@Component
public class TestBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
        log.info("******************** TestBeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry ****************");
        log.info("******************** bean的数量：[{}] ****************", beanDefinitionRegistry.getBeanDefinitionCount());
        AbstractBeanDefinition beanDefinition = BeanDefinitionBuilder.rootBeanDefinition(User.class).getBeanDefinition();
        MutablePropertyValues propertyValues = beanDefinition.getPropertyValues();
        propertyValues.addPropertyValue("username", "alvin");
        propertyValues.addPropertyValue("age", 12);
        // 注册bean信息
        beanDefinitionRegistry.registerBeanDefinition("beanDefineDemo", beanDefinition);
    }
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {
        log.info("******************** TestBeanDefinitionRegistryPostProcessor#postProcessBeanFactory ****************");
        log.info("******************** bean的数量：[{}] ****************", configurableListableBeanFactory.getBeanDefinitionCount());
    }
}

/**
 * 扩展到标准的{@link BeanFactoryPostProcessor} SPI
 * 允许在常规BeanFactoryPostProcessor检测开始之前注册更多的bean定义。
 * 特别是，BeanDefinitionRegistryPostProcessor可以注册更多的bean定义，
 * 这些定义反过来又定义了BeanFactoryPostProcessor实例。
 * 
 * Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
 * the registration of further bean definitions <i>before</i> regular
 * BeanFactoryPostProcessor detection kicks in. In particular,
 * BeanDefinitionRegistryPostProcessor may register further bean definitions
 * which in turn define BeanFactoryPostProcessor instances.
 *
 * @author Juergen Hoeller
 * @since 3.0.1
 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor
 */
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
  /**
     * 在标准初始化之后修改应用程序上下文的内部bean定义注册表。所有常规bean定义都已加载，
     * 但还没有实例化bean。这允许在进入下一个后处理阶段之前添加更多的bean定义。
     * 
   * 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;

public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
      StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
      // Prepare this context for refreshing.
            // 刷新上下文的准备阶段，跳过
      prepareRefresh();
      // Tell the subclass to refresh the internal bean factory.
      // 创建beanFacotry工厂
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
      // Prepare the bean factory for use in this context.
            // beanFactory准备工作， 跳过
      prepareBeanFactory(beanFactory);
      try {
        // Allows post-processing of the bean factory in context subclasses.
        postProcessBeanFactory(beanFactory);
        StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
        // Invoke factory processors registered as beans in the context.
        // 调用注册的BeanFactoryPostProcessors对象
                invokeBeanFactoryPostProcessors(beanFactory);
    }
  }

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    // 注册的核心代码， beanFactory代表当前的容器， 第二个参数为内置的参数处理器
        PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
      .....
}

public static void invokeBeanFactoryPostProcessors(
      ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
    // WARNING: Although it may appear that the body of this method can be easily
    // refactored to avoid the use of multiple loops and multiple lists, the use
    // of multiple lists and multiple passes over the names of processors is
    // intentional. We must ensure that we honor the contracts for PriorityOrdered
    // and Ordered processors. Specifically, we must NOT cause processors to be
    // instantiated (via getBean() invocations) or registered in the ApplicationContext
    // in the wrong order.
    //
    // Before submitting a pull request (PR) to change this method, please review the
    // list of all declined PRs involving changes to PostProcessorRegistrationDelegate
    // to ensure that your proposal does not result in a breaking change:
    // https://github.com/spring-projects/spring-framework/issues?q=PostProcessorRegistrationDelegate+is%3Aclosed+label%3A%22status%3A+declined%22
    // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        // 如果存在BeanDefinitionRegistryPostProcessors，那么就会首先调用。
    Set<String> processedBeans = new HashSet<>();
    if (beanFactory instanceof BeanDefinitionRegistry) {
      BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
      // 常规的BeanFactory后置处理器集合
            List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
      // Bean定义注册的后置处理器
            List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
            // 遍历容器优先内置的后置处理器
      for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
        // 判断是否为Bean定义注册的后置处理器
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
          BeanDefinitionRegistryPostProcessor registryProcessor =
              (BeanDefinitionRegistryPostProcessor) postProcessor;
                    // 直接调用postProcessBeanDefinitionRegistry方法
          registryProcessor.postProcessBeanDefinitionRegistry(registry);
          registryProcessors.add(registryProcessor);
        }
        else {
          regularPostProcessors.add(postProcessor);
        }
      }
      // Do not initialize FactoryBeans here: We need to leave all regular beans
      // uninitialized to let the bean factory post-processors apply to them!
      // Separate between BeanDefinitionRegistryPostProcessors that implement
      // PriorityOrdered, Ordered, and the rest.
            // 定义外部实现了BeanDefinitionRegistryPostProcessor扩展的集合
      List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
      // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
      // 首先，调用实现PriorityOrdered的 BeanDefinitionRegistryPostProcessor。
            // 此处获取的postProcessorNames必须是已经进行注册过的BeanDefinition。
            // 第一次我们无法找到我们写的BeanDefinitionRegistryPostProcessor，是因为它本身并未注册到bean定义注册中心。
            // 但是会发现有ConfigurationClassPostProcessor，它会帮我们写的PostProcessor注册到bean定义注册中心。
            String[] postProcessorNames =
          beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
      for (String ppName : postProcessorNames) {
        // 判断BeanDefinitionRegistryPostProcessor是否实现了PriorityOrdered接口
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
          currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
          processedBeans.add(ppName);
        }
      }
            // 根据优先级排序
      sortPostProcessors(currentRegistryProcessors, beanFactory);
      registryProcessors.addAll(currentRegistryProcessors);
            // 真正执行后置处理器
      invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
      currentRegistryProcessors.clear();
      // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
      // 再次获取BeanDefinitionRegistryPostProcessor后置处理器，此时可以看到我们写的后置处理器，
            // 那是因为我们写的后置处理器bean定义经过ConfigurationClassPostProcessor注册
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
      for (String ppName : postProcessorNames) {
        // 判断是否实现了Ordered接口
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
          currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
          processedBeans.add(ppName);
        }
      }
      sortPostProcessors(currentRegistryProcessors, beanFactory);
      registryProcessors.addAll(currentRegistryProcessors);
            // 真正执行后置处理器
      invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
      currentRegistryProcessors.clear();
      // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
      // 最后执行没有实现Orderd接口的后置处理器
            boolean reiterate = true;
      while (reiterate) {
        reiterate = false;
        postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        for (String ppName : postProcessorNames) {
          if (!processedBeans.contains(ppName)) {
            currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
            processedBeans.add(ppName);
            reiterate = true;
          }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
        currentRegistryProcessors.clear();
      }
      // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
      // 因为
            invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
      invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
    }
    else {
      // Invoke factory processors registered with the context instance.
      invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
    }
        // 后面的内容是执行BeanFactoryPostProcessor的后置处理器，后面再说
        .....
  }