BeanPostProcessor接口的作用是在Spring容器完成Bean实例化前后可以添加一些自己的逻辑处理，我们可以定义一个或者多个BeanPostProcessor接口的实现。

BeanPostProcessor接口提供了两个方法：

1、postProcessBeforeInitialization  可以对Bean在实例化之前添加一些逻辑处理

2、postProcessAfterInitialization  可以对bean在实例化之后添加一些逻辑处理

public interface BeanPostProcessor {

  //可以对Bean在实例化之前添加一些逻辑处理
  Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

  //可以对bean在实例化之后添加一些逻辑处理
  Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

简单示例MyBeanPostProcessor：

public class MyBeanPostProcessor implements BeanPostProcessor {
 
    //Bean 实例化之前进行的处理
    public Object postProcessBeforeInitialization(Object bean, String beanName)
           throws BeansException {
       System.out.println("对象" + beanName + "开始实例化");
       return bean;
    }
 
    //Bean 实例化之后进行的处理
    public Object postProcessAfterInitialization(Object bean, String beanName)
           throws BeansException {
       System.out.println("对象" + beanName + "实例化完成");
       return bean;
    }
}

在applicationContext.xml中添加配置

<bean class="com.spring.test.di.MyBeanPostProcessor"/>

1、BeanPostProcessor保存到Spring容器

这样MyBeanPostProcessor就会添加到Spring的容器中，在Spring容器中会将所有实现BeanPostProcessor的bean保存起来，具体实现在AbstractApplicationContext的registerBeanPostProcessors中

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    //获取所有实现BeanPostProcessor的实现类
    String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

    
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
    List<String> orderedPostProcessorNames = new ArrayList<String>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
    for (String ppName : postProcessorNames) {
      if (isTypeMatch(ppName, PriorityOrdered.class)) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        //保存所有的BeanPostProcessor
        priorityOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
          internalPostProcessors.add(pp);
        }
      }
      else if (isTypeMatch(ppName, Ordered.class)) {
        orderedPostProcessorNames.add(ppName);
      }
      else {
        nonOrderedPostProcessorNames.add(ppName);
      }
    }

    OrderComparator.sort(priorityOrderedPostProcessors);
    //注册所有的BeanPostProcessor
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : orderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      orderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
        internalPostProcessors.add(pp);
      }
    }
    OrderComparator.sort(orderedPostProcessors);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : nonOrderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      nonOrderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
        internalPostProcessors.add(pp);
      }
    }
    registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

    OrderComparator.sort(internalPostProcessors);
    registerBeanPostProcessors(beanFactory, internalPostProcessors);

    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector());
  }
//在容器中添加BeanPostProcessor
private void registerBeanPostProcessors(
      ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {

    for (BeanPostProcessor postProcessor : postProcessors) {
      beanFactory.addBeanPostProcessor(postProcessor);
    }
  }


2、执行所有的BeanPostProcessor

在Spring容器在初始化Bean前后都会执行所有的BeanPostProcessor的实现类提供的方法。 

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

    Object result = existingBean;
    //getBeanPostProcessors()获取所有的BeanPostProcessor的实现类
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
      //执行BeanPostProcessor实现类的方法
      result = beanProcessor.postProcessAfterInitialization(result, beanName);
      if (result == null) {
        return result;
      }
    }
    return result;
  }


Spring源码学习--BeanPostProcessor_spring