概述

BeanFactoryPostProcessor是Spring中一个相当重要的扩展点，扩展点就是能让我们在Spring容器以及Bean生命周期的各个阶段中可以进行修改扩展。

什么是BeanFactoryPostProcessor

BeanFactoryPostProcessor, 翻译过来大致是Bean的工厂处理器，顾名思义，可以理解为它对Bean工厂中Bean定义（BeanDefintion）进行修改，它的执行时机：BeanFactory标准初始化之后，所有的Bean定义已经被加载，但标准Bean的实例还没被创建（不包括BeanFactoryPostProcessor类型）。该方法通常用于修改bean的定义，Bean的属性值等，甚至可以在此快速初始化Bean。

而另外一个相关的扩展接口的BeanDefinitionRegistryPostProcessor，继承自BeanFactoryPostProcessor，所有的Bean定义即将被加载，但Bean的实例还没被创建时，也就是说，BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法执行时机先于BeanFactoryPostProcessor的postProcessBeanFactory方法。

区别于一个很类似的扩展接口BeanPostProcessor, 它的执行时机实在Bean初始化前后，添加一些自己想要的逻辑。

小结一下，上面关联的扩展接口执行顺序如下：1.BeanDefinitionRegistryPostProcessor，2.BeanFactoryPostProcessor，3.BeanPostProcessor。而BeanFactoryPostProcessor主要是在标准的BeanDefinition已经准备完毕，可以去修改已有的BeanDefinition的相关属性等。

如何使用BeanFactoryPostProcessor

新建一个测试bean

@Data@Componentpublic class Student { @Value(“${user.username:alvin}”) private String username; @Value(“${user.age:12}”) private int age;}

新建处理器实现BeanFactoryPostProcessor

@Component@Slf4jpublic 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()); }}

验证结论

bean的属性被成功被修改了。

源码解析

接口定义

/** * 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. * *

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. * *

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. * *

Registration

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}. * *

Ordering

{@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 */@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;}

接口的注释很清楚的说明了它的作用和细节，大致有下面几点：

该接口允许用户自定义修改工厂bean中的BeanDefinition, 调整BeanDefinition的属性值，甚至初始化Bean。比如内置的PropertyResourceConfigurer，就是修改beanDefinition的属性为配置文件的属性。
该接口主要是用于修改BeanDefinition, 虽然也可以直接进行实例化Bean, 但是不建议这么做，可能会造成其他未知的错误。

执行流程

其中核心逻辑是PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors方法，该方法的前面部分逻辑主要是处理BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法，也就是想BeanDefinition注册中中心添加新的BeanDefinition。

public static void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory, List 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processorList priorityOrderedPostProcessors = new ArrayList(); // 存放实现了Ordered接口的processorList orderedPostProcessorNames = new ArrayList(); // 存放没有实现排序的processorList 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 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processorList 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 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();}}

小结

整个执行流程的源码还是比较清晰并且简单的。重点提下下面两个点：

我们自定义的BeanFactoryProcessor需要加上 @Component等注解，为什么？

看源码得知，String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);获取所有实现了BeanFactoryPostProcessor的Bean Name, 前提所有的Bean都要被注册到BeanDefinitionRegistry, 通过添加@Component, @Service等注解，可以将对应的Bean定义信息注册到BeanFactory中，方便后面实例化Bean。那么它是在什么地方注册的呢？可以看下ConfigurationClassPostProcessor类，它实现了BeanDefinitionRegistryPostProcessor,会扫描所有@Component, @Service等注解，将对应的Bean Definition注册到BeanFactory中。

执行顺序问题

我们可以通过实现PriorityOrdered, Ordered接口，控制BeanFactoryProcessor的执行顺序,，优先执行实现了PriorityOrdered接口，其次是Ordered，最后是没有实现任何排序接口的processor。