SpringBoot扩展篇：Spring注入 @Autowired & @Resource

1. 概述

1.1 职责

@Autowired注入是由AutowiredAnnotationBeanPostProcessor完成的。
@Resource注入是由CommonAnnotationBeanPostProcessor完成的。
由于两种方式代码重复率太高，原理基本一致，下面我们主要以AutowiredAnnotationBeanPostProcessor源码分析为主。

1.2 流程概述

AutowiredAnnotationBeanPostProcessor负责属性、方法的注入。
分为两个流程：

1. 注册元数据：先将要注入的字段和方法组成元数据，放入缓存中。
2. 赋值：通过缓存中的描述信息从beanFactory中获取bean，字段和方法注入。

2. Demo

AutoObjectA

@Component
public class AutoObjectA {
   
   

    @Autowired
    private AutoObjectB autoObjectB;
    
}

AutoObjectB

@Component
public class AutoObjectB {
   
   

}

3. AutowiredAnnotationBeanPostProcessor注册

在创建AnnotationConfigServletWebServerApplicationContext的时候，就已经注册了AutowiredAnnotationBeanPostProcessor和CommonAnnotationBeanPostProcessor。
SpringBoot 源码解析2：启动流程

AnnotationConfigUtils#registerAnnotationConfigProcessors

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
		BeanDefinitionRegistry registry, @Nullable 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<>(8);

	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));
	}

	// 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 {
   
   
			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;
}

4. 注册元数据

SpringBoot 源码解析7：Bean的创建② AbstractAutowireCapableBeanFactory#createBean

在doCreateBean方法源码中，我们可以知道，在实例化bean之后，会通过applyMergedBeanDefinitionPostProcessors方法回调MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition。

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
   
   
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
   
   
		if (bp instanceof MergedBeanDefinitionPostProcessor) {
   
   
			MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
			bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
		}
	}
}

AutowiredAnnotationBeanPostProcessor继承了MergedBeanDefinitionPostProcessor

元数据的注册的核心逻辑就在AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition

4.1 AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?