/**
 * 封装了用于xml解析的类XPath、Document和EntityResolver
 */
public class XPathParser {
    /**
     * 将xml文件读入内存，并构建一棵树，
     * 通过树结构对各个节点node进行操作
     */
    private Document document;
    /**
     * 是否开启xml文本格式验证
     */
    private boolean validation;
    /**
     * 用于加载本地DTD文件
     *
     * 默认情况下，对xml文档进行验证时，会根据xml文件开始
     * 位置指定的网址加载对应的dtd或xsd文件，当由于网络原因，易导致验证过慢，
     * 在实践中往往会提前设置EntityResolver接口对象加载本地的dtd文件，
     * 从而避免联网加载；
     *
     * XMLMapperEntityResolver implement EntityResolver
     */
    private EntityResolver entityResolver;
    /**
     * mybatis核心配置文件中标签<properties>定义的键值对
     */
    private Properties variables;
    /**
     * 为查询xml文本而设计的语言，配合Document使用
     * XPath之于xml好比Sql之于database
     */
    private XPath xpath;

    public XPathParser(String xml) {
        commonConstructor(false, null, null);
        this.document = createDocument(new InputSource(new StringReader(xml)));
    }
    //...一系列构造方法（略）

    public void setVariables(Properties variables) {
        this.variables = variables;
    }

    /**
     * XPath提供的一系列eval*()方法用于解析boolean、short、int、String、Node等类型信息，
     * 通过调用XPath.evaluate()方法查找指定路径的节点或者属性，并进行相应的类型转换
     */
    public String evalString(String expression) {
        return evalString(document, expression);
    }

    public String evalString(Object root, String expression) {
        String result = (String) evaluate(expression, root, XPathConstants.STRING);
        result = PropertyParser.parse(result, variables);
        return result;
    }

    public Boolean evalBoolean(String expression) {
        return evalBoolean(document, expression);
    }

    public Boolean evalBoolean(Object root, String expression) {
        return (Boolean) evaluate(expression, root, XPathConstants.BOOLEAN);
    }

    public Short evalShort(String expression) {
        return evalShort(document, expression);
    }

    public Short evalShort(Object root, String expression) {
        return Short.valueOf(evalString(root, expression));
    }

    public Integer evalInteger(String expression) {
        return evalInteger(document, expression);
    }

    public Integer evalInteger(Object root, String expression) {
        return Integer.valueOf(evalString(root, expression));
    }

    public Long evalLong(String expression) {
        return evalLong(document, expression);
    }

    public Long evalLong(Object root, String expression) {
        return Long.valueOf(evalString(root, expression));
    }

    public Float evalFloat(String expression) {
        return evalFloat(document, expression);
    }

    public Float evalFloat(Object root, String expression) {
        return Float.valueOf(evalString(root, expression));
    }

    public Double evalDouble(String expression) {
        return evalDouble(document, expression);
    }

    public Double evalDouble(Object root, String expression) {
        return (Double) evaluate(expression, root, XPathConstants.NUMBER);
    }

    public List<XNode> evalNodes(String expression) {
        return evalNodes(document, expression);
    }

    public List<XNode> evalNodes(Object root, String expression) {
        List<XNode> xnodes = new ArrayList<XNode>();
        NodeList nodes = (NodeList) evaluate(expression, root, XPathConstants.NODESET);
        for (int i = 0; i < nodes.getLength(); i++) {
            xnodes.add(new XNode(this, nodes.item(i), variables));
        }
        return xnodes;
    }

    public XNode evalNode(String expression) {
        return evalNode(document, expression);
    }

    public XNode evalNode(Object root, String expression) {
        Node node = (Node) evaluate(expression, root, XPathConstants.NODE);
        if (node == null) {
            return null;
        }
        return new XNode(this, node, variables);
    }

    private Object evaluate(String expression, Object root, QName returnType) {
        try {
            return xpath.evaluate(expression, root, returnType);
        } catch (Exception e) {
            throw new BuilderException("Error evaluating XPath.  Cause: " + e, e);
        }
    }

    /**
     * 创建Dom对象，调用此方法之前必须调用commonConstructor方法完成XPathParser初始化
     */
    private Document createDocument(InputSource inputSource) {
        // important: this must only be called AFTER common constructor
        try {
            //创建DocumentBuilderFactory对象
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            /**
             * 对DocumentBuilderFactory对象进行一系列参数配置
             */
            factory.setValidating(validation);
            factory.setNamespaceAware(false);
            factory.setIgnoringComments(true);
            factory.setIgnoringElementContentWhitespace(false);
            factory.setCoalescing(false);
            factory.setExpandEntityReferences(true);

            //创建DocumentBuilder对象
            DocumentBuilder builder = factory.newDocumentBuilder();
            /**
             * 对DocumentBuilder对象进行一系列参数配置
             */
            builder.setEntityResolver(entityResolver);
            builder.setErrorHandler(new ErrorHandler() {
                @Override
                public void error(SAXParseException exception) throws SAXException {
                    throw exception;
                }

                @Override
                public void fatalError(SAXParseException exception) throws SAXException {
                    throw exception;
                }

                @Override
                public void warning(SAXParseException exception) throws SAXException {
                }
            });
            //加载xml文件
            return builder.parse(inputSource);
        } catch (Exception e) {
            throw new BuilderException("Error creating document instance.  Cause: " + e, e);
        }
    }

    /**
     * 初始化XPathParser
     */
    private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
        this.validation = validation;
        this.entityResolver = entityResolver;
        this.variables = variables;
        XPathFactory factory = XPathFactory.newInstance();
        this.xpath = factory.newXPath();
    }

}