关于@Autowired注解的作用
@Autowired 注解在Spring中的作用是实现依赖注入（Dependency Injection），它用于自动装配（autowiring）Spring Bean 的依赖关系。具体来说， @Autowired 注解有以下作用：
- 自动装配依赖 ：通过在类的字段、构造函数、方法参数等地方使用 @Autowired 注解，Spring 容器会自动识别需要注入的依赖，并将适当的 Bean 实例注入到目标组件中。
- 减少手动配置 ：使用 @Autowired 注解可以减少手动配置依赖关系的工作，因为它会自动发现并管理组件之间的依赖关系，从而降低了配置的复杂性。
- 提高可维护性 ： @Autowired 注解明确地标识了类的依赖关系，使代码更易于理解和维护，因为它清晰地表达了组件之间的关联。
- 解耦：通过将依赖项的注入交给 Spring 容器处理，实现了松散耦合，使组件更容易替换、扩展和测试，同时降低了组件之间的耦合度。
- 支持多种装配模式 ： @Autowired 提供了多种装配模式，包括按类型、按名称、按限定符（qualifier）等方式，以满足不同的装配需求。
Spring Framework与@Autowired
在Spring Framework框架中最重要的概念是IoC和DI，通过这两个特性可以实现对象间的依赖关系由框架管理，构造对象间的依赖关系，将依赖对象自动注入到需要它们的类中，在使用时无需手动创建或查找依赖对象，注入依赖关系主要有以下方法：
- 通过xml配置注入
- 通过@Autowired等注解注入
- 当前Spring Framework推荐的通过构造方法注入
无论何种注入方法，Spring都会获取该Bean配置的元数据（Bean定义和依赖关系），那么接下来我将从源码层面分析@Autowired注入依赖的过程。
Bean创建过程
doCreateBean-创建Bean实例
java
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
//省略其他代码，展示主要流程
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}创建Bean方法的主要逻辑在doCreateBean中，Spring Framework通过doCreateBean创建指定Bean，在该方法中，其中通过populateBean()遍历对应后置处理器，即：一个被注解标注的类被注入到Spring容器时，首先会创建Bean对象，创建后调用populateBean方法以遍历后置处理器通过后置处理器获取到需要的value，将@Autowired注解中的属性(元数据)赋值到Bean中。
populateBean-遍历后置处理器
java
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
//InstantiationAwareBeanPostProcessors可以为@Autowired注解提供后置处理，
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
//遍历所有相关后置处理器，获取需要的value
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
}@Autowired注解所需的后置处理器是：AutowiredAnnotationBeanPostProcessor。接下来Spring容器开始使用该注解的后置处理器去获取对应的属性value，假设我们不知道@Autowired注解对应后置处理器的逻辑，那么根据这个需求来猜测后置处理器中的相关逻辑的方法名：需要带有处理、属性，那么对应的单词就是：Process、Properties，对应找一下，postProcessProperties()便是目标方法。
java
@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
//根据BeanName获取注入的元数据
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
//元数据Value注入目标Bean中
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;
}获取注入的元数据
java
//用于缓存Spring解析过的Bean元数据
private final Map<String, InjectionMetadata> injectionMetadataCache = new ConcurrentHashMap<>(256);
java
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());
// 查询缓存中是否存在对应元数据-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);
}
//当缓存中不存在指定Bean的MetaData时，构建MetaData
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}如何构造Metadata
Spring Framework通过buildAutowiringMetadata()方法解析注解中的数据。
java
//需要解析的注解结合
private final Set<Class<? extends Annotation>> autowiredAnnotationTypes = new LinkedHashSet<>(4);
if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
return InjectionMetadata.EMPTY;
}
java
/**
* Create a new {@code AutowiredAnnotationBeanPostProcessor} for Spring's
* standard {@link Autowired @Autowired} and {@link Value @Value} annotations.
* <p>Also supports JSR-330's {@link javax.inject.Inject @Inject} annotation,
* if available.
* 为 Spring 的标准@Autowired和@Value注释创建一个新的AutowiredAnnotationBeanPostProcessor
*/
@SuppressWarnings("unchecked")
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.
}
}这段代码首先会调用 isCandidateClass 方法判断当前类是否为一个候选类，判断的依据就是 类、属性、方法上是否包含autowiredAnnotationTypes 集合中初始化的值（@Autowired、@Value、@Inject），当Bean的定义中包含集合中对应类型的注解时，被判定为候选类，再去获取该类对应注解中的元数据。
如果Bean中没有没有指定类型的注解时，返回一个空的元数据注入对象。如果有指定注解，则开始获取注解中的元数据。
获取元数据的方式，是通过反射实现的。以下是通过反射获取类、属性、方法中对应注解的逻辑。
以DruidDataSourceWrapper为例：
通过反射获取指定类的字段中的属性
java
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
return InjectionMetadata.EMPTY;
}
List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
Class<?> targetClass = clazz;
do {
final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
//通过反射获取指定类的字段中的属性
ReflectionUtils.doWithLocalFields(targetClass, field -> {
MergedAnnotation<?> 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));
}
});
}
以DruidDataSource为例
通过反射获取指定类的方法中的属性
java
//通过反射获取指定类的方法中的属性
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
MergedAnnotation<?> 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));
}
});
元数据注入
从缓存中获取值
AutowiredAnnotationBeanPostProcessor类中的inject方法用来注入元数据。
该方法会首先从缓存中获取元数据，如果缓存中没有，则执行resolvedCachedArgument解析字段值。
java
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
try {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}解析字段值
java
@Nullable
private ConfigurableListableBeanFactory beanFactory;
@Nullable
private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
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();
Object value;
try {
//解析核心方法
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
if (!this.cached) {
Object cachedFieldValue = null;
if (value != null || this.required) {
cachedFieldValue = desc;
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
this.cachedFieldValue = cachedFieldValue;
this.cached = true;
}
}
return value;
}DependencyDescriptor：表示和处理Bean之间的依赖关系。
resolveDependency方法是接口BeanFactory接口提供的，DefaultListableBeanFactory是BeanFactory的一个实现类。
resolveDependency方法用于解析和解决依赖关系，该方法的作用是根据给定的 DependencyDescriptor 对象，解析并返回不同类型的依赖对象，当前方法最终会走doResolveDependency。
java
@Override
@Nullable
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;
}
}doResolveDependency方法
java
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
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);
}
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);
}
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);
}- resolveMultipleBean：见名知意， 解析多个Bean，在注入时，当前类中能有不同类型的Bean，如Bean、数组、集合、Map等，该方法针对不同类型的Bean查找返回。
- findAutowireCandidates：查找满足条件的Bean，该方法查找出来的Bean可能有一个或多个。
以上，完成了满足条件的候选对象列表并注入。