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BeanFactoryPostProcessor的作用可不仅仅是修改Bean定义

一说到 BeanFactoryPostProcessor，很多人的第一反映就是他可以修改Bean定义，对，他是可以修改Bean定义，然而，他的作用仅限于修改Bean定义吗？显然不是，他是Spring提供的一个扩展点，解耦的一个手段，下面仔细说一下：

先说下他是怎么修改Bean定义的

@FunctionalInterface
public interface BeanFactoryPostProcessor {
   void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

上面就是 BeanFactoryPostProcessor 的源码，只有一个方法，然而修改Bean定义的并不是这个方法，而是参数ConfigurableListableBeanFactory，然而他也不能直接的修改Bean定义，我们进去看看

image-20200925174024474

里面有一个 getBeanDefinition，返回一个BeanDefinition，继续往下跟

BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;

ok，看到了最终的真相

image-20200925174236988

真正可以修改Bean定义的是 BeanDefinition，现在我们也知道了BeanFactoryPostProcessor之所以能修改Bean定义其实是返回了 BeanDefinition。

他是怎么充当扩展点的

看过源码的同学对这段代码肯定很熟悉了, 实例化一个容器嘛，重点看看this方法

public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
   // 初始化了Bean的扫描规则，扫描包含Component的|注册自带的Bf(一些BeanFactoryPostProcessor)
   this();
   // 把 componentClasses 的 bf 注册进去，上面这两步注册了6个bf，5个自带的，一个配置类
   register(componentClasses);
   // 开始扫描Bean，注册监听者，发布事件等
   refresh();
}

AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);

跟到这个方法

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
      BeanDefinitionRegistry registry, @Nullable Object source) {

   DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
   if (beanFactory != null) {
      if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
         beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
      }
      if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
         beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
      }
   }

   Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

   if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
   }

   if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
   }

   // Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
   if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
   }

   // Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
   if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition();
      try {
         def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
               AnnotationConfigUtils.class.getClassLoader()));
      }
      catch (ClassNotFoundException ex) {
         throw new IllegalStateException(
               "Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
      }
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
   }

   if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
   }

   if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
      RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
      def.setSource(source);
      beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
   }

   return beanDefs;
}

代码有点长，这个函数主要做了什么呢？其实就是注册Spring自带的后置处理器，我们看这个类 ConfigurationClassPostProcessor，从图上可以看出他是实现了BeanFactoryPostProcessor的，那这个类又是干嘛的呢？

image-20200925174852779****

还记得上面说的 BeanFactoryPostProcessor 有一个 postProcessBeanFactory方法吗？我们看看实现

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
   int factoryId = System.identityHashCode(beanFactory);
   if (this.factoriesPostProcessed.contains(factoryId)) {
      throw new IllegalStateException(
            "postProcessBeanFactory already called on this post-processor against " + beanFactory);
   }
   this.factoriesPostProcessed.add(factoryId);
   if (!this.registriesPostProcessed.contains(factoryId)) {
      // BeanDefinitionRegistryPostProcessor hook apparently not supported...
      // Simply call processConfigurationClasses lazily at this point then.
      processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
   }

   enhanceConfigurationClasses(beanFactory);
   beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}

进 processConfigBeanDefinitions

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
   List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
   String[] candidateNames = registry.getBeanDefinitionNames();

   do {
      // 这个方法开始真正扫描解析 @Component 以及其派生注解
      // 把bf 写到容器里 beanDefinitionMap 这个
      parser.parse(candidates);
   }
}

这里只保留Parse方法，主要看这个，一直往下跟，直到看到 doProcessConfigurationClass 这个方法

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
      throws IOException {

   if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
      // Recursively process any member (nested) classes first
      processMemberClasses(configClass, sourceClass);
   }

   // Process any @PropertySource annotations
   for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
         sourceClass.getMetadata(), PropertySources.class,
         org.springframework.context.annotation.PropertySource.class)) {
      if (this.environment instanceof ConfigurableEnvironment) {
         processPropertySource(propertySource);
      }
      else {
         logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
               "]. Reason: Environment must implement ConfigurableEnvironment");
      }
   }

   // ok， 扫描
   // Process any @ComponentScan annotations
   Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
         sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
   if (!componentScans.isEmpty() &&
         !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
      for (AnnotationAttributes componentScan : componentScans) {
         // The config class is annotated with @ComponentScan -> perform the scan immediately
         // 看这里
         Set<BeanDefinitionHolder> scannedBeanDefinitions =
               this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
         // Check the set of scanned definitions for any further config classes and parse recursively if needed
         for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
            BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
            if (bdCand == null) {
               bdCand = holder.getBeanDefinition();
            }
            if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
               parse(bdCand.getBeanClassName(), holder.getBeanName());
            }
         }
      }
   }
   // 处理 import的
   // Process any @Import annotations
   processImports(configClass, sourceClass, getImports(sourceClass), true);

   // Process any @ImportResource annotations
   AnnotationAttributes importResource =
         AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
   if (importResource != null) {
      String[] resources = importResource.getStringArray("locations");
      Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
      for (String resource : resources) {
         String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
         configClass.addImportedResource(resolvedResource, readerClass);
      }
   }

   // Process individual @Bean methods
   Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
   for (MethodMetadata methodMetadata : beanMethods) {
      configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
   }

   // Process default methods on interfaces
   processInterfaces(configClass, sourceClass);

   // Process superclass, if any
   if (sourceClass.getMetadata().hasSuperClass()) {
      String superclass = sourceClass.getMetadata().getSuperClassName();
      if (superclass != null && !superclass.startsWith("java") &&
            !this.knownSuperclasses.containsKey(superclass)) {
         this.knownSuperclasses.put(superclass, configClass);
         // Superclass found, return its annotation metadata and recurse
         return sourceClass.getSuperClass();
      }
   }

   // No superclass -> processing is complete
   return null;
}

看到这里就很明白了，这就是处理配置类上的注解的嘛，Spring 的 BeanFactoryPostProcessor 里的 postProcessBeanFactory 可以留给外部自己实现逻辑，你完全可以利用这个扩展点往Spring里织入自己业务逻辑

private static void invokeBeanFactoryPostProcessors(
			Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {

		for (BeanFactoryPostProcessor postProcessor : postProcessors) {
			postProcessor.postProcessBeanFactory(beanFactory);
		}
	}

Spring 会通过上面的方法，调用外部实现的postProcessor

所以，你明白 BeanFactoryPostProcessor到底是干嘛的了吗？

悦读

道可道，非常道；名可名，非常名。 无名，天地之始，有名，万物之母。 故常无欲，以观其妙，常有欲，以观其徼。 此两者，同出而异名，同谓之玄，玄之又玄，众妙之门。

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