@Autowired是一个用于注入容器的注解他是按照类型查找bean  NoSuchBeanDefinitionException如果么有相关类型；如果有两个相同类型的bean则按照名字查询bean如果按照名字也没有找到则会抛出NoUniqueBeanDefinitionException
 

@Autowired注解可以对类成员变量、方法和构造函数进行标注，完成自动装配的工作

autowire属性,自动装配（按照类型装配，通过set方法，且方法可以省略）

注解实现注入主要是通过 bean 后置处理器 BeanPostProcessor 接口的实现类来生效的。BeanPostProcessor 后置处理器是在 spring 容器启动时，创建 bean 对象实例后,马上执行的，对 bean 对象实例进行加工处理。@Autowired 是通过 BeanPostProcessor 接口的实现类AutowiredAnnotationBeanPostProcessor 来实现对 bean 对象对其他 bean 对象的依赖注入的。

底层原理
在Spring源代码当中，Autowired注解位于包org.springframework.beans.factory.annotation之中

1.核心逻辑在buildAutowiringMetadata中
通过反射获取该类的每一个字段和方法，并且分别用 findAutowiredAnnotation方法遍历每一个字段和方法，如果有@Autowired注解修饰，则返回注解相关属性。最后这个方法返回的就是包含所有带有autowire注解修饰的一个InjectionMetadata集合。
 

private InjectionMetadata buildAutowiringMetadata(Class clazz) {if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {return InjectionMetadata.EMPTY;} else {List elements = new ArrayList();Class targetClass = clazz;do {List currElements = new ArrayList();ReflectionUtils.doWithLocalFields(targetClass, (field) -> {MergedAnnotation ann = this.findAutowiredAnnotation(field);if (ann != null) {if (Modifier.isStatic(field.getModifiers())) {if (this.logger.isInfoEnabled()) {this.logger.info("Autowired annotation is not supported on static fields: " + field);}return;}boolean required = this.determineRequiredStatus(ann);currElements.add(new AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement(field, required));}});ReflectionUtils.doWithLocalMethods(targetClass, (method) -> {Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);if (BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {MergedAnnotation ann = this.findAutowiredAnnotation(bridgedMethod);if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {if (Modifier.isStatic(method.getModifiers())) {if (this.logger.isInfoEnabled()) {this.logger.info("Autowired annotation is not supported on static methods: " + method);}return;}if (method.getParameterCount() == 0 && this.logger.isInfoEnabled()) {this.logger.info("Autowired annotation should only be used on methods with parameters: " + method);}boolean required = this.determineRequiredStatus(ann);PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);currElements.add(new AutowiredAnnotationBeanPostProcessor.AutowiredMethodElement(method, required, pd));}}});elements.addAll(0, currElements);targetClass = targetClass.getSuperclass();} while(targetClass != null && targetClass != Object.class);return InjectionMetadata.forElements(elements, clazz);}}

2.InjectionMetadata类

这个类由两部分组成: targetClass目标类和我们要获得的injectedElements集合。

 public InjectionMetadata(Class targetClass, Collection elements) {this.targetClass = targetClass;this.injectedElements = elements;}

3.实现注入逻辑

 调用InjectionMetadata中的公共inject方法遍历调用protect的inject方法

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {InjectionMetadata metadata = this.findAutowiringMetadata(beanName, bean.getClass(), pvs);try {metadata.inject(bean, beanName, pvs);return pvs;} catch (BeanCreationException var6) {throw var6;} catch (Throwable var7) {throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", var7);}}

4.调用InjectionMetadata中的公共inject 

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {Collection checkedElements = this.checkedElements;Collection elementsToIterate = checkedElements != null ? checkedElements : this.injectedElements;if (!((Collection)elementsToIterate).isEmpty()) {Iterator var6 = ((Collection)elementsToIterate).iterator();while(var6.hasNext()) {InjectionMetadata.InjectedElement element = (InjectionMetadata.InjectedElement)var6.next();element.inject(target, beanName, pvs);}}}

5.遍历调用protect的inject方法 

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, this.getResourceToInject(target, requestingBeanName));} else {if (this.checkPropertySkipping(pvs)) {return;}try {Method method = (Method)this.member;ReflectionUtils.makeAccessible(method);method.invoke(target, this.getResourceToInject(target, requestingBeanName));} catch (InvocationTargetException var5) {throw var5.getTargetException();}}}