Spring源码入门——XmlBeanDefinitionReader解析

　　接上篇【】 ，我们看到BeanDefinitionReader解决的是从资源文件（xml,propert）到BeanDefinition集合的过程。所以BeanDefinitionReader接口有两个实现版本。

　

　　BeanDefinitionReader的接口声明，ResourceLoader是spring中解决Resource加载的操作。四个loadBeanDefinitions就是重载解决单个或者多个资源文件的处理问题。

　　

　　loadBeanDefinitions 是加载BeanDefinition接口的核心入口，AbstractBeanDefinitionReader中剩下了loadBeanDefinitions(Resource resource)这个方法给子类实现。

　　开始加载资源文件，这一步会将Resource构造为一个EncodedResource，添加了编码的相关控制信息。

 //XmlBeanDefinitionReader.java
     public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
         return loadBeanDefinitions(new EncodedResource(resource));
     }

　　下一步需要去判断当前配置文件是否被循环加载。

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }
        //没有明白为什么需要使用ThreadLoacal?
        //private final ThreadLocal<Set<EncodedResource>> resourcesCurrentlyBeingLoaded = new NamedThreadLocal<Set<EncodedResource>>("XML bean definition resources currently being loaded");
        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<EncodedResource>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }    

　　　真正执行加载操作的是doLoadBeanDefinitions方法。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
　　　　　　　//获取xml的校验格式，dtd或者xsd的校验模式
            int validationMode = getValidationModeForResource(resource);
　　　　　　　//解析xml为dom，用了sax解析的
            Document doc = this.documentLoader.loadDocument(
                    inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
　　　　　　　//真正的主角来了
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }    

　　spring解析xml文件为org.w3c.dom.Document使用了SAX的实现方法，除了SAX，常用的xml解析方法还有dom，dom4j，jdom等方法。

　　registerBeanDefinitions是正式注册BeanDefinition的地方，这里又引入了一个新的接口BeanDefinitionDocumentReader（定义如何解析dom为BeanDefinition）。

    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
　　　　　//创建一个解析器
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
　　　　　//设置上下文环境
        documentReader.setEnvironment(this.getEnvironment());
        int countBefore = getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

　　　　解析的实现为reader.registerBeanDefinitions方法。

    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

　　缓存readerContext，目的不清楚。直接parse

    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 = createHelper(this.readerContext, root, parent);

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

        this.delegate = parent;
    }

　　preProcessXml和postProcessXml方法暂时留空，目的为了需要增加新的实现版本时候方便扩展。

protected void preProcessXml(Element root) {
    }
protected void postProcessXml(Element root) {
    }

　　parseBeanDefinitions方法内将

    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
　　　　　//判断解析的BeanDefinition是spring定义的还是自定义扩展的
        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);
        }
    }

　　对于spring定义的BeanDefinition的解析以beans , bean , import , alias开头的四种bean，其他标示开始认为是自定义bean。自定义bean的解析机制都要从NamespaceHandler接口出发。Spring-core和Spring-beans包内就提供了如下这些实现。

　　

　　　　下面是BeanDefinitionParserDelegate类中具体处理自定义bean的代码，允许根据不同的nameSpaceUri查找当前应用下面定义的解析类

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
        String namespaceUri = getNamespaceURI(ele);
        NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
        if (handler == null) {
            error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
            return null;
        }
        return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
    }

　　至此解析部分完成了。但是解析完成的BeanDefinition放到哪里了呢？看一个例子：

　　

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.
　　　　　　　　　//真正执行注册BeanDefinition的类
                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));
        }
    }

　　这样解析加载注册BeanDefinition 的任务就结束了。