JAXB: how to unmarshal a List of objects of different types but with common parent?

Question

There is a fairly common pattern in our applications. We configure a configure a set (or list) of objects in Xml, which all implement a common interface. On start-up, the

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

If you are using MOXy as your JAXB provider then you could use the MOXy's @XmlPaths annotation to extend the standard JAXB @XmlElements annotation to do the following:

Fees

import java.util.List;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.*;

@XmlRootElement
public class Fees {

    @XmlElements({
        @XmlElement(type=Commission.class),
        @XmlElement(type=FINRAPerShare.class),
        @XmlElement(type=SEC.class),
        @XmlElement(type=Route.class)
    })
    @XmlPaths({
        @XmlPath("fee[@type='Commission']"),
        @XmlPath("fee[@type='FINRAPerShare']"),
        @XmlPath("fee[@type='SEC']"),
        @XmlPath("fee[@type='Route']")
    })
    private List<Fee> fees;

}

Commission

The implementations of the Fee interface would be annotated normally.

import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
public class Commission implements Fee {

    @XmlAttribute
    private String name;

    @XmlAttribute
    private String rate;

}

For More Information

• http://blog.bdoughan.com/2011/03/map-to-element-based-on-attribute-value.html
• http://blog.bdoughan.com/2010/10/jaxb-and-xsd-choice-xmlelements.html
• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html

Answer 2

I don't think this is possible if all the elements are named <fee>. Even if it were(or is) it would be very confusing from maintenance point of view.

Do you have the ability to rename various fee elements based on type (e.g. <tradeFee> instead of <fee>)?

Otherwise you can create a BaseFee class that has all the fields for every possible type of <fee>. You can unmarshall data into a list of BaseFee objects and convert them into a more specific type at runtime, e.g.

List<BaseFee> fees = ...;
for (BaseFee fee : fees) {
    if (isTradeFee(fee)) {
        TradeFee tradeFee = toTradeFee(fee);
        // do something with trade fee...
    }
}

A bit of a hack but given the requirements it should do the job.

Answer 3

You could use an XmlAdapter for this use case. The impl bleow handles just the Commission type but could be easily extended to support all the types. You need to ensure that AdaptedFee contains the combined properties from all the implementations of the Fee interface.

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class FeeAdapter extends XmlAdapter<FeeAdapter.AdaptedFee, Fee>{

    public static class AdaptedFee {

        @XmlAttribute
        public String type;

        @XmlAttribute
        public String name;

        @XmlAttribute
        public String rate;

    }

    @Override
    public AdaptedFee marshal(Fee fee) throws Exception {
        AdaptedFee adaptedFee = new AdaptedFee();
        if(fee instanceof Commission) {
            Commission commission = (Commission) fee;
            adaptedFee.type = "Commission";
            adaptedFee.name = commission.name;
            adaptedFee.rate = commission.rate;
        }
        return adaptedFee;
    }

    @Override
    public Fee unmarshal(AdaptedFee adaptedFee) throws Exception {
        if("Commission".equals(adaptedFee.type)) {
            Commission commission = new Commission();
            commission.name = adaptedFee.name;
            commission.rate = adaptedFee.rate;
            return commission;
        }
        return null;
    }

}

An XmlAdapter is configured using the @XmlJavaTypeAdapter annotation:

import java.util.List;
import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Fees {

    @XmlElement(name="fee")
    @XmlJavaTypeAdapter(FeeAdapter.class)
    private List<Fee> fees;

}

For More Information

• http://blog.bdoughan.com/2012/01/jaxb-and-inhertiance-using-xmladapter.html