Spring 注解原理（一）组件注册

Spring 注解原理（一）组件注册

Spring 系列目录(https://www.cnblogs.com/binarylei/p/10198698.html)

当我们需要使用 Spring 提供的注解开发时，必须在 Spring 容器中声明相关的组件。如 @Autowired 必须注册 AutowiredAnnotationBeanPostProcessor 组件，如果每个组件都需要手动注册未免太麻烦了吧，所以 Spring 为我们提供了自动注入这些组件的方式。

常用注解对应的组件如下：

序号	组件	功能
1	AnnotationAwareOrderComparator	排序用，@Order @Priority
2	ContextAnnotationAutowireCandidateResolver	判断一个给定的对象是否可以注入 @Qualifier @Value
3	ConfigurationClassPostProcessor	@Configuration
4	AutowiredAnnotationBeanPostProcessor	@Autowired
5	RequiredAnnotationBeanPostProcessor	@Required
6	CommonAnnotationBeanPostProcessor	@Resource、@PostConstruct、@PreDestroy
7	PersistenceAnnotationBeanPostProcessor	@PersistenceContext
8	EventListenerMethodProcessor	@EventListener
9	DefaultEventListenerFactory	@EventListener

一、xml 配置

在 Spring 的源码中搜索 annotation-config 关键字，找到 ContextNamespaceHandler 类。

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());
    }
}

annotation-config 对应的解析器为 AnnotationConfigBeanDefinitionParser，查看其源码发现

public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser {
    @Override
    public BeanDefinition parse(Element element, ParserContext parserContext) {
        Object source = parserContext.extractSource(element);
        // 注册 annotation-config 注解驱动相关的组件
        Set<BeanDefinitionHolder> processorDefinitions =
                AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);
        return null;
    }
}

根据上面代码可以看出真正的解析是在 AnnotationConfigUtils.registerAnnotationConfigProcessors() 方法中

二、注解配置 AnnotatedBeanDefinitionReader

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
    Assert.notNull(environment, "Environment must not be null");
    this.registry = registry;
    this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
    AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

可以看到注解的配置的类也调用了 AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry) 方法。

三、AnnotationConfigUtils

registerAnnotationConfigProcessors 中注册了许多相关的组件。

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

    DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
    if (beanFactory != null) {
        // 1. 排序用，解析 @Order @Priority 注解或 PriorityOrdered Ordered 接口
        if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
            beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
        }
        // 2. 判断一个给定的对象是否可以注入 @Qualifier @Value
        if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
            beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
        }
    }

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

    // 3. @Configuration -> ConfigurationClassPostProcessor
    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));
    }
    // 4. @Autowired -> 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));
    }
    // 5. @Required -> 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));
    }

    // 6. @Resource、@PostConstruct、@PreDestroy -> 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));
    }

    // 7. @Persistence -> 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( ex);
        }
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
    }

    // 8. @Persistence -> EventListenerMethodProcessor
    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));
    }
    // 9. @EventListener -> DefaultEventListenerFactory
    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;
}

参考：

1. 《＜context:annotation-config／＞自动扫描标签详解》：https://www.jianshu.com/p/89f55286cf21

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