关于<context:component-scan/>与<context:annotation-config>问题

   最近做项目发现Spring核心配置文件中的<context:component-scan/>与<context:annotation-config/>配置经常一起出现，但有时候又单独出现，而且没有出现问题。今天抽空看了看源码，发现原来这俩个标签的处理逻辑及其相似(在处理我们经常使用的几个注解上基本如出一辙)。

   起初，添加<context:annotation-config/>标签，想必是为了我们大家使用像@Autowired、@Resource、@PostConstruct、@PreDestroy等等标签方便，以为之前我们注册BeanPostProcessor都是直接在Spring配置文件中使用<bean/>标签声明的方式,如果使用的注解一旦多起来，像我这样的懒人实在是难以接受。像我们常用的几个：

@Autowired

<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor "/>

 @Resource/@PostConstruct/@PreDestroy

<bean class="org.springframework.context.annotation.CommonAnnotationBeanPostProcessor"/>

 @Required

<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>

 @PersistenceContext

<bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/>

 

而<context:annotation-config/>的出现解放了我们的体力，只需一行配置就搞定了。

上源码：

<context:annotation-config/>由AnnotationConfigBeanDefinitionParser解析：

public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser {

	public BeanDefinition parse(Element element, ParserContext parserContext) {
		Object source = parserContext.extractSource(element);

		// Obtain bean definitions for all relevant BeanPostProcessors.
		Set<BeanDefinitionHolder> processorDefinitions =
				AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);//此处为BeanPostProcessor 注册部分

		// Register component for the surrounding <context:annotation-config> element.
		CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);
		parserContext.pushContainingComponent(compDefinition);

		// Nest the concrete beans in the surrounding component.
		for (BeanDefinitionHolder processorDefinition : processorDefinitions) {
			parserContext.registerComponent(new BeanComponentDefinition(processorDefinition));
		}

		// Finally register the composite component.
		parserContext.popAndRegisterContainingComponent();

		return null;
	}

}

 AnnotationConfigUtils.registerAnnotationConfigProcessor(..)

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, Object source) {

		Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<BeanDefinitionHolder>(4);

		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));
		}

		if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                        //@Autowired等
			RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {                   //@Required等
			RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
		if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {    //@PostConstruct、@PreDestroy、@Resource等
			RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
		if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {  //@PersistenceContext
			RootBeanDefinition def = new RootBeanDefinition();
			try {
				ClassLoader cl = AnnotationConfigUtils.class.getClassLoader();
				def.setBeanClass(cl.loadClass(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME));
			}
			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));
		}

		return beanDefs;
	}

 

 以上我们可以清晰的看到标签<context:annotation-config/>的解析过程以及注解相关的BeanPostProcessor的注册过程。接下来我们来看看<context:component-scan/>的逻辑：

ComponentScanBeanDefinitionParser

public class ComponentScanBeanDefinitionParser implements BeanDefinitionParser {

public BeanDefinition parse(Element element, ParserContext parserContext) {
		String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
				ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);

		// Actually scan for bean definitions and register them.
		ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);         //扫描指定路径下的类，并注册BeanPostProcessor
		Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
		registerComponents(parserContext.getReaderContext(), beanDefinitions, element);

		return null;
	}




}

 

 

protected void registerComponents(
			XmlReaderContext readerContext, Set<BeanDefinitionHolder> beanDefinitions, Element element) {
	 ... ...
		// Register annotation config processors, if necessary.
		boolean annotationConfig = true;
		if (element.hasAttribute(ANNOTATION_CONFIG_ATTRIBUTE)) {
			annotationConfig = Boolean.valueOf(element.getAttribute(ANNOTATION_CONFIG_ATTRIBUTE));
		}
		if (annotationConfig) {
			Set<BeanDefinitionHolder> processorDefinitions =
					AnnotationConfigUtils.registerAnnotationConfigProcessors(readerContext.getRegistry(), source);
			for (BeanDefinitionHolder processorDefinition : processorDefinitions) {
				compositeDef.addNestedComponent(new BeanComponentDefinition(processorDefinition));
			}
		}
		... ...
	}

     我们可以发现Spring默认<context:component-scan/>的annotation-config的属性值为true，当其为true时，才对一些BeanPostProcessor进行注册，也就是完成<context:annotation-config/>的任务。

  从以上源码我们可以看出如果我们在使用<context:component-scan/>时不将其annotation-config属性设置为false的话，我们就可以不配置<context:annotation-config/>。