spring 3源码解析之如何解析"import", "alias", "bean"标签

解析的步骤：
1、加载web.xml、加载监听器
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
2、ContextLoaderListener
初始化initWebApplicationContext方法创建
org.springframework.web.context.support. XmlWebApplicationContext对象
3、XmlWebApplicationContext
调用loadBeanDefinitions方法，该方法主要做两件事情：初始化XmlBeanDefinitionReader、获取applicationContext.xml配置文件的路径、然后把事情交给XmlBeanDefinitionReader来处理
4、XmlBeanDefinitionReader
获取到applicationContext.xml配置文件的路径、读取配置文件的内容得到一个输入流、对输入流转码操作、然后封装成一个inputSource对象、再然后封装成一个document对象；在生成document对象的同事也生成了一个Resource对象、这两个对象分部是：document对象承载配置文件的主要内容信息、Resource承载配置文件的描述信息以及一些验证信息。
再由Resource对象创建一个XmlReaderContext。完成了以上操作XmlBeanDefinitionReader就把document对象和XmlReaderContext对象交给DefaultBeanDefinitionDocumentReader来处理
5、DefaultBeanDefinitionDocumentReader
1）、对XmlReaderContext装饰成一个BeanDefinitionParserDelegate对象；
2）、迭代document对象、把document对象拆分成Element元素逐个逐个解析；
3）、使用BeanDefinitionParserDelegate装饰对象解析Element元素或者说标签。
这里的Element元素有两种：一种是DefaultElement、另一种是CustomElement；DefaultElement包括alias、import、bean，CustomElement包括DefaultElement以外的所有元素
我们所关心的应该是第一中元素DefaultElement，看下DefaultBeanDefinitionDocumentReader解析该元素的方法：

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);
		}
	}

解析import标签的方法：

protected void importBeanDefinitionResource(Element ele) {
		String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
		if (!StringUtils.hasText(location)) {
			getReaderContext().error("Resource location must not be empty", ele);
			return;
		}

		// Resolve system properties: e.g. "${user.dir}"
		location = SystemPropertyUtils.resolvePlaceholders(location);

		Set<Resource> actualResources = new LinkedHashSet<Resource>(4);

		// Discover whether the location is an absolute or relative URI 
		boolean absoluteLocation = false;
		try {
			absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
		}
		catch (URISyntaxException ex) {
			// cannot convert to an URI, considering the location relative
			// unless it is the well-known Spring prefix "classpath*:"
		}

		// Absolute or relative?
		if (absoluteLocation) {
			try {
				int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
				if (logger.isDebugEnabled()) {
					logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
				}
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error(
						"Failed to import bean definitions from URL location [" + location + "]", ele, ex);
			}
		}
		else {
			// No URL -> considering resource location as relative to the current file.
			try {
				int importCount;
				Resource relativeResource = getReaderContext().getResource().createRelative(location);
				if (relativeResource.exists()) {
					importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
					actualResources.add(relativeResource);
				}
				else {
					String baseLocation = getReaderContext().getResource().getURL().toString();
					importCount = getReaderContext().getReader().loadBeanDefinitions(
							StringUtils.applyRelativePath(baseLocation, location), actualResources);
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
				}
			}
			catch (IOException ex) {
				getReaderContext().error("Failed to resolve current resource location", ele, ex);
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
						ele, ex);
			}
		}
		Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
		getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
	}

解析alias标签的方法：

	protected void processAliasRegistration(Element ele) {
		String name = ele.getAttribute(NAME_ATTRIBUTE);
		String alias = ele.getAttribute(ALIAS_ATTRIBUTE);
		boolean valid = true;
		if (!StringUtils.hasText(name)) {
			getReaderContext().error("Name must not be empty", ele);
			valid = false;
		}
		if (!StringUtils.hasText(alias)) {
			getReaderContext().error("Alias must not be empty", ele);
			valid = false;
		}
		if (valid) {
			try {
				getReaderContext().getRegistry().registerAlias(name, alias);
			}
			catch (Exception ex) {
				getReaderContext().error("Failed to register alias '" + alias +
						"' for bean with name '" + name + "'", ele, ex);
			}
			getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
		}
	}

解析bean标签的方法：

	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));
		}
	}

方法调用关系图：