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/

如何使用 Java 中的 xpath 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。

<?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。

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中。
• 使用其newInstance()静态方法创建XPathFactory。
• 从XPathFactory获取XPath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。

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：

[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

@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 + "]";
    }
}

@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 + "]";
    }
}

@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！