                if (configClass != enhancedClass) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(String.format("Replacing bean definition '%s' existing class '%s' with " +
                                "enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
                    }
                    //替换class，将原来的替换为CGLIB代理的class
                    beanDef.setBeanClass(enhancedClass);
                }
            }
            catch (Throwable ex) {
                throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
            }
        }
    }
判断是否为配置类的代码如下：

//是否为配置类
public static boolean isConfigurationCandidate(AnnotationMetadata metadata) {
return (isFullConfigurationCandidate(metadata) || isLiteConfigurationCandidate(metadata));
}
 
//是否为完整配置类
public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
return metadata.isAnnotated(Configuration.class.getName());
}
//是否为精简配置类
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;
    }
}
static {
    candidateIndicators.add(Component.class.getName());
    candidateIndicators.add(ComponentScan.class.getName());
    candidateIndicators.add(Import.class.getName());
    candidateIndicators.add(ImportResource.class.getName());
}