Component和Configuration注解区别实例详解

第Component和Configuration注解区别实例详解目录引言AnnotationBean.javaAnnotationTest.javaSpring-source-5.2.8两个注解声明增强逻辑

引言

第一眼看到这个题目，我相信大家都会脑子里面弹出来一个想法：这不都是Spring的注解么，加了这两个注解的类都会被最终封装成BeanDefinition交给Spring管理，能有什么区别？

首先先给大家看一段示例代码：

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，这个test7()的执行结果究竟是成功呢还是失败呢？

答案是失败的。如果将AnnotationBean的注解从@Component换成@Configuration，那test7()就会执行成功。

究竟是为什么呢？通常Spring管理的bean不都是单例的么？

别急，让笔者慢慢道来~~~

Spring-source-5.2.8两个注解声明

以下是摘自Spring-source-5.2.8的两个注解的声明

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Indexed

public@interfaceComponent{

*Thevaluemayindicateasuggestionforalogicalcomponentname,

*tobeturnedintoaSpringbeanincaseofanautodetectedcomponent.

*@returnthesuggestedcomponentname,ifany(oremptyStringotherwise)

Stringvalue()default"";

----------------------------------这是分割线-----------------------------------

@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;

}

从这两个注解的定义中，可能大家已经看出了一点端倪：@Configuration比@Component多一个成员变量booleanproxyBeanMethods()默认值是true.从这个成员变量的注释中，我们可以看到一句话

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

其实从这句话，我们就可以初步得到我们想要的答案了：在带有@Configuration注解的类中，一个带有@Bean注解的方法显式调用另一个带有@Bean注解的方法，返回的是共享的单例对象.下面我们从Spring源码实现角度来看看这中间的原理.

从Spring源码实现中可以得出一个规律，Spring作者在实现注解时，通常是先收集解析，再调用。@Configuration是基于@Component实现的，在@Component的解析过程中，我们可以看到下面一段逻辑：

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);

}

默认情况下，Spring在将带有@Configuration注解的类封装成BeanDefinition的时候，会设置一个属性CONFIGURATION_CLASS_ATTRIBUTE，属性值为CONFIGURATION_CLASS_FULL,反之，如果只有@Component注解，那该属性值就会是CONFIGURATION_CLASS_LITE(这个属性值很重要).在@Component注解的调用过程当中，有下面一段逻辑：

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属性值为ConfigurationClassUtils.CONFIGURATION_CLASS_FULL,则该BeanDefinition对象会被加入到MapString,AbstractBeanDefinitionconfigBeanDefs容器中。

如果Spring发现该Map是空的，则认为不需要进行代理增强，立即返回；反之，则为该类(本文中，被代理类即为AnnotationBean,以下简称该类)创建代理。

所以如果该类的注解是@Component，调用带有@Bean注解的innerBean1()方法时，this对象为Spring容器中的真实单例对象，例如AnnotationBean@4149.

@Bean

publicInnerBeanFactoryinnerBeanFactory(){

InnerBeanFactoryfactory=newInnerBeanFactory();

factory.setInnerBean(innerBean1());

returnfactory;

}

那在上述方法中每调用一次innerBean1()方法时，势必会返回一个新创建的InnerBean对象。如果该类的注解为@Configuration时，this对象为Spring生成的AnnotationBean的代理对象，例如AnnotationBean$$EnhancerBySpringCGLIB$$90f8540c@4296,

增强逻辑

//Thecallbackstouse.Notethatthesecallbacksmustbestateless.

privatestaticfinalCallback[]CALLBACKS=newCallback[]{

newBeanMethodInterceptor(),

newBeanFactoryAwareMethodInterceptor(),

NoOp.INSTANCE

-----------------------------------这是分割线-------------------------------

*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;

}

当在上述方法中调用innerBean1()时，ConfigurationClassEnhancer遍历3种回调方法判断当前调用应该使用哪个回调方法时，第一个回调类型匹配成功org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#isMatch匹配过程如下所示：

@Override

publicbooleanisMatch(MethodcandidateMethod){

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