在 Java 中使用 xpath 查找具有属性值的 xml 元素 · HowToDoInJava 中文系列教程

# 在 Java 中使用 xpath 查找具有属性值的 xml 元素 > 原文： [https://howtodoinjava.com/xml/xpath-attribute-evaluate/](https://howtodoinjava.com/xml/xpath-attribute-evaluate/) 如何使用 Java 中的 [xpath](https://howtodoinjava.com/xml/java-xpath-tutorial-example/) 获取 xml 中的属性值的简单示例。我们将学习获取信息以匹配*属性值*，*属性值在*范围内，xpath 属性`contains()`等。 ## 1\. XPath 属性表达式 #### 1.1 输入 XML 文件首先查看我们将读取的 XML 文件，然后使用 **xpath 查询**从中获取信息。 ```java <?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> <employee id="3"> <firstName>Alex</firstName> <lastName>Kolenchisky</lastName> <department> <id>103</id> <name>FINANCE</name> </department> </employee> <employee id="4"> <firstName>Amit</firstName> <lastName>Jain</lastName> <department> <id>104</id> <name>HR</name> </department> </employee> <employee id="5"> <firstName>David</firstName> <lastName>Beckham</lastName> <department> <id>105</id> <name>DEVOPS</name> </department> </employee> <employee id="6"> <firstName>Virat</firstName> <lastName>Kohli</lastName> <department> <id>106</id> <name>DEVOPS</name> </department> </employee> <employee id="7"> <firstName>John</firstName> <lastName>Wick</lastName> <department> <id>107</id> <name>IT</name> </department> </employee> <employee id="8"> <firstName>Mike</firstName> <lastName>Anderson</lastName> <department> <id>108</id> <name>HR</name> </department> </employee> <employee id="9"> <firstName>Bob</firstName> <lastName>Sponge</lastName> <department> <id>109</id> <name>FINANCE</name> </department> </employee> <employee id="10"> <firstName>Gary</firstName> <lastName>Kasporov</lastName> <department> <id>110</id> <name>IT</name> </department> </employee> </employees> ``` #### 1.2 XPath 属性表达式示例现在来看几个有关如何构建 xpath 以便基于属性获取信息的示例。 | 描述 | XPath | 结果 | | --- | --- | --- | | 获取所有员工 ID | `/employees/employee/@id` | `[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]` | | 获取人力资源部门的所有员工 ID | `/employees/employee[department/name='HR']/@id` | `[2, 4, 8]` | | 获取员工编号“ Alex” | `/employees/employee[firstName='Alex']/@id` | `[3]` | | 获取大于 5 的员工 ID | `/employees/employee/@id[. > 5]` | `[6, 7, 8, 9, 10]` | | 获取其 ID 包含“1”的员工 | `/employees/employee[contains(@id,'1')]/firstName/text()` | `[Lokesh, Gary]` | | 获取其 ID 包含 1 的员工 | `descendant-or-self::*[contains(@id,'1')]/firstName/text()` | `[Lokesh, Gary]` | ## 2\. 使用 xpath 查找具有属性值的 xml 元素的 Java 示例我们来看一下用于求值以上 xpath 表达式以选择具有特定属性值的节点的代码。 #### 2.1 XPath 求值示例要**在 Java** 中求值 xpath，您需要执行以下步骤： * 将 XML 文件读取到[`org.w3c.dom.Document`](https://docs.oracle.com/javase/7/docs/api/org/w3c/dom/Document.html)中。 * 使用其`newInstance()`静态方法创建[`XPathFactory`](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPathFactory.html)。 * 从`XPathFactory`获取[`XPath`](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPath.html)实例。该对象提供对 xpath 求值环境和表达式的访问。 * 创建 xpath 表达式字符串。使用`xpath.compile()`方法将 xpath 字符串转换为[`XPathExpression`](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPathExpression.html)对象。 * 针对第一步中创建的文档实例求值 xpath。它将返回文档中的 DOM 节点列表。 * 使用`getNodeValue()`方法迭代节点并获得测试值。 > **XPath 表达式不是线程安全的**。确保在任何给定时间不从多个线程使用一个`XPathExpression`对象是应用的责任，并且在调用求值方法时，应用可能不会递归调用求值方法。 ```java package com.howtodoinjava.demo; import java.util.ArrayList; import java.util.List; 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 attribute values using xpath******/ //Get all employee ids xpathExpression = "/employees/employee/@id"; System.out.println( evaluateXPath(document, xpathExpression) ); //Get all employee ids in HR department xpathExpression = "/employees/employee[department/name='HR']/@id"; System.out.println( evaluateXPath(document, xpathExpression) ); //Get employee id of 'Alex' xpathExpression = "/employees/employee[firstName='Alex']/@id"; System.out.println( evaluateXPath(document, xpathExpression) ); //Get employee ids greater than 5 xpathExpression = "/employees/employee/@id[. > 5]"; System.out.println( evaluateXPath(document, xpathExpression) ); //Get employee whose id contains 1 xpathExpression = "/employees/employee[contains(@id,'1')]/firstName/text()"; System.out.println( evaluateXPath(document, xpathExpression) ); //Get employee whose id contains 1 xpathExpression = "descendant-or-self::*[contains(@id,'1')]/firstName/text()"; System.out.println( evaluateXPath(document, xpathExpression) ); } private static List<String> evaluateXPath(Document document, String xpathExpression) throws Exception { // Create XPathFactory object XPathFactory xpathFactory = XPathFactory.newInstance(); // Create XPath object XPath xpath = xpathFactory.newXPath(); List<String> values = new ArrayList<>(); try { // Create XPathExpression object XPathExpression expr = xpath.compile(xpathExpression); // Evaluate expression result on XML document NodeList nodes = (NodeList) expr.evaluate(document, XPathConstants.NODESET); for (int i = 0; i < nodes.getLength(); i++) { values.add(nodes.item(i).getNodeValue()); } } catch (XPathExpressionException e) { e.printStackTrace(); } return values; } 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： ```java [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] [2, 4, 8] [3] [6, 7, 8, 9, 10] [Lokesh, Gary] [Lokesh, Gary] ``` #### 2.2 模型类 ```java @XmlRootElement(name="employees") @XmlAccessorType(XmlAccessType.FIELD) public class Employees implements Serializable { private static final long serialVersionUID = 1L; @XmlElement(name="employee") private List<Employee> employees; public List<Employee> getEmployees() { if(employees == null) { employees = new ArrayList<Employee>(); } return employees; } public void setEmployees(List<Employee> employees) { this.employees = employees; } @Override public String toString() { return "Employees [employees=" + employees + "]"; } } ``` ```java @XmlRootElement(name="employee") @XmlAccessorType(XmlAccessType.FIELD) public class Employee implements Serializable { private static final long serialVersionUID = 1L; @XmlAttribute private Integer id; private String firstName; private String lastName; private Department department; public Employee() { super(); } public Employee(int id, String fName, String lName, Department department) { super(); this.id = id; this.firstName = fName; this.lastName = lName; this.department = department; } //Setters and Getters @Override public String toString() { return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + ", department=" + department + "]"; } } ``` ```java @XmlRootElement(name="department") @XmlAccessorType(XmlAccessType.FIELD) public class Department implements Serializable { private static final long serialVersionUID = 1L; Integer id; String name; public Department() { super(); } public Department(Integer id, String name) { super(); this.id = id; this.name = name; } //Setters and Getters @Override public String toString() { return "Department [id=" + id + ", name=" + name + "]"; } } ``` 向我提供有关如何使用 xpath 查找具有属性值的 xml 元素的问题。学习愉快！