springMVC之<context:annotation-config />标签

springMVC的配置文件中经常见到<context:annotation-config/>，那么这句话的作用到底是什么呢？

现在的注解非常方便，但是系统如何才能识别注解呢，这就需要相应的处理程序了，springMVC使用AnnotationBeanPostProcessor让系统能够识别相应的注解

例如：如果你想使用Autowired注解，那么必须先在spring容器中声明AutoWiredAnnotationBeanPostProccessor的实例，如要使用@Required注解就必须先声明RequiredAnnotationBeanPostProccessor，一般的方法是在配置文件挨个定义：

//@AutoWired注解处理器
<bean class="org.springframework.beans.factory.annotation. AutowiredAnnotationBeanPostProcessor "/> 
//@Required注解处理器
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/> 

然而对于这些常见的注解，每次显示定义很麻烦，所以springMVC给我们提供了隐式定义的方法，即<context:annotation-config/>标签。

下面看看<context:annotation-config/>标签注册过程及注册了哪些实例到容器。

首先找到实例定义接口BeanDefinitionParser

public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser {
    public BeanDefinition parse(Element element, ParserContext parserContext) {
        Object source = parserContext.extractSource(element);
　　　　　//得到一个set集合，此集合中就是各个注解的处理器(AnnotationBeanPostProcessor)和一些事件监听处理器
        Set 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));
        }

        parserContext.popAndRegisterContainingComponent();

        return null;
    }
}

查看registerAnnotationConfigProcessors方法

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);
            }
            if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
                beanFactory
                        .setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
            }
        }

        Set beanDefs = new LinkedHashSet(4);

        if (!(registry
                .containsBeanDefinition("org.springframework.context.annotation.internalConfigurationAnnotationProcessor"))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    ConfigurationClassPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(
                    registry,
                    def,
                    "org.springframework.context.annotation.internalConfigurationAnnotationProcessor"));
        }
　　　　　//Autowired注解处理器
        if (!(registry
                .containsBeanDefinition("org.springframework.context.annotation.internalAutowiredAnnotationProcessor"))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    AutowiredAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.annotation.internalAutowiredAnnotationProcessor"));
        }
　　　　　//Required注解处理器　　
        if (!(registry
                .containsBeanDefinition("org.springframework.context.annotation.internalRequiredAnnotationProcessor"))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    RequiredAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.annotation.internalRequiredAnnotationProcessor"));
        }
　　　　　//@Resource、@PostConstruct、@PreDestroy等注解处理器
        if ((jsr250Present)
                && (!(registry
                        .containsBeanDefinition("org.springframework.context.annotation.internalCommonAnnotationProcessor")))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    CommonAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.annotation.internalCommonAnnotationProcessor"));
        }

        if ((jpaPresent)
                && (!(registry
                        .containsBeanDefinition("org.springframework.context.annotation.internalPersistenceAnnotationProcessor")))) {
            RootBeanDefinition def = new RootBeanDefinition();
            try {
                def.setBeanClass(ClassUtils
                        .forName(
                                "org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor",
                                AnnotationConfigUtils.class.getClassLoader()));
            } catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Cannot load optional framework class: org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor",
                        ex);
            }

            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.annotation.internalPersistenceAnnotationProcessor"));
        }

        if (!(registry
                .containsBeanDefinition("org.springframework.context.event.internalEventListenerProcessor"))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    EventListenerMethodProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.event.internalEventListenerProcessor"));
        }
        if (!(registry
                .containsBeanDefinition("org.springframework.context.event.internalEventListenerFactory"))) {
            RootBeanDefinition def = new RootBeanDefinition(
                    DefaultEventListenerFactory.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def,
                    "org.springframework.context.event.internalEventListenerFactory"));
        }

        return beanDefs;
    }

for循环逐个注册实例，registerComponent方法

public void registerComponent(ComponentDefinition component) {
        CompositeComponentDefinition containingComponent = getContainingComponent();
        if (containingComponent != null) {
            containingComponent.addNestedComponent(component);
        } else
            this.readerContext.fireComponentRegistered(component);
    }

addNestedComponent方法（省略部分代码）

public class CompositeComponentDefinition extends AbstractComponentDefinition {

　　private final List<ComponentDefinition> nestedComponents = new LinkedList();
　　public void addNestedComponent(ComponentDefinition component) {
        Assert.notNull(component, "ComponentDefinition must not be null");
        this.nestedComponents.add(component);
    }

}

可看到，最终实例存到了一个链表中，整个bean注册过程基本完成。 

不过，该标签的功能被

<context:component-scan base-package=”**.**”/>

标签所包含，及扫描时也会注入上述实例，所以，在配置了自动扫描包配置之后，本标签可省略不配置。