JAXB - 仅为元素的特定子类型设置 XmlAdapter

JAXB（Java Architecture for XML Binding）是Java编程语言中用于将XML数据与Java对象之间进行转换的技术。通过使用JAXB，可以将XML文档转换为Java对象，反之亦然。在JAXB中，可以使用XmlAdapter将Java对象与XML元素之间进行映射。本文将介绍如何使用XmlAdapter为元素的特定子类型设置映射，并提供相应的案例代码。

java
public class Animal {
    // 父类Animal的属性和方法
}
public class Dog extends Animal {
    // 子类Dog的属性和方法
}
public class Cat extends Animal {
    // 子类Cat的属性和方法
}

java
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class AnimalTypeAdapter extends XmlAdapter {
    @Override
    public Animal unmarshal(String value) throws Exception {
        if (value.equals("dog")) {
            return new Dog();
        } else if (value.equals("cat")) {
            return new Cat();
        }
        return null;
    }
    @Override
    public String marshal(Animal animal) throws Exception {
        if (animal instanceof Dog) {
            return "dog";
        } else if (animal instanceof Cat) {
            return "cat";
        }
        return null;
    }
}

java
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
@XmlAccessorType(XmlAccessType.FIELD)
public class Animal {
    @XmlElement
    @XmlJavaTypeAdapter(AnimalTypeAdapter.class)
    private Animal type;
    
    // 其他属性和方法
}

xml
    dog

java
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;
import java.io.File;
public class Main {
    public static void main(String[] args) {
        try {
            File file = new File("animal.xml");
            JAXBContext jaxbContext = JAXBContext.newInstance(Animal.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Animal animal = (Animal) jaxbUnmarshaller.unmarshal(file);
            
            if (animal.getType() instanceof Dog) {
                System.out.println("This is a dog.");
            } else if (animal.getType() instanceof Cat) {
                System.out.println("This is a cat.");
            } else {
                System.out.println("This is an unknown animal.");
            }
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}