Spring组件扫描原理解析
Spring组件扫描原理解析
一、概述
用一幅图来对两种方式配置Bean的组件扫描进行一个总结：
二、原理剖析
通过之前的学习我们知道只要是将Bean放到了BeanDefinitionMap中，那么就势必会经过Spring的那套Bean的生命周期，就势必会创建Bean。
2.1xml组件扫描方式
首先我们在xml中是通过这段代码来进行组件扫描的，扫描指定的基本包及其子包下的类，识别使用@Component注解：
<context:component-scan base-package="com.itheima"/>依据之前学习的命名空间的知识我们知道，通过命名空间是可以找到它对应的Handler处理器的，命名空间：
xmlns:context="http://www.springframework.org/schema/context"对应的Handler处理器：
http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandlerhttp\://www.springframework.org/schema/jee=org.springframework.ejb.config.JeeNamespaceHandlerhttp\://www.springframework.org/schema/lang=org.springframework.scripting.config.LangNamespaceHandlerhttp\://www.springframework.org/schema/task=org.springframework.scheduling.config.TaskNamespaceHandlerhttp\://www.springframework.org/schema/cache=org.springframework.cache.config.CacheNamespaceHandler进入ContextNamespaceHandler：
//// Source code recreated from a .class file by IntelliJ IDEA// (powered by FernFlower decompiler)//packageorg.springframework.context.config;importorg.springframework.beans.factory.xml.NamespaceHandlerSupport;importorg.springframework.context.annotation.AnnotationConfigBeanDefinitionParser;importorg.springframework.context.annotation.ComponentScanBeanDefinitionParser;publicclassContextNamespaceHandlerextendsNamespaceHandlerSupport{publicContextNamespaceHandler(){}publicvoidinit(){this.registerBeanDefinitionParser("property-placeholder",newPropertyPlaceholderBeanDefinitionParser());this.registerBeanDefinitionParser("property-override",newPropertyOverrideBeanDefinitionParser());this.registerBeanDefinitionParser("annotation-config",newAnnotationConfigBeanDefinitionParser());this.registerBeanDefinitionParser("component-scan",newComponentScanBeanDefinitionParser());this.registerBeanDefinitionParser("load-time-weaver",newLoadTimeWeaverBeanDefinitionParser());this.registerBeanDefinitionParser("spring-configured",newSpringConfiguredBeanDefinitionParser());this.registerBeanDefinitionParser("mbean-export",newMBeanExportBeanDefinitionParser());this.registerBeanDefinitionParser("mbean-server",newMBeanServerBeanDefinitionParser());}}进入component-scan对应的ComponentScanBeanDefinitionParser，看它的parse解析方法：
@NullablepublicBeanDefinitionparse(Elementelement,ParserContextparserContext){StringbasePackage=element.getAttribute("base-package");basePackage=parserContext.getReaderContext().getEnvironment().resolvePlaceholders(basePackage);String[]basePackages=StringUtils.tokenizeToStringArray(basePackage,",; \t\n");ClassPathBeanDefinitionScannerscanner=this.configureScanner(parserContext,element);Set<BeanDefinitionHolder>beanDefinitions=scanner.doScan(basePackages);this.registerComponents(parserContext.getReaderContext(),beanDefinitions,element);returnnull;}进入ClassPathBeanDefinitionScanner的doScan方法，重点看这：
if(this.checkCandidate(beanName,candidate)){BeanDefinitionHolderdefinitionHolder=newBeanDefinitionHolder(candidate,beanName);definitionHolder=AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata,definitionHolder,this.registry);beanDefinitions.add(definitionHolder);this.registerBeanDefinition(definitionHolder,this.registry);}执行到BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);时可以看到BeanName是OtherBean3，BeanDefinitionMap为空；执行完this.registerBeanDefinition(definitionHolder, this.registry);，OtherBean3就注入了BeanDefinitionMap：
由此可见xml组件扫描方式是将Bean直接放到BeanDefinitionMap中实现创建Bean的，并没有通过BeanFactoryPostProcessor。
2.2注解方式主键扫描
首先先是进入AnnotationConfigApplicationContext类：
publicclassAnnotationConfigApplicationContextextendsGenericApplicationContextimplementsAnnotationConfigRegistry{privatefinalAnnotatedBeanDefinitionReaderreader;privatefinalClassPathBeanDefinitionScannerscanner;publicAnnotationConfigApplicationContext(){StartupStepcreateAnnotatedBeanDefReader=this.getApplicationStartup().start("spring.context.annotated-bean-reader.create");this.reader=newAnnotatedBeanDefinitionReader(this);createAnnotatedBeanDefReader.end();this.scanner=newClassPathBeanDefinitionScanner(this);}publicAnnotationConfigApplicationContext(DefaultListableBeanFactorybeanFactory){super(beanFactory);this.reader=newAnnotatedBeanDefinitionReader(this);this.scanner=newClassPathBeanDefinitionScanner(this);}publicAnnotationConfigApplicationContext(Class<?>...componentClasses){this();this.register(componentClasses);this.refresh();}publicAnnotationConfigApplicationContext(String...basePackages){this();this.scan(basePackages);this.refresh();}直接进入this()，
publicAnnotationConfigApplicationContext(){StartupStepcreateAnnotatedBeanDefReader=this.getApplicationStartup().start("spring.context.annotated-bean-reader.create");this.reader=newAnnotatedBeanDefinitionReader(this);createAnnotatedBeanDefReader.end();this.scanner=newClassPathBeanDefinitionScanner(this);}进入AnnotatedBeanDefinitionReader，之前xml的形式是XmlBeanDefinitionReader：
publicAnnotatedBeanDefinitionReader(BeanDefinitionRegistryregistry){this(registry,getOrCreateEnvironment(registry));}publicAnnotatedBeanDefinitionReader(BeanDefinitionRegistryregistry,Environmentenvironment){this.beanNameGenerator=AnnotationBeanNameGenerator.INSTANCE;this.scopeMetadataResolver=newAnnotationScopeMetadataResolver();Assert.notNull(registry,"BeanDefinitionRegistry must not be null");Assert.notNull(environment,"Environment must not be null");this.registry=registry;this.conditionEvaluator=newConditionEvaluator(registry,environment,(ResourceLoader)null);AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);}再是进入**AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);**中的registerAnnotationConfigProcessors：
publicstaticSet<BeanDefinitionHolder>registerAnnotationConfigProcessors(BeanDefinitionRegistryregistry,@NullableObjectsource){DefaultListableBeanFactorybeanFactory=unwrapDefaultListableBeanFactory(registry);if(beanFactory!=null){if(!(beanFactory.getDependencyComparator()instanceofAnnotationAwareOrderComparator)){beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);}if(!(beanFactory.getAutowireCandidateResolver()instanceofContextAnnotationAutowireCandidateResolver)){beanFactory.setAutowireCandidateResolver(newContextAnnotationAutowireCandidateResolver());}}Set<BeanDefinitionHolder>beanDefs=newLinkedHashSet(8);if(!registry.containsBeanDefinition("org.springframework.context.annotation.internalConfigurationAnnotationProcessor")){RootBeanDefinitiondef=newRootBeanDefinition(ConfigurationClassPostProcessor.class);def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.annotation.internalConfigurationAnnotationProcessor"));}if(!registry.containsBeanDefinition("org.springframework.context.annotation.internalAutowiredAnnotationProcessor")){RootBeanDefinitiondef=newRootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.annotation.internalAutowiredAnnotationProcessor"));}if(jsr250Present&&!registry.containsBeanDefinition("org.springframework.context.annotation.internalCommonAnnotationProcessor")){RootBeanDefinitiondef=newRootBeanDefinition(CommonAnnotationBeanPostProcessor.class);def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.annotation.internalCommonAnnotationProcessor"));}if(jpaPresent&&!registry.containsBeanDefinition("org.springframework.context.annotation.internalPersistenceAnnotationProcessor")){RootBeanDefinitiondef=newRootBeanDefinition();try{def.setBeanClass(ClassUtils.forName("org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor",AnnotationConfigUtils.class.getClassLoader()));}catch(ClassNotFoundExceptionex){thrownewIllegalStateException("Cannot load optional framework class: org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor",ex);}def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.annotation.internalPersistenceAnnotationProcessor"));}if(!registry.containsBeanDefinition("org.springframework.context.event.internalEventListenerProcessor")){RootBeanDefinitiondef=newRootBeanDefinition(EventListenerMethodProcessor.class);def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.event.internalEventListenerProcessor"));}if(!registry.containsBeanDefinition("org.springframework.context.event.internalEventListenerFactory")){RootBeanDefinitiondef=newRootBeanDefinition(DefaultEventListenerFactory.class);def.setSource(source);beanDefs.add(registerPostProcessor(registry,def,"org.springframework.context.event.internalEventListenerFactory"));}returnbeanDefs;}在这里会注册六个if中的Bean进入BeanDefinitionMap当中：
每执行完一次return beanDefs就是注册一个。再进入ConfigurationClassPostProcessor.class（因为是注解形式的组件扫描），看到postProcessBeanDefinitionRegistry中的parser.parse(candidates);的parse方法，关注与parse相关的：
for(BeanDefinitionHolderholder:this.componentScanParser.parse(componentScan,sourceClass.getMetadata().getClassName())){继续推进，看componentScanParser的parse方法：
ClassPathBeanDefinitionScannerscanner=newClassPathBeanDefinitionScanner(this.registry,componentScan.getBoolean("useDefaultFilters"),this.environment,this.resourceLoader);是否有一种似曾相识燕归来的感觉呢？哈哈，实际上这是和xml配置组件扫描是一致的啦，接下来就是老套路了，继续看doscan，由此进入：
returnscanner.doScan(StringUtils.toStringArray(basePackages));if(this.checkCandidate(beanName,candidate)){BeanDefinitionHolderdefinitionHolder=newBeanDefinitionHolder(candidate,beanName);definitionHolder=AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata,definitionHolder,this.registry);beanDefinitions.add(definitionHolder);this.registerBeanDefinition(definitionHolder,this.registry);一样的逻辑定义definitionHolder，使用BeanDefinitionRegistry的registerBeanDefinition(definitionHolder进行注册Bean到BeanDefinitionMap当中，完结了。