注解@Autowired和@Resource都可以实现Bean的自动注入。
其中@Autowired是通过Bean后处理器 BeanPostProcessor实现类AutowiredAnnotationBeanPostProcessor来完成。
而@Resource是通过另外一个CommonAnnotationBeanPostProcessor来实现的。
那么Spring是怎么做到区分的呢？
先看AbstractAutowireCapableBeanFactory，实例化bean会调用这个类的doCreateBean方法
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);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
进入applyMergedBeanDefinitionPostProcessors方法
protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class beanType, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof MergedBeanDefinitionPostProcessor) {
MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
}
}
}
getBeanPostProcessors()包含很多个BeanPostProcessor，就包含上面提到的两个，并且实现了MergedBeanDefinitionPostProcessor接口
进入org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#postProcessMergedBeanDefinition
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName) {
super.postProcessMergedBeanDefinition(beanDefinition, beanType, beanName);
if (beanType != null) {
Injectionmetadata metadata = findResourcemetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
}
private Injectionmetadata findResourcemetadata(String beanName, final Class clazz, 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);
}
try {
metadata = buildResourcemetadata(clazz);
this.injectionmetadataCache.put(cacheKey, metadata);
}
catch (NoClassDefFoundError err) {
throw new IllegalStateException("Failed to introspect bean class [" + clazz.getName() +
"] for resource metadata: could not find class that it depends on", err);
}
}
}
}
return metadata;
}
再进入buildResourcemetadata方法
private Injectionmetadata buildResourcemetadata(final Class clazz) {
linkedList elements = new linkedList();
Class targetClass = clazz;
do {
final linkedList currElements =
new linkedList();
ReflectionUtils.doWithLocalFields(targetClass, new ReflectionUtils.FieldCallback() {
@Override
public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
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 (!ignoredResourceTypes.contains(field.getType().getName())) {
currElements.add(new ResourceElement(field, field, null));
}
}
}
});
ReflectionUtils.doWithLocalMethods(targetClass, new ReflectionUtils.MethodCallback() {
@Override
public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
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.getParameterTypes().length != 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.getParameterTypes().length != 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 (!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);
}
这个方法有点长，只要看这句：field.isAnnotationPresent(Resource.class)，属性field是被Resource注解的就添加到currElements中，最终生成Injectionmetadata对象，这个对象包含的就是bean待注入的所有属性。最后统一放到injectionmetadataCache这个Map中，为后面填充属性使用。
我们再看类org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition
@Override
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);
}
metadata = buildAutowiringmetadata(clazz);
this.injectionmetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}
private Injectionmetadata buildAutowiringmetadata(final Class clazz) {
List elements = new ArrayList<>();
Class targetClass = clazz;
do {
final List currElements = new ArrayList<>();
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));
}
});
findAutowiredAnnotation方法是关键
@Nullable
private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
if (ao.getAnnotations().length > 0) { // autowiring annotations have to be local
for (Class type : this.autowiredAnnotationTypes) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
if (attributes != null) {
return attributes;
}
}
}
return null;
}
其中autowiredAnnotationTypes是一个Set集合，类初始化的时候就add进去了：
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
public AutowiredAnnotationBeanPostProcessor() {
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {
this.autowiredAnnotationTypes.add((Class)
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.
}
}
所以Bean的属性包含@Autowired或者@Value的都会满足条件，最终也是生成Injectionmetadata对象，也是存在一个map里面injectionmetadataCache。
回到AbstractAutowireCapableBeanFactory类的doCreateBean方法，接着上面往下走，populateBean方法，填充属性，也就是注入属性。
其中有一段：依然是遍历getBeanPostProcessors这个集合
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;
}
}
进入postProcessProperties方法
@Override
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;
}
是不是看到了熟悉的Injectionmetadata ，就是上面事先存好放在Map中的，这里重新拿出来，遍历BeanPostProcessors，其中CommonAnnotationBeanPostProcessor和AutowiredAnnotationBeanPostProcessor都会分别进入postProcessProperties方法中，从map拿出来Injectionmetadata对象，拿到Injectionmetadata对象中的injectedElements集合，这里存的就是待注入的field，跟着遍历metadata.inject(bean, beanName, pvs)，最终通过反射方式实现注入。
如果使用@Autowired注解，AutowiredAnnotationBeanPostProcessor拿到的injectedElements集合就不为空，AutowiredAnnotationBeanPostProcessor来完成注入的工作。
相反使用@Resource注解，CommonAnnotationBeanPostProcessor拿到集合injectedElements就不为空，由CommonAnnotationBeanPostProcessor来完成注入工作。
以上就是@Autowired 和 @Resource实现原理。
