當(dāng)我們需要使用BeanPostProcessor時(shí)，直接在Spring配置文件中定義這些Bean顯得比較笨拙，例如：使用@Autowired注解酿炸，必須事先在Spring容器中聲明AutowiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor "/>
　　使用 @Required注解，就必須聲明RequiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
　　類似地宾添，使用@Resource、@PostConstruct、@PreDestroy等注解就必須聲明 CommonAnnotationBeanPostProcessor缕陕；使用@PersistenceContext注解粱锐，就必須聲明 PersistenceAnnotationBeanPostProcessor的Bean。
　　這樣的聲明未免太麻煩了吧扛邑，所以Spring為我們提供了一種極為方便注冊(cè)這些BeanPostProcessor的方式怜浅，即使用<context:annotation- config/>隱式地向 Spring容器注冊(cè)AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor蔬崩、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor這4個(gè)BeanPostProcessor恶座。如下：
<context:annotation-config/>

官方文檔的說明：

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/> 這個(gè)標(biāo)簽時(shí)何時(shí)注入這些BeanPostProcessor的呢？

下面進(jìn)行源碼分析

在Spring容器解析配置文件中時(shí)舱殿，會(huì)加載beans標(biāo)簽中配置的URL,如下圖

<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）根據(jù)beans標(biāo)簽中配置的xsi:schemaLocation的URL到spring.handlers文件中奥裸，找到對(duì)應(yīng)的NamespaceHandler

Paste_Image.png

比如我上面的是http://www.springframework.org/schema/context
那么將對(duì)應(yīng)spring.handlers文件中的ContextNamespaceHandler

http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandler

（2）由上面的可見，根據(jù)這個(gè)http://www.springframework.org/schema/context可以找到ContextNamespaceHandler類沪袭，這個(gè)類是干啥用的呢？

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());
    }

}

就一個(gè)方法樟氢，在唯一一個(gè)方法中冈绊，可以看到一些很熟悉的單詞。沒錯(cuò)埠啃，它的作用就是創(chuàng)建這些標(biāo)簽所對(duì)應(yīng)的解析類死宣。
何為解析類？也就是所有的自定義命名空間（像mvc碴开，context等）下的標(biāo)簽解析都是由BeanDefinitionParser接口的子類來完成的毅该。
可以看到nnotation-config"的解析類是 AnnotationConfigBeanDefinitionParser
，那么進(jìn)入這個(gè)類中看它是如何解析這個(gè)標(biāo)簽的
（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 =
//這里將注冊(cè)獲取到所有相關(guān)BeanPostProcessors的bean定義潦牛。    所以解析過程是在這個(gè)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;
    }

根據(jù)上面代碼可以看出真正的解析是在AnnotationConfigUtils.registerAnnotationConfigProcessors（）方法中進(jìn)行的
進(jìn)去這個(gè)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));
        }
//實(shí)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));
        }
//實(shí)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));
        }

        // 實(shí)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));
        }

        // 實(shí)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;
    }

代碼可以看到眶掌，分別注冊(cè)了AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor巴碗、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor這4個(gè)BeanPostProcessor朴爬。

后記

這是我自己看源碼總結(jié)的，錯(cuò)誤之處請(qǐng)幫忙指出橡淆，大家一起進(jìn)步召噩。

[朝陽區(qū)尼克楊]
轉(zhuǎn)載請(qǐng)注明原創(chuàng)出處，謝謝啦逸爵！