深入理解Spring系列之五：BeanDefinition装载

接上篇《深入理解Spring系列之四：BeanDefinition装载前奏曲》，进入AbstractXmlApplicationContext类的loadBeanDefinitions方法，代码如下所示。

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException { // Create a new XmlBeanDefinitionReader for the given BeanFactory. XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory); // Configure the bean definition reader with this context's // resource loading environment. beanDefinitionReader.setEnvironment(this.getEnvironment()); beanDefinitionReader.setResourceLoader(this); beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this)); // Allow a subclass to provide custom initialization of the reader, // then proceed with actually loading the bean definitions. initBeanDefinitionReader(beanDefinitionReader); loadBeanDefinitions(beanDefinitionReader); }

关注其中的最后一行的loadBeanDefinitions方法，具体实现也在AbstractXmlApplicationContext类中，代码如下所示。

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException { Resource[] configResources = getConfigResources(); if (configResources != null) { reader.loadBeanDefinitions(configResources); } String[] configLocations = getConfigLocations(); if (configLocations != null) { reader.loadBeanDefinitions(configLocations); } }

这个方法里就是根据给定的xml配置文件进行后续的解析及装载。继续跟踪代码，进入reader.loadBeanDefinitions(configLocations)，这个XmlBeanDefinitionReader用于从xml文件中读取Bean的定义。接下来的代码有很多跳转，大部分都是不重要的，直接追踪到重要代码，代码如下。

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()); } 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(); } } }

这里只要关注doLoadBeanDefinitions方法，进入其实现，代码如下。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException { try { Document doc = doLoadDocument(inputSource, resource); 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); } }

直接看try块中的代码，可以看到，Spring将xml配置文件转成Document，这里使用了SAX对XML的解析，至于里面是如何解析可以后续针对性的研究。接着进入registerBeanDefinitions方法，后续又有很多代码的跳转，先不一一关注，直接进入重要代码，代码如下。

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); }else { delegate.parseCustomElement(ele); } } } }else { delegate.parseCustomElement(root); } }

可以看到就是对Document中元素、节点的不断解析。这里的解析分成了两条路线，一个是默认标签的解析，如Spring自己定义的标签；一个是对自定义标签的解析，如自定义的标签。这里先关注默认标签的解析，对于自定义标签的解析可以后续分析，进入parseDefaultElement方法，代码如下。

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) { if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) { importBeanDefinitionResource(ele); }else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) { processAliasRegistration(ele); }else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) { processBeanDefinition(ele, delegate); }else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) { // recurse doRegisterBeanDefinitions(ele); } }

代码中分别对import、alias、bean、beans标签进行解析，最终将解析得到的BeanDefinition存入DefaultListableBeanFactory中的beanDefinitionMap中。