@Resource和@Autowired 区别和源码详解

一、前言

我们知道在Spring注入Bean对象的时候，可以在成员变量上写@Resource和@Autowired两种，那么他们的区别是什么呢？

结论：

@Resource 按照Bean对象的名称去查找，并判断Spring工厂中的Bean对象类型与要注入的成员变量的类型是否匹配。如果按名称找不到，则按照类型去匹配。

@Autowired 按照Bean对象的类型去匹配。

二、源码分析

首先要知道什么时候时候去注入，就是在执行AbstractAutowireCapableBeanFactory类创建Bean方法的时候，doCreateBean方法中的populateBean中

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

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		..................

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
    }

 populateBean中的 ibp.postProcessProperties()方法

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
		...........................
		PropertyDescriptor[] filteredPds = null;
		if (hasInstAwareBpps) {
			if (pvs == null) {
				pvs = mbd.getPropertyValues();
			}
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof InstantiationAwareBeanPostProcessor) {
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					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;
				}
			}
		}
		if (needsDepCheck) {
			if (filteredPds == null) {
				filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
			}
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}

		if (pvs != null) {
			applyPropertyValues(beanName, mbd, bw, pvs);
		}
	}

ibp.postProcessProperties()方法有很多实现类，其中处理@Resource的在CommonAnnotationBeanPostProcessor类中。

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

    private InjectionMetadata findResourceMetadata(String beanName, final 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 = buildResourceMetadata(clazz);
					this.injectionMetadataCache.put(cacheKey, metadata);
				}
			}
		}
		return metadata;
	}

在buildResourceMetadata方法里

private InjectionMetadata buildResourceMetadata(final Class<?> clazz) {
		List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
		Class<?> targetClass = clazz;

		do {
			final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();

			ReflectionUtils.doWithLocalFields(targetClass, field -> {
				if (webServiceRefClass != null && field.isAnnotationPresent(webServiceRefClass)) {
					if (Modifier.isStatic(field.getModifiers())) {
						throw new IllegalStateException("@WebServiceRef annotation is not supported on static fields");
					}
					currElements.add(new WebServiceRefElement(field, field, null));
				}
				else if (ejbRefClass != null && field.isAnnotationPresent(ejbRefClass)) {
					if (Modifier.isStatic(field.getModifiers())) {
						throw new IllegalStateException("@EJB annotation is not supported on static fields");
					}
					currElements.add(new EjbRefElement(field, field, null));
				}
				else if (field.isAnnotationPresent(Resource.class)) {
					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));
					}
				}
			});

			ReflectionUtils.doWithLocalMethods(targetClass, method -> {
				Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
				if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
					return;
				}
				if (method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
					if (webServiceRefClass != null && bridgedMethod.isAnnotationPresent(webServiceRefClass)) {
						if (Modifier.isStatic(method.getModifiers())) {
							throw new IllegalStateException("@WebServiceRef annotation is not supported on static methods");
						}
						if (method.getParameterCount() != 1) {
							throw new IllegalStateException("@WebServiceRef annotation requires a single-arg method: " + method);
						}
						PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
						currElements.add(new WebServiceRefElement(method, bridgedMethod, pd));
					}
					else if (ejbRefClass != null && bridgedMethod.isAnnotationPresent(ejbRefClass)) {
						if (Modifier.isStatic(method.getModifiers())) {
							throw new IllegalStateException("@EJB annotation is not supported on static methods");
						}
						if (method.getParameterCount() != 1) {
							throw new IllegalStateException("@EJB annotation requires a single-arg method: " + method);
						}
						PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
						currElements.add(new EjbRefElement(method, bridgedMethod, pd));
					}
					else if (bridgedMethod.isAnnotationPresent(Resource.class)) {
						if (Modifier.isStatic(method.getModifiers())) {
							throw new IllegalStateException("@Resource annotation is not supported on static methods");
						}
						Class<?>[] paramTypes = method.getParameterTypes();
						if (paramTypes.length != 1) {
							throw new IllegalStateException("@Resource annotation requires a single-arg method: " + method);
						}
						if (!this.ignoredResourceTypes.contains(paramTypes[0].getName())) {
							PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
							currElements.add(new ResourceElement(method, bridgedMethod, pd));
						}
					}
				}
			});

			elements.addAll(0, currElements);
			targetClass = targetClass.getSuperclass();
		}
		while (targetClass != null && targetClass != Object.class);

		return new InjectionMetadata(clazz, elements);
	}

会循环判断类中的Field和Method，找到上面有Resource注解的属性。之后创建ResourceElement节点 并add到currElements的List中等待处理。ResourceElement类初始化。

public ResourceElement(Member member, AnnotatedElement ae, @Nullable PropertyDescriptor pd) {
			super(member, pd);
			Resource resource = ae.getAnnotation(Resource.class);
			String resourceName = resource.name();
			Class<?> resourceType = resource.type();
            // ** 1 **
			this.isDefaultName = !StringUtils.hasLength(resourceName);
			if (this.isDefaultName) {
				resourceName = this.member.getName();
				if (this.member instanceof Method && resourceName.startsWith("set") && resourceName.length() > 3) {
					resourceName = Introspector.decapitalize(resourceName.substring(3));
				}
			}
			else if (embeddedValueResolver != null) {
				resourceName = embeddedValueResolver.resolveStringValue(resourceName);
			}
            // ** 2 **
			if (Object.class != resourceType) {
				checkResourceType(resourceType);
			}
			else {
				// No resource type specified... check field/method.
				resourceType = getResourceType();
			}
			this.name = (resourceName != null ? resourceName : "");
			this.lookupType = resourceType;
			String lookupValue = resource.lookup();
			this.mappedName = (StringUtils.hasLength(lookupValue) ? lookupValue : resource.mappedName());
			Lazy lazy = ae.getAnnotation(Lazy.class);
			this.lazyLookup = (lazy != null && lazy.value());
		}

ResourceElement节点初始化时，在1处，先判断@Resource的name是否为空，如果为空则使用Field的默认名字。

@Component
public class BeanA {
    @Resource(type = BeanC.class)
    private BeanFather beanB;
}

在2处，判断@Resource注解标识的type类型与定义类型的是否一致。如示例，判断BeanC是否属于BeanFather类型。

之后进入postProcessProperties的metadata.inject(bean, beanName, pvs);方法。在InjectionMetadata类的inject方法

    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);
				}
				element.inject(target, beanName, pvs);
			}
		}
	}

	protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
				throws Throwable {

			if (this.isField) {
				Field field = (Field) this.member;
				ReflectionUtils.makeAccessible(field);
				field.set(target, getResourceToInject(target, requestingBeanName));
			}
			else {
				if (checkPropertySkipping(pvs)) {
					return;
				}
				try {
					Method method = (Method) this.member;
					ReflectionUtils.makeAccessible(method);
					method.invoke(target, getResourceToInject(target, requestingBeanName));
				}
				catch (InvocationTargetException ex) {
					throw ex.getTargetException();
				}
			}
		}

判断是节点是Field或是Method。进入getResourceToInject方法，回到CommonAnnotationBeanPostProcessor类的ResourceElement内部类的getResourceToInject方法

		protected Object getResourceToInject(Object target, @Nullable String requestingBeanName) {
			return (this.lazyLookup ? buildLazyResourceProxy(this, requestingBeanName) :
					getResource(this, requestingBeanName));
		}

进入getResource方法。

	protected Object getResource(LookupElement element, @Nullable String requestingBeanName)
			throws NoSuchBeanDefinitionException {

		if (StringUtils.hasLength(element.mappedName)) {
			return this.jndiFactory.getBean(element.mappedName, element.lookupType);
		}
		if (this.alwaysUseJndiLookup) {
			return this.jndiFactory.getBean(element.name, element.lookupType);
		}
		if (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 beanFactory = (AutowireCapableBeanFactory) factory;
			DependencyDescriptor descriptor = element.getDependencyDescriptor();
            // ** 1 **
			if (this.fallbackToDefaultTypeMatch && element.isDefaultName && !factory.containsBean(name)) {
				autowiredBeanNames = new LinkedHashSet<>();
				resource = beanFactory.resolveDependency(descriptor, requestingBeanName, autowiredBeanNames, null);
				if (resource == null) {
					throw new NoSuchBeanDefinitionException(element.getLookupType(), "No resolvable resource object");
				}
			}
			else {
                // ** 2 **
				resource = beanFactory.resolveBeanByName(name, descriptor);
				autowiredBeanNames = Collections.singleton(name);
			}
		}
		else {
			resource = factory.getBean(name, element.lookupType);
			autowiredBeanNames = Collections.singleton(name);
		}

		if (factory instanceof ConfigurableBeanFactory) {
			ConfigurableBeanFactory beanFactory = (ConfigurableBeanFactory) factory;
			for (String autowiredBeanName : autowiredBeanNames) {
				if (requestingBeanName != null && beanFactory.containsBean(autowiredBeanName)) {
					beanFactory.registerDependentBean(autowiredBeanName, requestingBeanName);
				}
			}
		}

		return resource;
	}

两种情况：

（1）：autowireResource方法在1处，会根据定义判断是否使用默认名字，以及BeanFactory中是否含有这个名字的Bean，如果不含这个name的bean，则调用beanFactory.resolveDependency(descriptor, requestingBeanName, autowiredBeanNames, null);方法。

	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;
		}
	}

 通过doResolveDependency方法

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

	        .....................

			Class<?> type = descriptor.getDependencyType();
			......................

			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;

			if (matchingBeans.size() > 1) {
				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);
			}
		  ..................
	}

该方法通过 findAutowireCandidates(beanName, type, descriptor);去查找。因为根据beanName已经不存在了，所以主要是根据type，如果找到两个相同类型的Bean，又没有标识@Primary或优先级，则报错（在if (matchingBeans.size() > 1)中判断）。

（2）：autowireResource方法在1处，会根据定义判断是否使用默认名字，以及BeanFactory中是否含有这个名字的Bean，如果包含这个name的bean，则调用 beanFactory.resolveBeanByName(name, descriptor); 在 // ** 2 ** 处

	public Object resolveBeanByName(String name, DependencyDescriptor descriptor) {
		InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
		try {
			return getBean(name, descriptor.getDependencyType());
		}
		finally {
			ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
		}
	}

	public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
		return doGetBean(name, requiredType, null, false);
	}

该方法会根据name找到bean，并判断bean的类型，和@Resource定义的是否一样。在doGetBean中最后一段判断。如下代码：

		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				if (convertedBean == null) {
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				return convertedBean;
			}
			catch (TypeMismatchException ex) {
				if (logger.isTraceEnabled()) {
					logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}

这里可能有人会问，在生成@Resource注解的ResourceElement初始化时，不是已经判断了@Resource的type和Field的类型是否一致了吗。其实主要是为了解决注入接口类型的Bean对象问题。

比如我有BeanB和BeanC两个类，都实现了BeanFather这个接口，那么在BeanA中，如果我这样写

@Component
public class BeanA {
    @Resource(type = BeanC.class)
    private BeanFather beanB;
}

在生成ResourceElement时，BeanC确实属于BeanFather，但是到doGetBean实际注入中，因为在BeanFactory中有BeanB这个对象，通过@Resource获取时优先根据名字，那么获取到的BeanB这个类型就和要注入的BeanC不符合了，就会抛错，因为BeanB和BeanC并不能互相转换。

报错如图

 @Resource就分析完了，@Autowired的处理类是AutowiredAnnotationBeanPostProcessor。内容基本类似，也是在postProcessProperties中，生成的节点是AutowiredFieldElement。在InjectionMetadata类的inject方法时

		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 {
					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);
			}
		}
	}

直接执行的value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);和@Resource的beanFactory中不包含name执行的一样，根据类型注入。

三、总结

@Resource 按照Bean对象的名称去查找，并判断Spring工厂中的Bean对象类型与要注入的成员变量的类型是否匹配。如果按名称找不到，则按照类型去匹配。

@Autowired 按照Bean对象的类型去匹配。