你所不知道的Spring的@Autowired实现细节

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：

	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：

	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：

	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AnnotationAttributes
			ReflectionUtils.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AnnotationAttributes
			ReflectionUtils.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：

	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 local
			for (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：

	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 5
		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;
				}
			}
		}
		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);
		}
	}

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：

<?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：

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：

	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：

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：

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：

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：

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Map
		Object 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：

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：

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。