一文吃透Spring Boot扩展之BeanFactoryPostProcessor
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
@Component
public class Student {
@Value("${user.username:alvin}")
private String username;
@Value("${user.age:12}")
private int age;
}
复制代码
- 新建处理器实现
BeanFactoryPostProcessor
@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());
}
}
复制代码
- 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 */ @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; } 复制代码
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：
PropertyResourceConfigurer
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, ListbeanFactoryPostProcessors) { .......... 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 priorityOrderedPostProcessors = new ArrayList<>(); // 存放实现了Ordered接口的processor List orderedPostProcessorNames = new ArrayList<>(); // 存放没有实现排序的processor List 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processor 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(); } //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(); } } 复制代码
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。
内置的BeanFactoryPostProcessor
Spring内置了一个比较重要的 BeanFactoryPostProcessor 是 PlaceholderConfigurerSupport , 实现从配置文件中获取属性。
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