Spring 源码解析（四）解析配置文件1

通过前边分析，我们知道Spring配置文件已经被加载成了Document对象实例，接上一章，我们来分析下DefaultBeanDefinitionDocumentReader如何解析Document，进入registerBeanDefinitions方法

 //org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader
 1     /**
     * {@inheritDoc}
     * <p>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);
    }

第12行，获取spring配置文件跟节点，即<benas>节点

第13行，解析<benas>节点，进入方法

    /**
     * Register each bean definition within the given root {@code <beans/>} element.
     * @throws IllegalStateException if {@code <beans profile="..."} attribute is present
     * and Environment property has not been set
     * @see #setEnvironment
     */
    protected void doRegisterBeanDefinitions(Element root) {
        String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
        if (StringUtils.hasText(profileSpec)) {
            Assert.state(this.environment != null, "Environment must be set for evaluating profiles");
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                    profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            if (!this.environment.acceptsProfiles(specifiedProfiles)) {
                return;
            }
        }

        // 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(this.readerContext, root, parent);

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

        this.delegate = parent;
    }

第25行，创建了BeanDefinitionParserDelegate实例（这个类的描述为--Stateful delegate class used to parse XML bean definitions.）

第28行，处理<benas>节点，进入方法

    /**
     * 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);//处理默认命名空间的Element
                    }
                    else {
                        delegate.parseCustomElement(ele);//处理用户定义命名空间的Element
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

 第8行，获取<benas></beans>所有的子节点。然后循环解析这些子节点（节点是Elemet则解析）。

 第14行，解析处理默认命名空间的Element，比如最常用的<bean></bean>节点。

 第17行，解析用户自定义的命名空间的Element，比如常用的<context:component-scan />、<tx:advice  />、<mvc:annotation-driven />。

 如何解析Element下节介绍，我们来大致简单地总结下Spring是如何加载解析配置文件的，其实是把配置文件路径交给XmlBeanDefinitionReader实例处理最终生成BeanDefiniton

 1、XmlBeanDefinitionReader实例将配置文件转换成Document对象。

 2、XmlBeanDefinitionReader创建了BeanDefinitionDocumentReader实例、XmlReaderContext实例。

 3、将XmlReaderContext实例、Document对象作为入参调用BeanDefinitionDocumentReader的registerBeanDefinitions方法解析Document对象。

 4、BeanDefinitionDocumentReader获取Document对象的<benas></beans>节点，创建代理类实例BeanDefinitionParserDelegate来处理<beans></beans>节点。