Marshalling/Unmarshalling Java superclass and subclasses using JAXB

问题

I've been experimenting with JAXB tutorials and have managed to get code working that generates an XML file from a Java object and then is able to use the XML to generate a Java object. At the moment it reads multiple instances of the same class to create an XML file similar to the one below

<Car>
    <regplate>TR54</regplate>
    <colour>red</colour>
    <energyrating>5</energyrating>
</Car>
<Car>
    <regplate>BN04 THY</regplate>
    <colour>yellow</colour>
    <energyrating>3</energyrating>
</Car>
<Car>
    <regplate>BN05 THY</regplate>
    <colour>yellow</colour>
    <energyrating>5</energyrating>
</Car>

I would like to be able to use the JAXB technology to work with subclasses. For example: Say I have a Car, Van and Bicycle objects that are subclasses of Vehicle. Is it possible for me to manipulate my JAXB class to write an XML file that would produce something similar to this? I have provided the code I am working with below.

<Vehicle>
    <Car>
        <regplate>TR54</regplate>
        <colour>red</colour>
        <energyrating>5</energyrating>
    </Car>
    <Van>
        <regplate>MN05 RFD</regplate>
        <colour>red</colour>
        <energyrating>5</energyrating>
    </Van>
    <Car>
        <regplate>ZX54 UJK</regplate>
        <colour>red</colour>
        <energyrating>1</energyrating>
    </Car>
</Vehicle>

Main Class

package basictransport2;

public class Main
{
    public static void main(String[] args)
    {
        JAXB parser = new JAXB();
        parser.marshall();
        //parser.unmarshallList();
    }
}

Vehicle Class

package basictransport2;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

//@XmlRootElement(name = "Vehicle")
public class Vehicle
{
    private int ownerId;

    public Vehicle(int ownerId)
    {
        this.setOwnerId(ownerId);
    }

    //@XmlElement (name = "Owner ID")
    public int getOwnerId()
    {
        return ownerId;
    }

    public void setOwnerId(int ownerId)
    {
        this.ownerId = ownerId;
    }


    public int getEnergyRating()
    {
        return (Integer) null;
    }


    public String getColour()
    {
        return null;
    }

    public String getRegPlate()
    {
        return null;
    }
}

Car Class

package basictransport2;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

//@XmlRootElement(name = "Car")
public class Car extends Vehicle
{
    private String regPlate;
    private int energyRating;
    private String colour;

    public Car(String regPlate, int energyRating, String colour, int ownerId)
    {
        super(ownerId);
        this.regPlate = regPlate;
        this.energyRating = energyRating;
        this.colour = colour;
    } 

    public Car(int ownerId)
    {
        super(ownerId);
    }

    //@XmlElement (name = "Registration")
    public String getRegPlate()
    {
        return regPlate;
    }

    public void setRegPlate(String regPlate)
    {
        if(this.regPlate == null)
        {
            this.regPlate = regPlate;
        }
    }

    //@XmlElement (name = "Energy Rating")
    public int getEnergyRating()
    {
        return energyRating;
    }

    public void setEnergyRating(int energyRating)
    {
        this.energyRating = energyRating;
    }

    //@XmlElement (name = "Colour")
    public String getColour()
    {
        return colour;
    }

    public void setColour(String colour)
    {
        this.colour = colour;
    }
}

JAXB Class

package basictransport2;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class JAXB
{   
    public void marshall()
    {
        try
        {
            List<Vehicle> vehicleList = new ArrayList<Vehicle>();

            vehicleList.add(new Car("SG09 TYH", 4, "Yellow", 1));
            vehicleList.add(new Car("XX09 VVV", 3, "Red", 2));
            vehicleList.add(new Car("BL09 TYZ", 4, "Blue", 3));

            Garage listOfVehicles = new Garage();
            listOfVehicles.setListOfVehicles(vehicleList);

            JAXBContext context = JAXBContext.newInstance(Garage.class);
            Marshaller marshaller = context.createMarshaller();
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            marshaller.marshal(listOfVehicles, System.out);
            marshaller.marshal(listOfVehicles, new File("src\\data\\listcar.xml"));
        }

        catch(Exception e)
        {
            System.out.println(e.getMessage());
        }
    }

    public void unmarshall()
    {
        try
        {
            JAXBContext context = JAXBContext.newInstance(Garage.class);
            Unmarshaller unmarhsaller = context.createUnmarshaller();
            Garage listOfVehicles = (Garage)unmarhsaller.unmarshal(new File("src\\data\\listcar.xml"));
            System.out.println("List Car information");

            for(Vehicle vehicle : listOfVehicles.getListOfVehicles())
            {
                System.out.println("Reg Plate: " + vehicle.getRegPlate());
                System.out.println("Energy Rating: " + vehicle.getEnergyRating());
                System.out.println("Colour: " + vehicle.getColour());
                System.out.println("================");
            }    
        }

        catch (Exception e)
        {
            System.out.println(e.getMessage());
        }
    }
}

List class

package basictransport2;

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElements;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSeeAlso;
import javax.xml.bind.annotation.XmlType;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name="Vehicle")
public class Garage
{
    @XmlElements
    ({
        @XmlElement(name = "Car", type = Car.class, required = false)
    })    

    private List<Vehicle> vehicleCollection = new ArrayList<Vehicle>();

    public List<Vehicle> getListOfVehicles()
    {
        return vehicleCollection;
    }

    public void setListOfVehicles(List<Vehicle> listOfVehicles)
    {
        this.vehicleCollection = listOfVehicles;
    }
}

回答1:

Thanks everyone for your input. I used feedback from all your answers but ultimately it was a combination of them that worked which is why I created a seperate answer for anyone who may have this problem in the future.

To get this to work I had to ensure that all getter methods within the super and sub classes being marhsalled/unmarshalled were annotated with @XmlElement. This would determine the XML tag for the corresponding variable.

@XmlElement (name = "OwnerID")
    public int getOwnerId()
    {
        return ownerId;
    }

The superclass had to be annotated with @XmlSeeAlso to bind the subclasses to it. i.e In my code RoadVehicle was the superclass and both the Car and Van classes extended it.

@XmlSeeAlso({Car.class, Van.class})
public class Vehicle
{

With the super and subclasses now annotated the only other class that required annotations was the list class (Garage in my code). The changes here would determine what the XML tags were populated with.

The root XML tag was set by applying the @XmlRootElement annotation to the top of the class. i.e. "Vehicle" would be the root XML tag in my example.

@XmlRootElement(name = "Vehicle")
public class Garage
{

Finally an @XmlElements list had to be declared with an @XmlElements annotation for each sub class that required an XML tag with the name supplying the name of the XML tag. This list had to be declared above the getter method for the collection.

@XmlElements
    ({
        @XmlElement(name = "Car", type = Car.class, required = false),
        @XmlElement(name = "Van", type = Van.class, required = false)
    })    
    public List<Vehicle> getListOfVehicles()
    {
        return vehicleCollection;
    }

回答2:

you are on right track. May something below will help

@XmlRootElement(name = "car")
public class Car extends BasicType{

}

@XmlRootElement(name = "van")
public class Van extends BasicType{

}

@XmlRootElement(name = "vehicle")
    public class Vehicle {
         List<BasicType> basicType;


    }

回答3:

The simplest solution is to have different subclasses for cars and vans, even it they don't add anything to the base classes. Then, the root element class contains a list of the base class, with element QNames identifying the actual class.

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Vehicle")
public class Vehicle {
    @XmlElements({
        @XmlElement(name = "Car", type = Car.class, required = false),
        @XmlElement(name = "Van", type = Van.class, required = false)
    })
    protected List carOrVan;
    public List getCarOrVan() {
        if (carOrVan == null) {
            carOrVan = new ArrayList();
        }
        return this.carOrVan;
    }
}

Here's the base class and the subclasses:

public class Basic {
    private String regplate;
    private String color;
    private String energyrating;

    public String getRegplate(){ return regplate; }
    public void setRegplate( String v ){ regplate = v; }
    public String getColor(){ return color; }
    public void setColor( String v ){ color = v; }
    public String getEnergyrating(){ return energyrating; }
    public void setEnergyrating( String v ){ energyrating = v; }
}

public class Car extends Basic {}

public class Van extends Basic {}

This will go smoothly if cars and vans develop into distinct subclasses.

来源：https://stackoverflow.com/questions/33369790/marshalling-unmarshalling-java-superclass-and-subclasses-using-jaxb

标签

java

xml

jaxb

unmarshalling