@Autowried entry and source code analysis

Man of few words said, directly on the code:

Declare an interface userDao:

package ioc.hello;

public interface UserDao {
    public void test();
}

2 implementation class:

package ioc.hello;

import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;
@Component("userdao")
public class UserDaoImpl implements  UserDao {
    public void test() {
        System.out.println("userDaoImpl...");
    }
}

package ioc.hello;

import org.springframework.stereotype.Component;

@Component("userdao2")
public class UserDaoImpl2 implements  UserDao {
    public void test() {
        System.out.println("userDaoImpl2...");
    }
}

CHARGING UserService Interface:

package ioc.hello;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component("userservice")
public class UserService {
    @Autowired
    private UserDao dao;
    public void test() {
        dao.test();
    }
}

Test categories:

package ioc.hello;
import ioc.AppConfig;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class TestIOC {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext();
        annotationConfigApplicationContext.register(AppConfig.class);
        annotationConfigApplicationContext.refresh();
        UserService service = annotationConfigApplicationContext.getBean(UserService.class);
        service.test();
    }
}

我们直接运行以上代码的话讲道理是会报错的，为什么呢？因为我们的UserDao接口有2个实现类，但是我们在注入的时候注入的是接口类型，而且注入的名称是dao；
看效果吧！

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'ioc.hello.UserDao' available: expected single matching bean but found 2: userdao,userdao2
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:215)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1139)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1088)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:581)
    ... 13 more

上面的报错相信大家都很熟悉吧。

如果我们把注入的接口的名称修改为userdao或者是userdao2的话就会不报错了，当然我们也可以注入dao，但是在注入的时候我们需要做一下处理，可以在要注入的实现类加一个@Primary注解，也可以在注入的时候加一个@Qualifier（或者是Resource）注解，指定注入的对象，这个知识点此处不做介绍；

package ioc.hello;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component("userservice")
public class UserService {
    @Autowired
    private UserDao userdao;
    public void test() {
        userdao.test();
    }
}

运行结果：

userDaoImpl...

下面我们就此看以下Spring中的源码吧！

首先不得不说，我们的@Autowired注入是在对象创建的时候一起注入的，换句话说就是比如我们上面的案例中，在UserService中注入接口，是在UserService对象创建的时候注入的，看下面的代码，对象创建之前初始化环境的代码太多，这里就不分析了：

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

调用了doCreateBean方法；在此处打上条件断点，当beanName.equals("userservice")，然后调用了populateBean方法；

// 调用doCreateBean 创建bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);

//设置属性，非常重要
populateBean(beanName, mbd, instanceWrapper);

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

getBeanPostProcessors()方法会得到所有的后置处理器，遍历以后，当后置处理器为AutowriedAnnotationPoetProcessor的时候完成了@Autowried注入

if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
   for (BeanPostProcessor bp : getBeanPostProcessors()) {
      if (bp instanceof InstantiationAwareBeanPostProcessor) {
         InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
         if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
            continueWithPropertyPopulation = false;
            break;
         }
      }
   }
}

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

//findAutowiringMetadata方法会找到注入的所有类的属性
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
   metadata.inject(bean, beanName, pvs);
}

private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
   // Fall back to class name as cache key, for backwards compatibility with custom callers.
   String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
   //injectionMetadataCache可以理解为，是所有的bean，从所有的bean中得到service
   InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
   if (InjectionMetadata.needsRefresh(metadata, clazz)) {
      synchronized (this.injectionMetadataCache) {
         metadata = this.injectionMetadataCache.get(cacheKey);
         if (InjectionMetadata.needsRefresh(metadata, clazz)) {
            if (metadata != null) {
               metadata.clear(pvs);
            }
            metadata = buildAutowiringMetadata(clazz);
            this.injectionMetadataCache.put(cacheKey, metadata);
         }
      }
   }
   return metadata;
}

org.springframework.beans.factory.annotation.InjectionMetadata#inject

@Override
   protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
       //得到dao的属性名称
       Field field = (Field) this.member;
      Object value;
      if (this.cached) {
         value = resolvedCachedArgument(beanName, this.cachedFieldValue);
      }
      else {
         DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
         desc.setContainingClass(bean.getClass());
         Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
         Assert.state(beanFactory != null, "No BeanFactory available");
         TypeConverter typeConverter = beanFactory.getTypeConverter();
         try {
             //得到注入的对象，resolveDependency方法看下面
            value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
         }
         catch (BeansException ex) {
            throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
         }
         synchronized (this) {
            if (!this.cached) {
               if (value != null || this.required) {
                  this.cachedFieldValue = desc;
                  registerDependentBeans(beanName, autowiredBeanNames);
                  if (autowiredBeanNames.size() == 1) {
                     String autowiredBeanName = autowiredBeanNames.iterator().next();
                     if (beanFactory.containsBean(autowiredBeanName) &&
                           beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                        this.cachedFieldValue = new ShortcutDependencyDescriptor(
                              desc, autowiredBeanName, field.getType());
                     }
                  }
               }
               else {
                  this.cachedFieldValue = null;
               }
               this.cached = true;
            }
         }
      }
      if (value != null) {
         ReflectionUtils.makeAccessible(field);
         //用field.set方法注入,把value注入到了bean里面
         field.set(bean, value);
      }
   }
}

org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveDependency

if (result == null) {
   result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}

org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency

@Nullable
    public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
            @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

        InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
        try {
            Object shortcut = descriptor.resolveShortcut(this);
            if (shortcut != null) {
                return shortcut;
            }

            Class<?> type = descriptor.getDependencyType();
            Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
            if (value != null) {
                if (value instanceof String) {
                    String strVal = resolveEmbeddedValue((String) value);
                    BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
                    value = evaluateBeanDefinitionString(strVal, bd);
                }
                TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                return (descriptor.getField() != null ?
                        converter.convertIfNecessary(value, type, descriptor.getField()) :
                        converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
            }

            Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
            if (multipleBeans != null) {
                return multipleBeans;
            }
　　　　//从Spring工厂中得到该接口下所有的实现类的bean；这个方法最终是通过beanDefinitionNames得到的bean

               Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor); if (matchingBeans.isEmpty()) { if (isRequired(descriptor)) { raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor); } return null; } String autowiredBeanName; Object instanceCandidate; 　　　　　　 //如果有多个实现类的话就走这边，要遍历这些bean找到适合的对象返回 if (matchingBeans.size() > 1) { autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor); if (autowiredBeanName == null) { if (isRequired(descriptor) || !indicatesMultipleBeans(type)) { return descriptor.resolveNotUnique(type, matchingBeans); } else { // In case of an optional Collection/Map, silently ignore a non-unique case: // possibly it was meant to be an empty collection of multiple regular beans // (before 4.3 in particular when we didn't even look for collection beans). return null; } } instanceCandidate = matchingBeans.get(autowiredBeanName); } else {
　　　　　　　　   //如果接口只有一个实现类的话，就直接得到第一个实现类作为bean返回了
                // We have exactly one match.
                Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
                autowiredBeanName = entry.getKey();
                instanceCandidate = entry.getValue();
            }

            if (autowiredBeanNames != null) {
                autowiredBeanNames.add(autowiredBeanName);
            }
            if (instanceCandidate instanceof Class) {
                instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
            }
            Object result = instanceCandidate;
            if (result instanceof NullBean) {
                if (isRequired(descriptor)) {
                    raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
                }
                result = null;
            }
            if (!ClassUtils.isAssignableValue(type, result)) {
                throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
            }
            return result;
        }
        finally {
            ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
        }
    }

以上代码中：如果接口只有一个实现类的话就已经结束返回了，返回了接口的实现类，返回以后通过 field.set(bean, value);实现了注入；所以如果接口只有一个实现类的话，我们在注入接口的时候，属性名称可以随便写，他都可以注入成功；

下面就是有多个实现类的逻辑了；
我们来看下matchingBeans.size() > 1这段逻辑代码，调用了determineAutowireCandidate(matchingBeans, descriptor);

org.springframework.beans.factory.support.DefaultListableBeanFactory#determineAutowireCandidate

 

protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {
        Class<?> requiredType = descriptor.getDependencyType();
        //次处就是上面提到的处理@Primary注解的地方啦
        String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
        if (primaryCandidate != null) {
            return primaryCandidate;
        }
        String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
        if (priorityCandidate != null) {
            return priorityCandidate;
        }
        // Fallback
        for (Map.Entry<String, Object> entry : candidates.entrySet()) {
            String candidateName = entry.getKey();
            Object beanInstance = entry.getValue();
　　　　　　　//主要看这个方法：matchesBeanName； descriptor.getDependencyName()得到的是service类里面注入的接口的名称
            if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
                    matchesBeanName(candidateName, descriptor.getDependencyName())) {
                return candidateName;
            }
        }
        return null;
    }

org.springframework.beans.factory.support.DefaultListableBeanFactory#matchesBeanName

根据注入的属性的名称和bean的name做对比，是否equals，如果是的话就直接返回这个bean了，如果不是的话报错啦。

所以这个地方就解释了我们上面报错的问题了，注入了dao，会分别通过用userdao和userdao2和dao做字符串比较，如果两个都满足的话就会找不到bean，继而就报错，如果找到了就直接注入了找到的这个bean了。

 

protected boolean matchesBeanName(String beanName, @Nullable String candidateName) {
        return (candidateName != null &&
                (candidateName.equals(beanName) || ObjectUtils.containsElement(getAliases(beanName), candidateName)));
    }

到此就结束了。。。