springMVC之 標簽

springMVC的配置文件中經常見到<context:annotation-config/>，那么這句話的作用到底是什么呢？

現在的注解非常方便，但是系統如何才能識別注解呢，這就需要相應的處理程序了，springMVC使用AnnotationBeanPostProcessor讓系統能夠識別相應的注解

例如：如果你想使用Autowired注解，那么必須先在spring容器中聲明AutoWiredAnnotationBeanPostProccessor的實例，如要使用@Required注解就必須先聲明RequiredAnnotationBeanPostProccessor，一般的方法是在配置文件挨個定義：

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

然而對於這些常見的注解，每次顯示定義很麻煩，所以springMVC給我們提供了隱式定義的方法，即<context:annotation-config/>標簽。

下面看看<context:annotation-config/>標簽注冊過程及注冊了哪些實例到容器。

首先找到實例定義接口BeanDefinitionParser

public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser { public BeanDefinition parse(Element element, ParserContext parserContext) { Object source = parserContext.extractSource(element); 　　　　　//得到一個set集合，此集合中就是各個注解的處理器(AnnotationBeanPostProcessor)和一些事件監聽處理器 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注解處理器 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注解處理器　　 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等注解處理器 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循環逐個注冊實例，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（省略部分代碼）

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，最終實例存到了一個鏈表中，整個bean注冊過程基本完成。 

不過，該標簽的功能被

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

標簽所包含，及掃描時也會注入上述實例，所以，在配置了自動掃描包配置之后，本標簽可省略不配置。