第基于springbootbean的實例化過程和屬性注入過程目錄bean的實例化過程和屬性注入過程bean實例化流程說明

bean的實例化過程和屬性注入過程

了解過springboot的加載流程的都知道springboot初始化bean都在refresh方法中。

這個方法代碼如下：

//Preparethiscontextforrefreshing.

prepareRefresh();

//Tellthesubclasstorefreshtheinternalbeanfactory.新建beanFactory

ConfigurableListableBeanFactorybeanFactory=obtainFreshBeanFactory();

//Preparethebeanfactoryforuseinthiscontext.

prepareBeanFactory(beanFactory);

try{

//Allowspost-processingofthebeanfactoryincontextsubclasses.

postProcessBeanFactory(beanFactory);

//Invokefactoryprocessorsregisteredasbeansinthecontext.

//加載實現beanFactoryPostProcessor的bean，bean定義的時候執行

invokeBeanFactoryPostProcessors(beanFactory);

//Registerbeanprocessorsthatinterceptbeancreation.

//加載實現了beanPostProcessor，在bean實例化前、后執行

registerBeanPostProcessors(beanFactory);

//Initializemessagesourceforthiscontext.

initMessageSource();

//Initializeeventmulticasterforthiscontext.

initApplicationEventMulticaster();

//Initializeotherspecialbeansinspecificcontextsubclasses.

onRefresh();

//Checkforlistenerbeansandregisterthem.

registerListeners();

//Instantiateallremaining(non-lazy-init)singletons.

//實例化非懶加載的bean、bean封裝、屬性注入、注解注入(主要使用BeanPostProcessor或子類實現)等

finishBeanFactoryInitialization(beanFactory);

//Laststep:publishcorrespondingevent.

finishRefresh();

這里我們主要看下finishBeanfactoryInitialization方法。此方法實現了bean的實例和屬性注入等。進入此方法的最后一行。

//Instantiateallremaining(non-lazy-init)singletons.

beanFactory.preInstantiateSingletons();

注釋寫的很清楚，實例化剩余非懶加載的bean。此方法的實現是核心類DefaultLisListableBeanFactory實現。這個方法中有個判斷：bean是否是beanFactory的實現類。

如果是則獲取bean的時候會從beanFactory實現類的getObject中獲取，我們重點看看getBean這個方法。

getBean是spring中最重要、最牛逼的方法之一，具體的邏輯是通過doGetBean方法處理的。

我們看下doGetBean方法，方法很長。我們分成幾個部分去看。

1、先判斷是否已經存在緩存中，代碼如下：

if(sharedInstance!=nullargs==null){

if(logger.isDebugEnabled()){

if(isSingletonCurrentlyInCreation(beanName)){

logger.debug("Returningeagerlycachedinstanceofsingletonbean'"+beanName+

"'thatisnotfullyinitializedyet-aconsequenceofacircularreference");

else{

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

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

2、從父beanfactory中獲取

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);

else{

//Noargs-delegatetostandardgetBeanmethod.

returnparentBeanFactory.getBean(nameToLookup,requiredType);

}

3、直接創建RootBeanDefinition

//mark給此bean馬克一下。防止重復創建

if(!typeCheckOnly){

markBeanAsCreated(beanName);

try{

finalRootBeanDefinitionmbd=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);

getBean(dep);

4、是singleton還是prototype類型的，根據不同類型去實例化bean,代碼只貼了單例的類型：

//Createbeaninstance.

if(mbd.isSingleton()){

sharedInstance=getSingleton(beanName,()-{

try{

returncreateBean(beanName,mbd,args);

catch(BeansExceptionex){

//Explicitlyremoveinstancefromsingletoncache:Itmighthavebeenputthere

//eagerlybythecreationprocess,toallowforcircularreferenceresolution.

//Alsoremoveanybeansthatreceivedatemporaryreferencetothebean.

destroySingleton(beanName);

throwex;

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

5、檢查bean的類型是否匹配

//Checkifrequiredtypematchesthetypeoftheactualbeaninstance.

if(requiredType!=null!requiredType.isInstance(bean)){

try{

TconvertedBean=getTypeConverter().convertIfNecessary(bean,requiredType);

if(convertedBean==null){

thrownewBeanNotOfRequiredTypeException(name,requiredType,bean.getClass());

returnconvertedBean;

catch(TypeMismatchExceptionex){

if(logger.isDebugEnabled()){

logger.debug("Failedtoconvertbean'"+name+"'torequiredtype'"+

ClassUtils.getQualifiedName(requiredType)+"'",ex);

thrownewBeanNotOfRequiredTypeException(name,requiredType,bean.getClass());

}

整個doGetBean方法改成這5個部分，重點看下第4個部分中的createBean和getObjectForBeanInstance方法。

1、createBean方法，里面主要是2個部分,bean直接是通過BeanPostProcessor的postProcessBeforeIntantiation方法獲取的。

注釋也是描述的很清楚：GiveBeanPostProcessorsachancetoreturnaproxyinsteadofthebeaninstance(英語渣渣不敢瞎翻譯誤導大家，理解就好)

代碼如下：

try{

//GiveBeanPostProcessorsachancetoreturnaproxyinsteadofthetargetbeaninstance.

Objectbean=resolveBeforeInstantiation(beanName,mbdToUse);

if(bean!=null){

returnbean;

catch(Throwableex){

thrownewBeanCreationException(mbdToUse.getResourceDescription(),beanName,

"BeanPostProcessorbeforeinstantiationofbeanfailed",ex);

第一個部分自己實現，那么createBean方法第2個部分毋庸置疑肯定是spring去實例化bean，代碼如下：

try{

ObjectbeanInstance=doCreateBean(beanName,mbdToUse,args);

if(logger.isDebugEnabled()){

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

returnbeanInstance;

}

又是do開頭的方法，說明這個也是一個創建方法，spring中一般do開頭的都是用于創建某個對象。跟著代碼走，看下doCreateBean方法，在查看此方法之前，可能需要了解下BeanWarpper這個封裝類。bean的封裝、屬性注入等都是用BeanWarpper去完成的。

看下代碼：

//Instantiatethebean.

BeanWrapperinstanceWrapper=null;

if(mbd.isSingleton()){

instanceWrapper=this.factoryBeanInstanceCache.remove(beanName);

if(instanceWrapper==null){

instanceWrapper=createBeanInstance(beanName,mbd,args);

}

進入createBeanInstance方法中，就是調用構造器去實例化bean，返回beanWrapper。通過構造器獲取實例代碼如下：

//Needtodeterminetheconstructor...

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

if(ctors!=null||

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

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

returnautowireConstructor(beanName,mbd,ctors,args);

//Nospecialhandling:simplyuseno-argconstructor.

returninstantiateBean(beanName,mbd);

這里實例化我一開始以為就是簡單的反射，后面我點進去時候發現里面一直提策略實例化，我目前了解的是可以解決Override的問題等。如果有興趣的可以自行查看。到這里為止。我們終于實例化bean了。下面看下第二步bean的屬性注入等。

代碼如下：

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);

}

populateBean方法名稱就暴露他是干啥的：填充bean。我放一段比較重要的部分：

if(hasInstAwareBpps||needsDepCheck){

if(pvs==null){

pvs=mbd.getPropertyValues();

PropertyDescriptor[]filteredPds=filterPropertyDescriptorsForDependencyCheck(bw,mbd.allowCaching);

if(hasInstAwareBpps){

for(BeanPostProcessorbp:getBeanPostProcessors()){

if(bpinstanceofInstantiationAwareBeanPostProcessor){

InstantiationAwareBeanPostProcessoribp=(InstantiationAwareBeanPostProcessor)bp;

pvs=ibp.postProcessPropertyValues(pvs,filteredPds,bw.getWrappedInstance(),beanName);

if(pvs==null){

return;

if(needsDepCheck){

checkDependencies(beanName,mbd,filteredPds,pvs);

}

這段代碼貌似沒有什么注入、填充什么的，我們看下InstantiationAwareBeanPostProcessor這個接口，發現這個接口有個很熟悉的實現類是：AutowiredAnnotationBeanPostProcessor。這個類的方法中我們看到終于看到了jnject方法。但是在inject之前需要調用下findAutowiringMatedata方法獲取一下元數據：

//Fallbacktoclassnameascachekey,forbackwardscompatibilitywithcustomcallers.

StringcacheKey=(StringUtils.hasLength(beanName)beanName:clazz.getName());

//Quickcheckontheconcurrentmapfirst,withminimallocking.

InjectionMetadatametadata=this.injectionMetadataCache.get(cacheKey);

if(InjectionMetadata.needsRefresh(metadata,clazz)){

synchronized(this.injectionMetadataCache){

metadata=this.injectionMetadataCache.get(cacheKey);

if(InjectionMetadata.needsRefresh(metadata,clazz)){

if(metadata!=null){

metadata.clear(pvs);

metadata=buildAutowiringMetadata(clazz);

this.injectionMetadataCache.put(cacheKey,metadata);

returnmetadata;

那么問題又來了，這個injectionMetadataCache集合值是從那里來的呢？AutowiredAnnotationBeanPostProcessor實現了MergeBeandefinitionPostProcessor,那么就好了，肯定是在bean實例的時候調用了postProcessMergedBeanDefintion這個方法。

果然在doCreateBean方法中有這么一段：

//Allowpost-processorstomodifythemergedbeandefinition.

synchronized(mbd.postProcessingLock){

if(!mbd.postProcessed){

try{

//通過BeanPostProcessor將需要注解的元數據放到Map中

applyMergedBeanDefinitionPostProcessors(mbd,beanType,beanName);

catch(Throwableex){

thrownewBeanCreationException(mbd.getResourceDescription(),beanName,

"Post-processingofmergedbeandefinitionfailed",ex);

mbd.postProcessed=true;

}

哎，spring縝密啊。元數據有了，下面我們看下element.inject()方法是如何操作的：

if(this.isField){

Fieldfield=(Field)this.member;

ReflectionUtils.makeAccessible(field);

field.set(target,getResourceToInject(target,requestingBeanName));

else{

if(checkPropertySkipping(pvs)){

return;

try{

Methodmethod=(Method)this.member;

ReflectionUtils.makeAccessible(method);

method.invoke(target,getResourceToInject(target,requestingBeanName));

catch(InvocationTargetExceptionex){

throwex.getTargetException();

}

方法簡單明了，直接使用反射將值set到屬性中，至此bean的實例、屬性注入基本完成，下面我們回頭來看doGetBean的另一個方法getObjectForBeanInstance。

2、getObjectForBeanInstance方法。對于是FactoryBean類型的bean通過getObject獲取到bean的代理實例，跟著方法一直走下去會到getObejct()方法中。

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

AccessControlContextacc=getAccessControlContext();

try{

object=AccessController.doPrivileged((PrivilegedExceptionActionObject)factory::getObject,acc);

catch(PrivilegedActionExceptionpae){

throwpae.getException();

else{

//從FactoryBean實現bean中getObejct獲取到bean

object=factory.getObject();

}

到此finishBeanfactoryInitialization方法執行結束！

bean實例化流程說明

bean實例化流程流程是在onRefresh方法的finishBeanFactoryInitialization中。進入該方法

protectedvoidfinishBeanFactoryInitialization(ConfigurableListableBeanFactorybeanFactory){

//Initializeconversionserviceforthiscontext.

if(beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME)

beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME,ConversionService.class)){

beanFactory.setConversionServi