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一、BeanDefinitionParserDelegate

Spring在解析xml文件的时候，在遇到<bean>标签的时候，我们会使用BeanDefinitionParserDelegate对象类解析<bean>标签的内容，包括<bean>标签的多个属性，例如 id name class init-method destory-method等，还包括各种子标签 例如 <properties> <constructor>

org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#doRegisterBeanDefinitions

/*** Register each bean definition within the given root {@code <beans/>} element.*/@SuppressWarnings("deprecation")  // for Environment.acceptsProfiles(String...)protected void doRegisterBeanDefinitions(Element root) {// Any nested <beans> elements will cause recursion in this method. In// order to propagate and preserve <beans> default-* attributes correctly,// keep track of the current (parent) delegate, which may be null. Create// the new (child) delegate with a reference to the parent for fallback purposes,// then ultimately reset this.delegate back to its original (parent) reference.// this behavior emulates a stack of delegates without actually necessitating one.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);// We cannot use Profiles.of(...) since profile expressions are not supported// in XML config. See SPR-12458 for details.if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {if (logger.isDebugEnabled()) {logger.debug("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;}

 可以看到这里 this.delegate 是一个 BeanDefinitionParserDelegate 对象

 接下来的几层调用中，传递的都是 this.delegate 这个BeanDefinitionParserDelegate 对象

org.springframework.beans.factory.xml.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)) {parseDefaultElement(ele, delegate);}else {delegate.parseCustomElement(ele);}}}}else {delegate.parseCustomElement(root);}}

二、NamespaceHandler