第Component和Configuration注解區別實例詳解目錄引言AnnotationBean.javaAnnotationTest.javaSpring-source-5.2.8兩個注解聲明增強邏輯

引言

第一眼看到這個題目，我相信大家都會腦子里面彈出來一個想法：這不都是Spring的注解么，加了這兩個注解的類都會被最終封裝成BeanDefinition交給Spring管理，能有什么區別？

首先先給大家看一段示例代碼：

AnnotationBean.java

importlombok.Data;

importorg.springframework.beans.factory.annotation.Qualifier;

importorg.springframework.context.annotation.Bean;

importorg.springframework.context.annotation.Configuration;

importorg.springframework.stereotype.Component;

@Component

//@Configuration

publicclassAnnotationBean{

@Qualifier("innerBean1")

@Bean()

publicInnerBeaninnerBean1(){

returnnewInnerBean();

@Bean

publicInnerBeanFactoryinnerBeanFactory(){

InnerBeanFactoryfactory=newInnerBeanFactory();

factory.setInnerBean(innerBean1());

returnfactory;

publicstaticclassInnerBean{

@Data

publicstaticclassInnerBeanFactory{

privateInnerBeaninnerBean;

}

AnnotationTest.java

@Test

voidtest7(){

AnnotationConfigApplicationContextapplicationContext=newAnnotationConfigApplicationContext(BASE_PACKAGE);

Objectbean1=applicationContext.getBean("innerBean1");

ObjectfactoryBean=applicationContext.getBean("innerBeanFactory");

inthashCode1=bean1.hashCode();

InnerBeaninnerBeanViaFactory=((InnerBeanFactory)factoryBean).getInnerBean();

inthashCode2=innerBeanViaFactory.hashCode();

Assertions.assertEquals(hashCode1,hashCode2);

}

大家可以先猜猜看，這個test7()的執行結果究竟是成功呢還是失敗呢？

答案是失敗的。如果將AnnotationBean的注解從@Component換成@Configuration，那test7()就會執行成功。

究竟是為什么呢？通常Spring管理的bean不都是單例的么？

別急，讓筆者慢慢道來~~~

Spring-source-5.2.8兩個注解聲明

以下是摘自Spring-source-5.2.8的兩個注解的聲明

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Indexed

public@interfaceComponent{

*Thevaluemayindicateasuggestionforalogicalcomponentname,

*tobeturnedintoaSpringbeanincaseofanautodetectedcomponent.

*@returnthesuggestedcomponentname,ifany(oremptyStringotherwise)

Stringvalue()default"";

----------------------------------這是分割線-----------------------------------

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Component

public@interfaceConfiguration{

*ExplicitlyspecifythenameoftheSpringbeandefinitionassociatedwiththe

*{@code@Configuration}class.Ifleftunspecified(thecommoncase),abean

*namewillbeautomaticallygenerated.

*pThecustomnameappliesonlyifthe{@code@Configuration}classispicked

*upviacomponentscanningorsupplieddirectlytoan

*{@linkAnnotationConfigApplicationContext}.Ifthe{@code@Configuration}class

*isregisteredasatraditionalXMLbeandefinition,thename/idofthebean

*elementwilltakeprecedence.

*@returntheexplicitcomponentname,ifany(oremptyStringotherwise)

*@seeAnnotationBeanNameGenerator

@AliasFor(annotation=Component.class)

Stringvalue()default"";

*Specifywhether{@code@Bean}methodsshouldgetproxiedinordertoenforce

*beanlifecyclebehavior,e.g.toreturnsharedsingletonbeaninstanceseven

*incaseofdirect{@code@Bean}methodcallsinusercode.Thisfeature

*requiresmethodinterception,implementedthrougharuntime-generatedCGLIB

*subclasswhichcomeswithlimitationssuchastheconfigurationclassand

*itsmethodsnotbeingallowedtodeclare{@codefinal}.

*pThedefaultis{@codetrue},allowingfor'inter-beanreferences'viadirect

*methodcallswithintheconfigurationclassaswellasforexternalcallsto

*thisconfiguration's{@code@Bean}methods,e.g.fromanotherconfigurationclass.

*Ifthisisnotneededsinceeachofthisparticularconfiguration's{@code@Bean}

*methodsisself-containedanddesignedasaplainfactorymethodforcontaineruse,

*switchthisflagto{@codefalse}inordertoavoidCGLIBsubclassprocessing.

*pTurningoffbeanmethodinterceptioneffectivelyprocesses{@code@Bean}

*methodsindividuallylikewhendeclaredonnon-{@code@Configuration}classes,

*a.k.a."@BeanLiteMode"(see{@linkBean@Bean'sjavadoc}).Itistherefore

*behaviorallyequivalenttoremovingthe{@code@Configuration}stereotype.

*@since5.2

booleanproxyBeanMethods()defaulttrue;

}

從這兩個注解的定義中，可能大家已經看出了一點端倪：@Configuration比@Component多一個成員變量booleanproxyBeanMethods()默認值是true.從這個成員變量的注釋中，我們可以看到一句話

Specifywhether{@code@Bean}methodsshouldgetproxiedinordertoenforcebeanlifecyclebehavior,e.g.toreturnsharedsingletonbeaninstancesevenincaseofdirect{@code@Bean}methodcallsinusercode.

其實從這句話，我們就可以初步得到我們想要的答案了：在帶有@Configuration注解的類中，一個帶有@Bean注解的方法顯式調用另一個帶有@Bean注解的方法，返回的是共享的單例對象.下面我們從Spring源碼實現角度來看看這中間的原理.

從Spring源碼實現中可以得出一個規律，Spring作者在實現注解時，通常是先收集解析，再調用。@Configuration是基于@Component實現的，在@Component的解析過程中，我們可以看到下面一段邏輯：

org.springframework.context.annotation.ConfigurationClassUtils#checkConfigurationClassCandidate

MapString,Objectconfig=metadata.getAnnotationAttributes(Configuration.class.getName());

if(config!=null!Boolean.FALSE.equals(config.get("proxyBeanMethods"))){

beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE,CONFIGURATION_CLASS_FULL);

elseif(config!=null||isConfigurationCandidate(metadata)){

beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE,CONFIGURATION_CLASS_LITE);

}

默認情況下，Spring在將帶有@Configuration注解的類封裝成BeanDefinition的時候，會設置一個屬性CONFIGURATION_CLASS_ATTRIBUTE，屬性值為CONFIGURATION_CLASS_FULL,反之，如果只有@Component注解，那該屬性值就會是CONFIGURATION_CLASS_LITE(這個屬性值很重要).在@Component注解的調用過程當中，有下面一段邏輯：

org.springframework.context.annotation.ConfigurationClassPostProcessor#enhanceConfigurationClasses

for(StringbeanName:beanFactory.getBeanDefinitionNames()){

......

if((configClassAttr!=null||methodMetadata!=null)beanDefinstanceofAbstractBeanDefinition){

......

if(ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(configClassAttr)){

if(!(beanDefinstanceofAbstractBeanDefinition)){

thrownewBeanDefinitionStoreException("Cannotenhance@Configurationbeandefinition'"+

beanName+"'sinceitisnotstoredinanAbstractBeanDefinitionsubclass");

elseif(logger.isInfoEnabled()beanFactory.containsSingleton(beanName)){

("Cannotenhance@Configurationbeandefinition'"+beanName+

"'sinceitssingletoninstancehasbeencreatedtooearly.Thetypicalcause"+

"isanon-static@BeanmethodwithaBeanDefinitionRegistryPostProcessor"+

"returntype:Considerdeclaringsuchmethodsas'static'.");

configBeanDefs.put(beanName,(AbstractBeanDefinition)beanDef);

if(configBeanDefs.isEmpty()){

//nothingtoenhance-returnimmediately

return;

ConfigurationClassEnhancerenhancer=newConfigurationClassEnhancer();

......

如果BeanDefinition的ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE屬性值為ConfigurationClassUtils.CONFIGURATION_CLASS_FULL,則該BeanDefinition對象會被加入到MapString,AbstractBeanDefinitionconfigBeanDefs容器中。

如果Spring發現該Map是空的，則認為不需要進行代理增強，立即返回；反之，則為該類(本文中，被代理類即為AnnotationBean,以下簡稱該類)創建代理。

所以如果該類的注解是@Component，調用帶有@Bean注解的innerBean1()方法時，this對象為Spring容器中的真實單例對象，例如AnnotationBean@4149.

@Bean

publicInnerBeanFactoryinnerBeanFactory(){

InnerBeanFactoryfactory=newInnerBeanFactory();

factory.setInnerBean(innerBean1());

returnfactory;

}

那在上述方法中每調用一次innerBean1()方法時，勢必會返回一個新創建的InnerBean對象。如果該類的注解為@Configuration時，this對象為Spring生成的AnnotationBean的代理對象，例如AnnotationBean$$EnhancerBySpringCGLIB$$90f8540c@4296,

增強邏輯

//Thecallbackstouse.Notethatthesecallbacksmustbestateless.

privatestaticfinalCallback[]CALLBACKS=newCallback[]{

newBeanMethodInterceptor(),

newBeanFactoryAwareMethodInterceptor(),

NoOp.INSTANCE

-----------------------------------這是分割線-------------------------------

*CreatesanewCGLIB{@linkEnhancer}instance.

privateEnhancernewEnhancer(ClassconfigSuperClass,@NullableClassLoaderclassLoader){

Enhancerenhancer=newEnhancer();

enhancer.setSuperclass(configSuperClass);

enhancer.setInterfaces(newClass[]{EnhancedConfiguration.class});

enhancer.setUseFactory(false);

enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);

enhancer.setStrategy(newBeanFactoryAwareGeneratorStrategy(classLoader));

enhancer.setCallbackFilter(CALLBACK_FILTER);

enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());

returnenhancer;

}

當在上述方法中調用innerBean1()時，ConfigurationClassEnhancer遍歷3種回調方法判斷當前調用應該使用哪個回調方法時，第一個回調類型匹配成功org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#isMatch匹配過程如下所示：

@Override

publicbooleanisMatch(MethodcandidateMethod){

return(candidateMethod.getDeclaringClass()!=Object.class!BeanFactoryAwareMethodInterceptor.