Spring IOC容器初始化底层代码分析

IOC容器案例：XmlBeanFactory

XmlBeanFactory容器初始化步骤：

XmlBeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("com/zhiwei/ioc/applicationContext.xml"));

步骤1：XmlBeanFactory这里使用ClassPathResource来映射Spring XML配置文件

new ClassPathResource(“com/zhiwei/ioc/applicationContext.xml”）

这里写代码片

步骤2：初始化空白的IOC容器，然后使用XmlBeanDefinitionReader解析Spring XML配置文件，并注入到IOC容器

public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws        BeansException {
super(parentBeanFactory);
this.reader.loadBeanDefinitions(resource);
}

XmlBeanDefinitionReader解析XML文件的一般步骤：

步骤1： 查看XmlBeanDefinitionReader.loadBeanDefinitions方法实现

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<EncodedResource>(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());
}
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();
}
}
}

方法解读：

resourcesCurrentlyBeingLoaded：通过ThreadLocal绑定EncodedResource,这样XmlBeanDefinitionReader就持有外部资源的映射，然后为方便DOM XML解析，将配置资源封装为

inputSource对象，然后传递给doLoadBeanDefinitions方法

步骤3：查看XmlBeanDefinitionReader.doLoadBeanDefinitions方法实现

源码：

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {

Document doc = doLoadDocument(inputSource, resource);
return registerBeanDefinitions(doc, resource);
}

方法解读：

doLoadDocument方法底层通过DOM解析的方法解析Spring XML配置文件，并返回Document对象，接着调用registerBeanDefinitions进行Bean的注册工作

步骤4：查看XmlBeanDefinitionReader.registerBeanDefinitions方法实现

源码：

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}

方法解读：

getRegistry()：可用于获取IOC容器，原因XmlBeanFactory实例化

private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);

将XML的Bean定义阅读器与IOC容器绑定在一起

registerBeanDefinitions内部调用registerBeanDefinitions注册Bean，注意createReaderContext方法会创建XmlReaderContext，该对象会与XmlBeanDefinitionReader 绑定，从而实现与IOC容器的间接绑定，这一点对于后续的Bean实际IOC容器注册非常重要

步骤5：查看BeanDefinitionDocumentReader.registerBeanDefinitions方法

注意：BeanDefinitionDocumentReader主要作用就是将Spring XMl配置文件元素映射为IOC容器中的BeanDefinition数据结构，实现真正意义上的解析、注册Bean的功能

源码：

@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}

方法解读：将readerContext作为BeanDefinitionDocumentReader的内置组件，然后获取XML文件的Root节点，接着调用doRegisterBeanDefinitions方法处理

步骤6：查看BeanDefinitionDocumentReader.doRegisterBeanDefinitions方法代码

源码：

protected void doRegisterBeanDefinitions(Element root) {

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;
}
}
}
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);

this.delegate = parent;
}

方法解读：实际解析XMl文件的时BeanDefinitionParserDelegate对象，查看createDelegate方法：

protected BeanDefinitionParserDelegate createDelegate(
XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {

BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
delegate.initDefaults(root, parentDelegate);
return delegate;
}

该方法执行时不仅仅返回BeanDefinitionParserDelegate 对象，还会在创建后进行Beans标签的属性信息的初始化工作，该方法核心代码：

preProcessXml(root); //自定义标签前置处理

parseBeanDefinitions(root, this.delegate); //Spring原始标签处理

postProcessXml(root); //自定义标签后置处理

步骤7：查看BeanDefinitionDocumentReader.parseBeanDefinitions方法代码

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);
}
}

方法解读：该方法遍历XML文件映射的DOM树的节点，delegate.isDefaultNamespace(root)主要用于XML文件命名空间校验，delegate.isDefaultNamespace(ele)用于区别配置文件中的元素是否时Spring自带的，若是Spring自身元素则parseDefaultElement方法处理，否则parseCustomElement处理（例如自定义标签）

步骤8：查看BeanDefinitionDocumentReader.parseDefaultElement方法代码

源码：

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}

方法解读：该方法作为一个中转接口，更具标签元素的不同，选择对应的方法处理，这里主要探索Bean标签的处理方法processBeanDefinition(ele, delegate);

步骤9：查看BeanDefinitionDocumentReader.processBeanDefinition方法代码：

源码：

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
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(new BeanComponentDefinition(bdHolder));
}
}

方法解读：

BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);

主要时根据Root元素节点遍历，然后将数据封装到BeanDefinitionHolder 这个数据结构，真正IOC容器的Bean注册在

BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());方法

步骤10：查看BeanDefinitionReaderUtils.registerBeanDefinition方法代码

源码：

public static void registerBeanDefinition(
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 (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}

方法解读：这里Bean注册分为Bean注册和Bean别名注册，主要看registry.registerBeanDefinition方法

步骤11：查看BeanDefinitionRegistry.registerBeanDefinition（这里使用DefaultListableBeanFactory基础IOC容器）

源码：

@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<String>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> updatedSingletons = new LinkedHashSet<String>(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);
}
}

方法解读：

首先校验生成的BeanDefinition类型，接着校验新创建的Bean是否在IOC容器中注册过，如果注册过则按照Bean的相应配置策略进行处理，例如重写，接着根据hasBeanCreationStarted方法判断IOC容器是否启动过，如果未启动则beanDefinitionMap Map记录BeanName与BeanDefinition对象的关系，beanDefinitionNames记录注册的Bean名称，这里Map的key为BeanName这就时IOC容器中BeanName唯一的原因，如果IOC容器实现启动则beanDefinitionMap直接存放，manualSingletonNames需要已有的BeanName进行校验，最终等于beanDefinitionNames数据，至此整个XMl解析与IOC容器的初始化工作全部完成。