spring 源码解读与设计详解：6 BeanDefinitionParserDelegate与资源解析

上篇文章中提到了spring资源加载的核心代码为；

DefaultBeanDefinitionDocumentReader类中的void registerBeanDefinitions(Document doc, XmlReaderContext readerContext)方法。

实质上XML文件的解析是由XMLBeanDefinitionReader对象交由BeanDefinitionParserDelegate委托来完成的，实质上这里面使用到了委托模式。

下面继续讲解registerBeanDefinitions方法，首先看代码：

/** * This implementation parses bean definitions according to the "spring-beans" XSD * (or DTD, historically). * <p>Opens a DOM Document; then initializes the default settings * specified at the {@code <beans/>} level; then parses the contained bean definitions. */@Overridepublic void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {this.readerContext = readerContext;logger.debug("Loading bean definitions");Element root = doc.getDocumentElement();doRegisterBeanDefinitions(root);}

解析过程为doRegisterBeanDefinitions(root)

/** * Register each bean definition within the given root {@code <beans/>} element. */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);if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {return;}}}preProcessXml(root);parseBeanDefinitions(root, this.delegate);postProcessXml(root);this.delegate = parent;}

看最后几行代码：

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

这里的preProcessXml和postProcessXml都是空实现，这个doRegisterBeanDefinitions方法使用了经典的模板方法设计模式，子类可以重写preProcessXml和postProcessXml，实现对XML配置文件的自定义扩展。类似于Junit的setUp、testXXX、tearDown方法

而Parent这里，则是实现了beans的嵌套。