后置处理器BeanPostProcessor

BeanPostProcessor：回调机制，在 bean 的初始化前后做一些额外的处理，扩展功能。
可以配置多个 BeanPostProcessor 实例，并且可以通过实现Ordered接口，设置order属性来控制这些BeanPostProcessor实例的运行顺序。

//原接口
public interface BeanPostProcessor {
    
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

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

//编写后置处理器，对每个bean的创建都会生效
@Component
public class AnimalBeanPostProcessor implements BeanPostProcessor {
    
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("拦截到Bean的初始化之前：" + beanName);
        return bean;
    }
    
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("拦截到Bean的初始化之后：" + beanName);
        return bean;
    }
}

通过后置处理器修改bean的属性

@Component
public class CatBeanPostProcessor implements BeanPostProcessor {
    
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
            Cat cat = (Cat) bean;
            System.out.println("初始化之前，cat的name为：" + cat.getName());
            cat.setName("zhangsan");
        }
        return bean;
    }
    
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
            Cat cat = (Cat) bean;
            System.out.println("初始化之后，cat的name为：" + cat.getName());
        }
        return bean;
    }
}

bean的初始化流程：

对于factorybean，后置处理器会在factorybean创建时执行Before和after方法，在用factorybean去getbean时，执行after方法。