spring針對Bean之間的循環依賴，有自己的處理方案。關鍵點就是三級緩存。當然這種方案不能解決所有的問題，他只能解決Bean單例模式下非構造函數的循環依賴。
我們就從A->B->C-A這個初始化順序，也就是A的Bean中需要B的實例，B的Bean中需要C的實例，C的Bean中需要A的實例，當然這種需要不是構造函數那種依賴。前提條件有了，我們就可以開始了。毫無疑問，我們會先初始化A.初始化的方法是org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { final String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. Object sharedInstance = getSingleton(beanName); //關注點1 if (sharedInstance != null && args == null) { if (logger.isDebugEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.debug("Returning cached instance of singleton bean '" + beanName + "'"); } } bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent. String nameToLookup = originalBeanName(name); if (args != null) { // Delegation to parent with explicit args. return (T) parentBeanFactory.getBean(nameToLookup, args); } else { // No args -> delegate to standard getBean method. return parentBeanFactory.getBean(nameToLookup, requiredType); } } if (!typeCheckOnly) { markBeanAsCreated(beanName); } try { final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on. String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { for (String dependsOnBean : dependsOn) { if (isDependent(beanName, dependsOnBean)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'"); } registerDependentBean(dependsOnBean, beanName); getBean(dependsOnBean); } } // Create bean instance. if (mbd.isSingleton()) { //關注點2 sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { try { return createBean(beanName, mbd, args); } catch (BeansException ex) { // Explicitly remove instance from singleton cache: It might have been put there // eagerly by the creation process, to allow for circular reference resolution. // Also remove any beans that received a temporary reference to the bean. destroySingleton(beanName); throw ex; } } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } else if (mbd.isPrototype()) { // It's a prototype -> create a new instance. Object prototypeInstance = null; try { beforePrototypeCreation(beanName); prototypeInstance = createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } else { String scopeName = mbd.getScope(); final Scope scope = this.scopes.get(scopeName); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { beforePrototypeCreation(beanName); try { return createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } } }); bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider " + "defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex); } } } catch (BeansException ex) { cleanupAfterBeanCreationFailure(beanName); throw ex; } } // Check if required type matches the type of the actual bean instance. if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) { try { return getTypeConverter().convertIfNecessary(bean, requiredType); } catch (TypeMismatchException ex) { if (logger.isDebugEnabled()) { logger.debug("Failed to convert bean '" + name + "' to required type [" + ClassUtils.getQualifiedName(requiredType) + "]", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; } 這個方法很長我們一點點說。先看我們的關注點1 Object sharedInstance = getSingleton(beanName)根據名稱從單例的集合中獲取單例對象，我們看下這個方法,他最終是org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean)
protected Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return (singletonObject != NULL_OBJECT ? singletonObject : null); } 大家一定要注意這個方法，很關鍵，我們開篇提到了三級緩存，使用點之一就是這里。到底是哪三級緩存呢，第一級緩存singletonObjects里面放置的是實例化好的單例對象。第二級earlySingletonObjects里面存放的是提前曝光的單例對象（沒有完全裝配好）。第三級singletonFactories里面存放的是要被實例化的對象的對象工廠。解釋好了三級緩存，我們再看看邏輯。第一次進來this.singletonObjects.get(beanName)返回的肯定是null。然后isSingletonCurrentlyInCreation決定了能否進入二級緩存中獲取數據。
public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); } 從singletonsCurrentlyInCreation這個Set中有沒有包含傳入的BeanName，前面沒有地方設置，所以肯定不包含，所以這個方法返回false,后面的流程就不走了。getSingleton這個方法返回的是null。
下面我們看下關注點2.也是一個getSingleton只不過他是真實的創建Bean的過程，我們可以看到傳入了一個匿名的ObjectFactory的對象，他的getObject方法中調用的是createBean這個真正的創建Bean的方法。當然我們可以先擱置一下，繼續看我們的getSingleton方法
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { Assert.notNull(beanName, "'beanName' must not be null"); synchronized (this.singletonObjects) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { if (this.singletonsCurrentlyInDestruction) { throw new BeanCreationNotAllowedException(beanName, "Singleton bean creation not allowed while the singletons of this factory are in destruction " + "(Do not request a bean from a BeanFactory in a destroy method implementation!)"); } if (logger.isDebugEnabled()) { logger.debug("Creating shared instance of singleton bean '" + beanName + "'"); } beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) { this.suppressedExceptions = new LinkedHashSet<Exception>(); } try { singletonObject = singletonFactory.getObject(); newSingleton = true; } catch (IllegalStateException ex) { // Has the singleton object implicitly appeared in the meantime -> // if yes, proceed with it since the exception indicates that state. singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { throw ex; } } catch (BeanCreationException ex) { if (recordSuppressedExceptions) { for (Exception suppressedException : this.suppressedExceptions) { ex.addRelatedCause(suppressedException); } } throw ex; } finally { if (recordSuppressedExceptions) { this.suppressedExceptions = null; } afterSingletonCreation(beanName); } if (newSingleton) { addSingleton(beanName, singletonObject); } } return (singletonObject != NULL_OBJECT ? singletonObject : null); } } 這個方法的第一句Object singletonObject = this.singletonObjects.get(beanName)從一級緩存中取數據，肯定是null。隨后就調用的beforeSingletonCreation方法。
protected void beforeSingletonCreation(String beanName) { if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } } 其中就有往singletonsCurrentlyInCreation這個Set中添加beanName的過程，這個Set很重要，后面會用到。隨后就是調用singletonFactory的getObject方法進行真正的創建過程，下面我們看下剛剛上文提到的真正的創建的過程createBean,它里面的核心邏輯是doCreateBean.
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) { // 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 != null ? instanceWrapper.getWrappedInstance() : null); Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null); // Allow post-processors to modify the merged bean definition. synchronized (mbd.postProcessingLock) { if (!mbd.postProcessed) { applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName); mbd.postProcessed = true; } } // Eagerly cache singletons to be able to resolve circular references // even when triggered by lifecycle interfaces like BeanFactoryAware. //關注點3 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. 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; } createBeanInstance利用反射創建了對象，下面我們看看關注點3 earlySingletonExposure屬性值的判斷，其中有一個判斷點就是isSingletonCurrentlyInCreation(beanName)
public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); } 發現使用的是singletonsCurrentlyInCreation這個Set,上文的步驟中已經將BeanName已經填充進去了，所以可以查到，所以earlySingletonExposure這個屬性是結合其他的條件綜合判斷為true,進行下面的流程addSingletonFactory,這里是為這個Bean添加ObjectFactory,這個BeanName(A)對應的對象工廠，他的getObject方法的實現是通過getEarlyBeanReference這個方法實現的。首先我們看下addSingletonFactory的實現
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) { Assert.notNull(singletonFactory, "Singleton factory must not be null"); synchronized (this.singletonObjects) { if (!this.singletonObjects.containsKey(beanName)) { this.singletonFactories.put(beanName, singletonFactory); this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName); } } } 往第三級緩存singletonFactories存放數據，清除第二級緩存根據beanName的數據。這里有個很重要的點，是往三級緩存里面set了值，這是Spring處理循環依賴的核心點。getEarlyBeanReference這個方法是getObject的實現，可以簡單認為是返回了一個為填充完畢的A的對象實例。設置完三級緩存后，就開始了填充A對象屬性的過程。下面這段描述，沒有源碼提示，只是簡單的介紹一下。
填充A的時候，發現需要B類型的Bean，于是繼續調用getBean方法創建，記性的流程和上面A的完全一致，然后到了填充C類型的Bean的過程，同樣的調用getBean(C)來執行，同樣到了填充屬性A的時候，調用了getBean(A),我們從這里繼續說，調用了doGetBean中的Object sharedInstance = getSingleton(beanName),相同的代碼，但是處理邏輯完全不一樣了。
protected Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return (singletonObject != NULL_OBJECT ? singletonObject : null); } 還是從singletonObjects獲取對象獲取不到，因為A是在singletonsCurrentlyInCreation這個Set中，所以進入了下面的邏輯，從二級緩存earlySingletonObjects中取，還是沒有查到，然后從三級緩存singletonFactories找到對應的對象工廠調用getObject方法獲取未完全填充完畢的A的實例對象，然后刪除三級緩存的數據，填充二級緩存的數據，返回這個對象A。C依賴A的實例填充完畢了，雖然這個A是不完整的。不管怎么樣C式填充完了，就可以將C放到一級緩存singletonObjects同時清理二級和三級緩存的數據。同樣的流程B依賴的C填充好了，B也就填充好了，同理A依賴的B填充好了，A也就填充好了。Spring就是通過這種方式來解決循環引用的。
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