当我们需要使用BeanPostProcessor时，直接在Spring配置文件中定义这些Bean显得比较笨拙，例如：使用@Autowired注解，必须事先在Spring容器中声明AutowiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor "/>
使用 @Required注解，就必须声明RequiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
类似地，使用@Resource、@PostConstruct、@PreDestroy等注解就必须声明 CommonAnnotationBeanPostProcessor；使用@PersistenceContext注解，就必须声明 PersistenceAnnotationBeanPostProcessor的Bean。
这样的声明未免太麻烦了吧，所以Spring为我们提供了一种极为方便注册这些BeanPostProcessor的方式，即使用<context:annotation- config/>隐式地向 Spring容器注册AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor这4个BeanPostProcessor。如下：
<context:annotation-config/>
官方文档的说明：
As always, you can register them as individual bean definitions, but they can also be implicitly registered by including the following tag in an XML-based Spring configuration (notice the inclusion of the context namespace):
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
</beans>
那么<context:annotation-config/> 这个标签时何时注入这些BeanPostProcessor的呢？
下面进行源码分析
在Spring容器解析配置文件中时，会加载beans标签中配置的URL,如下图
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.1.xsd
" >
（1）根据beans标签中配置的xsi:schemaLocation的URL到spring.handlers文件中，找到对应的NamespaceHandler
Paste_Image.png
比如我上面的是http://www.springframework.org/schema/context
那么将对应spring.handlers文件中的ContextNamespaceHandler
http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandler
（2）由上面的可见，根据这个http://www.springframework.org/schema/context可以找到ContextNamespaceHandler类，这个类是干啥用的呢？
public class ContextNamespaceHandler extends NamespaceHandlerSupport {
@Override
public void init() {
registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
}
}
就一个方法，在唯一一个方法中，可以看到一些很熟悉的单词。没错，它的作用就是创建这些标签所对应的解析类。
何为解析类？也就是所有的自定义命名空间（像mvc，context等）下的标签解析都是由BeanDefinitionParser接口的子类来完成的。
可以看到nnotation-config"的解析类是 AnnotationConfigBeanDefinitionParser
，那么进入这个类中看它是如何解析这个标签的
（3）
解析的方法是在parse方法中
@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
Object source = parserContext.extractSource(element);
// Obtain bean definitions for all relevant BeanPostProcessors.
Set<BeanDefinitionHolder> processorDefinitions =
//这里将注册获取到所有相关BeanPostProcessors的bean定义。 所以解析过程是在这个方法中
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;
}
根据上面代码可以看出真正的解析是在AnnotationConfigUtils.registerAnnotationConfigProcessors（）方法中进行的
进去这个方法看看
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<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));
}
//实例化AutowiredAnnotationBeanPostProcessor
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));
}
//实例化RequiredAnnotationBeanPostProcessor
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));
}
// 实例化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));
}
// 实例化PersistenceAnnotationBeanPostProcessor.
if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition();
try {
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));
}
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));
}
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;
}
代码可以看到，分别注册了AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor这4个BeanPostProcessor。
后记
这是我自己看源码总结的，错误之处请帮忙指出，大家一起进步。
[朝阳区尼克杨]
转载请注明原创出处，谢谢啦！
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