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spring解决循环依赖主要通过三级缓存去完成。
/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256);
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);
/** Cache of early singleton objects: bean name --> bean instance */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);在doGetBean中，它首先会通过getSingleton方法去尝试获取bean。
Object sharedInstance = getSingleton(beanName);protected Object getSingleton(String beanName, boolean allowEarlyReference) {
/** 首先从singletonObjects获取bean实例 */
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
/**
* 如果singletonObjects还没有此bean，有两种情况
* 1.这个bean正在创建状态，先从earlySingletonObjects获取
* 2.这个bean还没开始创建
*/
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
/**
* 如果earlySingletonObjects还没有此bean，有两种情况
* 1.说明还未被其他bean注入，正在创建状态，先从singletonFactories获取
* 2.该bean还没开始创建
*/
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
//将此bean放到提前缓存到earlySingletonObjects中
this.earlySingletonObjects.put(beanName, singletonObject);
//从singletonObject删除bean
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}这个方法做了这么一件事，逐步从三级缓存中获取bean：
- 先到singletonObjects中获取，如果有表示实例化已经完成；
- 否则到earlySingletonObjects获取，如果有表示已经有bean，且存在循环依赖，将此bean作为属性注入了
- 否则到singletonFactories获取，如果存在循环依赖，且此属性是第一次被其他bean作为属性注入
相应的在doCreateBean中有这么一串代码：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//BeanWrapper是用来持有创建出来的Bean对象的
BeanWrapper instanceWrapper = null;
//如果是Singleton先把缓存中同名的清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//这是创建Bean的地方由createBeanInstance来完成
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
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);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
//这里是对Bean的初始化，依赖注入往往在这里发生，
// 这个exposedObject在初始化处理完后会返回作为依赖注入完成后的bean
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}核心在这里：
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
//将beanName存放入singletonFactories中
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}这串代码会在创建bean的时候将自身注册进singletonFactories中。所以可以想象当两个bean互相依赖时，A会将自身注册进singletonFactories中，当它创建B时它可以直接从singletonFactories中获取A的引用所以B能成功创建，当B成功创建后那A也可以从singletonObjects中获取B的引用，最后A也成功创建。
总结
Spring通过三级缓存解决了循环依赖的问题，它会在创建bean的时候将自身注册进一个singletonFactory中，然后创建它的依赖bean时会从这个缓存中获取它的引用使那个依赖bean先进行创造，最后完成自身的依赖。
举个列子：
- 创建A单例时，放入缓存，然后创建它的依赖B单例。
- 创建B单例时，发现需要依赖注入A单例，然后去加载A单例，此时从缓存中发现A单例正在创建中，于是直接从缓存得到A单例，完成自身的依赖注入。
- B单例依赖完成后，返回给A单例。
- A单例再把B单例注入到自身域中完成依赖注入。