JAXB `Unmarshaller`（解组器）示例

原文： https://howtodoinjava.com/jaxb/jaxb-unmarshaller-example/

JAXB Unmarshaller接口负责管理将 XML 数据反序列化为 Java 对象的过程。 可以将对象解组到各种输入源。

1.如何将 XML 解组到对象

1.1 创建解组器

通常，要创建Unmarshaller，可以重用此代码。

JAXBContext jaxbContext     = JAXBContext.newInstance( Employee.class );
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
//Overloaded methods to unmarshal from different xml sources
Employee employee = (Employee) jaxbUnmarshaller.unmarshal( xmlSource );

1.2 从InputStream解组

InputStream inStream = new FileInputStream( "employee.xml" );
Employee employee = (Employee) jaxbUnmarshaller.unmarshal( inStream );

1.3 从 URL 解组

URL url = new URL( "http://localhost:8080/employee.xml" );
Employee employee = (Employee) jaxbUnmarshaller.unmarshal( url );

1.4 解组字符串内容

String xmlString = "...";
Employee employee = (Employee) jaxbUnmarshaller.unmarshal(new StringReader(xmlString));

1.5 从org.w3c.dom.Node解组

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);
DocumentBuilder db = dbf.newDocumentBuilder();
Document document = db.parse(new File( "employee.xml")); //Node
Employee employee = (Employee) jaxbUnmarshaller.unmarshal( document );

2. JAXB 解组器属性

当前，Unmarshaller上的所有 JAXB 供应器都不需要所有属性。 但是，某些供应器可能支持它们自己的一组供应器特定属性。

3.解组事件回调

您可以通过在 JAXB 注解的类中添加这些回调方法来自定义解组操作，例如Employee.java。 您需要定义两个方法，这些方法将在Unmarshaller处理该类之前和之后监听。

• void beforeUnmarshal(Unmarshaller unmarshaller, Object parent) {}
• void afterUnmarshal(Unmarshaller unmarshaller, Object parent) {}

package com.howtodoinjava.demo.model;
import java.io.Serializable;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement(name = "employee")
@XmlAccessorType(XmlAccessType.PROPERTY)
public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;
    private Integer id;
    private String firstName;
    private String lastName;
    private Department department;
    public Employee() {
        super();
    }
    //Setters and Getters
    @Override
    public String toString() {
        return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + ", department="
                + department + "]";
    }
    // It is called immediately after the object is created and before the unmarshalling begins.
    // The callback provides an opportunity to initialize JavaBean properties prior to unmarshalling.
    void beforeUnmarshal(Unmarshaller unmarshaller, Object parent) {
        System.out.println("Before Unmarshaller Callback");
    }
    // It is called after all the properties are unmarshalled for this object,
    // but before this object is set to the parent object.
    void afterUnmarshal(Unmarshaller unmarshaller, Object parent) {
        System.out.println("After Unmarshaller Callback");
    }
}

4. JAXB 解组器示例

将 XML 文件解组到 Java 对象的示例。

package com.howtodoinjava.demo;
import java.io.File;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;
import com.howtodoinjava.demo.model.Employee;
public class JaxbExample 
{
    public static void main(String[] args) 
    {
        String fileName = "employee.xml";
        jaxbXmlFileToObject(fileName);
    }
    private static void jaxbXmlFileToObject(String fileName) {
        File xmlFile = new File(fileName);
        JAXBContext jaxbContext;
        try 
        {
            jaxbContext = JAXBContext.newInstance(Employee.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Employee employee = (Employee) jaxbUnmarshaller.unmarshal(xmlFile);
            System.out.println(employee);
        }
        catch (JAXBException e) 
        {
            e.printStackTrace();
        }
    }
}

程序输出。

Before Unmarshaller Callback
After Unmarshaller Callback
Employee [id=1, firstName=Lokesh, lastName=Gupta, department=Department [id=101, name=IT]]

其中employee.xml文件是：

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

向我提出您的有关使用 JAXB 注解在 Java 中解组的问题。

学习愉快！