第Spring循環(huán)引用失敗問題源碼解析目錄前言:例子啟動容器加載circulationaAbstractBeanFactory最終調(diào)用BeanDefinitionValueResolvercirculationb加載分析

前言:

之前我們有分析過Spring是怎么解決循環(huán)引用的問題,主要思路就是三級緩存；

Spring在加載beanA的時候會先調(diào)用默認的空構(gòu)造函數(shù)(在沒有指定構(gòu)造函數(shù)實例化的前提下)得到一個空的實例引用對象，這個時候沒有設置任何值，但是Spring會用緩存把它給提前暴露出來，讓其他依賴beanA的bean可以持有它提前暴露的引用；

比如a依賴b，b依賴a，并且他們都是通過默認方法實例化，那么簡單流程是這樣的：

ioc實例化a,a提前暴露自己的，然后填充屬性值，在填充屬性值的時候發(fā)現(xiàn)有個對象b,這個時候去容器里面取到b的引用，發(fā)現(xiàn)b還沒有被創(chuàng)建，那么就走實例化b的流程；實例化b；流程跟a一樣；但是不同的是b填充屬性的時候，發(fā)現(xiàn)有引用a的實例，這個時候a已經(jīng)提前暴露了自己了，所以b可以直接在容器里面拿到a的引用；那么b就實例化并且也初始化完成了;拿到b了之后，a就可以持有b的引用，整個流程就走完了;

具體詳細一點可以看這篇文章Spring-bean的循環(huán)依賴以及解決方式

Spring不能解決A的構(gòu)造方法中依賴了B的實例對象，同時B依賴了A的實例對象這類問題

這篇文章我想從源碼的角度來分析一下整個流程；

并且分析一下Spring為什么不能解決A的構(gòu)造方法中依賴了B的實例對象，同時B依賴了A的實例對象這類問題

例子

首先創(chuàng)建兩個bean類;CirculationA有個屬性circulationB，并且有個構(gòu)造函數(shù)給circulationB賦值；

publicclassCirculationA{

privateCirculationBcirculationB;

publicCirculationA(CirculationBcirculationB){

this.circulationB=circulationB;

CirculationB有個屬性circulationA，然后set方法

publicclassCirculationB{

privateCirculationAcirculationA;

publicCirculationAgetCirculationA(){

returncirculationA;

publicvoidsetCirculationA(CirculationAcirculationA){

this.circulationA=circulationA;

SpringContextConfig.xmlcirculationa用給定的構(gòu)造函數(shù)實例化;

circulationb就用默認的實例化方法(默認的空構(gòu)造函數(shù))

beanid="circulationa"

constructor-argname="circulationB"ref="circulationb"/

/bean

beanid="circulationb"

propertyname="circulationA"ref="circulationa"/

/bean

好，例子準完畢，上面的例子是circulationa的構(gòu)造函數(shù)里面有circulationb；

然后circulationb屬性里面有circulationa;

啟動容器

結(jié)果如下:

警告:Exceptionencounteredduringcontextinitialization-cancellingrefreshattempt:org.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithnamecirculationadefinedinclasspathresource[config.xml]:Cannotresolvereferencetobeancirculationbwhilesettingconstructorargument;nestedexceptionisorg.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithnamecirculationbdefinedinclasspathresource[config.xml]:CannotresolvereferencetobeancirculationawhilesettingbeanpropertycirculationAnestedexceptionisorg.springframework.beans.factory.BeanCurrentlyInCreationException:Errorcreatingbeanwithnamecirculationa:Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference

Exceptioninthreadmainorg.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithnamecirculationadefinedinclasspathresource[config.xml]:Cannotresolvereferencetobeancirculationbwhilesettingconstructorargument;nestedexceptionisorg.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithnamecirculationbdefinedinclasspathresource[config.xml]:CannotresolvereferencetobeancirculationawhilesettingbeanpropertycirculationAnestedexceptionisorg.springframework.beans.factory.BeanCurrentlyInCreationException:Errorcreatingbeanwithnamecirculationa:Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference

DisconnectedfromthetargetVM,address:127.0.0.1:64128,transport:socket

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)

atorg.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648)

atorg.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1193)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1095)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)

atorg.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)

atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)

atorg.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)

atorg.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)

atorg.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)

atorg.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)

atorg.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)

atorg.springframework.context.support.ClassPathXmlApplicationContext.init(ClassPathXmlApplicationContext.java:139)

atorg.springframework.context.support.ClassPathXmlApplicationContext.init(ClassPathXmlApplicationContext.java:83)

atStartIOCUseDefaultListAbleBeanFactory.main(StartIOCUseDefaultListAbleBeanFactory.java:30)

Causedby:org.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithnamecirculationbdefinedinclasspathresource[config.xml]:CannotresolvereferencetobeancirculationawhilesettingbeanpropertycirculationAnestedexceptionisorg.springframework.beans.factory.BeanCurrentlyInCreationException:Errorcreatingbeanwithnamecirculationa:Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1531)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1276)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)

atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)

atorg.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)

atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)

atorg.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)

atorg.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)

...17more

Causedby:org.springframework.beans.factory.BeanCurrentlyInCreationException:Errorcreatingbeanwithnamecirculationa:Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference

atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347)

atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)

atorg.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)

atorg.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)

atorg.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)

...27more

報錯了,Spring它解決不了這種情況Ok,源碼走起來：為了節(jié)省篇幅我只貼重要代碼第一步

加載circulationaAbstractBeanFactory

publicabstractclassAbstractBeanFactoryextendsFactoryBeanRegistrySupportimplementsConfigurableBeanFactory{

protectedTTdoGetBean(

finalStringname,finalClassTrequiredType,finalObject[]args,booleantypeCheckOnly)

throwsBeansException{

if(mbd.isSingleton()){

sharedInstance=getSingleton(beanName,newObjectFactoryObject(){

@Override

publicObjectgetObject()throwsBeansException{

try{

returncreateBean(beanName,mbd,args);

publicObjectgetSingleton(StringbeanName,ObjectFactorysingletonFactory){

//在創(chuàng)建之前把beanName加入到正在創(chuàng)建中的屬性中singletonsCurrentlyInCreation；

//但是這個是一個set，如果之前已經(jīng)加進去了，再進去就拋異常BeanCurrentlyInCreationException

//Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference")提示可能存在循環(huán)引用

beforeSingletonCreation(beanName);

protectedvoidbeforeSingletonCreation(StringbeanName){

if(!this.inCreationCheckExclusions.contains(beanName)!this.singletonsCurrentlyInCreation.add(beanName)){

thrownewBeanCurrentlyInCreationException(beanName);

@Override

protectedObjectcreateBean(StringbeanName,RootBeanDefinitionmbd,Object[]args)throwsBeanCreationException{

ObjectbeanInstance=doCreateBean(beanName,mbdToUse,args);

protectedObjectdoCreateBean(finalStringbeanName,finalRootBeanDefinitionmbd,finalObject[]args)

throwsBeanCreationException{

instanceWrapper=createBeanInstance(beanName,mbd,args);

//.......

//Initializethebeaninstance.

ObjectexposedObject=bean;

try{

populateBean(beanName,mbd,instanceWrapper);

if(exposedObject!=null){

exposedObject=initializeBean(beanName,exposedObject,mbd);

protectedBeanWrappercreateBeanInstance(StringbeanName,RootBeanDefinitionmbd,Object[]args){

Constructor[]ctors=determineConstructorsFromBeanPostProcessors(beanClass,beanName);

//因為circulationa是有構(gòu)造函數(shù)的，所以使用autowireConstructor

if(ctors!=null||

mbd.getResolvedAutowireMode()==RootBeanDefinition.AUTOWIRE_CONSTRUCTOR||

mbd.hasConstructorArgumentValues()||!ObjectUtils.isEmpty(args)){

returnautowireConstructor(beanName,mbd,ctors,args);

//最終執(zhí)行

publicBeanWrapperautowireConstructor(finalStringbeanName,finalRootBeanDefinitionmbd,

Constructor[]chosenCtors,finalObject[]explicitArgs){

//解析構(gòu)造函數(shù)參數(shù)值

minNrOfArgs=resolveConstructorArguments(beanName,mbd,bw,cargs,resolvedValues);

//......

//選擇對應的策略來實例化對象；這里是生成正在的實例了。

//但是在這之前，構(gòu)造參數(shù)要拿到

beanInstance=this.beanFactory.getInstantiationStrategy().instantiate(

mbd,beanName,this.beanFactory,constructorToUse,argsToUse);

省略....

最終調(diào)用BeanDefinitionValueResolver

/**

*Resolveareferencetoanotherbeaninthefactory.

privateObjectresolveReference(ObjectargName,RuntimeBeanReferenceref){

try{

StringrefName=ref.getBeanName();

refName=String.valueOf(doEvaluate(refName));

if(ref.isToParent()){

if(this.beanFactory.getParentBeanFactory()==null){

thrownewBeanCreationException(

this.beanDefinition.getResourceDescription(),this.beanName,

"Can'tresolvereferencetobean'"+refName+

"'inparentfactory:noparentfactoryavailable");

//!!!這里,要先去查找refName的實例

returnthis.beanFactory.getParentBeanFactory().getBean(refName);

else{

Objectbean=this.beanFactory.getBean(refName);

this.beanFactory.registerDependentBean(refName,this.beanName);

returnbean;

catch(BeansExceptionex){

thrownewBeanCreationException(

this.beanDefinition.getResourceDescription(),this.beanName,

"Cannotresolvereferencetobean'"+ref.getBeanName()+"'whilesetting"+argName,ex);

跟著上面的順序我們整理一下;

啟動容器,加載circulationa,因為是構(gòu)造函數(shù)生成，所以要先解析構(gòu)造函數(shù)的屬性，這時候發(fā)現(xiàn)有引用circulationb，那么通過getBean(circulationb)先拿到circulationb的實例；如果拿到了，則生成circulationa的實例對象返回;但是這個時候代碼執(zhí)行circulationb的加載過程了；

circulationb加載分析

然后我們分析一下circulationb加載circulationb跟circulationa差不多加載circulationb,把它加入到正在創(chuàng)建的屬性中

protectedvoidbeforeSingletonCreation(StringbeanName){

if(!this.inCreationCheckExclusions.contains(beanName)amp;amp;!this.singletonsCurrentlyInCreation.add(beanName)){

thrownewBeanC