Zhuoli's Blog之前的文章详细介绍了Bean的实例化和初始化的过程，但其实Bean能实例化、初始化的前提是Spring容器在启动时，间配置文件（配置类）中的配置加载成BeanDefinition，这一点我们再之前的文章Spring源码解读『IOC容器1-自定义实现IOC容器』一文中看得比较清楚。关于Spring对配置文件（配置类）的处理，我们还没有介绍，本篇文章我们就来介绍一下Spring容器启动时是如何加载xml配置文件并加载成BeanDefinition的，在下篇文章来介绍@Configuration配置类。首先来看一下我们之前定义的spring xml配置文件：
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.1.xsd">
<bean id="testBean0" class="com.zhuoli.service.spring.explore.beanpostprocessor.TestBean" init-method="initMethod" destroy-method="destroyMethod">
<property name="testName" value="testName0"/>
</bean>
<bean id="testBean1" class="com.zhuoli.service.spring.explore.beanpostprocessor.TestBean" init-method="initMethod" destroy-method="destroyMethod">
<property name="testName" value="testName1"/>
</bean>
<bean id="myBeanPostProcessor" class="com.zhuoli.service.spring.explore.beanpostprocessor.MyBeanPostProcessor" />
</beans>按照之前文章的介绍，我们知道Spring IOC容器实例化Bean的前置条件是，Spring将上述配置文件转化为BeanDefinition，完成这一转化的方法是在refresh方法中调用obtainFreshBeanFactory方法。
1. BeanDefinition
在文章Spring源码解读『IOC容器1-自定义实现IOC容器』中，我们定义了一个BeanDefinition，并且有了一个大致的概念——BeanDefinition类就是用来存储Bean的属性信息的（比如bean名称，bean类信息，bean属性依赖），从而在bean实例化阶段可以根据BeanDefinition实例化对象。实际上Spring中的BeanDefinition类比我们之前定义的复杂的多（因为配置的Bean有很多属性可选，比如lazyInit、protoType等，这些信息都需要存储在BeanDefinition中），但是理念是相通的，BeanDefinition就是为实例化Bean服务的，所以最重要的肯定还是要存储bean名称、bean类信息、bean属性依赖这些信息。
BeanDefinition是一个接口，Bean实例化所需要的信息都是定义在BeanDefinition的实现类——AbstractBeanDefinition抽象类中的。
通过BeanDefinition，就可以实例化Bean对象。而Spring容器解析得到的BeanDefinition都会注册到Spring容器中，这里的“容器”其实就是DefaultListableBeanFactory，该类有两个相关的成员变量。
/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
/** List of bean definition names, in registration order */
private volatile List<String> beanDefinitionNames = new ArrayList<>(256);分别用来存储所有的BeanDefinition定义及所有的BeanDefinition对应的名称。
2. XML配置文件解析流程
2.1 refresh()方法
之前的文章，我们介绍到，通过xml启动Spring容器的入口在ClassPathXmlApplicationContext类的构造方法，而该方法的核心就是调用org.springframework.context.support.AbstractApplicationContext#refresh。配置文件读取、Bean的实例化、初始化都是在refresh方法中完成的。由于之前已经介绍过Bean实例化、初始化流程，关于这部分内容，下面的源码解读部分都会被省略。
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 1. 准备刷新Spring上下文，主要用来记录Spring上下文加载开始时间，设置一些基础成员变量value
prepareRefresh();
// 2. 刷新BeanFactory，此方法内完成配置文件中配置的Bean到BeanDefinition的转化及注册
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 3. 准备Bean工厂，主要用来配置BeanFactory的基础信息，例如上下文的ClassLoader和后处理器
prepareBeanFactory(beanFactory);
// ……
}2.2 obtainFreshBeanFactory()
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}getBeanFactory()方法其实就是类的getter方法，获取类的beanFactory成员变量，所以可以推测肯定在refreshBeanFactory()方法中，已经完成了beanFactory的初始化。
2.3 refreshBeanFactory()
refreshBeanFactory()最终调用的是AbstractRefreshableApplicationContext类的refreshBeanFactory方法。
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}该方法的核心在createBeanFactory()和loadBeanDefinitions(beanFactory)方法的调用上。
其中createBeanFactory()方法创建了DefaultListableBeanFactory实例，这里我们可以想一下为什么是DefaultListableBeanFactory而不是其他的BeanFactory。因为BeanDefinition是存储在Map<String, BeanDefinition>中的，而这个Map是DefaultListableBeanFactory的成员变量。通过createBeanFactory()方法获取的DefaultListableBeanFactory实例会通过方法参数的方式传入到loadBeanDefinitions方法，最终完成在加载BeanDefinition后，将BeanDefinition注册到DefaultListableBeanFactory（注册就是将BeanDefinition put到Map中）。
loadBeanDefinitions方法则完成了BeanDefinition的加载和注册。
2.4 loadBeanDefinitions
loadBeanDefinitions方法，是AbstractRefreshableApplicationContext子类AbstractXmlApplicationContext中的一个方法：
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// 1. 通过BeanFactory构建XmlBeanDefinitionReader
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
initBeanDefinitionReader(beanDefinitionReader);
// 2. 加载Bean定义
loadBeanDefinitions(beanDefinitionReader);
}第1步，构建XmlBeanDefinitionReader的逻辑没什么好说的，继续来看loadBeanDefinitions方法，该方法最终会调用到AbstractBeanDefinitionReader#loadBeanDefinitions(java.lang.String, java.util.Set)方法。
2.5 AbstractBeanDefinitionReader#loadBeanDefinitions
public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int loadCount = loadBeanDefinitions(resources);
if (actualResources != null) {
for (Resource resource : resources) {
actualResources.add(resource);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
}
return loadCount;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// Can only load single resources by absolute URL.
Resource resource = resourceLoader.getResource(location);
int loadCount = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
}
return loadCount;
}
}该方法的第一个参数其实就是通过ClassPathXmlApplicationContext构造函数指定的配置文件位置，第二个参数这里是空。另外ResourceLoader是在org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.support.DefaultListableBeanFactory)方法中通过调用setResourceLoader方法指定的，传入的参数是ClassPathXmlApplicationContext，该类是ResourcePatternResolver的实现类，所以上述方法会进如下的判断分支
if (resourceLoader instanceof ResourcePatternResolver)该判断分支下，最终会执行到org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.support.EncodedResource)方法。
2.6 XmlBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.support.EncodedResource)
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
// 加载BeanDefinition定义
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}按照经验，核心代码肯定在doLoadBeanDefinitions中。
2.7 BeanDefinitionReader#doLoadBeanDefinitions
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
// 1. 通过xml配置文件DOM得到org.w3c.dom.Document对象
Document doc = doLoadDocument(inputSource, resource);
// 2. 解析并注册BeanDefinition
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}2.8 XmlBeanDefinitionReader#registerBeanDefinitions
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
// 1. 获取documentReader，用于读取通过xml获得的Document
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
// 2. 获取当前xml文件（Document）解析前，已经注册的BeanDefinition数目
int countBefore = getRegistry().getBeanDefinitionCount();
// 3. 解析并注册当前配置文件的BeanDefinition
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
// 4. 用当前注册的BeanDefinition数目减去之前注册的数目，返回该配置文件注册的BeanDefinition数目
return getRegistry().getBeanDefinitionCount() - countBefore;
}2.9 DefaultBeanDefinitionDocumentReader#registerBeanDefinitions
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}2.10 DefaultBeanDefinitionDocumentReader#doRegisterBeanDefinitions
protected void doRegisterBeanDefinitions(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.
// 1. 造出Bean定义解析器的委托类，该委托类对象中的defaults属性，设置了BeanDefinition加载过程中一些列的默认属性，比如"lazy-init"等
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);
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isInfoEnabled()) {
logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
// 2. 空方法，留给子类扩展
preProcessXml(root);
// 3. 使用解析器委托类解析并注册BeanDefinition
parseBeanDefinitions(root, this.delegate);
// 4. 空方法，留给子类扩展
postProcessXml(root);
this.delegate = parent;
}关于上述第1点，可以跟进去看一下，其实就是为了解决有嵌套的<beans>标签，这样子标签可以从父标签继承一些属性。这也是为什么第4步执行结束后，要将delegate重新置为parent。
第2、4点，适用于给子类扩展的，DefaultBeanDefinitionDocumentReader类中的实现是空的。
第3点，就是我们要关注的核心内容，解析并注册BeanDefinition。
2.11 DefaultBeanDefinitionDocumentReader#parseBeanDefinitions
/**
* Parse the elements at the root level in the document:
* "import", "alias", "bean".
* @param root the DOM root element of the document
*/
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}该方法的作用就是遍历xml文件，解析并注册xml中所有的Bean定义。我们这里只关心<Bean>定义，<beans>和<bean>都是DefaultNamespace标签，所以会调用parseDefaultElement方法。
2.12 DefaultBeanDefinitionDocumentReader#parseDefaultElement
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
// 解析<import>
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
// 解析<alias>
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
// 解析<bean>标签
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// <beans>嵌套标签，所以调用doRegisterBeanDefinitions嵌套处理
doRegisterBeanDefinitions(ele);
}
}这里我们关心的是<bean>标签的解析，所以我们继续跟processBeanDefinition方法。
2.13 DefaultBeanDefinitionDocumentReader#processBeanDefinition
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 1. 解析<Bean>标签为BeanDefinitionHolder
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
// 2. 装饰Bean定义，比如AOP
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// 3. 注册Bean
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(new BeanComponentDefinition(bdHolder));
}
}我们的继续关注<Bean>标签的解析，继续看parseBeanDefinitionElement，第2步我们接下来先略过，在讲AOP的时候再讲。然后来看第3步Bean注册过程。
2.14 BeanDefinitionParserDelegate#parseBeanDefinitionElement
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
// 1. 获取<Bean>标签id属性value
String id = ele.getAttribute(ID_ATTRIBUTE);
// 2. 获取<Bean>标签name属性value
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
// 3. 通过分隔符",;"阶段nameAttr，获取Bean别名List
List<String> aliases = new ArrayList<>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
// 4. beanName默认为id
String beanName = id;
// 5. 如果beanId未定义，以name属性中的第一个别名作为beanName
if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
beanName = aliases.remove(0);
if (logger.isDebugEnabled()) {
logger.debug("No XML 'id' specified - using '" + beanName +
"' as bean name and " + aliases + " as aliases");
}
}
// 6. 检查别名唯一性
if (containingBean == null) {
checkNameUniqueness(beanName, aliases, ele);
}
// 7. 解析Bean定义
AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
if (beanDefinition != null) {
if (!StringUtils.hasText(beanName)) {
try {
if (containingBean != null) {
beanName = BeanDefinitionReaderUtils.generateBeanName(
beanDefinition, this.readerContext.getRegistry(), true);
}
else {
beanName = this.readerContext.generateBeanName(beanDefinition);
// Register an alias for the plain bean class name, if still possible,
// if the generator returned the class name plus a suffix.
// This is expected for Spring 1.2/2.0 backwards compatibility.
String beanClassName = beanDefinition.getBeanClassName();
if (beanClassName != null &&
beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
!this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
aliases.add(beanClassName);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Neither XML 'id' nor 'name' specified - " +
"using generated bean name [" + beanName + "]");
}
}
catch (Exception ex) {
error(ex.getMessage(), ele);
return null;
}
}
String[] aliasesArray = StringUtils.toStringArray(aliases);
return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
}
return null;
}public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, @Nullable BeanDefinition containingBean) {
this.parseState.push(new BeanEntry(beanName));
// 1. 获取Bean对应的类路径
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
// 2. 获取<bean>标签中的parent属性
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
try {
// 3. 根据Bean对应的类路径以及parent，生成一个AbstractBeanDefinition，AbstractBeanDefinition是一个抽象类，生成出来的具体实例为GenericBeanDefinition
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
// 4. 解析Bean定义的属性，包括scope、sington、abstract、lazy-init等，如果配置文件中未定义，会使用BeanDefinitionParserDelegate的成员变量defaults
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
// 5. 设置Bean描述
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
// 6. 解析<bean>标签meta属性
parseMetaElements(ele, bd);
// 7. 解析<bean>标签下<lookup-method>标签
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
// 8. 解析<bean>标签下的<replaced-method>标签
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
// 9. 解析<bean>标签下的<constructor-arg>标签，<constructor-arg>标签用于实现构造函数注入Bean属性
parseConstructorArgElements(ele, bd);
// 10. 解析<bean>标签下的<property>标签，<property>标签是最常见的Bean属性注入的方式
parsePropertyElements(ele, bd);
// 11. <bean>标签下的<qualifier>标签，用于设置存在同一类型的多个Bean时，依赖的bean名称
parseQualifierElements(ele, bd);
bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));
return bd;
}
catch (ClassNotFoundException ex) {
error("Bean class [" + className + "] not found", ele, ex);
}
catch (NoClassDefFoundError err) {
error("Class that bean class [" + className + "] depends on not found", ele, err);
}
catch (Throwable ex) {
error("Unexpected failure during bean definition parsing", ele, ex);
}
finally {
this.parseState.pop();
}
return null;
}2.15 BeanDefinitionReaderUtils#registerBeanDefinition
bean解析结束后，我们来看最后一步，Bean注册。该过程在上述方法中有调用：
org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#processBeanDefinitionpublic static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// 1. 注册BeanDefinition
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// 2. 通过别名注册BeanDefinition
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}注册BeanDefinition方法最终会调用到DefaultListableBeanFactory中的registerBeanDefinition方法，如下：
public void 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) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + existingDefinition + "] bound.");
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isWarnEnabled()) {
logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
existingDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(existingDefinition)) {
if (logger.isInfoEnabled()) {
logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
}
else {
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}注册过程就是像DefaultListableBeanFactory的成员变量beanDefinitionMap添加Entry的过程，key为beanName，value为BeanDefinition。
以上就是xml配置文件的解析过程，文章中只介绍了<bean>标签的解析为BeanDefinition的过程，其他比如AOP相关的标签可以去跟一下，道理类似。
参考链接：
1. Spring源码
2. spring5 源码深度解析—– IOC 之 默认标签解析（下）
2. 配置文件读取流程