Spring4.3.x 浅析xml配置的解析过程（7）——解析context命名空间之annotation-config标签

概述

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Spring context命名空间有property-placeholder、property-override、annotation-config、component-scan、load-time-weaver、spring-configured、mbean-export和mbean-server 8个标签。

上一节探讨了 property-placeholder和property-override标签的解析，这一节来探讨一下annotation-config的解析与用途。

解析annotation-config标签

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annotation-config标签用于向BeanDefinitionRegistry对象（BeanDefinition注册表）中注册处理注解配置的工厂后处理器类和bean后处理器类以及其他一些特定类对应的BeanDefinition对象。这些类和注解有

• 处理@Configuration注解的ConfigurationClassPostProcessor工厂后处理器；

• 处理@Autowired、@Value、@Inject以及@Lookup注解的AutowiredAnnotationBeanPostProcessor bean后处理器；

• 处理@Required注解的RequiredAnnotationBeanPostProcessor bean后处理器；

• 处理JSR-250的@Resource、@PostConstruct、@PreDestroy、@EJB和@WebServiceRef注解的CommonAnnotationBeanPostProcessor bean后处理器；

• 处理JPA注解的PersistenceAnnotationBeanPostProcessor bean后处理器

• 处理@EventListener注解的EventListenerMethodProcessor后处理器和创建ApplicationListener对象的EventListenerFactory对象。

annotation-config标签的解析器类为AnnotationConfigBeanDefinitionParser，它直接实现了BeanDefinitionParser接口，下面是它实现的parse方法的源代码。

    @Override    public BeanDefinition parse(Element element, ParserContext parserContext) {        Object source = parserContext.extractSource(element);        // 注册所有相关的BeanPostProcessor.        Set<BeanDefinitionHolder> processorDefinitions =                AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);        CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);        parserContext.pushContainingComponent(compDefinition);        for (BeanDefinitionHolder processorDefinition : processorDefinitions) {            parserContext.registerComponent(new BeanComponentDefinition(processorDefinition));        }        // 最后注册 composite component.        parserContext.popAndRegisterContainingComponent();        return null;    }

parse方法主要调用注解配置工具类AnnotationConfigUtils的registerAnnotationConfigProcessors静态方法来注册于注解配置有关的bean后处理器，这个方法的源码如下。

    private static final boolean jsr250Present =            ClassUtils.isPresent("javax.annotation.Resource", AnnotationConfigUtils.class.getClassLoader());            // 定义常量PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME            // = "org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"    private static final boolean jpaPresent =            ClassUtils.isPresent("javax.persistence.EntityManagerFactory", AnnotationConfigUtils.class.getClassLoader()) &&            ClassUtils.isPresent(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, AnnotationConfigUtils.class.getClassLoader());    /**    * 注册用于注解配置的bean后处理器    **/    public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(            BeanDefinitionRegistry registry, Object source) {        DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);        if (beanFactory != null) {            if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {                beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);            }            // 设置自动注入候选对象的解析器，用于判断BeanDefinition是否为候选对象            if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {                beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());            }        }        Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<BeanDefinitionHolder>(4);        // 设置处理@Configuration注解的工厂后处理器        // 常量CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME        // ="org.springframework.context.annotation.internalConfigurationAnnotationProcessor";        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));        }        // 设置用于处理@Autowired、@Value、@Inject以及@Lookup注解        // 定义常量AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME        // = "org.springframework.context.annotation.internalAutowiredAnnotationProcessor"        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));        }        // 设置处理@Required注解的bean后处理器        // 定义常量REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME        // = "org.springframework.context.annotation.internalRequiredAnnotationProcessor"        if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {            RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);            def.setSource(source);            beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));        }        // 设置处理JSR-250提供的注解的bean后处理器。        // 比如，@Resource, @PostConstruct, @PreDestroy, @EJB, @WebServiceRef        // 检查系统是否支持JSR-250        // 定义常量COMMON_ANNOTATION_PROCESSOR_BEAN_NAME        // = "org.springframework.context.annotation.internalCommonAnnotationProcessor";        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));        }        // 注册用于处理JPA注解的bean后处理器        // 检查系统是否支持JPA        // 定义常量PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME        // = "org.springframework.context.annotation.internalPersistenceAnnotationProcessor"        if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {            RootBeanDefinition def = new RootBeanDefinition();            try {                // 定义常量PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME                // = "org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"                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));        }        // 设置用于处理@EventListener注解的bean后处理器        // 定义常量EVENT_LISTENER_PROCESSOR_BEAN_NAME        // = "org.springframework.context.event.internalEventListenerProcessor"        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));        }        // 设置用于生产ApplicationListener对象的EventListenerFactory对象        // 定义常量EVENT_LISTENER_FACTORY_BEAN_NAME        // = "org.springframework.context.event.internalEventListenerFactory"        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;    }

AnnotationConfigUtils用于注册BeanDefinition的registerPostProcessor静态方法的代码如下。

    /**    * 注册RootBeanDefinition对象    **/    private static BeanDefinitionHolder registerPostProcessor(            BeanDefinitionRegistry registry, RootBeanDefinition definition, String beanName) {        definition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);        registry.registerBeanDefinition(beanName, definition);        return new BeanDefinitionHolder(definition, beanName);    }

总结

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（1）annotation-config用于提供bean生命周期管理、bean的依赖注入以及动态注册事件监听器的注解处理器。

（2）annotation-config简化了我们的xml配置。在xml文件中，我们只需要通过bean标签定义bean，并不需要设置bean的依赖，也不需要设置bean的init和destroy方法。