基于XML文件读取bean定义：XmlBeanDefinitionReader

基于XML文件读取bean定义
除了基于Properties文件读取bean的定义，这里介绍另一种读取bean的定义方式。这中方式更为常见。
XmlBeanDefinitionReader
主要方法介绍：
loadBeanDefinitions();//加载bean的定义，同样是通过用被指定的编码方式来读取流xml资源文件。同样返回定义的bean的数量

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());
		}

		try {
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				InputSource inputSource = new InputSource(inputStream);
				if (encodedResource.getEncoding() != null) {//如果指定了编码方式，就用指定的编码为流的编码					inputSource.setEncoding(encodedResource.getEncoding());
				}//这类调用该方式真正的做到了加载bean定义
				return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
			}
			finally {//关闭流
				inputStream.close();
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(
					"IOException parsing XML document from " + encodedResource.getResource(), ex);
		}
	}

doLoadBeanDefinitions();该方法真正完成加载bean定义的工作

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {
		try {//取得xml文件的文档验证方式，比如dtd，xsd。
			int validationMode = getValidationModeForResource(resource);//加载器加载xml文件。
			Document doc = this.documentLoader.loadDocument(
					inputSource, this.entityResolver, this.errorHandler, validationMode, this.namespaceAware);//调用下面介绍的方法注册bean到工厂
			return registerBeanDefinitions(doc, resource);
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (ParserConfigurationException ex) {
			throw new BeanDefinitionStoreException(
					"Parser configuration exception parsing XML from " + resource, ex);
		}
		catch (SAXParseException ex) {
			throw new BeanDefinitionStoreException(
					"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
		}
		catch (SAXException ex) {
			throw new BeanDefinitionStoreException("XML document from " + resource + " is invalid", ex);
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException("IOException parsing XML document from " + resource, ex);
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException("Unexpected exception parsing XML document from " + resource, ex);
		}
	}

registerBeanDefinitions();这里完成注册bean到工厂的工作。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
				if (this.parserClass != null) {//拿到解析器负责从xml解析定义的bean，并负责将它们注册到工厂
			XmlBeanDefinitionParser parser =
					(XmlBeanDefinitionParser) BeanUtils.instantiateClass(this.parserClass);
			return parser.registerBeanDefinitions(this, doc, resource);
		}//若解析器为null，就创建一个documentReader ，把解析bean定义并注册bean到工厂的工作交给它完成
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
		int countBefore = getBeanFactory().getBeanDefinitionCount();
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
		return getBeanFactory().getBeanDefinitionCount() - countBefore;
	}