Spring DefaultDocumentLoader

public class DefaultDocumentLoader implements DocumentLoader {

	/**
	 * JAXP attribute used to configure the schema language for validation.
	 */
	private static final String SCHEMA_LANGUAGE_ATTRIBUTE = "http://java.sun.com/xml/jaxp/properties/schemaLanguage";

	/**
	 * JAXP attribute value indicating the XSD schema language.
	 */
	private static final String XSD_SCHEMA_LANGUAGE = "http://www.w3.org/2001/XMLSchema";


	private static final Log logger = LogFactory.getLog(DefaultDocumentLoader.class);


	/**
	 * Load the {@link Document} at the supplied {@link InputSource} using the standard JAXP-configured
	 * XML parser.
	 */
	@Override
	public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
			ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

		// 创建 xml document 构建工具
		DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
		if (logger.isTraceEnabled()) {
			logger.trace("Using JAXP provider [" + factory.getClass().getName() + "]");
		}

		// documentBuilder 类创建
		DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
		return builder.parse(inputSource);
	}

	/**
	 * Create the {@link DocumentBuilderFactory} instance.
	 * @param validationMode the type of validation: {@link XmlValidationModeDetector#VALIDATION_DTD DTD}
	 * or {@link XmlValidationModeDetector#VALIDATION_XSD XSD})
	 * @param namespaceAware whether the returned factory is to provide support for XML namespaces
	 * @return the JAXP DocumentBuilderFactory
	 * @throws ParserConfigurationException if we failed to build a proper DocumentBuilderFactory
	 */
	protected DocumentBuilderFactory createDocumentBuilderFactory(int validationMode, boolean namespaceAware)
			throws ParserConfigurationException {

		// 创建出 DocumentBuilderFactory 对象
		DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
		// 设置命名空间支持情况
		factory.setNamespaceAware(namespaceAware);

		// 验证规则
		if (validationMode != XmlValidationModeDetector.VALIDATION_NONE) {
			factory.setValidating(true);
			if (validationMode == XmlValidationModeDetector.VALIDATION_XSD) {
				// Enforce namespace aware for XSD...
				factory.setNamespaceAware(true);
				try {
					// 设置 scheme 信息
					factory.setAttribute(SCHEMA_LANGUAGE_ATTRIBUTE, XSD_SCHEMA_LANGUAGE);
				}
				catch (IllegalArgumentException ex) {
					ParserConfigurationException pcex = new ParserConfigurationException(
							"Unable to validate using XSD: Your JAXP provider [" + factory +
									"] does not support XML Schema. Are you running on Java 1.4 with Apache Crimson? " +
									"Upgrade to Apache Xerces (or Java 1.5) for full XSD support.");
					pcex.initCause(ex);
					throw pcex;
				}
			}
		}

		return factory;
	}

	/**
	 * Create a JAXP DocumentBuilder that this bean definition reader
	 * will use for parsing XML documents. Can be overridden in subclasses,
	 * adding further initialization of the builder.
	 * @param factory the JAXP DocumentBuilderFactory that the DocumentBuilder
	 * should be created with
	 * @param entityResolver the SAX EntityResolver to use
	 * @param errorHandler the SAX ErrorHandler to use
	 * @return the JAXP DocumentBuilder
	 * @throws ParserConfigurationException if thrown by JAXP methods
	 */
	protected DocumentBuilder createDocumentBuilder(DocumentBuilderFactory factory,
			@Nullable EntityResolver entityResolver, @Nullable ErrorHandler errorHandler)
			throws ParserConfigurationException {

		DocumentBuilder docBuilder = factory.newDocumentBuilder();
		if (entityResolver != null) {
			docBuilder.setEntityResolver(entityResolver);
		}
		if (errorHandler != null) {
			docBuilder.setErrorHandler(errorHandler);
		}
		return docBuilder;
	}

}