关于Spring 中的context:annotation-config 推荐

我们在Spring配置文件中会习惯性的协商context:annotation-config.今天我们就来研究下这个配置的作用。

我们从http://www.springframework.org/schema/context/spring-context-3.0.xsd 中找到了Spring 3.0对这个标注的解释：

<xsd:element name="annotation-config">
<xsd:annotation>
<xsd:documentation>
<![CDATA[
Activates various annotations to be detected in bean classes: Spring's @Required and @Autowired, as well as JSR 250's @PostConstruct, @PreDestroy and @Resource (if available), JAX-WS's @WebServiceRef (if available), EJB3's @EJB (if available), and JPA's @PersistenceContext and @PersistenceUnit (if available). Alternatively, you may choose to activate the individual BeanPostProcessors for those annotations. Note: This tag does not activate processing of Spring's @Transactional or EJB3's @TransactionAttribute annotation. Consider the use of the <tx:annotation-driven> tag for that purpose.
]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>

从这里可以看出它的描述是用来激活各种对于bean类的注解标注，这些标注可以是Spring的 (如@Required)， 也可以是其他框架的,比如EJB3的，JPA的。

那么我们肯定要具体研究下这个注解如何影响这些注解的呢？

对于这个配置文件的java处理类是org.springframework.context.annotation.AnnotationConfigBeanDefinitionParser 类

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);

// 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;
}

}

从这里我们可以很明显的看出来，在第04行，它会让ParseContext来提取这个配置元素。其中ParseContext是Spring框架中一个专门用于解析Spring的xml-based上下文配置文件的解析器。

然后第07-08行它会去读取所有Bean的用annotation标注的处理器，我们看下它到底分析了哪些标注，在AnnotationConfigUtils类中。

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)) {
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)) {
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)) {
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)) {
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;
}

正如开始我们说的，它会除了处理Spring中的@Autowired,@Required，还会处理JSR-250，JPA等注解。

然后在第10-11行，它会吧所有用annotation标注的bean类向CompositeComponentDefinition注册。

然后在第14-15行，它会遍历这些处理器的定义，并且嵌套在具体bean上。