1. 扫描当前类中标注@Autowired的属性和方法；
2. 再查找父类中注@Autowired的属性和方法，依次遍历；

1 @Autowired 干嘛的?

用来执行依赖注入.每当一个Spring管理的bean发现有该注解时,会直接注入相应的另一个Spring管理的bean.

• 属性
  Spring将通过扫描自定义的package或通过在配置文件中直接查找bean
• 方法
  使用@Autowired注解的每个方法都要用到依赖注入
  但要注意的是,签名中呈现的所有对象都必须是Spring所管理的bean
  如果你有一个方法，比如setTest(Article article, NoSpringArticle noSpringArt) ,其中只有一个参数 (Article article)是由Spring管理的，那么就将抛出一个org.springframework.beans.factory.BeanCreationException异常
  这是由于Spring容器里并没有指定的一个或多个参数所指向的bean，所以也就无法解析它们

如下相同类型的bean

<bean name="article1" class="com.sss.Article"><property name="text" value="Content of the 1st Article" />
</bean><bean name="article2" class="com.sss.Article"><property name="text" value="Content of the 2nd Article" />
</bean>

假如只是一个简单的@Autowired，Spring根本不知道你要注入哪个bean。这就需要@Qualifier(value =“beanName”)协作.

譬如,要从 com.javaedge.Article类型的bean中区分article1,article2:

@Qualifier(value="article1")
@Autowired
private Article firstArticle;@Qualifier(value="article2")
@Autowired
private Article secondArticle;

启动自动注入

<context:annotation-config />

放在应用程序上下文配置。可以使在遇到@Autowired注解时启用依赖注入

• bean

// beans first
public class Comment {private String content;public void setContent(String content) {this.content = content;}public String getContent() {return this.content;}}// sample controller
@Controller
public class TestController {@Qualifier(value="comment1")@Autowiredprivate Comment firstComment;@Qualifier(value="comment2")@Autowiredprivate Comment secondComment;@RequestMapping(value = "/test", method = RequestMethod.GET)public String test() {System.out.println("1st comment text: "+firstComment.getText());System.out.println("2nd comment text: "+secondComment.getText());return "test";}}// no-Spring managed class
public class TestNoSpring {@Autowiredprivate Comment comment;public void testComment(String content) {if (comment == null) {System.out.println("Comment's instance wasn't autowired because this class is not Spring-managed bean");} else {comment.setContent(content);System.out.println("Comment's content: "+comment.getContent());}}}

•  
• 配置

<bean name="comment1" class="com.sss.exchanger.Comment"><property name="content" value="Content of the 1st comment" />
</bean><bean name="comment2" class="com.sss.exchanger.Comment"><property name="content" value="Content of the 2nd comment" />
</bean>

打开http://localhost:8080/test来运行TestController
TestController的注解字段正确地自动注入，而TestNoSpring的注解字段并没有注入进去

1st comment text: Content of the 1st comment
2nd comment text: Content of the 2nd comment
Comment's instance wasn't autowired because this class is not Spring-managed bean

哪里不对 ？TestNoSpring类不由Spring所管理
这就是为什么Spring不能注入Comment实例的依赖

Spring管理可用于整个应用程序的Java对象bean,我们不需要处理他们的生命周期(初始化，销毁)。该任务由此容器来完成。
该上下文具有入口点，在Web应用程序中，是dispatcherservlet。
容器(也就是该上下文)会在它那里被启动并且所有的bean都会被注入

看<context:annotation-config />的定义

<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), EJB 3's @EJB (if available), and JPA's@PersistenceContext and @PersistenceUnit (if available). Alternatively, you maychoose to activate the individual BeanPostProcessors for those annotations.Note: This tag does not activate processing of Spring's @Transactional or EJB 3's@TransactionAttribute annotation. Consider the use of the <tx:annotation-driven>tag for that purpose.See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContextfor information on code-based alternatives to bootstrapping annotation-driven support.]]></xsd:documentation></xsd:annotation></xsd:element>

类内部的注解，如@Autowired、@Value、@Required、@Resource以及Web Serivce相关的注解，是容器对Bean对象实例化和依赖注入时，通过容器中注册的Bean后置处理器处理这些注解的

所以配置了上面这个配置(<context:component-scan>假如有配置这个，那么就可以省略<context:annotation-config />)后，将隐式地向Spring容器注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及这4个专门用于处理注解的Bean后置处理器。

当 Spring 容器启动时，AutowiredAnnotationBeanPostProcessor 将扫描 Spring 容器中所有 Bean
当发现 Bean 中拥有@Autowired 注解时就找到和其匹配（默认按类型匹配）的 Bean
并注入到对应的地方中去。

通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入
通过这个类来处理@Autowired @Value Spring
它也可以管理JSR-303的@Inject

• 在AutowiredAnnotationBeanPostProcessor构造函数中定义要处理的注解
  
  

之后,有几种方法对@Autowired处理

第一个，private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz)解析等待自动注入类的所有属性。它通过分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata类的实例来实现。

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>();Class<?> targetClass = clazz;do {final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>();//分析所有字段ReflectionUtils.doWithLocalFields(targetClass, field -> {//findAutowiredAnnotation(field)此方法后面会解释AnnotationAttributes ann = findAutowiredAnnotation(field);if (ann != null) {if (Modifier.isStatic(field.getModifiers())) {if (logger.isWarnEnabled()) {logger.warn("Autowired annotation is not supported on static fields: " + field);}return;}boolean required = determineRequiredStatus(ann);currElements.add(new AutowiredFieldElement(field, required));}});//分析所有方法ReflectionUtils.doWithLocalMethods(targetClass, method -> {Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {return;}AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {if (Modifier.isStatic(method.getModifiers())) {if (logger.isWarnEnabled()) {logger.warn("Autowired annotation is not supported on static methods: " + method);}return;}if (method.getParameterCount() == 0) {if (logger.isWarnEnabled()) {logger.warn("Autowired annotation should only be used on methods with parameters: " +method);}}boolean required = determineRequiredStatus(ann);PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);currElements.add(new AutowiredMethodElement(method, required, pd));}});elements.addAll(0, currElements);targetClass = targetClass.getSuperclass();}while (targetClass != null && targetClass != Object.class);//返回一个InjectionMetadata初始化的对象实例return new InjectionMetadata(clazz, elements);}
.../*** 'Native' processing method for direct calls with an arbitrary target instance,* resolving all of its fields and methods which are annotated with {@code @Autowired}.* @param bean the target instance to process* @throws BeanCreationException if autowiring failed*/public void processInjection(Object bean) throws BeanCreationException {Class<?> clazz = bean.getClass();InjectionMetadata metadata = findAutowiringMetadata(clazz.getName(), clazz, null);try {metadata.inject(bean, null, null);}catch (BeanCreationException ex) {throw ex;}catch (Throwable ex) {throw new BeanCreationException("Injection of autowired dependencies failed for class [" + clazz + "]", ex);}}

InjectionMetadata类包含要注入的元素的列表

注入是通过Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj，Object … args)方法完成的
此过程直接在AutowiredAnnotationBeanPostProcessor的方法中调用
public void processInjection(Object bean) throws BeanCreationException
它将所有可注入的bean检索为InjectionMetadata实例，并调用它们的inject()方法

public class InjectionMetadata {...
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {Collection<InjectedElement> checkedElements = this.checkedElements;Collection<InjectedElement> elementsToIterate =(checkedElements != null ? checkedElements : this.injectedElements);if (!elementsToIterate.isEmpty()) {boolean debug = logger.isDebugEnabled();for (InjectedElement element : elementsToIterate) {if (debug) {logger.debug("Processing injected element of bean '" + beanName + "': " + element);}//看下面静态内部类的方法element.inject(target, beanName, pvs);}}}...public static abstract class InjectedElement {protected final Member member;protected final boolean isField;.../*** Either this or {@link #getResourceToInject} needs to be overridden.*/protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)throws Throwable {if (this.isField) {Field field = (Field) this.member;ReflectionUtils.makeAccessible(field);field.set(target, getResourceToInject(target, requestingBeanName));}else {if (checkPropertySkipping(pvs)) {return;}try {//具体的注入看此处咯Method method = (Method) this.member;ReflectionUtils.makeAccessible(method);method.invoke(target, getResourceToInject(target, requestingBeanName));}catch (InvocationTargetException ex) {throw ex.getTargetException();}}}...}
}

分析属于一个字段或一个方法的所有注解来查找@Autowired注解。如果未找到@Autowired注解，则返回null，字段或方法也就视为不可注入。