Spring 源码分析四-parseBeanDefinitions&BeanDefinitionParserDelegate

上一篇博客说到，BeanDefinition的解析,已经走到了DefaultBeanDefinitionDocumentR
eader里，这时候配置文件已经被加载，并解析成w3c的Document对象。这篇博客就接着介绍，DefaultBeanDefinitionDocumentReader和BeanDefinitionParserDelegate类，是怎么协同完成bean的解析和注册的。

BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createHelper(readerContext, root, parent);
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;

这段代码，创建了一个BeanDefinitionParserDelegate组件，然后就是preProcessXml()、parseBeanDefinitions()、postProcessXml()方法
其中preProcessXml()和postProcessXml()默认是空方法，接下来就看下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);
}
}

从这个方法开始，BeanDefinitionParserDelegate就开始发挥作用了，判断当前解析元素是否属于默认的命名空间，如果是的话，就调用parseDefaultElement()方法，否则调用delegate上parseCustomElement()方法

public boolean isDefaultNamespace(String namespaceUri) {
return (!StringUtils.hasLength(namespaceUri) || BEANS_NAMESPACE_URI.equals(namespaceUri));
}
public boolean isDefaultNamespace(Node node) {
return isDefaultNamespace(getNamespaceURI(node));
}

只有http://www.springframework.org/schema/beans，会被认为是默认的命名空间。也就是说，beans、bean这些元素，会认为属于默认的命名空间，而像task:scheduled这些，就认为不属于默认命名空间。
根节点beans的一个子节点bean，是属于默认命名空间的，所以会进入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);
}
}

这里可能会有4种情况，import、alias、bean、beans，分别有一个方法与之对应，这里解析的是bean元素，所以会进入processBeanDefinition()方法

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));
}
}

这里主要有3个步骤，先是委托delegate对bean进行解析，然后委托delegate对bean进行装饰，最后由一个工具类来完成BeanDefinition的注册
可以看出来，DefaultBeanDefinitionDocumentReader不负责任何具体的bean解析，它面向的是xml Document对象，根据其元素的命名空间和名称，起一个类似路由的作用（不过，命名空间的判断，也是委托给delegate来做的）。所以这个类的命名，是比较贴切的，突出了其面向Document的特性。具体的工作，是由BeanDefinitionParserDelegate来完成的
下面就看下parseBeanDefinitionElement()方法

public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
String id = ele.getAttribute(ID_ATTRIBUTE);
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
List<String> aliases = new ArrayList<String>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
String beanName = id;
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");
}
}
if (containingBean == null) {
checkNameUniqueness(beanName, aliases, ele);
}
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;
}

这个方法很长，可以分成三段来看

String id = ele.getAttribute(ID_ATTRIBUTE);
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
List<String> aliases = new ArrayList<String>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
String beanName = id;
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");
}
}
if (containingBean == null) {
checkNameUniqueness(beanName, aliases, ele);
}

这一段，主要是处理一些跟alias，id等标识相关的东西

AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);

这一行是核心，进行实际的解析

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);
}

这段是后置处理，对beanName进行处理
前置处理和后置处理，不是核心，就不细看了，重点看下核心的那一行调用

public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, BeanDefinition containingBean) {
this.parseState.push(new BeanEntry(beanName));
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
try {
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
parseMetaElements(ele, bd);
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele,   bd.getMethodOverrides());
parseConstructorArgElements(ele, bd);
parsePropertyElements(ele, bd);
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;
}

这个方法也挺长的，拆开看看

this.parseState.push(new BeanEntry(beanName));
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}

这段是从配置中抽取出类名。接下来的长长一段，把异常处理先抛开，看看实际的业务

String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);                  
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
parseMetaElements(ele, bd);
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
parseConstructorArgElements(ele, bd);
parsePropertyElements(ele, bd);
parseQualifierElements(ele, bd);
bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));
return bd;

这里每个方法的命名，就说明了是要干什么，可以一个个跟进去看，本文就不细说了。总之，经过这里的解析，就得到了一个完整的BeanDefinitionHolder。只是说明一下，如果在配置文件里，没有对一些属性进行设置，比如autowire-candidate等，那么这个解析生成的BeanDefinition，都会得到一个默认值
然后，对这个Bean做一些必要的装饰

public BeanDefinitionHolder decorateBeanDefinitionIfRequired(
Element ele, BeanDefinitionHolder definitionHolder, BeanDefinition containingBd) {
BeanDefinitionHolder finalDefinition = definitionHolder;
// Decorate based on custom attributes first.
NamedNodeMap attributes = ele.getAttributes();
for (int i = 0; i < attributes.getLength(); i++) {
Node node = attributes.item(i);
finalDefinition = decorateIfRequired(node, finalDefinition, containingBd);
}
// Decorate based on custom nested elements.
NodeList children = ele.getChildNodes();
for (int i = 0; i < children.getLength(); i++) {
Node node = children.item(i);
if (node.getNodeType() == Node.ELEMENT_NODE) {
finalDefinition = decorateIfRequired(node, finalDefinition, containingBd);
}
}
return finalDefinition;
}

持续单步调试，代码继续运行到DefaultBeanDefinitionDocumentReader中的processBeanDefinition中的registerBeanDefinition()

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

单步进入代码发现BeanDefinitionReaderUtils静态方法registerBeanDefinition()

public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
// 其实调用的是DefaultListableBeanFactory中的registerBeanDefinition方法
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String aliase : aliases) {
registry.registerAlias(beanName, aliase);
}
}
}

解释一下其实调用的是DefaultListableBeanFactory中的registerBeanDefinition方法这句话，因为DefaultListableBeanFactory实现BeanDefinitionRegistry接口，BeanDefinitionRegistry接口中定义了registerBeanDefinition()方法
看下DefaultListableBeanFactory中registerBeanDefinition()实例方法的具体实现：

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);
}
}
synchronized (this.beanDefinitionMap) {
Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else {
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
}
this.beanDefinitionMap.put(beanName, beanDefinition);
resetBeanDefinition(beanName);
}
}

代码追溯之后发现这个方法里，最关键的是以下2行：

this.beanDefinitionNames.add(beanName);
this.beanDefinitionMap.put(beanName, beanDefinition);

前者是把beanName放到队列里，后者是把BeanDefinition放到map中，到此注册就完成了。在后面实例化的时候，就是把beanDefinitionMap中的BeanDefinition取出来，逐一实例化
BeanFactory准备完毕之后，代码又回到了ClassPathXmlApplicationContext里

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
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) {
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
}
}

也就是obtainFreshBeanFactory()方法执行之后，再进行下面的步骤。
总结来说，ApplicationContext将解析配置文件的工作委托给BeanDefinitionReader，然后BeanDefinitionReader将配置文件读取为xml的Document文档之后，又委托给BeanDefinitionDocumentReader
BeanDefinitionDocumentReader这个组件是根据xml元素的命名空间和元素名，起到一个路由的作用，实际的解析工作，是委托给BeanDefinitionParserDelegate来完成的
BeanDefinitionParserDelegate的解析工作完成以后，会返回BeanDefinitionHolder给BeanDefinitionDocumentReader，在这里，会委托给DefaultListableBeanFactory完成bean的注册
XmlBeanDefinitionReader（计数、解析XML文档），BeanDefinitionDocumentReader（依赖xml文档，进行解析和注册），BeanDefinitionParserDelegate（实际的解析工作）。可以看出，在解析bean的过程中，这3个组件的分工是比较清晰的，各司其职，这种设计思想值得学习
到此为止，bean的解析、注册、spring ioc 容器的实例化过程就基本分析结束了。