Spring中三种后置处理器对比

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BeanPostProcessor BeanFactoryPostProcessor BeanDefinitionRegistryPostProcessor
处理目标 bean 实例 BeanDefinition BeanDefinition、.class 文件等
执行时机 bena 的初始化阶段前后（已创建出 bean 对象） BeanDefinition解析完毕并注册进 BeanFactory 之后（此时 bean 未实例化） 配置文件、配置类已解析完毕并注册进 BeanFactory，但还未被 BeanFactoryPostProcessor 处理
可操作空间 对 bean 的属性赋值、创建代理对象等 给 BeanDefinition 中增删属性，移除 BeanDefinition 等 向 BeanFactory 中注册新的 BeanDefinition 和 BeanFactoryPostProcessor

BeanPostProcessor

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public interface BeanPostProcessor {

   @Nullable
   default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
      return bean;
   }

   @Nullable
   default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
      return bean;
   }

}

入参中的 bean 为根据 BeanDefinition 生成的 bean

出参中的 bean，如果不为 null 则会替换掉 BeanFactory 中传入的 bean

如果该 bean，是通过 factoryBean 生成的，则不会执行 postProcesserBeforeInitialization 方法

BeanFactoryPostProcessor

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public interface BeanFactoryPostProcessor {

	/**
	 * Modify the application context's internal bean factory after its standard
	 * initialization. All bean definitions will have been loaded, but no beans
	 * will have been instantiated yet. This allows for overriding or adding
	 * properties even to eager-initializing beans.
	 * @param beanFactory the bean factory used by the application context
	 * @throws org.springframework.beans.BeansException in case of errors
	 */
	void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

通过入参 beanfactory 就可以获取到对应的 beanDefinition

BeanDefinitionRegistryPostProcessor

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public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {

	/**
	 * Modify the application context's internal bean definition registry after its
	 * standard initialization. All regular bean definitions will have been loaded,
	 * but no beans will have been instantiated yet. This allows for adding further
	 * bean definitions before the next post-processing phase kicks in.
	 * @param registry the bean definition registry used by the application context
	 * @throws org.springframework.beans.BeansException in case of errors
	 */
	void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

}

通过 BeanDefinitionRegistry 的 registerBeanDefinition 方法就可以向容器中注入 beanDefinition