使用案例
前置条件： 现在有一个 Vehicle 接口，它有两个实现类 Bus 和 Car ，现在还有一个类 VehicleService 需要注入一个 Vehicle 类型的 Bean：
public interface Vehicle {}@Component
public class Car implements Vehicle {}@Component
public class Bus implements Vehicle {}
使用 @Autowired 注解注入 Bean：
@Autowired 注解可以和 @Qualifier 注解一起使用，在有多个符合条件的 Bean 的情况下限制注入特定名称的 Bean：
@Component
public class VehicleService {@Autowired@Qualifier("car") //假设这里是想要注入Bean名称为car的这个Beanprivate Vehicle vehicle;
}
使用 @Inject 注解注入 Bean：
@Inject 注解可以和 @Qualifier或者 @Named 注解一起使用，在有多个符合条件的 Bean 的情况下限制注入特定名称的 Bean：
@Component
public class VehicleService {@Inject@Qualifier("car") //假设这里是想要注入Bean名称为car的这个Beanprivate Vehicle vehicle;@Inject@Named("bus") //假设这里是想要注入Bean名称为bus的这个Beanprivate Vehicle anotherVehicle;
}
使用 @Resource 注解注入 Bean：
@Component
public class VehicleService {@Resource(name = "car")private Vehicle vehicle;
}
虽然以上三种使用方法都能够实现注入 Bean 的需求，但是它们在底层实现上有什么区别呢？
注解体系
在 Java EE 和 Spring 体系中定义了几套注解：
JSR 250：定义了 @PostConstruct，@PreDestroy，@Resource 注解，其中 @Resource 注解默认是按照名称进行注入。
JSR 330：定义了 @Inject，@Qualifier, @Named 注解，其中 @Inject 注解默认是按照类型进行注入，可以搭配 @Qualifier 或者@Named 注解实现按照名称注入。
Spring：定义了 @Autowired，@Qualifier注解，其中 @Autowired 注解默认是按照类型进行注入，可以搭配 @Qualifier 注解实现按照名称注入。
当前 JSR 250 定义的注解属于 jakarta.annotation-api，而 JSR 330 定义的注解属于 jakarta.inject-api。
实现原理
InstantiationAwareBeanPostProcessor 方法调用触发的位置：
Spring 中提供了 InstantiationAwareBeanPostProcessor 接口，它有一个 postProcessProperties() 负责实现对 Bean 的属性进行处理。
Spring 中提供了实现类 CommonAnnotationBeanPostProcessor 负责处理 @Resource 注解；提供了实现类 AutowiredAnnotationBeanPostProcessor 负责处理 @Autowired 注解和 @Inject 注解。
InstantiationAwareBeanPostProcessor的 postProcessProperties() 方法是在 AbstractAutowireCapableBeanFactory 中的 doCreateBean() 创建 Bean 的方法中触发调用的，在这个方法中的主要实现逻辑是实例化 Bean -> 填充 Bean 属性 -> 初始化 Bean。 代码如下：
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {//实例化Bean对象instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));if (earlySingletonExposure) {addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}Object exposedObject = bean;try {//填充Bean属性populateBean(beanName, mbd, instanceWrapper);//初始化BeanexposedObject = initializeBean(beanName, exposedObject, mbd);}
}
在填充 Bean 属性的方法 populateBean() 中实现了对 postProcessProperties() 方法的调用，在该方法实现对注解修饰的需要注入的字段进行赋值，即自动注入。 代码如下：
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) { //省略部分代码PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null); if (hasInstantiationAwareBeanPostProcessors()) { if (pvs == null) { pvs = mbd.getPropertyValues(); } //这里获取所有InstantiationAwareBeanPostProcessor接口的实现类for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) { //调用postProcessProperties()方法PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName); if (pvsToUse == null) { return; } pvs = pvsToUse; } }
}
InstantiationAwareBeanPostProcessor 注册的时机：
既然 InstantiationAwareBeanPostProcessor 是负责处理 Bean 的属性的自动注入的，那么它一定是在业务 Bean 创建之前就已经完成初始化了，这样在业务 Bean 创建的时候才能调用它的实例方法。它的初始化是在 Spring 上下文的基类 AbstractApplicationContext 的 refresh() 方法中完成的。代码如下：
public void refresh() throws BeansException, IllegalStateException {//省略其它代码//这里注册了InstantiationAwareBeanPostProcessorregisterBeanPostProcessors(beanFactory);//省略其它代码//这里创建所有的单例BeanfinishBeanFactoryInitialization(beanFactory);finishRefresh();
}
而在 registerBeanPostProcessors() 方法中又调用了 PostProcessorRegistrationDelegate 的 registerBeanPostProcessors() 方法来完成注册的。代码如下：
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}
在PostProcessorRegistrationDelegate 的 registerBeanPostProcessors() 方法真正实现注册逻辑。代码如下：
public static void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {//这里获取到所有实现了BeanPostProcessor接口的Bean名称//InstantiationAwareBeanPostProcessor接口继承了BeanPostProcessor接口String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);//遍历Bean名称调用BeanFactory.getBean()方法触发BeanPostProcessor Bean的创建//然后根据是否实现了PriorityOrdered接口、Ordered接口和其它分为三大类//分别将这三大类的BeanPostProcessor实例进行注册List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();List<String> orderedPostProcessorNames = new ArrayList<>();List<String> nonOrderedPostProcessorNames = new ArrayList<>();for (String ppName : postProcessorNames) {if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {//这里调用BeanFactory.getBean()方法触发BeanPostProcessor Bean的创建BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);priorityOrderedPostProcessors.add(pp);if (pp instanceof MergedBeanDefinitionPostProcessor) {internalPostProcessors.add(pp);}}else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {orderedPostProcessorNames.add(ppName);}else {nonOrderedPostProcessorNames.add(ppName);}}//首先注册实现了PriorityOrdered接口的BeanPostProcessorsortPostProcessors(priorityOrderedPostProcessors, beanFactory);registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);//然后触发实现了Ordered接口的BeanPostProcessor Bean的创建并注册List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());for (String ppName : orderedPostProcessorNames) {BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);orderedPostProcessors.add(pp);if (pp instanceof MergedBeanDefinitionPostProcessor) {internalPostProcessors.add(pp);}}sortPostProcessors(orderedPostProcessors, beanFactory);registerBeanPostProcessors(beanFactory, orderedPostProcessors);//最后触发其它BeanPostProcessor Bean的创建并注册List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());for (String ppName : nonOrderedPostProcessorNames) {BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);nonOrderedPostProcessors.add(pp);if (pp instanceof MergedBeanDefinitionPostProcessor) {internalPostProcessors.add(pp);}}registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);sortPostProcessors(internalPostProcessors, beanFactory);registerBeanPostProcessors(beanFactory, internalPostProcessors);
}
CommonAnnotationBeanPostProcessor 实现逻辑（以修饰字段为例）
首先在 CommonAnnotationBeanPostProcessor 的静态初始化块中初始化了它要处理的注解。代码如下：
static {//这里是为了适配不同版本@Resource注解在不同的包路径下jakartaResourceType = loadAnnotationType("jakarta.annotation.Resource");if (jakartaResourceType != null) {resourceAnnotationTypes.add(jakartaResourceType);}//这里是为了适配不同版本@Resource注解在不同的包路径下javaxResourceType = loadAnnotationType("javax.annotation.Resource");if (javaxResourceType != null) {resourceAnnotationTypes.add(javaxResourceType);}
}
在它的 postProcessProperties() 方法中主要实现逻辑为找到 @Resource 注解修饰的字段 -> 通过反射给字段赋值。代码如下：
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {//找@Resource注解修饰的字段InjectionMetadata metadata = findResourceMetadata(beanName, bean.getClass(), pvs);try {//给字段赋值metadata.inject(bean, beanName, pvs);}catch (Throwable ex) {throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);}return pvs;
}
找 @Resource 注解修饰的字段是在 findResourceMetadata() 方法中实现的，在该方法中又调用了 buildResourceMetadata() 来进行实际的查找，在这个方法中通过反射的方式遍历字段看它是否有 @Resource 注解修饰，如果是的话把它包装为一个 ResourceElement 对象放到列表中。最后基于列表构造一个 InjectionMetadata 对象返回。代码如下：
private InjectionMetadata findResourceMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());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);}//这里调用buildResourceMetadata()方法metadata = buildResourceMetadata(clazz);this.injectionMetadataCache.put(cacheKey, metadata);}}}return metadata;
}private InjectionMetadata buildResourceMetadata(Class<?> clazz) {List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();Class<?> targetClass = clazz;//省略部分代码do {final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();//这里就会遍历每个字段看字段是否有@Resource注解修饰有的话就加入到列表中ReflectionUtils.doWithLocalFields(targetClass, field -> {//省略部分代码if (jakartaResourceType != null && field.isAnnotationPresent(jakartaResourceType)) {if (Modifier.isStatic(field.getModifiers())) {throw new IllegalStateException("@Resource annotation is not supported on static fields");}if (!this.ignoredResourceTypes.contains(field.getType().getName())) {currElements.add(new ResourceElement(field, field, null));}}else if (javaxResourceType != null && field.isAnnotationPresent(javaxResourceType)) {if (Modifier.isStatic(field.getModifiers())) {throw new IllegalStateException("@Resource annotation is not supported on static fields");}if (!this.ignoredResourceTypes.contains(field.getType().getName())) {currElements.add(new LegacyResourceElement(field, field, null));}}});elements.addAll(0, currElements);targetClass = targetClass.getSuperclass();}while (targetClass != null && targetClass != Object.class);return InjectionMetadata.forElements(elements, clazz);
}
实际触发赋值的操作是在 InjectionMetadata 的 inject() 方法中实现的，在它的方法中又会循环调用 InjectedElement 的 inject() 方法。代码如下：
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()) {for (InjectedElement element : elementsToIterate) {element.inject(target, beanName, pvs);}}
}
在 InjectedElement 的 inject() 方法中通过反射的方式将找到的 Bean 赋值给字段。代码如下：
protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)throws Throwable {if (!shouldInject(pvs)) {return;}if (this.isField) {Field field = (Field) this.member;ReflectionUtils.makeAccessible(field);//这里通过反射的方式设置值，设置的值就是根据Bean名称获取到的Beanfield.set(target, getResourceToInject(target, requestingBeanName));} else {//省略其它代码}
}
在 ResourceElement 的 getResourceToInject() 方法中实现了查找逻辑：如果 BeanFactory 中包含这个 Bean 名称对应的 Bean 则直接根据名称查找，否则会根据类型进行匹配，这个就是常说的 @Resource 注解默认是按照名称进行匹配的，名称匹配不到的情况下再按照类型进行匹配。代码如下：
protected Object getResource(LookupElement element, @Nullable String requestingBeanName)throws NoSuchBeanDefinitionException {//省略代码// Regular resource autowiringif (this.resourceFactory == null) {throw new NoSuchBeanDefinitionException(element.lookupType,"No resource factory configured - specify the 'resourceFactory' property");}return autowireResource(this.resourceFactory, element, requestingBeanName);
}protected Object autowireResource(BeanFactory factory, LookupElement element, @Nullable String requestingBeanName)throws NoSuchBeanDefinitionException {Object resource;Set<String> autowiredBeanNames;String name = element.name;if (factory instanceof AutowireCapableBeanFactory autowireCapableBeanFactory) {//如果根据Bean名称找不到Bean且允许按照类型匹配的情况下走第一个分支if (this.fallbackToDefaultTypeMatch && element.isDefaultName && !factory.containsBean(name)) {autowiredBeanNames = new LinkedHashSet<>();resource = autowireCapableBeanFactory.resolveDependency(element.getDependencyDescriptor(), requestingBeanName, autowiredBeanNames, null);if (resource == null) {throw new NoSuchBeanDefinitionException(element.getLookupType(), "No resolvable resource object");}} else { //如果根据名称找得到Bean则直接根据名称获取Beanresource = autowireCapableBeanFactory.resolveBeanByName(name, element.getDependencyDescriptor());autowiredBeanNames = Collections.singleton(name);}} else {//省略代码}//省略代码return resource;
}
按照类型匹配的逻辑是在 DefaultListableBeanFactory 的 doResolveDependency() 方法中实现的，在该方法中会根据类型找到所有是当前类型的 Bean，然后构造一个 Map，key 是 Bean 的名称，value 是对应的 Bean 对象，如果找到的 Bean 个数大于 1 则会选择一个最符合条件的返回（选择的依据后面会讲到），如果等于 1 则直接返回这个 Bean。代码如下：
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);try {//省略代码//这里根据类型找到所有的Bean，然后Bean的名称作为key，Bean作为ValueMap<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);if (matchingBeans.isEmpty()) {// Step 4c (fallback): custom Collection / Map declarations for collecting multiple beansmultipleBeans = resolveMultipleBeansFallback(descriptor, beanName, autowiredBeanNames, typeConverter);if (multipleBeans != null) {return multipleBeans;}// Raise exception if nothing found for required injection pointif (isRequired(descriptor)) {raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);}return null;}String autowiredBeanName;Object instanceCandidate;//如果根据类型找到多个Bean则需要选择一个合适的Bean返回if (matchingBeans.size() > 1) {autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);if (autowiredBeanName == null) {if (isRequired(descriptor) || !indicatesArrayCollectionOrMap(type)) {// Raise exception if no clear match found for required injection pointreturn descriptor.resolveNotUnique(descriptor.getResolvableType(), 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 {//如果只有一个Bean则直接返回这个BeanMap.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();autowiredBeanName = entry.getKey();instanceCandidate = entry.getValue();}// Step 6: validate single resultif (autowiredBeanNames != null) {autowiredBeanNames.add(autowiredBeanName);}if (instanceCandidate instanceof Class) {instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);}return resolveInstance(instanceCandidate, descriptor, type, autowiredBeanName);}finally {ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);}
}
AutowiredAnnotationBeanPostProcessor 实现逻辑（以修饰字段为例）
首先在构造函数中初始化了需要处理的注解包括 @Autowired 和 @Inject 注解。代码如下：
public AutowiredAnnotationBeanPostProcessor() {//添加要处理@Autowired注解this.autowiredAnnotationTypes.add(Autowired.class);this.autowiredAnnotationTypes.add(Value.class);ClassLoader classLoader = AutowiredAnnotationBeanPostProcessor.class.getClassLoader();try {//这里是为了适配不同版本@Inject注解在不同的包路径下this.autowiredAnnotationTypes.add((Class<? extends Annotation>)ClassUtils.forName("jakarta.inject.Inject", classLoader));} catch (ClassNotFoundException ex) {// jakarta.inject API not available - simply skip.}try {//这里是为了适配不同版本@Inject注解在不同的包路径下this.autowiredAnnotationTypes.add((Class<? extends Annotation>)ClassUtils.forName("javax.inject.Inject", classLoader));} catch (ClassNotFoundException ex) {// javax.inject API not available - simply skip.}
}
在它的 postProcessProperties() 方法中主要实现逻辑为找到 @Autowired 或者 @Inject 注解修饰的字段 -> 通过反射给字段赋值。代码如下：
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;
}
找@Autowired 或者 @Inject 注解修饰的字段是在 findAutowiringMetadata() 方法中实现的，在该方法中又调用了 buildAutowiringMetadata() 来进行实际的查找，在这个方法中通过反射的方式遍历字段看它是否有 @Autowired 或者 @Inject 注解修饰，如果是的话把它包装为一个AutowiredFieldElement 对象放到列表中。最后基于列表构造一个 InjectionMetadata 对象返回。代码如下：
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());// Quick check on the concurrent map first, with minimal locking.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;
}private InjectionMetadata buildAutowiringMetadata(Class<?> clazz) {if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {return InjectionMetadata.EMPTY;}final List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();Class<?> targetClass = clazz;do {final List<InjectionMetadata.InjectedElement> fieldElements = new ArrayList<>();ReflectionUtils.doWithLocalFields(targetClass, field -> {//这里找到是否有@Autowired或者@Inject注解修饰MergedAnnotation<?> ann = findAutowiredAnnotation(field);if (ann != null) {if (Modifier.isStatic(field.getModifiers())) {return;}boolean required = determineRequiredStatus(ann);fieldElements.add(new AutowiredFieldElement(field, required));}});}
}
实际触发赋值的操作是在 InjectionMetadata 的 inject() 方法中实现的，在它的方法中又会循环调用 AutowiredFieldElement 的 inject() 方法。代码如下：
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()) {for (InjectedElement element : elementsToIterate) {element.inject(target, beanName, pvs);}}
}
在 InjectedElement 的 inject() 方法中通过反射的方式将找到的 Bean 赋值给字段。代码如下：
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {Field field = (Field) this.member;Object value;if (this.cached) {//省略代码} else {//找到对应的Beanvalue = resolveFieldValue(field, bean, beanName);}if (value != null) {ReflectionUtils.makeAccessible(field);//通过反射的方式赋值field.set(bean, value);}
}@Nullable
private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {DependencyDescriptor desc = new DependencyDescriptor(field, this.required);desc.setContainingClass(bean.getClass());Set<String> autowiredBeanNames = new LinkedHashSet<>(2);TypeConverter typeConverter = beanFactory.getTypeConverter();Object value;try {//调用beanFactory的resolveDependency()方法value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);} catch (BeansException ex) {throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);}return value;
}
然后会调用到 DefaultListableBeanFactory 的 doResolveDependency() 方法，和上面 @Resource 注解根据名称找不到 Bean 需要根据类型进行匹配的调用的是一个方法，只是它会多一个分支。在这个分支里面判断 Bean 名称对应的 Bean 是否存在，如果存在则直接返回，如果不存在才会按照类型去匹配，这里实际上还是先按照名称匹配的，名称匹配不上再走的类型匹配的逻辑。代码如下：
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);try {//省略代码//如果是@Autowired注解或者@Inject注解会先走到下面这个分支//在这个分支里面也会先判断对应Bean名称的Bean是否存在，如果存在//则直接获取返回，如果不存在才会按照类型去匹配if (descriptor.usesStandardBeanLookup()) {String dependencyName = descriptor.getDependencyName();if (dependencyName == null || !containsBean(dependencyName)) {String suggestedName = getAutowireCandidateResolver().getSuggestedName(descriptor);dependencyName = (suggestedName != null && containsBean(suggestedName) ? suggestedName : null);}if (dependencyName != null) {dependencyName = canonicalName(dependencyName); // dependency name can be alias of target nameif (isTypeMatch(dependencyName, type) && isAutowireCandidate(dependencyName, descriptor) &&!isFallback(dependencyName) && !hasPrimaryConflict(dependencyName, type) &&!isSelfReference(beanName, dependencyName)) {if (autowiredBeanNames != null) {autowiredBeanNames.add(dependencyName);}Object dependencyBean = getBean(dependencyName);return resolveInstance(dependencyBean, descriptor, type, dependencyName);}}}//这里根据类型找到所有的Bean，然后Bean的名称作为key，Bean作为ValueMap<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);if (matchingBeans.isEmpty()) {// Step 4c (fallback): custom Collection / Map declarations for collecting multiple beansmultipleBeans = resolveMultipleBeansFallback(descriptor, beanName, autowiredBeanNames, typeConverter);if (multipleBeans != null) {return multipleBeans;}// Raise exception if nothing found for required injection pointif (isRequired(descriptor)) {raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);}return null;}String autowiredBeanName;Object instanceCandidate;//如果根据类型找到多个Bean则需要选择一个合适的Bean返回if (matchingBeans.size() > 1) {autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);if (autowiredBeanName == null) {if (isRequired(descriptor) || !indicatesArrayCollectionOrMap(type)) {// Raise exception if no clear match found for required injection pointreturn descriptor.resolveNotUnique(descriptor.getResolvableType(), 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 {//如果只有一个Bean则直接返回这个BeanMap.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();autowiredBeanName = entry.getKey();instanceCandidate = entry.getValue();}// Step 6: validate single resultif (autowiredBeanNames != null) {autowiredBeanNames.add(autowiredBeanName);}if (instanceCandidate instanceof Class) {instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);}return resolveInstance(instanceCandidate, descriptor, type, autowiredBeanName);}finally {ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);}
}
当有多个类型匹配的 Bean 选择返回一个 Bean 的原则
当根据类型找到多个 Bean 时需要根据一些规则返回一个Bean。常见的可以通过 @Qualifer 限定名称或者通过 @Primary 来表示优先注入。在DefaultListableBeanFactor 的 determineAutowireCandidate() 方法中就实现了这些逻辑：
首先遍历找到的所有符合类型的 Bean，然后看是否有 @Primary 注解修饰，如果有的话，则优先返回有该 Bean；
否则再次尝试根据字段的名称匹配看是否有匹配的 Bean，如果有则返回；
否则尝试获取 @Qualifier注解定义的名称（对于 @Named 注解来说它本身上面也有 @Qualifer 注解修饰），然后看是否有名称匹配的 Bean，如果有则返回；
否则遍历 Bean 看是否有 @Priority 注解修饰，如果有则找最高优先级的 Bean 返回，值越小优先级越高；
否则看 resolvableDependencies 是否有注册对应的实例，如果有则返回，它的使用场景一般是有用户自己的 new 的对象可以注册到这里面，然后在一个 Spring 管理的 Bean 中可以把它注入进来。代码如下：
protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {Class<?> requiredType = descriptor.getDependencyType();//首先处理@Primary注解，如果某个Bean有@Primary注解修饰则优先返回它String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);if (primaryCandidate != null) {return primaryCandidate;}//否则再次根据字段的名称进行匹配，看找到的Bean里面有没有和字段名称相同的Bean，有的话则优先返回String dependencyName = descriptor.getDependencyName();if (dependencyName != null) {for (String beanName : candidates.keySet()) {if (matchesBeanName(beanName, dependencyName)) {return beanName;}}}//否则尝试获取@Qualifier注解定义的名称，看找打的Bean里面有没有和该名称相同的Bean，有的话则优先返回String suggestedName = getAutowireCandidateResolver().getSuggestedName(descriptor);if (suggestedName != null) {for (String beanName : candidates.keySet()) {if (matchesBeanName(beanName, suggestedName)) {return beanName;}}}//否则看找到的Bean是否有@Priority注解修饰，有的话取优先级最高的返回即值最小的String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);if (priorityCandidate != null) {return priorityCandidate;}//否则自定义注册的非Spring管理生命周期的对象中是否有匹配，resolvableDependencies里面可以放//一些对象，这些对象不是由Spring创建的而是用户自己创建放入的且需要在一个Spring的Bean中注入它for (Map.Entry<String, Object> entry : candidates.entrySet()) {String candidateName = entry.getKey();Object beanInstance = entry.getValue();if (beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) {return candidateName;}}return null;
}@Named 注解定义中使用了 @Qualifer 注解修饰。代码如下：
@Qualifier // 这里使用了@Qualifer注解修饰
@Documented
@Retention(RUNTIME)
public @interface Named {String value() default "";
}
