@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 elements = new ArrayList<>();
Class targetClass = clazz;
do {
final List 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 checkedElements = this.checkedElements;
Collection 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 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 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 autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
.....................
Class type = descriptor.getDependencyType();
......................
Map 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 getBean(String name, Class 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 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。