第Spring三級(jí)緩存解決循環(huán)依賴這樣拿到A的對(duì)象之后，B的屬性填充完畢，B初始化完成，方法return到DefaultSingletonBeanRegistry#getSingleton的重載方法時(shí)，調(diào)用DefaultSingletonBeanRegistry#addSingleton方法，將B對(duì)象放入一級(jí)緩存，并將B從二三級(jí)緩存中移除（雖然已經(jīng)沒(méi)有了）。

這樣在return回A的流程，第11步，將A依賴的B屬性填充完整，此時(shí)A也填充完畢，初始化完成，方法繼續(xù)return到A流程的DefaultSingletonBeanRegistry#getSingleton的重載方法時(shí)，調(diào)用DefaultSingletonBeanRegistry#addSingleton方法，將A對(duì)象放入一級(jí)緩存，并將A從二三級(jí)緩存中移除（此時(shí)只有二級(jí)緩存中有）。

這樣A和B就初始化完成了。

如果A或者B存在AOP，需要返回代理對(duì)象，這操作是在第9步的AbstractAutowireCapableBeanFactory#getEarlyBeanReference中完成的，B嘗試獲取A的時(shí)候，觸發(fā)了這個(gè)方法，如果A需要被代理，則是在這個(gè)方法中執(zhí)行的，這個(gè)方法最終返回了一個(gè)代理對(duì)象，并將這個(gè)對(duì)象以A的名義放入了二級(jí)緩存。

打完收工。

源碼

AbstractApplicationContext#refresh#finishBeanFactoryInitialization入口類方法刷新上下文，初始化BeanFactory，完成工廠初始化。

DefaultListableBeanFactory#preInstantiateSingletons準(zhǔn)備實(shí)例化單例對(duì)象

@Override

publicvoidpreInstantiateSingletons()throwsBeansException{

if(logger.isTraceEnabled()){

logger.trace("Pre-instantiatingsingletonsin"+this);

//Iterateoveracopytoallowforinitmethodswhichinturnregisternewbeandefinitions.

//Whilethismaynotbepartoftheregularfactorybootstrap,itdoesotherwiseworkfine.

ListStringbeanNames=newArrayList(this.beanDefinitionNames);

//Triggerinitializationofallnon-lazysingletonbeans...

for(StringbeanName:beanNames){

RootBeanDefinitionbd=getMergedLocalBeanDefinition(beanName);

if(!bd.isAbstract()bd.isSingleton()!bd.isLazyInit()){

if(isFactoryBean(beanName)){

Objectbean=getBean(FACTORY_BEAN_PREFIX+beanName);

if(beaninstanceofFactoryBean){

FactoryBeanfactory=(FactoryBean)bean;

booleanisEagerInit;

if(System.getSecurityManager()!=nullfactoryinstanceofSmartFactoryBean){

isEagerInit=AccessController.doPrivileged(

(PrivilegedActionBoolean)((SmartFactoryBean)factory)::isEagerInit,

getAccessControlContext());

else{

isEagerInit=(factoryinstanceofSmartFactoryBean

((SmartFactoryBean)factory).isEagerInit());

if(isEagerInit){

getBean(beanName);

else{

getBean(beanName);

//Triggerpost-initializationcallbackforallapplicablebeans...

for(StringbeanName:beanNames){

ObjectsingletonInstance=getSingleton(beanName);

if(singletonInstanceinstanceofSmartInitializingSingleton){

StartupStepsmartInitialize=this.getApplicationStartup().start("spring.beans.smart-initialize")

.tag("beanName",beanName);

SmartInitializingSingletonsmartSingleton=(SmartInitializingSingleton)singletonInstance;

if(System.getSecurityManager()!=null){

AccessController.doPrivileged((PrivilegedActionObject)()-{

smartSingleton.afterSingletonsInstantiated();

returnnull;

},getAccessControlContext());

else{

smartSingleton.afterSingletonsInstantiated();

smartInitialize.end();

AbstractBeanFactory#getBean(java.lang.String)獲取bean對(duì)象

@Override

publicObjectgetBean(Stringname)throwsBeansException{

returndoGetBean(name,null,null,false);

AbstractBeanFactory#doGetBean實(shí)際執(zhí)行獲取bean對(duì)象

*Returnaninstance,whichmaybesharedorindependent,ofthespecifiedbean.

*@paramnamethenameofthebeantoretrieve

*@paramrequiredTypetherequiredtypeofthebeantoretrieve

*@paramargsargumentstousewhencreatingabeaninstanceusingexplicitarguments

*(onlyappliedwhencreatinganewinstanceasopposedtoretrievinganexistingone)

*@paramtypeCheckOnlywhethertheinstanceisobtainedforatypecheck,

*notforactualuse

*@returnaninstanceofthebean

*@throwsBeansExceptionifthebeancouldnotbecreated

@SuppressWarnings("unchecked")

protectedTTdoGetBean(

Stringname,@NullableClassTrequiredType,@NullableObject[]args,booleantypeCheckOnly)

throwsBeansException{

StringbeanName=transformedBeanName(name);

ObjectbeanInstance;

//Eagerlychecksingletoncacheformanuallyregisteredsingletons.

ObjectsharedInstance=getSingleton(beanName);

if(sharedInstance!=nullargs==null){

if(logger.isTraceEnabled()){

if(isSingletonCurrentlyInCreation(beanName)){

logger.trace("Returningeagerlycachedinstanceofsingletonbean'"+beanName+

"'thatisnotfullyinitializedyet-aconsequenceofacircularreference");

else{

logger.trace("Returningcachedinstanceofsingletonbean'"+beanName+"'");

beanInstance=getObjectForBeanInstance(sharedInstance,name,beanName,null);

else{

//Failifwe'realreadycreatingthisbeaninstance:

//We'reassumablywithinacircularreference.

if(isPrototypeCurrentlyInCreation(beanName)){

thrownewBeanCurrentlyInCreationException(beanName);

//Checkifbeandefinitionexistsinthisfactory.

BeanFactoryparentBeanFactory=getParentBeanFactory();

if(parentBeanFactory!=null!containsBeanDefinition(beanName)){

//Notfound-checkparent.

StringnameToLookup=originalBeanName(name);

if(parentBeanFactoryinstanceofAbstractBeanFactory){

return((AbstractBeanFactory)parentBeanFactory).doGetBean(

nameToLookup,requiredType,args,typeCheckOnly);

elseif(args!=null){

//Delegationtoparentwithexplicitargs.

return(T)parentBeanFactory.getBean(nameToLookup,args);

elseif(requiredType!=null){

//Noargs-delegatetostandardgetBeanmethod.

returnparentBeanFactory.getBean(nameToLookup,requiredType);

else{

return(T)parentBeanFactory.getBean(nameToLookup);

if(!typeCheckOnly){

markBeanAsCreated(beanName);

StartupStepbeanCreation=this.applicationStartup.start("spring.beans.instantiate")

.tag("beanName",name);

try{

if(requiredType!=null){

beanCreation.tag("beanType",requiredType::toString);

RootBeanDefinitionmbd=getMergedLocalBeanDefinition(beanName);

checkMergedBeanDefinition(mbd,beanName,args);

//Guaranteeinitializationofbeansthatthecurrentbeandependson.

String[]dependsOn=mbd.getDependsOn();

if(dependsOn!=null){

for(Stringdep:dependsOn){

if(isDependent(beanName,dep)){

thrownewBeanCreationException(mbd.getResourceDescription(),beanName,

"Circulardepends-onrelationshipbetween'"+beanName+"'and'"+dep+"'");

registerDependentBean(dep,beanName);

try{

getBean(dep);

catch(NoSuchBeanDefinitionExceptionex){

thrownewBeanCreationException(mbd.getResourceDescription(),beanName,

"'"+beanName+"'dependsonmissingbean'"+dep+"'",ex);

//Createbeaninstance.

if(mbd.isSingleton()){

sharedInstance=getSingleton(beanName,()-{

try{

returncreateBean(beanName,mbd,args);

catch(BeansExceptionex){

//Explicitlyremoveinstancefromsingletoncache:Itmighthavebeenputthere

//eagerlybythecreationprocess,toallowforcircularreferenceresolution.

//Alsoremoveanybeansthatreceivedatemporaryreferencetothebean.

destroySingleton(beanName);

throwex;

beanInstance=getObjectForBeanInstance(sharedInstance,name,beanName,mbd);

elseif(mbd.isPrototype()){

//It'saprototype-createanewinstance.

ObjectprototypeInstance=null;

try{

beforePrototypeCreation(beanName);

prototypeInstance=createBean(beanName,mbd,args);

finally{

afterPrototypeCreation(beanName);

beanInstance=getObjectForBeanInstance(prototypeInstance,name,beanName,mbd);

else{

StringscopeName=mbd.getScope();

if(!StringUtils.hasLength(scopeName)){

thrownewIllegalStateException("Noscopenamedefinedforbean′"+beanName+"'");

Scopescope=this.scopes.get(scopeName);

if(scope==null){

thrownewIllegalStateException("NoScoperegisteredforscopename'"+scopeName+"'");

try{

ObjectscopedInstance=scope.get(beanName,()-{

beforePrototypeCreation(beanName);

try{

returncreateBean(beanName,mbd,args);

finally{

afterPrototypeCreation(beanName);

beanInstance=getObjectForBeanInstance(scopedInstance,name,beanName,mbd);

catch(IllegalStateExceptionex){

thrownewScopeNotActiveException(beanName,scopeName,ex);

catch(BeansExceptionex){

beanCreation.tag("exception",ex.getClass().toString());

beanCreation.tag("message",String.valueOf(ex.getMessage()));

cleanupAfterBeanCreationFailure(beanName);

throwex;

finally{

beanCreation.end();

returnadaptBeanInstance(name,beanInstance,requiredType);

AbstractAutowireCapableBeanFactory.javacreateBean、doCreateBean、getEarlyBeanReference

@Override

protectedObjectcreateBean(StringbeanName,RootBeanDefinitionmbd,@NullableObject[]args)

throwsBeanCreationException{

if(logger.isTraceEnabled()){

logger.trace("Creatinginstanceofbean'"+beanName+"'");

RootBeanDefinitionmbdToUse=mbd;

//Makesurebeanclassisactuallyresolvedatthispoint,and

//clonethebeandefinitionincaseofadynamicallyresolvedClass

//whichcannotbestoredinthesharedmergedbeandefinition.

ClassresolvedClass=resolveBeanClass(mbd,beanName);

if(resolvedClass!=null!mbd.hasBeanClass()mbd.getBeanClassName()!=null){

mbdToUse=newRootBeanDefinition(mbd);

mbdToUse.setBeanClass(resolvedClass);

//Preparemethodoverrides.

try{

mbdToUse.prepareMethodOverrides();

catch(BeanDefinitionValidationExceptionex){

thrownewBeanDefinitionStoreException(mbdToUse.getResourceDescription(),

beanName,"Validationofmethodoverridesfailed",ex);

try{

//GiveBeanPostProcessorsachancetoreturnaproxyinsteadofthetargetbeaninstance.

Objectbean=resolveBeforeInstantiation(beanName,mbdToUse);

if(bean!=null){

returnbean;

catch(Throwableex){

thrownewBeanCreationException(mbdToUse.getResourceDescription(),beanName,

"BeanPostProcessorbeforeinstantiationofbeanfailed",ex);

try{

ObjectbeanInstance=doCreateBean(beanName,mbdToUse,args);

if(logger.isTraceEnabled()){

logger.trace("Finishedcreatinginstanceofbean'"+beanName+"'");

returnbeanInstance;

catch(BeanCreationException|ImplicitlyAppearedSingletonExceptionex){

//Apreviouslydetectedexceptionwithproperbeancreationcontextalready,

//orillegalsingletonstatetobecommunicateduptoDefaultSingletonBeanRegistry.

throwex;

catch(Throwableex){

thrownewBeanCreationException(

mbdToUse.getResourceDescription(),beanName,"Unexpectedexceptionduringbeancreation",ex);

protectedObjectdoCreateBean(StringbeanName,RootBeanDefinitionmbd,@NullableObject[]args)

throwsBeanCreationException{

//Instantiatethebean.

BeanWrapperinstanceWrapper=null;

if(mbd.isSingleton()){

instanceWrapper=this.factoryBeanInstanceCache.remove(beanName);

if(instanceWrapper==null){

instanceWrapper=createBeanInstance(beanName,mbd,args);

Objectbean=instanceWrapper.getWrappedInstance();

ClassbeanType=instanceWrapper.getWrappedClass();

if(beanType!=NullBean.class){

mbd.resolvedTargetType=beanType;

//Allowpost-processorstomodifythemergedbeandefinition.

synchronized(mbd.postProcessingLock){

if(!mbd.postProcessed){

try{

applyMergedBeanDefinitionPostProcessors(mbd,beanType,beanName);

catch(Throwableex){

thrownewBeanCreationException(mbd.getResourceDescription(),beanName,

"Post-processingofmergedbeandefinitionfailed",ex);

mbd.postProcessed=true;

//Eagerlycachesingletonstobeabletoresolvecircularreferences

//evenwhentriggeredbylifecycleinterfaceslikeBeanFactoryAware.

booleanearlySingletonExposure=(mbd.isSingleton()this.allowCircularReferences

isSingletonCurrentlyInCreation(beanName));

if(earlySingletonExposure){

if(logger.isTraceEnabled()){

logger.trace("Eagerlycachingbean'"+beanName+

"'toallowforresolvingpotentialcircularreferences");

addSingletonFactory(beanName,()-getEarlyBeanReference(beanName,mbd,bean));

//Initializethebeaninstance.

ObjectexposedObject=bean;

try{

populateBean(beanName,mbd,instanceWrapper);

exposedObject=initializeBean(beanName,exposedObject,mbd);

catch(Throwableex){

if(exinstanceofBeanCreationExceptionbeanName.equals(((BeanCreationException)ex).getBeanName())){

throw(BeanCreationException)ex;

else{

thrownewBeanCreationException(

mbd.getResourceDescription(),beanName,"Initializationofbeanfailed",ex);

if(earlySingletonExposure){

ObjectearlySingletonReference=getSingleton(beanName,false);

if(earlySingletonReference!=null){

if(exposedObject==bean){

exposedObject=earlySingletonReference;

elseif(!this.allowRawInjectionDespiteWrappinghasDependentBean(beanName)){

String[]dependentBeans=getDependentBeans(beanName);

SetStringactualDependentBeans=newLinkedHashSet(dependentBeans.length);

for(StringdependentBean:dependentBeans){

if(!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)){

actualDependentBeans.add(dependentBean);

if(!actualDependentBeans.isEmpty()){

thrownewBeanCurrentlyInCreationException(beanName,

"Beanwithname'"+beanName+"'hasbeeninjectedintootherbeans["+

StringUtils.collectionToCommaDelimitedString(actualDependentBeans)+

"]initsrawversionaspartofacircularreference,buthaseventuallybeen"+

"wrapped.Thismeansthatsaidotherbeansdonotusethefinalversionofthe"+

"bean.Thisisoftentheresultofover-eagertypematching-considerusing"+

"'getBeanNamesForType'withthe'allowEagerInit'flagturnedoff,forexample.");

//Registerbeanasdisposable.

try{

registerDisposableBeanIfNecessary(beanName,bean,mbd);

catch(BeanDefinitionValidationExceptionex){

thrownewBeanCreationException(

mbd.getResourceDescription(),beanName,"Invaliddestructionsignature",ex);

returnexposedObject;

protectedObjectgetEarlyBeanReference(StringbeanName,RootBeanDefinitionmbd,Objectbean){

ObjectexposedObject=bean;

if(!mbd.isSynthetic()hasInstantiationAwareBeanPostProcessors()){

for(SmartInstantiationAwareBeanPostProcessorbp:getBeanPostProcessorCache().smartInstantiationAware){

exposedObject=bp.getEarlyBeanReference(exposedObject,beanName);

returnexposedObject;

DefaultSingletonBeanRegistry.java三級(jí)緩存

publicclassDefaultSingletonBeanRegistryextendsSimpleAliasRegistryimplementsSingletonBeanRegistry{

/**Cacheofsingletonobjects:beannametobeaninstance.一級(jí)緩存*/

privatefinalMapString,ObjectsingletonObjects=newConcurrentHashMap(256);

/**Cacheofsingletonfactories:beannametoObjectFactory.三級(jí)緩存*/

privatefinalMapString,ObjectFactorysingleton