Spring源码解析（三）Bean定义注册的后置处理（BeanDefinitionRegistryPostProcessor）

1.简介

BeanDefinitionRegistryPostProcessor是BeanDefinition注册到容器的后置处理，他继承了bean工厂的后置处理器BeanFactoryPostProcessor，在执行顺序上它要比BeanFactoryPostProcessor先执行。此时bean还没有初始化。

2.应用Demo

自定义一个BeanDefinitionRegistryPostProcessor，在bean定义注册完以后，手动注册一个实例进去，代码如下：

@Component
@Slf4j
public class DemoBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        log.info("当前注册到容器中的bean的数量为:{}", registry.getBeanDefinitionCount());
        registry.registerBeanDefinition("demoEntity", new RootBeanDefinition(DemoEntity.class));
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        log.info("当前容器中bean的数量为:{}", beanFactory.getBeanDefinitionCount());
        log.info(JSON.toJSONString(beanFactory.getBeanDefinition("demoEntity")));
    }
}

打印结果为：

---

2020-01-14 17:15:57.915 INFO 19808 — [ main] .DemoBeanDefinitionRegistryPostProcessor : 当前注册到容器中的bean的数量为:129
2020-01-14 17:15:57.960 INFO 19808 — [ main] .DemoBeanDefinitionRegistryPostProcessor : 当前容器中bean的数量为:130
2020-01-14 17:15:58.024 INFO 19808 — [ main] .DemoBeanDefinitionRegistryPostProcessor : {“abstract”:false,“autowireCandidate”:true,“autowireMode”:0,“beanClass”:“com.example.demo.entity.DemoEntity”,“beanClassName”:“com.example.demo.entity.DemoEntity”,“constructorArgumentValues”:{“argumentCount”:0,“empty”:true,“genericArgumentValues”:[],“indexedArgumentValues”:{}},“dependencyCheck”:0,“enforceDestroyMethod”:true,“enforceInitMethod”:true,“lenientConstructorResolution”:true,“methodOverrides”:{“empty”:true,“overrides”:[]},“nonPublicAccessAllowed”:true,“primary”:false,“propertyValues”:{“converted”:false,“empty”:true,“propertyValueList”:[],“propertyValues”:[]},“prototype”:false,“qualifiers”:[],“resolvableType”:{“array”:false,“componentType”:{“array”:false,“componentType”:{“KaTeX parse error: Expected 'EOF', got '}' at position 9: ref":"@"}̲,"generics":[],…EmptyType@1d3e6d34”},“superType”:{“KaTeX parse error: Expected 'EOF', got '}' at position 9: ref":"@"}̲,"type":{"ref”:“KaTeX parse error: Expected 'EOF', got '}' at position 38: …entType.source"}̲},"generics":[]…ref”:“KaTeX parse error: Expected 'EOF', got '}' at position 31: ….componentType"}̲,"generics":[],…ref”:"$.resolvableType.componentType"},“type”:“java.lang.Object”},“type”:“com.example.demo.entity.DemoEntity”},“resolvedAutowireMode”:0,“role”:0,“scope”:"",“singleton”:true,“synthetic”:false}

3.源码执行过程

org.springframework.context.support.AbstractApplicationContext#refresh
>org.springframework.context.support.AbstractApplicationContext#invokeBeanFactoryPostProcessors
>org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors

public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
       // 这里存储处理过的bean的名称，避免重复处理
		Set processedBeans = new HashSet<>();
       
       // AnnotationConfigServletWebServerApplicationContext继承了BeanDefinitionRegistry
		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            // BeanFactoryPostProcessor 需要调用postProcessBeanFactory
			List regularPostProcessors = new ArrayList<>();
          //   BeanDefinitionRegistryPostProcessor既需要执行postProcessBeanFactory，还需要先执行postProcessBeanDefinitionRegistry
          //  这里的postProcessBeanDefinitionRegistry既包括框架硬编码写入的，也包括用户注册的
			List registryProcessors = new ArrayList<>();

           // 这里调用系统硬编码写入的registryProcessor的postProcessBeanDefinitionRegistry方法
           // registryProcessor和regularPostProcessors的postProcessBeanFactory放在后面统一处理
			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
					BeanDefinitionRegistryPostProcessor registryProcessor =
							(BeanDefinitionRegistryPostProcessor) postProcessor;
					registryProcessor.postProcessBeanDefinitionRegistry(registry);
					registryProcessors.add(registryProcessor);
				}
				else {
					regularPostProcessors.add(postProcessor);
				}
			}

            // 把注册到容器中的BeanDefinitionRegistryPostProcessor取出来，按照PriorityOrdered、Ordered和其他的顺序来执行
			// 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.
          // 创建临时列表，列表内的对象按照Order值从小到大的顺序来执行
			List currentRegistryProcessors = new ArrayList<>();

			// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                  // 标记为已处理
					processedBeans.add(ppName);
				}
			}
            
          // 排序调用，然后清空临时列表
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			currentRegistryProcessors.clear();

			// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
          // 因为后置处理里面可以注册同类型的BeanDefinitionRegistryPostProcessor，所以这里重新查一遍，下面相同
			postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				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);
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			currentRegistryProcessors.clear();

			// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
			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);
				currentRegistryProcessors.clear();
			}

			// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
			invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
			invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
		}

		else {
			// Invoke factory processors registered with the context instance.
			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
		}
    
       // 用户注册到容器中的BeanFactoryPostProcessor（不包括BeanDefinitionRegistryPostProcessors）还没有调用
       // 这里同样以PriorityOrdered、Ordered、其他的顺序在临时队列里面排序然后执行
		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let the bean factory post-processors apply to them!
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		List priorityOrderedPostProcessors = new ArrayList<>();
       // 同样的，这里因为前面执行的处理器可以修改后面的处理器，所以这里只保存beanName
		List orderedPostProcessorNames = new ArrayList<>();
		List nonOrderedPostProcessorNames = new ArrayList<>();
		for (String ppName : postProcessorNames) {
			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.
		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

		// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
		List orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
		for (String postProcessorName : orderedPostProcessorNames) {
			orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		sortPostProcessors(orderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

		// Finally, invoke all other BeanFactoryPostProcessors.
		List 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();
}