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 application reads the Xml and uses JAXB to create/configure a List of objects. I have never figured out (after reading various posts many times) the "right way" to do this using only JAXB.

For example, we have an interface Fee, and multiple concrete implementing classes which have some common properties, as well as some diverging properties, and very different behaviors. The Xml we use to configure the List of Fees used by application is:

<fees>
   <fee type="Commission" name="commission" rate="0.000125" />
   <fee type="FINRAPerShare" name="FINRA" rate="0.000119" />
   <fee type="SEC" name="SEC" rate="0.0000224" />
   <fee type="Route" name="ROUTES">
       <routes>
        <route>
            <name>NYSE</name>
            <rates>
                <billing code="2" rate="-.0014" normalized="A" />
                <billing code="1" rate=".0029" normalized="R" />
            </rates>
        </route>        
        </routes>
          ...
    </fee>
  </fees>

In the above Xml, each <fee> element corresponds to a concrete subclass of a Fee interface. The type attribute gives information about which type to instantiate, and then once it is instantiated, the JAXB unmarshalling applies the properties from the remaining Xml.

I always have to resort to doing something like this:

private void addFees(TradeFeeCalculator calculator) throws Exception {
    NodeList feeElements = configDocument.getElementsByTagName("fee");
    for (int i = 0; i < feeElements.getLength(); i++) {
        Element feeElement = (Element) feeElements.item(i);
        TradeFee fee = createFee(feeElement);
        calculator.add(fee);
    }
}

private TradeFee createFee(Element feeElement) {
    try {
        String type = feeElement.getAttribute("type");
        LOG.info("createFee(): creating TradeFee for type=" + type);
        Class<?> clazz = getClassFromType(type);
        TradeFee fee = (TradeFee) JAXBConfigurator.createAndConfigure(clazz, feeElement);
        return fee;
    } catch (Exception e) {
        throw new RuntimeException("Trade Fees are misconfigured, xml which caused this=" + XmlUtils.toString(feeElement), e);
    }
}

In the above code, the JAXBConfigurator is just a simple wrapper around the JAXB objects for unmarshalling:

public static Object createAndConfigure(Class<?> clazz, Node startNode) {
    try {
        JAXBContext context = JAXBContext.newInstance(clazz);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        @SuppressWarnings("rawtypes")
        JAXBElement configElement = unmarshaller.unmarshal(startNode, clazz);
        return configElement.getValue();
    } catch (JAXBException e) {
        throw new RuntimeException(e);
    }
}

At the end, of the above code, we get a List which contains whichever types were configured in the Xml.

Is there a way to get JAXB to do this automatically without having to write the code to iterate the elements as above?

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