@Data
@Component
public class Student {
    @Value("${user.username:alvin}")
    private String username;
    @Value("${user.age:12}")
    private int age;
}

@Component
@Slf4j
public class TestBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        log.info("******************** TestBeanFactoryPostProcessor#postProcessBeanFactory ****************");
        log.info("******************** bean的数量：[{}] ****************", beanFactory.getBeanDefinitionCount());
        // 修改bean definition属性信息
        BeanDefinition userBeanDef = beanFactory.getBeanDefinition("student");
        userBeanDef.getPropertyValues().add("username", "cxw");
        // 快速初始化Bean
        User user = (User)beanFactory.getBean("student");
        log.info("student name: [{}]", user.getUsername());
    }
}

/**
 * Factory hook that allows for custom modification of an application context's
 * bean definitions, adapting the bean property values of the context's underlying
 * bean factory.
 *
 * <p>Useful for custom config files targeted at system administrators that
 * override bean properties configured in the application context. See
 * {@link PropertyResourceConfigurer} and its concrete implementations for
 * out-of-the-box solutions that address such configuration needs.
 *
 * <p>A {@code BeanFactoryPostProcessor} may interact with and modify bean
 * definitions, but never bean instances. Doing so may cause premature bean
 * instantiation, violating the container and causing unintended side-effects.
 * If bean instance interaction is required, consider implementing
 * {@link BeanPostProcessor} instead.
 *
 * <h3>Registration</h3>
 * <p>An {@code ApplicationContext} auto-detects {@code BeanFactoryPostProcessor}
 * beans in its bean definitions and applies them before any other beans get created.
 * A {@code BeanFactoryPostProcessor} may also be registered programmatically
 * with a {@code ConfigurableApplicationContext}.
 *
 * <h3>Ordering</h3>
 * <p>{@code BeanFactoryPostProcessor} beans that are autodetected in an
 * {@code ApplicationContext} will be ordered according to
 * {@link org.springframework.core.PriorityOrdered} and
 * {@link org.springframework.core.Ordered} semantics. In contrast,
 * {@code BeanFactoryPostProcessor} beans that are registered programmatically
 * with a {@code ConfigurableApplicationContext} will be applied in the order of
 * registration; any ordering semantics expressed through implementing the
 * {@code PriorityOrdered} or {@code Ordered} interface will be ignored for
 * programmatically registered post-processors. Furthermore, the
 * {@link org.springframework.core.annotation.Order @Order} annotation is not
 * taken into account for {@code BeanFactoryPostProcessor} beans.
 *
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @since 06.07.2003
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
@FunctionalInterface
public interface BeanFactoryPostProcessor {
  /**
   * Modify the application context's internal bean factory after its standard
   * initialization. All bean definitions will have been loaded, but no beans
   * will have been instantiated yet. This allows for overriding or adding
   * properties even to eager-initializing beans.
   * @param beanFactory the bean factory used by the application context
   * @throws org.springframework.beans.BeansException in case of errors
   */
  void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;
}

public static void invokeBeanFactoryPostProcessors(
      ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
        .......... 该部分是处理BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry相关逻辑，跳过， 可以看BeanDefinitionRegistryPostProcessor的解析
        // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let the bean factory post-processors apply to them!
        // 获取所有实现了BeanFactoryPostProcessor接口的bean name列表，前提是在BeanFactory的BeanDefinitions列表中包含对应的bean定义信息。
    String[] postProcessorNames =
        beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
    // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
        // 存放实现了PriorityOrdered接口的processor
    List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        // 存放实现了Ordered接口的processor
    List<String> orderedPostProcessorNames = new ArrayList<>();
        // 存放没有实现排序的processor
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        // 遍历前面全量的bean name，将他们归类，放到上面的容器中
    for (String ppName : postProcessorNames) {
            // 如果在第一阶段已经被调用过，就不调用，第一阶段主要是BeanDefinitionRegistryPostProcessor，它继承了BeanFactoryPostProcessor，它会在第一阶段调用。
      if (processedBeans.contains(ppName)) {
        // skip - already processed in first phase above
      }
      else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
        priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
      }
      else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
        orderedPostProcessorNames.add(ppName);
      }
      else {
        nonOrderedPostProcessorNames.add(ppName);
      }
    }
    // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        // 首先执行实现了PriorityOrdered接口的processor，对它们进行排序
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
        // 真实执行processor中的逻辑。
    invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
    // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        // 其次执行实现了Ordered接口的processor，对它们进行排序后执行processor中的逻辑。
    List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
    for (String postProcessorName : orderedPostProcessorNames) {
      orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
    // 最后执行，没有顺序要求的processor
    List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
    for (String postProcessorName : nonOrderedPostProcessorNames) {
      nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
    // Clear cached merged bean definitions since the post-processors might have
    // modified the original metadata, e.g. replacing placeholders in values...
    beanFactory.clearMetadataCache();
  }
//调用processors中的postProcessBeanFactory方法
private static void invokeBeanFactoryPostProcessors(
      Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
    for (BeanFactoryPostProcessor postProcessor : postProcessors) {
      StartupStep postProcessBeanFactory = beanFactory.getApplicationStartup().start("spring.context.bean-factory.post-process")
          .tag("postProcessor", postProcessor::toString);
      postProcessor.postProcessBeanFactory(beanFactory);
      postProcessBeanFactory.end();
    }
  }