# Java XPath 表達式示例 > 原文： [https://howtodoinjava.com/xml/java-xpath-expression-examples/](https://howtodoinjava.com/xml/java-xpath-expression-examples/) [Java xpath](https://howtodoinjava.com/xml/how-to-work-with-xpaths-in-java-with-examples/) 表達式示例，通過求值這些表達式從 XML 文檔中提取信息。 我們將學習獲取有關匹配屬性值，匹配字段值，`contains()`表達式等的信息。 ## 1\. XPath 查詢示例 #### 1.1 輸入 XML 文件 首先查看我們將讀取的 XML 文件，然后使用 **xpath 查詢**從中獲取信息。 ```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 表達式 現在來看幾個有關如何構建 xpath 以便基于字段和屬性的各種條件獲取信息的示例。 | 描述 | XPath | 結果 | | --- | --- | --- | | 獲取所有員工姓名 | `/employees/employee/firstName/text()` | `[Lokesh, Brian, Alex, Amit, David, Virat, John, Mike, Bob, Gary]` | | 獲取所有部門名稱 | `/employees/employee/department/name/text()` | `[IT, HR, FINANCE, HR, DEVOPS, DEVOPS, IT, HR, FINANCE, IT]` | | 獲取所有 IT 員工 | `/employees/employee[department/name='IT']/firstName/text()` | `[Lokesh, John, Gary]` | | 按編號獲取員工 | `/employees/employee[@id=4]/firstName/text()` | ` [Amit]` | | 取得 ID 大于 6 的員工 | `/employees/employee[@id>6]/firstName/text()` | `[John, Mike, Bob, Gary]` | | 獲取大衛的部門 | `/employees/employee[firstName = 'David']/department/name/text()` | `[DEVOPS]` | | 獲取所有員工 ID | `/employees/employee/@id` | ` [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]` | | 獲取人力資源部門的所有員工 ID | `/employees/employee[department/name='HR']/@id` | ` [2, 4, 8]` | | 獲取員工“`Alex`”編號 | `/employees/employee[firstName='Alex']/@id` | `[3]` | | 獲取大于 5 的員工 ID | `/employees/employee/@id[. > 5]` | `[6, 7, 8, 9, 10]` | | 獲取其 ID 包含“1”的員工 | `/employees/employee[contains(@id,'1')]/firstName/text()` | `[Lokesh, Gary]` | | 獲取其 ID 包含 1 的員工 | `descendant-or-self::*[contains(@id,'1')]/firstName/text()` | `[Lokesh, Gary]` | ## 2.求值 xpath 表達式的 Java 示例 讓我們看一下用于求值以上 xpath 表達式的代碼。 ```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; } } ``` 程序輸出： ```java [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] [2, 4, 8] [3] [6, 7, 8, 9, 10] [Lokesh, Gary] [Lokesh, Gary] ``` > **XPath 表達式不是線程安全的**。 確保在任何給定時間不從多個線程使用一個`XPathExpression`對象是應用的責任，并且在調用求值方法時，應用可能不會遞歸調用求值方法。 ## 3.模型類 ```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 + "]"; } } ``` 將我的問題放在評論部分。 學習愉快！ 參考文獻： [`XPathFactory` Java 文件](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPathFactory.html) [XPath Java 文件](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPath.html) [XPathExpression Java 文件](https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPathExpression.html) [org.w3c.dom.Document Java 文件](https://docs.oracle.com/javase/7/docs/api/org/w3c/dom/Document.html)