笔记：XML-解析文档-XPath 定位信息

如果需要定位某个XML文档中的一段特定信息，那么通过遍历DOM
树的众多节点来进行行查找显得有些麻烦，XPath语言使得访问树节点变得很容易，例如，下面的XML文档结构：

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

<root>

????????<title>

????????????????<font
enabled="false">

????????????????????????<!-- 字体名称 -->

????????????????????????<name>Helvetica</name>

????????????????????????<size>36</size>

????????????????</font>

????????????????<data>

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

????????????????</data>

????????</title>

</root>

可以通过对XPath表达式
/root/title/font/size
求值来得到
size
节点的值，示例代码如下：

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 表达式还提供了更丰富的语法来实现对XML
文档的筛选节点，节点是通过沿着路径 (path) 或者步 (steps) 来选取的：

• 选取节点：XPath 使用路径表达式在 XML 文档中选取节点。节点是通过沿着路径或者 step 来选取的

  下面列出了最有用的路径表达式：

示例代码如下：

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("----------------------------------------");

• 谓语：谓语用来查找某个特定的节点或者包含某个指定的值的节点，谓语被嵌在方括号中。

示例代码如下：

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("----------------------------------------");

• 运算符：

• 函数：

示例代码如下：

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[position()=2]/font/size/text()", xmlDocument);

System.out.println("XPath expression //title[position()=2]/font/size/text() value=" + xpathResult);

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

System.out.println("XPath expression concat(/root/title/font/name,root/title/font/size) value=" + xpathResult);

xpathResult = xPath.evaluate("concat((3.14),round(3.56),floor(3.14),floor(3.56))", xmlDocument);

System.out.println("XPath expression concat(round(3.14),round(3.56),floor(3.14),floor(3.56)) value=" + xpathResult);

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