BeanFactoryPostProcessor的作用

熟知BeanPostProcessor的知道（关于BeanPostProcessor见：），BeanPostProcessor是bean的后置处理器，用于在bean的生命周期来拓展对bean进行自定义处理。按照BeanPostProcessor的理解，BeanFactoryPostProcessor则是BeanFactory的后置处理器，用于对BeanFactory进行自定义处理，例如自定义注册bean，以及修改BeanFactory中注册的bean定义。

官方的解释是：

BeanFactoryPostProcessor operates on the bean configuration metadata. That is, the Spring IoC container lets a BeanFactoryPostProcessor read the configuration metadata and potentially change it before the container instantiates any beans other than BeanFactoryPostProcessor instances.

接口定义

@FunctionalInterfacepublic 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;}

调用入口

入口：AbstractApplicationContext#invokeBeanFactoryPostProcessors

而AbstractApplicationContext#invokeBeanFactoryPostProcessors是ApplicationContext执行refresh过程中调用的（spring容器启动过程中非常重要的一个过程）。

执行逻辑

执行过程非常简单，就是遍历AbstractApplicationContext#beanFactoryPostProcessors中的BeanFactoryPostProcessor依次执行postProcessBeanFactory方法，传递BeanFactory对象。

AbstractApplicationContext#invokeBeanFactoryPostProcessors
-->PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors
-->PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors

private static void invokeBeanFactoryPostProcessors(			Collection
    postProcessors, ConfigurableListableBeanFactory beanFactory) {		for (BeanFactoryPostProcessor postProcessor : postProcessors) {			postProcessor.postProcessBeanFactory(beanFactory);		}	}

如何添加BeanFactoryPostProcessor

方式一：手动添加，调用如下方法

AbstractApplicationContext#addBeanFactoryPostProcessor

即向beanFactoryPostProcessors变量中添加指定的BeanFactoryPostProcessor

@Override	public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {		Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");		this.beanFactoryPostProcessors.add(postProcessor);	}

方式二：按照添加一个普通bean的方式添加，例如通过spring的注解、xml、BeanDefinitionRegistry.registerBeanDefinition等

几个重要的实现类

PropertySourcesPlaceholderConfigurer：

用于处理属性中定义的${xxx}占位符，对这种格式的进行解析处理为真正的值。

EventListenerMethodProcessor：

处理@EventListener注解，即Spring的事件机制。

子接口BeanDefinitionRegistryPostProcessor

public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {	/**	 * 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;}

BeanDefinitionRegistryPostProcessor在BeanFactoryPostProcessor基础上增加了postProcessBeanDefinitionRegistry方法，用于回调传递BeanDefinitionRegistry对象，BeanDefinitionRegistry是注册bean的重要类

实现该方法可以向容器中注册bean。

重要实现类

ConfigurationClassPostProcessor：此类非常重要，执行优先级最高，用于处理@ComponentScan、@Configuration、@Import等注解指定的类，从而批量注册bean，此类后面单独分析。

具体分析见我这篇文章：

调用逻辑

调用入口同BeanFactoryPostProcessor一样都是在ApplicationContext执行refresh过程中的invokeBeanFactoryPostProcessors调用的。

调用过程在PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors方法中。

当初在多个BeanFactoryPostProcessor以及BeanDefinitionRegistryPostProcessor时，Spring时如何处理调用顺序的呢？

BeanFactoryPostProcessor的执行顺序

BeanFactoryPostProcessor执行的顺序逻辑都在 PostProcessorRegistrationDelegate的invokeBeanFactoryPostProcessors()方法中。

在保证先执行BeanDefinitionRegistryPostProcessor再执行BeanFactoryPostProcessor的原则上，再通过继承PriorityOrdered、Ordered可以自定义控制BeanFactory中注册的所有BeanFactoryPostProcessor的先后顺序，

源码

public static void invokeBeanFactoryPostProcessors(			ConfigurableListableBeanFactory beanFactory, List
   
     beanFactoryPostProcessors) {		// Invoke BeanDefinitionRegistryPostProcessors first, if any.		Set
    
      processedBeans = new HashSet<>();		if (beanFactory instanceof BeanDefinitionRegistry) {			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;			List
     
       regularPostProcessors = new ArrayList<>();			List
      
        registryProcessors = new ArrayList<>();			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {			    //1. 先调用通过AbstractApplicationContext#addBeanFactoryPostProcessor手动添加的BeanDefinitionRegistryPostProcessor				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {					BeanDefinitionRegistryPostProcessor registryProcessor =							(BeanDefinitionRegistryPostProcessor) postProcessor;					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.			List
       
         currentRegistryProcessors = new ArrayList<>();			// 2. 从beanFactory中获取自定义的BeanDefinitionRegistryPostProcessor，同时只执行实现了PriorityOrdered的BeanDefinitionRegistryPostProcessor 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));					processedBeans.add(ppName);				}			}			sortPostProcessors(currentRegistryProcessors, beanFactory);			registryProcessors.addAll(currentRegistryProcessors);			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);			currentRegistryProcessors.clear();			// 3. 继续从BeanFactory中获取BeanDefinitionRegistryPostProcessor，根据实现的Order排序，按顺序执行实现了Order的BeanDefinitionRegistryPostProcessor			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();			// 4. 继续从beanFactory中获取自定义的BeanDefinitionRegistryPostProcessor，最终执行剩余所有的BeanDefinitionRegistryPostProcessor			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();			}			// 5. 执行自定义BeanDefinitionRegistryPostProcessor的postProcessBeanFactory方法，也就是父接口的方法			invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);			//6. 执行手动添加的BeanFactoryPostProcessor#postProcessBeanFactory方法			invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);		}		else {			// Invoke factory processors registered with the context instance.			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);		}		// 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<>(); 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); } } // 7. 按顺序执行实现了PriorityOrdered的BeanFactoryPostProcessor sortPostProcessors(priorityOrderedPostProcessors, beanFactory); invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory); 8. 按顺序执行实现了Ordered的BeanFactoryPostProcessor List
           
             orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size()); for (String postProcessorName : orderedPostProcessorNames) { orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class)); } sortPostProcessors(orderedPostProcessors, beanFactory); invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory); // 9. 执行剩余其他的BeanFactoryPostProcessor 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(); }

执行过程

step 1： 调用通过AbstractApplicationContext#addBeanFactoryPostProcessor手动添加的BeanDefinitionRegistryPostProcessor

step 2： 从beanFactory中获取实现了PriorityOrdered的BeanDefinitionRegistryPostProcessor，然后按顺序依次执行

step 3： 继续从beanFactory中获取实现了Ordered的BeanDefinitionRegistryPostProcessor，然后按顺序依次执行

step 4： 继续从beanFactory中获取自定义的BeanDefinitionRegistryPostProcessor，最终执行剩余所有的BeanDefinitionRegistryPostProcessor

step 5： 按顺序执行所有BeanDefinitionRegistryPostProcessor的postProcessBeanFactory方法，也就是父接口BeanFactoryPostProcessor的方法

step 6： 执行手动添加的BeanFactoryPostProcessor#postProcessBeanFactory方法

step 7： 按顺序执行实现了PriorityOrdered的BeanFactoryPostProcessor

step 8： 按顺序执行实现了Ordered的BeanFactoryPostProcessor

step 9： 执行剩余其他的BeanFactoryPostProcessor

特别说明：对于顺序执行BeanDefinitionRegistryPostProcessor的几步顺序过程，每次都会重新从BeanFactory去获取，而BeanFactoryPostProcessor获取一次就不会再获取了，原因就是因为BeanDefinitionRegistryPostProcessor会向BeanFactory中注册bean，每执行一个BeanDefinitionRegistryPostProcessor后，BeanFactory中注册的bean都发生了变化，因此要重新获取，例如ConfigurationClassPostProcessor继承了PriorityOrdered，该类实现的postProcessBeanDefinitionRegistry方法会批量注册通过注解定义的bean，这里面同时也会添加很多BeanFactoryPostProcessor。

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