筆記：XML-解析文檔-XPath 定位信息

如果需要定位某個XML文檔中的一段特定信息，那麼通過遍歷DOM 樹的眾多節點來進行行查找顯得有些麻煩，XPath語言使得訪問樹節點變得很容易，例如，下麵的XML文檔結構：

<?xml version="1.0" encoding="utf-8" ?>

<root>

        <title>

                <font enabled="false">

                        <!-- 字體名稱 -->

                        <name>Helvetica</name>

                        <size>36</size>

                </font>

                <data>

                        <![CDATA[xml document root node <root.../>]]>

                </data>

        </title>

</root>

可以通過對XPath表達式 /root/title/font/size 求值來得到 size 節點的值，示例代碼如下：

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

DocumentBuilder builder = factory.newDocumentBuilder();

Path xmlPath = Paths.get("E:\\IDEA Workspace\\exampleiostream\\src\\main\\java\\org\\drsoft\\examples\\xml", "appParse.xml");

InputStream xmlStream = Files.newInputStream(xmlPath, StandardOpenOption.READ);

Document xmlDocument = builder.parse(xmlStream);

XPathFactory xPathFactory = XPathFactory.newInstance();

XPath xPath = xPathFactory.newXPath();

String xpathResult = xPath.evaluate("/root/title/font/size", xmlDocument);

System.out.println("XPath /root/title/font/size value " + xpathResult);

XPath 表達式還提供了更豐富的語法來實現對XML 文檔的篩選節點，節點是通過沿著路徑 (path) 或者步 (steps) 來選取的：

• 選取節點：XPath 使用路徑表達式在 XML 文檔中選取節點。節點是通過沿著路徑或者 step 來選取的

  下麵列出了最有用的路徑表達式：

示例代碼如下：

NodeList list = (NodeList) xPath.evaluate("root", xmlDocument, XPathConstants.NODESET);

for (int i = 0; i < list.getLength(); i++) {

        System.out.println("XPath root Node " + list.item(i).getNodeName());

}

System.out.println("----------------------------------------");

list = (NodeList) xPath.evaluate("//title", xmlDocument, XPathConstants.NODESET);

for (int i = 0; i < list.getLength(); i++) {

        System.out.println("XPath //title Node " + list.item(i).getNodeName());

}

System.out.println("----------------------------------------");

list = (NodeList) xPath.evaluate("//@enabled", xmlDocument, XPathConstants.NODESET);

for (int i = 0; i < list.getLength(); i++) {

        System.out.println("XPath //@enabled Node " + list.item(i).getNodeName() + " Value " + list.item(i).getNodeValue());

}

System.out.println("----------------------------------------");

• 謂語：謂語用來查找某個特定的節點或者包含某個指定的值的節點，謂語被嵌在方括弧中。

示例代碼如下：

list = (NodeList) xPath.evaluate("/root/title/font[size>10]", xmlDocument, XPathConstants.NODESET);

for (int i = 0; i < list.getLength(); i++) {

          System.out.println("XPath /root/title/font[size>10] Node " + list.item(i).getNodeName()

+ " Value " + list.item(i).getNodeValue());

}

System.out.println("----------------------------------------");

list = (NodeList) xPath.evaluate("/root/title/font[size>10]/name", xmlDocument, XPathConstants.NODESET);

for (int i = 0; i < list.getLength(); i++) {

          System.out.println("XPath /root/title/font[size>10]/name Node " + list.item(i).getNodeName()

+ " Value " + list.item(i).getNodeValue());

}

System.out.println("----------------------------------------");

• 運算符：

• 函數：

示例代碼如下：

xpathResult = xPath.evaluate("count(//title)", xmlDocument);

System.out.println("XPath expression count(//title) value=" + xpathResult);

xpathResult = xPath.evaluate("sum(//size/text())", xmlDocument);

System.out.println("XPath expression sum(//size/text()) value=" + xpathResult);

xpathResult = xPath.evaluate("//title[