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Java XPath – 检查节点或属性是否存在？

Java XPath – 检查节点或属性是否存在？

原文： https://howtodoinjava.com/xml/xpath-check-if-xml-tag-exists/

Java 示例使用 XPath 检查给定 XML 内容中的节点是否存在或检查 XML 中的属性是否存在。

1.如何检查 xml 节点是否存在？

要验证 XML 内容中是否存在节点或标记，您可以针对该 XML 的 DOM 文档执行 xpath 表达式，并计算匹配的节点。

  1. matching nodes > zero – XML 标记/属性存在。
  2. matching nodes <= zero – XML 标记/属性不存在。

1.1 XML 文件

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<employees>
    <employee id="1">
        <firstName>Lokesh</firstName>
        <lastName>Gupta</lastName>
        <department>
            <id>101</id>
            <name>IT</name>
        </department>
    </employee>
    <employee id="2">
        <firstName>Brian</firstName>
        <lastName>Schultz</lastName>
        <department>
            <id>102</id>
            <name>HR</name>
        </department>
    </employee>
</employees>

1.2 使用 XPath 计数 XML 标签以检查是否存在

package com.howtodoinjava.demo;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.NodeList;

public class XPathExample {
	public static void main(String[] args) throws Exception {
		// Get DOM Node for XML
		String fileName = "employees.xml";
		Document document = getDocument(fileName);

		String xpathExpression = "";

		// Get all employee names
		xpathExpression = "/employees/employee/firstName";
		System.out.println(checkIfNodeExists(document, xpathExpression));	//true

		// Get all employee ids
		xpathExpression = "/employees/employee/@id";
		System.out.println(checkIfNodeExists(document, xpathExpression));	//true

		// Get all employee age
		xpathExpression = "/employees/employee/@age";
		System.out.println(checkIfNodeExists(document, xpathExpression));	//false

		// Get all department names
		xpathExpression = "/employees/employee/department/name";
		System.out.println(checkIfNodeExists(document, xpathExpression));	//true

		// Get department locations
		xpathExpression = "/employees/employee/department/location";
		System.out.println(checkIfNodeExists(document, xpathExpression));	//false
	}

	private static boolean checkIfNodeExists(Document document, String xpathExpression) throws Exception 
	{
		boolean matches = false;

		// Create XPathFactory object
		XPathFactory xpathFactory = XPathFactory.newInstance();

		// Create XPath object
		XPath xpath = xpathFactory.newXPath();

		try {
			// Create XPathExpression object
			XPathExpression expr = xpath.compile(xpathExpression);

			// Evaluate expression result on XML document
			NodeList nodes = (NodeList) expr.evaluate(document, XPathConstants.NODESET);

			if(nodes != null  && nodes.getLength() > 0) {
				matches = true;
			}

		} catch (XPathExpressionException e) {
			e.printStackTrace();
		}
		return matches;
	}

	private static Document getDocument(String fileName) throws Exception {
		DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
		factory.setNamespaceAware(true);
		DocumentBuilder builder = factory.newDocumentBuilder();
		Document doc = builder.parse(fileName);
		return doc;
	}	
}

程序输出：

true
true
false
true
false

阅读更多： Xpath 示例

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