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。
1 //XmlBeanDefinitionReader.java 2 public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException { 3 return loadBeanDefinitions(new EncodedResource(resource)); 4 }
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。
