【spring源码分析】一、BeanPostProcessor

在spring-beans中org.springframework.beans.factory.config.BeanPostProcessor

BeanPostProcessor也称为Bean后置处理器，它是Spring中定义的接口，在Spring容器的创建过程中（具体为Bean初始化前后）会回调BeanPostProcessor中定义的两个方法。

public interface BeanPostProcessor {
  //在bean初始化之前执行
    @Nullable
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }
    //在bean的初始化之后执行
    @Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }
}

实现原理：探查源码

1）org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

在创建Bean时，会调起org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)进行初始化

这时会执行applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization，所以可以通过这两个方法对bean进行包装

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
        Object result = existingBean;
​
        Object current;
        //遍历当时容器中所有的BeanPostProcessor 一般都是遍历完所有的BeanPostProcessor
        for(Iterator var4 = this.getBeanPostProcessors().iterator(); var4.hasNext(); result = current) {
            BeanPostProcessor processor = (BeanPostProcessor)var4.next();
            current = processor.postProcessBeforeInitialization(result, beanName);
            //返回的是null那么我们通过getBean方法将得不到目标Bean ,退出遍历
            if (current == null) {
                return result;
            }
        }
​
        return result;
    }
​
    public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {
        Object result = existingBean;
​
        Object current;
        for(Iterator var4 = this.getBeanPostProcessors().iterator(); var4.hasNext(); result = current) {
            BeanPostProcessor processor = (BeanPostProcessor)var4.next();
            current = processor.postProcessAfterInitialization(result, beanName);
            if (current == null) {
                return result;
            }
        }
​
        return result;
    }

应用场景：

1、解析bean的注解，将注解中的字段转化为属性

2、统一将属性在执行前，注入bean中，如数据库访问的sqlMap，如严重服务，这样不需要每个bean都配置属性

3、打印日志，记录时间等。

常见BeanPostProcessor分析

ApplicationContextAwareProcessor

什么时候被注入的？

org.springframework.context.support.AbstractApplicationContext#refresh核心方法

//准备beanFactory
this.prepareBeanFactory(beanFactory);

org.springframework.context.support.AbstractApplicationContext#prepareBeanFactory

//往beanFactory添加ApplicationContextAwareProcessor
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
//往beanFactory添加ApplicationListenerDetector
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

作用：

org.springframework.context.support.ApplicationContextAwareProcessor#postProcessBeforeInitialization

org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces

实现了某种Aware接口，就会注入相关属性

例如：bean实现了ApplicationContextAware接口

if (bean instanceof ApplicationContextAware) {
  //会调用bean重写的setApplicationContext方法，注入applicationContext
    ((ApplicationContextAware)bean).setApplicationContext(this.applicationContext);
}