你所不知道的Spring的@Autowired实现细节

你所不知道的Spring的@Autowired实现细节
原文链接：https://cloud.tencent.com/developer/article/1692933

前言

相信很多Java开发都遇到过一个面试题：Resource和Autowired的区别是什么？这个问题的答案相信基本都清楚，但是这两者在Spring中是如何实现的呢？这就要分析Spring源码才能知道了。友情提示：本篇主要是讲解Autowired的实现原理，不会分析Spring初始化的过程，不熟悉的读者可以先阅读笔者之前的一篇文章《这一次搞懂Spring的Bean实例化原理》。

正文

在Spring Bean的整个创建过程中会调用很多BeanPostProcessor接口的的实现类：

上图是我整理的Spring Bean的创建过程及整个过程中各个BeanPostProcessor和回调的调用，右边相同颜色的连线代表是同一组调用，主要看到AutowiredAnnotationBeanPostProcessor和CommonAnnotationBeanPostProcessor，前者就是支持 @Autowired和@Value注解，后者则是支持@PostConstruct、@PreDestroy、@Resource注解。先了解这两个Processor的作用，下面从头分析。

从图中可以看到，在createBeanInstance方法中会调用SmartInstantiationAwareBeanPostProcessor类型的determineCandidateConstructors，这个方法是做什么的呢？看代码：

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	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {......// Candidate constructors for autowiring?//寻找当前正在实例化的bean中有@Autowired注解的构造函数Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {//如果ctors不为空，就说明构造函数上有@Autowired注解return autowireConstructor(beanName, mbd, ctors, args);}......// No special handling: simply use no-arg constructor.return instantiateBean(beanName, mbd);}	protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)throws BeansException {		if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);if (ctors != null) {return ctors;}}}}return null;}

createBeanInstance方法是去实例化Bean，而调用AutowiredAnnotationBeanPostProcessor.determineCandidateConstructors的目的就是先去找到带有@Autowired注解的构造方法（自动注入有三种模式：属性、构造方法、普通方法），也就是通过构造方法注入，如果没有找到则通过反射调用无参构造实例化。平时我们基本上都是使用的属性注入，所以一般都不会进入determineCandidateConstructors方法，所以这里也不详细阐述，感兴趣的读者可自行看看。

接着回到doCreateBean方法中，就可以看到调用了applyMergedBeanDefinitionPostProcessors方法：

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	protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof MergedBeanDefinitionPostProcessor) {/***	CommonAnnotationBeanPostProcessor  支持了@PostConstruct，@PreDestroy,@Resource注解* 	AutowiredAnnotationBeanPostProcessor 支持 @Autowired,@Value注解*/MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);}}}

这个方法本质上就是调用MergedBeanDefinitionPostProcessor类型的postProcessMergedBeanDefinition方法，通过这个方法去收集@Autowired、@Resource等注解，这里主要分析AutowiredAnnotationBeanPostProcessor的实现：

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	public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);metadata.checkConfigMembers(beanDefinition);}	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);}//主要看这个方法metadata = buildAutowiringMetadata(clazz);this.injectionMetadataCache.put(cacheKey, metadata);}}}return metadata;}	private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();Class<?> targetClass = clazz;		do {final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();			// 找到带有@Autowired注解的属性并封装为AnnotationAttributesReflectionUtils.doWithLocalFields(targetClass, field -> {AnnotationAttributes ann = findAutowiredAnnotation(field);if (ann != null) {if (Modifier.isStatic(field.getModifiers())) {if (logger.isInfoEnabled()) {logger.info("Autowired annotation is not supported on static fields: " + field);}return;}boolean required = determineRequiredStatus(ann);currElements.add(new AutowiredFieldElement(field, required));}});			// 找到带有@Autowired注解的方法并封装为AnnotationAttributesReflectionUtils.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.isInfoEnabled()) {logger.info("Autowired annotation is not supported on static methods: " + method);}return;}if (method.getParameterCount() == 0) {if (logger.isInfoEnabled()) {logger.info("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);		return new InjectionMetadata(clazz, elements);}

收集的逻辑主要在findAutowiringMetadata方法中，层层调用后可以看到是通过findAutowiredAnnotation这个方法去找到带有@Autowired和@Value注解的属性和方法：

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	private final Set<Class<? extends Annotation>> autowiredAnnotationTypes = new LinkedHashSet<>(4);		public AutowiredAnnotationBeanPostProcessor() {this.autowiredAnnotationTypes.add(Autowired.class);this.autowiredAnnotationTypes.add(Value.class);try {this.autowiredAnnotationTypes.add((Class<? extends Annotation>)ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));logger.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");}catch (ClassNotFoundException ex) {// JSR-330 API not available - simply skip.}}private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {if (ao.getAnnotations().length > 0) {  // autowiring annotations have to be localfor (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);if (attributes != null) {return attributes;}}}return null;}

最后将其封装为AutowiredFieldElement和AutowiredMethodElement对象的list并连同Class一起封装成InjectionMetadata返回，这就完成了相关注解的收集。

收集完成后在哪里使用呢？对Bean生命周期熟悉的读者都知道，之后就会进行依赖注入，自然相关的调用就在populateBean这个方法里：

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	protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {......		PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);		if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {MutablePropertyValues newPvs = new MutablePropertyValues(pvs);// Add property values based on autowire by name if applicable.if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {autowireByName(beanName, mbd, bw, newPvs);}// Add property values based on autowire by type if applicable.if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {autowireByType(beanName, mbd, bw, newPvs);}pvs = newPvs;}		boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);		PropertyDescriptor[] filteredPds = null;		//重点看这个if代码块，重要程度 5if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;// 依赖注入过程，@Autowired的支持PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}						// 老版本用这个完成依赖注入过程，@Autowired的支持pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;}}}if (needsDepCheck) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}checkDependencies(beanName, mbd, filteredPds, pvs);}		// xml中<property>标签的依赖注入if (pvs != null) {applyPropertyValues(beanName, mbd, bw, pvs);}}

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mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE

很多读者包括网上很多文章在看到这个判断时，都认为自动注入的逻辑就是这里，但实际上并不是，这里是自动注入没错，但却是针对以前xml配置，如下：

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<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans"         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="              http://www.springframework.org/schema/beans            http://www.springframework.org/schema/beans/spring-beans-2.5.xsd"default-autowire="byName"><bean id="a" class="com.A" autowire="byName"/><bean id="b" class="com.B" autowire="byType"/>
</beans> 

头文件中的default-autowire属性就是开启全局自动注入，而bean标签上的autowire则是特定针对当前bean的，会覆盖全局配置。这样我们配置后，bean标签就无需配置prototype子标签，也能自动注入对应的对象。这些属性包含了以下几个值：

• no：默认值。表示不进行自动注入。对应BeanDefinition中autowireMode的值为 0。
• byName：根据名称进行自动注入。对应BeanDefinition中autowireMode的值为1。
• byType：根据类型进行自动注入，如果容器中找到两个及以上符合该类型的Bean就将抛出异常。对应BeanDefinition中autowireMode的值为2。
• constructor：等同于byType，只是当指定autowire=”constructor”时表示将通过构造方法根据类型进行自动注入。对应BeanDefinition中autowireMode的值为3。

这就是xml配置中的自动注入，而我们使用@Autowired注解时，BeanDefinition中autowireMode的值为 0，即表示不进行自动注入。插一句题外话，网上很多人在争论@Autowired是自动注入还是手动注入，我个人认为都算自动注入，不能说它没有进入这段逻辑就不能叫自动注入，只是它以另外一种方式实现了，至少也没有让我们自己手动new并设置属性。

那这另外一种方式是什么呢？就是下面这个代码干的事：

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if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;// 依赖注入过程，@Autowired的支持PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}				// 老版本用这个完成依赖注入过程，@Autowired的支持pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;}}
}

可以看到这里又是调用了InstantiationAwareBeanPostProcessor类型的postProcessProperties和postProcessPropertyValues方法，后者是老版本中的实现，已经废弃，所以直接看postProcessProperties，还是进入到AutowiredAnnotationBeanPostProcessor类中：

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	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;}

findAutowiringMetadata这个方法不陌生的，刚刚已经分析了，是去收集对应注解并封装为InjectionMetadata对象放入到缓存，这里就是从缓存中拿到值，注入则是通过inject实现的：

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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) {if (logger.isTraceEnabled()) {logger.trace("Processing injected element of bean '" + beanName + "': " + element);}if(element.isField) {Field field = (Field)element.member;System.out.println("==IOC/DI===beanName==" + beanName + "==field[" + field.getName() +"]-> getBean(" + field.getName() + ")");}element.inject(target, beanName, pvs);}}
}

最后就是调用element.inject实现注入，element我们刚刚也看到了，就是AutowiredFieldElement和AutowiredMethodElement，分别实现属性注入和方法注入，这里我们看最常用的属性注入就行了：

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protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {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 {// 找到依赖对象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);field.set(bean, value);}
}

这段代码整体逻辑比较清晰，首先根据field创建一个依赖对象的抽象DependencyDescriptor对象，然后通过beanFactory.resolveDependency解析拿到对应的实例，最后通过反射注入即可。因此我们主要看resolveDependency方法中做了什么：

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public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {	descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());if (Optional.class == descriptor.getDependencyType()) {return createOptionalDependency(descriptor, requestingBeanName);}else if (ObjectFactory.class == descriptor.getDependencyType() ||ObjectProvider.class == descriptor.getDependencyType()) {return new DependencyObjectProvider(descriptor, requestingBeanName);}else if (javaxInjectProviderClass == descriptor.getDependencyType()) {return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);}else {Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);if (result == null) {result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);}return result;}
}

一般情况下，都是走的else分支并调用doResolveDependency方法：

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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()));}		// 有多个实现类需要注入，特指注入的是数组、集合或者MapObject multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);if (multipleBeans != null) {return multipleBeans;}		// 找到依赖对象的所有实现类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;		// 依赖的对象有多个实例if (matchingBeans.size() > 1) {// 根据@Primary、@Priority和名称依次进行匹配注入autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);if (autowiredBeanName == null) {if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {return 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 {// 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);}// 如果拿到的是class对象，通过getBean实例化返回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);}
}

这里面首先是通过 getAutowireCandidateResolver().getSuggestedValue(descriptor)拿到@Value注解的值，然后通过TypeConverter进行转换，默认可转换的类型是JDK和Spring内置的一些类型，自然不包含我们自定义的类，所以如果不进行扩展在@Autowired注入我们自定义类对象时同时使用@Value注解是会报错的。

接着是调用resolveMultipleBeans方法实现对Map、List、数组等属性的注入，本质上还是调用findAutowireCandidates方法找到所有的实现类的对象装入对应的集合数组中，所以直接看findAutowireCandidates：

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protected Map<String, Object> findAutowireCandidates(@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {	String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this, requiredType, true, descriptor.isEager());Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);......for (String candidate : candidateNames) {if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {addCandidateEntry(result, candidate, descriptor, requiredType);}}	......return result;
}private void addCandidateEntry(Map<String, Object> candidates, String candidateName,DependencyDescriptor descriptor, Class<?> requiredType) {	if (descriptor instanceof MultiElementDescriptor) {Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);if (!(beanInstance instanceof NullBean)) {candidates.put(candidateName, beanInstance);}}else if (containsSingleton(candidateName) || (descriptor instanceof StreamDependencyDescriptor &&((StreamDependencyDescriptor) descriptor).isOrdered())) {Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);candidates.put(candidateName, (beanInstance instanceof NullBean ? null : beanInstance));}else {candidates.put(candidateName, getType(candidateName));}
}

首先通过beanNamesForTypeIncludingAncestors方法拿到依赖类所有的实现类的beanName，然后调用addCandidateEntry将beanName及对应的实例或者Class对象放入到Map中。

接着回到doResolveDependency方法中：

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if (matchingBeans.size() > 1) {// 根据@Primary、@Priority和名称依次进行匹配注入autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);if (autowiredBeanName == null) {if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {return 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 {// 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);
}
// 如果拿到的是class对象，通过getBean实例化返回
if (instanceCandidate instanceof Class) {instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;

如果只有一个实例则直接返回该实例，如果实现类有多个则调用determineAutowireCandidate进行判断该使用哪一个实例对象，判断规则如下：

• 首先判断实现类上是否有@Primary注解，找到一个则返回当前实例，找到多个则报错。
• 若没有标注@Primary注解，则判断是否指定了优先级，且只能是通过@Priority注解指定的，@Order不支持。
• 上述都没有拿到合适的Bean则通过属性名称获取Bean。

拿到对应的实例后，最后进行反射注入即可。以上就是@Autowired的实现细节。

总结

本篇从源码角度详细分析了@Autowired的实现细节，只有真正阅读了源码才能了解更多的细节，在开发中更加清楚如何注入多个实例以及如何指定注入的优先级，同时在面试中也能更有理有据，而不是统一的大众回答，先根据byType，再根据byName。另外对于方法注入和@Resource注解的处理本篇没有涉及，但是相信看完本文读者自己也能轻松分析这部分源码。