Spring中具有多个实现类的接口集合是如何自动注入的？（List如何autowired）

与朋友聊到了一个博客上给出的demo，一个接口集合可以自动注入所有实现类，这种spring注入的写法在工作中还不太常见，一般这种我们见框架里使用SPI的形式居多

测试代码

// 定义接口
public interface MultiService {
}

// 实现类1
@Service
public class MultiServiceImpl1 implements MultiService {
}

// 实现类2
@Service
public class MultiServiceImpl2 implements MultiService {
}

// 注入测试
@Service
public class TestService {

    @Autowired
    private List<MultiService> multiServices;

}

经测试，multiServices 会成功被注入两个实现类，这是如何做到的呢？

了解SpringIOC过程的伙伴们都知道，multiServices作为TestService类的成员变量，会在创建TestService这个Bean的时候被作为属性进行填充，将这个“populateBean”作为debug的起点

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {

    ...
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
            // 进行属性填充
      populateBean(beanName, mbd, instanceWrapper);
      exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    catch (Throwable ex) {
      if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
        throw (BeanCreationException) ex;
      }
      else {
        throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
      }
    }

    ...

    return exposedObject;
  }

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {

        ...

        if (hasInstAwareBpps) {
            if (pvs == null) {
                pvs = mbd.getPropertyValues();
            }
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    // 使用AutowiredAnnotationBeanPostProcessor后置处理器填充autowired的属性
                    PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        if (filteredPds == null) {
                            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                        }
                        pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvsToUse == null) {
                            return;
                        }
                    }
                    pvs = pvsToUse;
                }
            }
        }
        
    ...

    }

org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessProperties

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

一路跟进metadata.inject方法：

1. org.springframework.beans.factory.annotation.InjectionMetadata#inject
2. org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject 进入 beanFactory.resolveDependency
3. org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveDependency
4. org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency
5. org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveMultipleBeans 进入 findAutowireCandidates
6. org.springframework.beans.factory.support.DefaultListableBeanFactory#findAutowireCandidates 进入 BeanFactoryUtils.beanNamesForTypeIncludingAncestors
7. org.springframework.beans.factory.BeanFactoryUtils#beanNamesForTypeIncludingAncestors(org.springframework.beans.factory.ListableBeanFactory, java.lang.Class<?>, boolean, boolean)
8. org.springframework.beans.factory.support.DefaultListableBeanFactory#getBeanNamesForType(java.lang.Class<?>, boolean, boolean)
9. org.springframework.beans.factory.support.DefaultListableBeanFactory#doGetBeanNamesForType 遍历所有bean定义名称，获取bean并校验是否符合传入的指定类型（是否为其子类、实现类）
10. org.springframework.beans.factory.support.AbstractBeanFactory#isTypeMatch(java.lang.String, org.springframework.core.ResolvableType)
11. org.springframework.core.ResolvableType#isAssignableFrom(java.lang.Class<?>)

方法中获取到了bean的依赖属性并进行了填充，完成了属性自动注入