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spring針對(duì)Bean之間的循環(huán)依賴,有自己的處理方案。關(guān)鍵點(diǎn)就是三級(jí)緩存。當(dāng)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(gòu)造函數(shù)的循環(huán)依賴。
我們就從A->B->C-A這個(gè)初始化順序,也就是A的Bean中需要B的實(shí)例,B的Bean中需要C的實(shí)例,C的Bean中需要A的實(shí)例,當(dāng)然這種需要不是構(gòu)造函數(shù)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(huì)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(guān)注點(diǎn)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(guān)注點(diǎn)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(gè)方法很長(zhǎng)我們一點(diǎn)點(diǎn)說。先看我們的關(guān)注點(diǎn)1 Object sharedInstance = getSingleton(beanName
)根據(jù)名稱從單例的集合中獲取單例對(duì)象,我們看下這個(gè)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(gè)方法,很關(guān)鍵,我們開篇提到了三級(jí)緩存,使用點(diǎn)之一就是這里。到底是哪三級(jí)緩存呢,第一級(jí)緩存singletonObjects
里面放置的是實(shí)例化好的單例對(duì)象。第二級(jí)earlySingletonObjects
里面存放的是提前曝光的單例對(duì)象(沒有完全裝配好)。第三級(jí)singletonFactories里面存放的是要被實(shí)例化的對(duì)象的對(duì)象工廠。解釋好了三級(jí)緩存,我們?cè)倏纯催壿?。第一次進(jìn)來this.singletonObjects.get(beanName)
返回的肯定是null。然后isSingletonCurrentlyInCreation
決定了能否進(jìn)入二級(jí)緩存中獲取數(shù)據(jù)。
public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); }
從singletonsCurrentlyInCreation
這個(gè)Set中有沒有包含傳入的BeanName,前面沒有地方設(shè)置,所以肯定不包含,所以這個(gè)方法返回false,后面的流程就不走了。getSingleton
這個(gè)方法返回的是null。
下面我們看下關(guān)注點(diǎn)2.也是一個(gè)getSingleton
只不過他是真實(shí)的創(chuàng)建Bean的過程,我們可以看到傳入了一個(gè)匿名的ObjectFactory的對(duì)象,他的getObject方法中調(diào)用的是createBean這個(gè)真正的創(chuàng)建Bean的方法。當(dāng)然我們可以先擱置一下,繼續(xù)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(gè)方法的第一句Object singletonObject = this.singletonObjects.get(beanName)
從一級(jí)緩存中取數(shù)據(jù),肯定是null。隨后就調(diào)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(gè)Set中添加beanName的過程,這個(gè)Set很重要,后面會(huì)用到。隨后就是調(diào)用singletonFactory的getObject方法進(jìn)行真正的創(chuàng)建過程,下面我們看下剛剛上文提到的真正的創(chuàng)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(guān)注點(diǎn)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(chuàng)建了對(duì)象,下面我們看看關(guān)注點(diǎn)3 earlySingletonExposure
屬性值的判斷,其中有一個(gè)判斷點(diǎn)就是isSingletonCurrentlyInCreation(beanName)
public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); }
發(fā)現(xiàn)使用的是singletonsCurrentlyInCreation這個(gè)Set,上文的步驟中已經(jīng)將BeanName已經(jīng)填充進(jìn)去了,所以可以查到,所以earlySingletonExposure
這個(gè)屬性是結(jié)合其他的條件綜合判斷為true,進(jìn)行下面的流程addSingletonFactory
,這里是為這個(gè)Bean添加ObjectFactory,這個(gè)BeanName(A)對(duì)應(yīng)的對(duì)象工廠,他的getObject方法的實(shí)現(xiàn)是通過getEarlyBeanReference
這個(gè)方法實(shí)現(xiàn)的。首先我們看下addSingletonFactory的實(shí)現(xiàn)
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(jí)緩存singletonFactories存放數(shù)據(jù),清除第二級(jí)緩存根據(jù)beanName的數(shù)據(jù)。這里有個(gè)很重要的點(diǎn),是往三級(jí)緩存里面set了值,這是Spring處理循環(huán)依賴的核心點(diǎn)。getEarlyBeanReference
這個(gè)方法是getObject的實(shí)現(xiàn),可以簡(jiǎn)單認(rèn)為是返回了一個(gè)為填充完畢的A的對(duì)象實(shí)例。設(shè)置完三級(jí)緩存后,就開始了填充A對(duì)象屬性的過程。下面這段描述,沒有源碼提示,只是簡(jiǎn)單的介紹一下。
填充A的時(shí)候,發(fā)現(xiàn)需要B類型的Bean,于是繼續(xù)調(diào)用getBean方法創(chuàng)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(diào)用getBean(C)來執(zhí)行,同樣到了填充屬性A的時(shí)候,調(diào)用了getBean(A),我們從這里繼續(xù)說,調(diào)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(duì)象獲取不到,因?yàn)锳是在singletonsCurrentlyInCreation
這個(gè)Set中,所以進(jìn)入了下面的邏輯,從二級(jí)緩存earlySingletonObjects
中取,還是沒有查到,然后從三級(jí)緩存singletonFactories
找到對(duì)應(yīng)的對(duì)象工廠調(diào)用getObject
方法獲取未完全填充完畢的A的實(shí)例對(duì)象,然后刪除三級(jí)緩存的數(shù)據(jù),填充二級(jí)緩存的數(shù)據(jù),返回這個(gè)對(duì)象A。C依賴A的實(shí)例填充完畢了,雖然這個(gè)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(jí)緩存singletonObjects
同時(shí)清理二級(jí)和三級(jí)緩存的數(shù)據(jù)。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(huán)引用的。
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