前言
提到Spring就会想到IOC、DI等概念，这是Spring的核心思想，只要使用过Spring框架的人都知道这些概念，但要问到Spring具体是怎么实现IOC的，恐怕只能看Spring的源码才能找到答案，在我看来只要搞清楚两个问题，就能对Spring的整体脉络有个整体的认识
Bean是如何注册到IOC注册中的？
Bean是如何从IOC容器中get出来的？
一个简单的例子
我们先从一个简单的例子开始，基本上在最开始学习Spring的时候都会从xml配置开始，把你需要交给Spring管理的类配置到xml文件中，你就可以不用管对象的创建了，下面来看一下代码
1、首先定义一个User类
很简单的一个JavaBean
2、增加一个xml配置
<?xml version="1.0" encoding="UTF-8"?>
3、读取配置，并运行
publicclassXmlBeanTest{publicstaticvoidmain(String[] args){ ClassPathXmlApplicationContext applicationContext =newClassPathXmlApplicationContext("bean.xml"); User user = (User) applicationContext.getBean("userBean"); System.out.println(user.getUserName()); }}
可以看到例子很简单，堪称Spring入门的HelloWorld
从上面几行代码可以看出，首先读取bean.xml中的配置，然后就可以从applicationContext中获取到User对象，那么肯定会有User对象注册到IOC容器中这个步骤
下面我们一起通过源码来看一下Bean是如何注册到Spring IOC容器中的
源码解析
从
ClassPathXmlApplicationContext开始
publicClassPathXmlApplicationContext(String configLocation)throwsBeansException{this(newString[] {configLocation},true,null);}publicClassPathXmlApplicationContext(String... configLocations)throwsBeansException{this(configLocations,true,null);}
从
ClassPathXmlApplicationContext的构造方法入手，构造方法传入xml配置文件的路径，这里可以传入单个或多个配置文件
publicClassPathXmlApplicationContext(String[] configLocations, boolean refresh,@NullableApplicationContext parent)throwsBeansException{super(parent);//将传入的xml配置位置信息设置到configLocationssetConfigLocations(configLocations);if(refresh) {//核心方法refresh();}}
这里看到，我们传入的配置文件设置到configLocations，然后调用了一个Spring最核心的方法refresh()，这个方法包括了容器启动的所有内容，是我们学习Spring源码的一个入口，可以说你只要把这个方法里面的内容研究清楚了，对于Spring框架的整个脉络会有一个全新的认识，下面我们来看看这个方法里面有些什么
publicvoidrefresh()throwsBeansException, IllegalStateException{synchronized(this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// 注意看这个方法ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try{// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);// Initialize message source for this context.initMessageSource();// Initialize event multicaster for this context.initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.onRefresh();// Check for listener beans and register them.registerListeners();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.finishRefresh();}catch(BeansException ex) {if(logger.isWarnEnabled()) {logger.warn("Exception encountered during context initialization - "+"cancelling refresh attempt: "+ ex);}// Destroy already created singletons to avoid dangling resources.destroyBeans();// Reset 'active' flag.cancelRefresh(ex);// Propagate exception to caller.throwex;}finally{// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}}
refresh()方法里面的内容很丰富，从各个方法名称就大致可以看出来其作用，这里我们主要看Bean注册的过程，将目光聚焦到
ConfigurableListableBeanFactorybeanFactory = obtainFreshBeanFactory();
这一行是获取BeanFactory，里面进行了Bean的注册逻辑
protectedConfigurableListableBeanFactoryobtainFreshBeanFactory(){refreshBeanFactory();returngetBeanFactory();}
这里调用的是
AbstractRefreshableApplicationContext类的refreshBeanFactory()方法，需要注意一下在看Spring源码的时候，同一个方法可能会有多个子类都实现了，需要注意区分一下，你当前实例化的是哪个子类
protectedfinalvoidrefreshBeanFactory()throwsBeansException{if(hasBeanFactory()) {destroyBeans();closeBeanFactory();}try{//创建一个BeanFactoryDefaultListableBeanFactory beanFactory = createBeanFactory();beanFactory.setSerializationId(getId());customizeBeanFactory(beanFactory);//这里进行Bean的加载loadBeanDefinitions(beanFactory);synchronized(this.beanFactoryMonitor) {this.beanFactory = beanFactory;}}catch(IOException ex) {thrownewApplicationContextException("I/O error parsing bean definition source for "+ getDisplayName(), ex);}}
因为我们使用的是xml的配置，所以这里调用的是
AbstractXmlApplicationContext这个抽象类中的loadBeanDefinitions方法
protectedvoidloadBeanDefinitions(DefaultListableBeanFactory beanFactory)throwsBeansException, IOException{// 构造一个XmlBeanDefinitionReader，用于读到xml中配置的beanXmlBeanDefinitionReader beanDefinitionReader =newXmlBeanDefinitionReader(beanFactory);// 配置XmlBeanDefinitionReaderbeanDefinitionReader.setEnvironment(this.getEnvironment());beanDefinitionReader.setResourceLoader(this);beanDefinitionReader.setEntityResolver(newResourceEntityResolver(this));//初始化XmlBeanDefinitionReaderinitBeanDefinitionReader(beanDefinitionReader);//加载BeanloadBeanDefinitions(beanDefinitionReader);}
protectedvoidloadBeanDefinitions(XmlBeanDefinitionReader reader)throwsBeansException, IOException{Resource[] configResources = getConfigResources();if(configResources !=null) {reader.loadBeanDefinitions(configResources);}String[] configLocations = getConfigLocations();if(configLocations !=null) {reader.loadBeanDefinitions(configLocations);}}
这里进行了两种不同方式的加载，调用的是不同的方法，我们传入的是configLocations
@OverridepublicintloadBeanDefinitions(String... locations)throwsBeanDefinitionStoreException{Assert.notNull(locations,"Location array must not be null");intcount =0;for(String location : locations) {count += loadBeanDefinitions(location);}returncount;}
publicintloadBeanDefinitions(String location, @Nullable Set<Resource> actualResources)throwsBeanDefinitionStoreException{ResourceLoader resourceLoader = getResourceLoader();if(resourceLoader ==null) {thrownewBeanDefinitionStoreException("Cannot load bean definitions from location ["+ location +"]: no ResourceLoader available");}if(resourceLoaderinstanceofResourcePatternResolver) {// Resource pattern matching available.try{Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);intcount = loadBeanDefinitions(resources);if(actualResources !=null) {Collections.addAll(actualResources, resources);}if(logger.isTraceEnabled()) {logger.trace("Loaded "+ count +" bean definitions from location pattern ["+ location +"]");}returncount;}catch(IOException ex) {thrownewBeanDefinitionStoreException("Could not resolve bean definition resource pattern ["+ location +"]", ex);}}else{// Can only load single resources by absolute URL.Resource resource = resourceLoader.getResource(location);intcount = loadBeanDefinitions(resource);if(actualResources !=null) {actualResources.add(resource);}if(logger.isTraceEnabled()) {logger.trace("Loaded "+ count +" bean definitions from location ["+ location +"]");}returncount;}}
这个里面主要方法是loadBeanDefinitions()，我们继续往下走
中间省略了一些简单调用
publicintloadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException{Assert.notNull(encodedResource,"EncodedResource must not be null");if(logger.isTraceEnabled()) {logger.trace("Loading XML bean definitions from "+ encodedResource);}Set currentResources =this.resourcesCurrentlyBeingLoaded.get();if(currentResources ==null) {currentResources =newHashSet<>(4);this.resourcesCurrentlyBeingLoaded.set(currentResources);}if(!currentResources.add(encodedResource)) {thrownewBeanDefinitionStoreException("Detected cyclic loading of "+ encodedResource +" - check your import definitions!");}try{InputStream inputStream = encodedResource.getResource().getInputStream();try{InputSource inputSource =newInputSource(inputStream);if(encodedResource.getEncoding() !=null) {inputSource.setEncoding(encodedResource.getEncoding());}//进行BeanDefinations加载returndoLoadBeanDefinitions(inputSource, encodedResource.getResource());}finally{inputStream.close();}}catch(IOException ex) {thrownewBeanDefinitionStoreException("IOException parsing XML document from "+ encodedResource.getResource(), ex);}finally{currentResources.remove(encodedResource);if(currentResources.isEmpty()) {this.resourcesCurrentlyBeingLoaded.remove();}}}
protectedintdoLoadBeanDefinitions(InputSource inputSource, Resource resource)throwsBeanDefinitionStoreException{try{//构造xml的Document结构，解析DOM结构Document doc = doLoadDocument(inputSource, resource);//注册BeanDefinitionintcount = registerBeanDefinitions(doc, resource);if(logger.isDebugEnabled()) {logger.debug("Loaded "+ count +" bean definitions from "+ resource);}returncount;}catch(BeanDefinitionStoreException ex) {throwex;}catch(SAXParseException ex) {thrownewXmlBeanDefinitionStoreException(resource.getDescription(),"Line "+ ex.getLineNumber() +" in XML document from "+ resource +" is invalid", ex);}catch(SAXException ex) {thrownewXmlBeanDefinitionStoreException(resource.getDescription(),"XML document from "+ resource +" is invalid", ex);}catch(ParserConfigurationException ex) {thrownewBeanDefinitionStoreException(resource.getDescription(),"Parser configuration exception parsing XML from "+ resource, ex);}catch(IOException ex) {thrownewBeanDefinitionStoreException(resource.getDescription(),"IOException parsing XML document from "+ resource, ex);}catch(Throwable ex) {thrownewBeanDefinitionStoreException(resource.getDescription(),"Unexpected exception parsing XML document from "+ resource, ex);}}
从上面的代码可以看出来，Spring是将xml的DOM结构解析后注册到IOC容器中的
publicintregisterBeanDefinitions(Document doc, Resource resource)throwsBeanDefinitionStoreException{BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();intcountBefore = getRegistry().getBeanDefinitionCount();documentReader.registerBeanDefinitions(doc, createReaderContext(resource));returngetRegistry().getBeanDefinitionCount() - countBefore;}
这个方法构造了一个
BeanDefinitionDocumentReader，进行注册BeanDefinition，并且返回了本次注册Bean的数量
protectedvoiddoRegisterBeanDefinitions(Element root){// Any nested <beans> elements will cause recursion in this method. In// order to propagate and preserve <beans> default-* attributes correctly,// keep track of the current (parent) delegate, which may be null. Create// the new (child) delegate with a reference to the parent for fallback purposes,// then ultimately reset this.delegate back to its original (parent) reference.// this behavior emulates a stack of delegates without actually necessitating one.BeanDefinitionParserDelegate parent =this.delegate;this.delegate= createDelegate(getReaderContext(), root, parent);if(this.delegate.isDefaultNamespace(root)) {String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);if(StringUtils.hasText(profileSpec)) {String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);// We cannot use Profiles.of(...) since profile expressions are not supported// in XML config. See SPR-12458 for details.if(!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {if(logger.isDebugEnabled()) {logger.debug("Skipped XML bean definition file due to specified profiles ["+ profileSpec +"] not matching: "+ getReaderContext().getResource());}return;}}}preProcessXml(root);//进行BeanDefinition转换，将DOM结构的对象转换成BeanDefinitionparseBeanDefinitions(root,this.delegate);postProcessXml(root);this.delegate= parent;}
protectedvoidparseBeanDefinitions(Element root, BeanDefinitionParserDelegatedelegate){if(delegate.isDefaultNamespace(root)) {NodeList nl = root.getChildNodes();for(inti =0; i < nl.getLength(); i++) {Node node = nl.item(i);if(node instanceof Element) {Element ele = (Element) node;if(delegate.isDefaultNamespace(ele)) {//Spring默认元素转换parseDefaultElement(ele,delegate);}else{//xml中自定义的Element进行解析delegate.parseCustomElement(ele);}}}}else{delegate.parseCustomElement(root);}}
privatevoidparseDefaultElement(Element ele, BeanDefinitionParserDelegatedelegate){if(delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {importBeanDefinitionResource(ele);}elseif(delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {processAliasRegistration(ele);}elseif(delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {processBeanDefinition(ele,delegate);}elseif(delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {// recursedoRegisterBeanDefinitions(ele);}}
可以看到xml中默认的配置元素包括import、alias、bean、beans，这些也是最常用的，我们主要看一个bean的转换
protectedvoidprocessBeanDefinition(Element ele, BeanDefinitionParserDelegatedelegate){BeanDefinitionHolder bdHolder =delegate.parseBeanDefinitionElement(ele);if(bdHolder !=null) {bdHolder =delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);try{// Register the final decorated instance.BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());}catch(BeanDefinitionStoreException ex) {getReaderContext().error("Failed to register bean definition with name '"+bdHolder.getBeanName() +"'", ele, ex);}// Send registration event.getReaderContext().fireComponentRegistered(newBeanComponentDefinition(bdHolder));}}
publicstaticvoidregisterBeanDefinition( BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)throws BeanDefinitionStoreException{// Register bean definition under primary name.String beanName = definitionHolder.getBeanName();//注册BeanDefinitionregistry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());// Register aliases for bean name, if any.String[] aliases = definitionHolder.getAliases();if(aliases !=null) {for(Stringalias: aliases) {registry.registerAlias(beanName,alias);}}}
// DefaultListableBeanFactorypublicvoid registerBeanDefinition(String beanName, BeanDefinition beanDefinition)throws BeanDefinitionStoreException {Assert.hasText(beanName,"Bean name must not be empty");Assert.notNull(beanDefinition,"BeanDefinition must not be null");if(beanDefinition instanceof AbstractBeanDefinition) {try{((AbstractBeanDefinition) beanDefinition).validate();}catch(BeanDefinitionValidationException ex) {thrownew BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,"Validation of bean definition failed", ex);}}BeanDefinition existingDefinition =this.beanDefinitionMap.get(beanName);if(existingDefinition !=null) {if(!isAllowBeanDefinitionOverriding()) {thrownew BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);}elseif(existingDefinition.getRole() < beanDefinition.getRole()) {// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTUREif(logger.isInfoEnabled()) {logger.info("Overriding user-defined bean definition for bean '"+ beanName +"' with a framework-generated bean definition: replacing ["+existingDefinition +"] with ["+ beanDefinition +"]");}}elseif(!beanDefinition.equals(existingDefinition)) {if(logger.isDebugEnabled()) {logger.debug("Overriding bean definition for bean '"+ beanName +"' with a different definition: replacing ["+ existingDefinition +"] with ["+ beanDefinition +"]");}}else{if(logger.isTraceEnabled()) {logger.trace("Overriding bean definition for bean '"+ beanName +"' with an equivalent definition: replacing ["+ existingDefinition +"] with ["+ beanDefinition +"]");}}//将beanDefinition放入beanDefinitionMap中this.beanDefinitionMap.put(beanName, beanDefinition);}else{if(hasBeanCreationStarted()) {// Cannot modify startup-time collection elements anymore (for stable iteration)synchronized (this.beanDefinitionMap) {//将beanDefinition放入beanDefinitionMap中this.beanDefinitionMap.put(beanName, beanDefinition);List updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() +1);updatedDefinitions.addAll(this.beanDefinitionNames);updatedDefinitions.add(beanName);this.beanDefinitionNames = updatedDefinitions;removeManualSingletonName(beanName);}}else{// Still in startup registration phase//将beanDefinition放入beanDefinitionMap中this.beanDefinitionMap.put(beanName, beanDefinition);this.beanDefinitionNames.add(beanName);removeManualSingletonName(beanName);}this.frozenBeanDefinitionNames =null;}if(existingDefinition !=null|| containsSingleton(beanName)) {resetBeanDefinition(beanName);}}
源码跟到这里，整个流程基本清楚了，最终beanDefinition存到一个beanDefinitionMap中，key为Bean的名称，value为beanDefinition对象
privatefinalMap beanDefinitionMap =newConcurrentHashMap<>(256);
beanDefinitionMap是一个ConcurrentHashMap，所以本质上Bean最终是被注册到一个Map中
上面我们说了很多次beanDefinition，最后注册到容器中的也是这个对象，那它到底是个啥对象?
A BeanDefinition describes a bean instance, which has property values,
constructor argument values, and further information supplied by
concrete implementations.
从注释中可以看出来BeanDefinition是一个用来描述带有属性值、构造方法、还有一些其他进一步信息的Bean实例（半调子英语，也不知道翻译的对不对~）
BeanDefinition是对Bean的抽象，因为配置文件中的Bean是多种多样的，BeanDefinition是对Bean的公共属性进行抽象，在BeanDefinition中很多属性是用来描述xml配置中bean的配置属性的
所以，下面来总结一下整个流程
注册xml配置文件到configLocations
调用refresh()进行整个Context的刷新，实际上就是整个Context的启动
Bean的加载会读到配置文件，解析成DOM对象
将DOM对象转换成beanDefinition
将beanDefinition存入beanDefinitionMap，完成整个Bean的注册
没看明白的同学，可以对照这个流程再回头去看看，整个流程下来还是挺清晰的
注解方式下Bean的注册
前面我们讲到xml配置文件进行Bean的注册，xml配置是Spring早期常用的配置方式，现在基本上大部分场景上都推荐使用注解的方式，尤其是SpringBoot时代的来临，进一步推动了注解方式的全面使用，下面我们来看看注解方式下的Bean注册，还是从个简单的例子入手
@ComponentpublicclassAnnotionConfig{@Bean(name ="userBean")publicUsergetUserBean(){ User user =newUser(); user.setUserName("Lucy");returnuser; }}
publicclassAnnotionBeanTest{publicstaticvoidmain(String[] args){ AnnotationConfigApplicationContext applicationContext =newAnnotationConfigApplicationContext("org.kxg.springDemo"); User user = (User) applicationContext.getBean("userBean"); System.out.println(user.getUserName()); }}
这里用到
AnnotationConfigApplicationContext，是另外一种容器的实现，传入一个包名，会自动扫描包下面的Spring注解，然后将其注册到容器中
publicAnnotationConfigApplicationContext(String... basePackages) {this();//主要是scan方法完成bean的注册scan(basePackages);//又到了这个方法，有没有很熟悉~~~refresh();}
下面我们重点看一下注解方式的Bean注册
publicint scan(String... basePackages) {int beanCountAtScanStart =this.registry.getBeanDefinitionCount();//扫描包，进行Bean注册doScan(basePackages);// Register annotation config processors, if necessary.if(this.includeAnnotationConfig) {AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);}return(this.registry.getBeanDefinitionCount() - beanCountAtScanStart);}
//ClassPathBeanDefinitionScannerprotectedSet doScan(String... basePackages) {Assert.notEmpty(basePackages,"At least one base package must be specified");Set beanDefinitions =newLinkedHashSet<>();for(StringbasePackage : basePackages) {//扫描包下打了注解的类，并将其转换成BeanDefinitionSet candidates = findCandidateComponents(basePackage);for(BeanDefinition candidate : candidates) {ScopeMetadata scopeMetadata =this.scopeMetadataResolver.resolveScopeMetadata(candidate);candidate.setScope(scopeMetadata.getScopeName());StringbeanName =this.beanNameGenerator.generateBeanName(candidate,this.registry);if(candidateinstanceofAbstractBeanDefinition) {postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);}if(candidateinstanceofAnnotatedBeanDefinition) {AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);}if(checkCandidate(beanName, candidate)) {BeanDefinitionHolder definitionHolder =newBeanDefinitionHolder(candidate, beanName);definitionHolder =AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder,this.registry);beanDefinitions.add(definitionHolder);//进行BeanDefinition注册registerBeanDefinition(definitionHolder,this.registry);}}}returnbeanDefinitions;}
publicstaticvoidregisterBeanDefinition( BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)throws BeanDefinitionStoreException{// Register bean definition under primary name.String beanName = definitionHolder.getBeanName();registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());// Register aliases for bean name, if any.String[] aliases = definitionHolder.getAliases();if(aliases !=null) {for(Stringalias: aliases) {registry.registerAlias(beanName,alias);}}}
看到这个方法，有没有点眼熟的感觉
上面xml方式进行Bean注册也调用到这个方法了，所以后面的流程都是一样的，注解方式和xml配置方式从本质上来讲，并没有什么不同，只是Bean的描述不同而已，最终都会被解析成BeanDefinition，注册到容器中，至此整个Bean的注册流程就已经完了。
当然在整个过程中，忽略了很多细节，只看了主线流程。
读源码的时候，很容易陷入细节中，尤其是像Spring这样通用的框架，它为了通用性和扩展性，会把代码写的很“绕”，如果你过于关注细节很容易让自己陷入实现的细节中，一开始看源码的时候，建议不用太关注细节，把主线功能先看完，知道大概的原理后再逐个去击破~
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