Spring XmlBeanFactory的Bean注册流程

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

引发的思考？？？

Spring xml文件中的Bean定义是怎么反映到BeanFactory的，中间经过什么样的过程，这里从源码分析稍微说明一下。

①：XmlBeanFactory :简单介绍

private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);

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

XmlBeanFactory是在DefaultListableBeanFactory真正意义的IOC容器添加XML的新特性的扩展工厂

XmlBeanDefinitionReader：从其使用方法来看，XmlBeanDefinitionReader会和XmlBeanFactory进行绑定，充当一个工具类的作用，主要用于XML文件的解析，类似这种形式的用法，例如tomcat里面的事件监听支持类：

protected final PropertyChangeSupport support =
new PropertyChangeSupport(this);

②：分析XmlBeanDefinitionReader的loadBeanDefinitions方法

本质注册BeanDefinition的方法

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;
}

值得注意的是内部调用的createReaderContext工具方法

public XmlReaderContext createReaderContext(Resource resource) {
return new XmlReaderContext(resource, this.problemReporter, this.eventListener,
this.sourceExtractor, this, getNamespaceHandlerResolver());
}

createReaderContext通过this这个属性，将XmlReaderContext和BeanFactory绑定在一起，这样就可以专注于xml文件的解析，而不用总是通过XmlBeanDefinitionReader去获取BeanFactory的信息

③：关注documentReader.registerBeanDefinitions方法：

本质调用DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions方法

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;
}

preProcessXml：解析xml文件的前期处理工作

postProcessXml：解析xml文件的后期处理工作

parseBeanDefinitions：真正意义上的解析xml文件内容

④：关注parseBeanDefinitions方法

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);
}
}

⑤：接着关注parseDefaultElement方法

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);
}
}

Spring xml文件解析真相大白，这里解析xml文件中各种标签元素

⑥：重点关注 processBeanDefinition方法

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}

分析：借助于BeanDefinitionParserDelegate的帮助，解析xml文件元素获取BeanDefinitionHolder对象，然后通过BeanDefinitionReaderUtils.registerBeanDefinition()将BeanDefinition和BeanFactory进行绑定

⑦：关注：BeanDefinitionReaderUtils.registerBeanDefinition方法

public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {

String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}

之前上面说到XmlReaderContext绑定了BeanFactory，因此这里是通过XmlReaderContext获取BeanFactory对象，然后进行Bean的注册的

⑧：关注registry.registerBeanDefinition方法

本质调用DefaultListableBeanFactory的registerBeanDefinition进行注册

@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);
}
}

分析：DefaultListableBeanFactory注册Bean的时候首先会判断Bean Factory是否存在同名的Bean,如果存在则进行BeanFactory是否允许重写、Bean的优先级等校验，然后在进行对应的处理，如果不能存在则直接创建，然后将数据填充到beanDefinitionMap这个Map的数据结构中，当需要使用getBean方法的时候，方便从beanDefinitionMap根据key(Bean名称)直接读取，如果是别名,IOC容器内部维护一个别名和Bean名称的Map，需要的时候会将别名转换为Bean名称，然后再按照正常流程处理。