实验环境：spring-framework-5.0.2、jdk8、gradle4.3.1- Spring源码-IOC容器简介【1】
- Spring源码-IOC容器初始化过程【2】
- Spring源码-Xml Bean解析注册过程【3】
- Spring源码-自定义IOC容器及Bean解析注册【4】
- Spring源码-Bean实例化过程【5】
- Spring源码-Spring是如何解决Bean循环依赖的【6】
- Spring源码-循环依赖-用实例证明去掉二级缓存会出现什么问题【7】
- Spring源码-AOP是如何实现代理的【8】
本文以Xml Bean解析为例，步骤如下
Bean解析过程发生在AbstractApplicationContext#refresh()方法的第2步：obtainFreshBeanFactory()
AbstractApplicationContext#obtainFreshBeanFactory方法
/**
* 初始化BeanFactory，在这一步完成了loadBeanDefinitions的加载
*/
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
// 抽象方法，交给子类实现。
// 子类刷新beanFactory，比如子类AbstractRefreshableApplicationContext会在refreshBeanFactory方法里createBeanFactory，并且执行loadBeanDefinitions加载资源配置。
// 而GenericApplicationContext只会对已经创建好的beanFactory实例设置一下id即可，loadBeanDefinitions放在了外部去做。
refreshBeanFactory();
// getBeanFactory也是抽象方法，由子类创建beanFactory
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}其中refreshBeanFactory()是个抽象方法，其子类AbstractRefreshableApplicationContext实现如下
AbstractRefreshableApplicationContext#refreshBeanFactory方法
/**
* 刷新BeanFactory，实现父类AbstractApplicationContext的抽象方法
*/
@Override
protected final void refreshBeanFactory() throws BeansException {
// 如果已经存在BeanFactory，销毁容器里的bean，关闭BeanFactory
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
// 创建beanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
// 对IOC容器进行定制化，如设置启动参数、开启注解对自动装配等
customizeBeanFactory(beanFactory);
// 调用载入bean定义对方法，loadBeanDefinitions(beanFactory)在本类是个抽象方法，交给子类实现
loadBeanDefinitions(beanFactory);
// 加锁，写的时候不允许读写
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}其中loadBeanDefinitions(beanFactory)是解析的入口，这里也是抽象方法，其子类AbstractXmlApplicationContext实现如下
AbstractXmlApplicationContext#loadBeanDefinitions方法
/**
* 实现AbstractRefreshableApplicationContext的抽象方法
*/
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
// 创建xml定义的bean读取器，容器使用它来读取bean配置资源
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
// 设置资源加载器
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
// 为bean读取器设置SAX xml解析器
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
// 当bean读取器读取bean定义的xml资源文件时，启动xml的校验机制
initBeanDefinitionReader(beanDefinitionReader);
// bean读取器真正实现加载的方法
loadBeanDefinitions(beanDefinitionReader);
}
/**
* bean读取器真正实现加载的方法
*/
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
// 获取bean配置资源的定位，比如 ClassPathXmlApplicationContext(String[] paths, Class<?> clazz)使用类加载资源，会走到这里
Resource[] configResources = getConfigResources();
if (configResources != null) {
// xml bean读取器调用其父类AbstractBeanDefinitionReader读取定位的bean配置资源
reader.loadBeanDefinitions(configResources);
}
// 获取bean配置资源的定位，比如 ClassPathXmlApplicationContext(String... configLocations)输入路径的构造，会走这里
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}这里的AbstractBeanDefinitionReader将资源路径转换成了Resource对象，然后传给子类XmlBeanDefinitionReader来具体解析
AbstractBeanDefinitionReader#loadBeanDefinitions方法
@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
Assert.notNull(locations, "Location array must not be null");
int counter = 0;
for (String location : locations) {
counter += loadBeanDefinitions(location);
}
return counter;
}
@Override
public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
return loadBeanDefinitions(location, null);
}
public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
// 获取在IOC容器初始化过程中设置的资源加载器
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 {
// 加载多个资源
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
// loadBeanDefinitions(resources)在本类没有实现，交给子类实现
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.
// 加载单个资源
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;
}
}XmlBeanDefinitionReader是具体的解析过程，在这里将Resource对象转成了Document对象，传给BeanDefinitionDocumentReader来解析注册
XmlBeanDefinitionReader#loadBeanDefinitions方法
/**
* 加载资源的入口方法
*/
@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
// 将读入的xml资源进行特殊编码处理
return loadBeanDefinitions(new EncodedResource(resource));
}
/**
* 加载xml形式的bean配置
*/
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.getResource());
}
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 {
// 资源文件转为io流
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
// 从InputStream得到xml的解析源
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
// 具体的读取流程
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();
}
}
}
/**
*
* 从xml文件加载资源，具体执行
*/
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
// xml文件转化为DOM对象
Document doc = doLoadDocument(inputSource, resource);
// 注册BeanDefinitions
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);
}
}
/**
* 将bean配置信息解析成容器内部数据结构
*/
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
// 获取一个解析器，这里用的是DefaultBeanDefinitionDocumentReader
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
// 获取已经注册的bean数量，getRegistry()获取的就是GenericApplicationContext
int countBefore = getRegistry().getBeanDefinitionCount();
// 解析过程的入口，具体解析过程交给DefaultBeanDefinitionDocumentReader
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
// 返回本次解析数量
return getRegistry().getBeanDefinitionCount() - countBefore;
}DefaultBeanDefinitionDocumentReader是专门针对Document对象的解析器。会从根节点递归的解析，封装成beanDefinition注册到beanDefinitionMap里
DefaultBeanDefinitionDocumentReader#registerBeanDefinitions方法
/**
* DefaultBeanDefinitionDocumentReader类 解析DOM资源
*/
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
// 具体解析注册过程
doRegisterBeanDefinitions(root);
}
/**
*
* 从根节点递归解析
*/
protected void doRegisterBeanDefinitions(Element root) {
// 具体解析过程由BeanDefinitionParserDelegate实现，其内部定义了xml文件的各种元素
// 父级指向当前
BeanDefinitionParserDelegate parent = this.delegate;
// 当前新建一个
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;
}
}
}
// 在解析之前，进行自定义解析，支持扩展
preProcessXml(root);
// 从文档的根元素进行bean定义的解析
parseBeanDefinitions(root, this.delegate);
// 在解析之后，进行自定义解析，支持扩展
postProcessXml(root);
this.delegate = parent;
}
/**
* 使用spring的bean规则从文档的根元素开始解析
*/
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)) {
// 使用spring的bean规则解析元素节点
parseDefaultElement(ele, delegate);
}
else {
// 使用用户自定义的bean规则解析元素节点
delegate.parseCustomElement(ele);
}
}
}
}
else {
// 使用用户自定义的bean规则解析元素节点
delegate.parseCustomElement(root);
}
}
// 具体解析元素，并注册beanDefinition
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
// import导入元素解析
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
// alias别名元素解析
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
// 普通bean元素解析
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// 递归调用
doRegisterBeanDefinitions(ele);
}
}
/**
*
* 封装成beanDefinition注册到beanDefinitionMap里
*/
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 会把解析出来的bean封装成BeanDefinitionHolder
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// 在这一步把beanDefinition注册到了DefaultListableBeanFactory的beanDefinitionMap里
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));
}
}具体的注册过程由BeanDefinitionReaderUtils类实现
BeanDefinitionReaderUtils#registerBeanDefinition方法
/**
* BeanDefinitionReaderUtils类 注册过程
*/
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
// BeanDefinition的具体注册，这个registry是个注册器，DefaultListableBeanFactory就是注册器的一种实现
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
// 注册别名
registry.registerAlias(beanName, alias);
}
}
}DefaultListableBeanFactory注册beanDefinition的具体逻辑，其实就是this.beanDefinitionMap.put(beanName, beanDefinition)
DefaultListableBeanFactory#registerBeanDefinition方法
/**
* DefaultListableBeanFactory类 注册beanDefinition实现
*/
@Override
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 oldBeanDefinition;
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(oldBeanDefinition)) {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + oldBeanDefinition +
"] 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 (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}总结：bean解析的代码相对来说比较多，各种调用关系链路比较长。总的来说就是父类先做抽象，把一些公共的逻辑实现，具体的解析细节交给子类来做。子类解析好后再调用注册器将BeanDefinition注册进去。
参考资料：
《Spring5核心原理与30个类手写》作者 谭勇德
《Spring源码深度解析》作者 郝佳
《Spring技术内幕》作者 计文柯