Bean后置处理器 BeanPostProcessor

1.BeanPostProcessor接口的作用

Bean后置处理器允许在调用初始化方法前后对Bean进行额外的处理，Bean后置处理器对IOC容器的所有bean实例逐一处理，而非单一实例。

我们可以定义一个或多个BeanPostProcessor接口实现类，然后注册到Spring IOC容器中。

2.BeanPostProcess接口的api

public interface BeanPostProcessor {

    /**
     * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
     * initialization callbacks (like InitializingBean‘s {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     */     //实例化、依赖注入完毕，在调用显示的初始化之前完成一些定制的初始化任务
    @Nullable
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    /**
     * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
     * initialization callbacks (like InitializingBean‘s {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     */
     //实例化、依赖注入、初始化完毕后执行     @Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

}