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.
     */
    @Override
    public 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。