小读spring ioc源码（五）——BeanDefinitionDocumentReader

上一篇博客说到，BeanDefinition的解析，已经走到了DefaultBeanDefinitionDocumentReader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上的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;
	}

完成了BeanDefinition的解析和装饰之后（装饰也是解析的一部分），就将BeanDefinition在BeanFactory里进行注册。

这个方法，是在BeanRegistry接口里定义的，前面提到过，DefaultListableBeanFactory实现了这个接口，所以这部分工作，也就是在DefaultListableBeanFactory里完成的

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 aliase : aliases) {
				registry.registerAlias(beanName, aliase);
			}
		}
	}

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);
		}
	}

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，就是从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XmlWebApplicationContext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。每个步骤的作用，在前面的博客里已经介绍过了

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>的解析过程就基本分析结束了。下一篇博客，会针对另外一些非Default Namespace的元素的处理过程，再更深入地分析一下spring的加载过程

评论

3 楼 nidetaidengma 2015-07-29  

给博主点个赞，确实很不错，应该置顶才对

2 楼 kyfxbl 2015-07-03  

lalahei 写道

写的这么好竟然没人顶

hehe，过奖了

1 楼 lalahei 2015-07-02  

写的这么好竟然没人顶