spring源码学习系列2.2-从document到beanDefinition

本章探讨从document到beanDefinition的过程- 注册beanDefinition到容器beanDefinitionMap中

从xml到document

从document到beanDefinition

从beanDefiniton到instance

涉及到的部分主要类或接口：
org.springframework.beans.factory.xml.BeanDefinitionDocumentReader

org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader

org.springframework.beans.factory.xml.BeanDefinitionParserDelegate

org.springframework.beans.factory.support.BeanDefinitionReaderUtils

org.springframework.beans.factory.config.BeanDefinition

org.springframework.beans.factory.xml.DefaultNamespaceHandlerResolver

从document到beanDefinition

<bean>的解析
从最直接的xml配置开始<bean>

对于像事务tx:annotation等标签在spring-tx.jar/META-INF/spring.handlers下有配置具体的处理类。bean是不是也有一样的配置呢，其实不然，我们找到解析注册标签的入口。

省略从web.xml加载到doc的过程,从解析Element root = doc.getDocumentElement()开始

解析根节点root的任务由DefaultBeanDefinitionDocumentReader开始

DefaultBeanDefinitionDocumentReader源码：
http://www.grepcode.com/file/repository.springsource.com/org.springframework/org.springframework.beans/3.2.2/org/springframework/beans/factory/xml/DefaultBeanDefinitionDocumentReader.java/

DefaultBeanDefinitionDocumentReader.parseBeanDefinitions

/**
	 * Parse the elements at the root level in the document:
	 * "import", "alias", "bean".
	 * @param root the DOM root element of the document
	 */
	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)) {
//1.处理默认标签
						parseDefaultElement(ele, delegate);
					}
					else {
						delegate.parseCustomElement(ele);
					}
				}
			}
		}
		else {
			delegate.parseCustomElement(root);
		}
	}

对于默认的标签，如"import", "alias", "bean" 在parseDefaultElement处开始解析。非默认的标签需定义schema/handlers进行处理。

查看默认标签的处理方法：
DefaultBeanDefinitionDocumentReader#parseDefaultElement

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//1.1 处理import标签
		if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
			importBeanDefinitionResource(ele);
		}
//1.2 处理alias标签
		else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
			processAliasRegistration(ele);
		}
//1.3 处理bean标签
		else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
			processBeanDefinition(ele, delegate);
		}
		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
			// recurse
			doRegisterBeanDefinitions(ele);
		}
	}

继续bean标签的处理BEAN_ELEMENT:
DefaultBeanDefinitionDocumentReader#processBeanDefinition

/**
	 * Process the given bean element, parsing the bean definition
	 * and registering it with the registry.
	 */
	protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//1.3.1 生成beanDefinitionHolder
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
//1.3.2 解析bean标签的自定义元素
//扩展BeanDefinition-自定义标签
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
//1.3.3 注册beanDefinitionMap
				// 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));
		}
	}

从以上代码可以看到,DefaultBeanDefinitionDocumentReader将解析生成beanDefinition的任务委托给了BeanDefinitionParserDelegate进行处理。

beanDefinitionDocumentReader
-------------------------------------------------------承上启下的分割线----------------------------------------------
beanDefinitionParserDelegate

BeanDefinitionParserDelegate源码：
http://grepcode.com/file/repository.springsource.com/org.springframework/org.springframework.beans/3.2.2/org/springframework/beans/factory/xml/BeanDefinitionParserDelegate.java#BeanDefinitionParserDelegate

由上可知,处理bean标签分3个部分：
一是解析bean标签生成BeanDefinitionHolder- 处理spring默认元素或属性
BeanDefinitionParserDelegate#parseBeanDefinitionElement

/**
	 * Parses the supplied {@code &lt;bean&gt;} element. May return {@code null}
	 * if there were errors during parse. Errors are reported to the
	 * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
	 */
	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;
	}

可以看到，先生成beanDefinition再包装成beanDefinitionHolder
(1)生成beanDefinition的过程
BeanDefinitionParserDelegate#createBeanDefinition

protected AbstractBeanDefinition createBeanDefinition(String className, String parentName)
			throws ClassNotFoundException {

		return BeanDefinitionReaderUtils.createBeanDefinition(
				parentName, className, this.readerContext.getBeanClassLoader());
	}

创建GenericBeanDefinition

BeanDefinitionReaderUtils#createBeanDefinition

public static AbstractBeanDefinition createBeanDefinition(
			String parentName, String className, ClassLoader classLoader) throws ClassNotFoundException {

		GenericBeanDefinition bd = new GenericBeanDefinition();
		bd.setParentName(parentName);
		if (className != null) {
			if (classLoader != null) {
				bd.setBeanClass(ClassUtils.forName(className, classLoader));
			}
			else {
				bd.setBeanClassName(className);
			}
		}
		return bd;
	}

这里可以发挥你的想象力。就像平时定义了一个model，然后设置各种属性的过程。这里处理的属性或子标签都是spring默认定义的属性或标签，自定义的标签会在第二步进行处理

bd.setBeanClass涉及到从benDefintion到instance的一个处理过程:resolveBeanClass
如果这里设置了beanClass，resolveBeanClass可以直接返回。

那这里会不会设置beanClass呢？
关键点是this.readerContext.getBeanClassLoader()，其里面继续调用XmlBeanDefinitionReader.getBeanClassLoader();
查看<spring源码学习系列2.1-从xml到document>创建XmlBeanDefinitionReader对象时，并没有设置beanClassLoader的值

这里定义的beanDefinition的实际类是GenericBeanDefinition,在从beanDefinition到instance的过程中，会转为RootBeanDefinition

平常我们只会设置几个熟悉的属性，其实beanDefinition中有很多属性,
例如：lookup-method replace-method等

spring是如何解析这些特殊属性或标签的来生成实例的呢？

其他属性可查看BeanDefinitionParserDelegate的源码，其用静态变量基本定义了所有属性

(2)此时生成的beanDefinitionHolder对象也很简单，只是存储了beanDefinition及其名称等

二是装饰第一步生成的BeanDefinitionHolder对象- 处理自定义标签或属性- 可以看做spring的一种扩展-对spring的默认标签进行扩展

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;
	}

// Decorate based on custom attributes first.
NamedNodeMap attributes = ele.getAttributes();

// Decorate based on custom nested elements.
NodeList children = ele.getChildNodes();

这里继续循环bean标签的属性或子标签，难道是继续对第一步的属性进行深入处理?其实不然,继续跟到decorateIfRequired方法中看看

private BeanDefinitionHolder decorateIfRequired(
			Node node, BeanDefinitionHolder originalDef, BeanDefinition containingBd) {

		String namespaceUri = getNamespaceURI(node);
		if (!isDefaultNamespace(namespaceUri)) {
			NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
			if (handler != null) {
				return handler.decorate(node, originalDef, new ParserContext(this.readerContext, this, containingBd));
			}
			else if (namespaceUri != null && namespaceUri.startsWith("http://www.springframework.org/")) {
				error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", node);
			}
			else {
				// A custom namespace, not to be handled by Spring - maybe "xml:...".
				if (logger.isDebugEnabled()) {
					logger.debug("No Spring NamespaceHandler found for XML schema namespace [" + namespaceUri + "]");
				}
			}
		}
		return originalDef;
	}

这段代码中，有个很重要的分子判断!isDefaultNamespace(namespaceUri)，即是否是自定义标签，如果不是则继续获取相应命名空间处理类进行解析

可见装饰主要是对自定义子标签或属性的解析过程- 对spring bean标签的扩展

NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);

this.readerContext.getNamespaceHandlerResolver()获取的是DefaultNamespaceHandlerResolver，其主要职责是管理命名空间处理器的。其数据来源主要是META-INF/spring.handlers下的内容,加载这些文件用到了懒加载机制(Load the specified NamespaceHandler mappings lazily)

resolve方法中,首先加载META-INF/spring.handler文件:

Properties mappings =
								PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);

并将其Properties类中的文件，转换为hashMap缓存起来:

CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);

再就是根据命名空间，读取相应命名空间处理器，并初始化返回:

NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);

将namespaceHandler放入handlerMappings中(之前将Properties转换为hashMap，value值只是string类型的)

handlerMappings.put(namespaceUri, namespaceHandler);

可跟踪源代码,借鉴spring是如何读取配置文件的

DefaultNamespaceHandlerResolver源码:
http://grepcode.com/file/repository.springsource.com/org.springframework/org.springframework.beans/3.2.2/org/springframework/beans/factory/xml/DefaultNamespaceHandlerResolver.java/

三是注册到beanDefinition容器

// Register the final decorated instance.
				BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());

/**
	 * Register the given bean definition with the given bean factory.
	 * @param definitionHolder the bean definition including name and aliases
	 * @param registry the bean factory to register with
	 * @throws BeanDefinitionStoreException if registration failed
	 */
	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);
			}
		}
	}

这里registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());实际上就是放到 DefaultListableBeanFactory.beanDefinitionMap中

至此,从document到beanDefinition分析结束

spring源码学习系列2.2-从document到beanDefinition

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