Question

Similar to this question asked on Stackoverflow, using JAXB to represent a list as the root element, I have the same issue. Though the solution outlined in the answer doesn’t work for me.

Note: I am using the Jackson fasterxml lib for doing the mappings.

Essentialy I am consuming some XML from an API (the recurly api). One of the messages returned by the API has the root element as a list, a demonstration is shown below:

<plans type="array">
    <plan href="...">
    ...
    </plan>
    <plan href="...">
    ...
    </plan>
</plans>

I created the following Java class + JAXB annotations to capture the above:

@XmlRootElement(name = "plans")
public class Plans extends RecurlyObject {

    @XmlElement(name = "plan", type = Plan.class)
    private List<Plan> plans;

    public List<Plan> getPlans() { return this.plans; }

    public void setPlans(final List<Plan> plans) { this.plans = plans; }
    ...
}

This doesn’t work. The docs & articles I have read all demo the above using Strings rather than custom types for the elements of the list.

Running the above generates the following error:

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value
of type [simple type, class com.ning.billing.recurly.model.Plan] from JSON String; no
single-String constructor/factory method (through reference chain:
com.ning.billing.recurly.model.Plans[“plan”])

I also tried the following – but generates a similar issue:

@XmlRootElement(name = "plans")
public class Plans extends RecurlyObject {
    @XmlTransient
    public static final String PLANS_RESOURCE = "/plans";

    @XmlElementWrapper(name = "plans")
    @XmlElement(name = "plan", type = Plan.class)
    private List<Plan> plans;

    public List<Plan> getPlans() { return this.plans; }

    public void setPlans(final List<Plan> plans) { this.plans = plans; }
    ...
}

The only way I can it to not error is if I create a ctor on my Plan class that takes a String. But then my List<Plan> is a list of Plan objects, one for every sub node of all of the Plans 🙁

BTW – the Plan Java class works fine – I use it to receive individual Plan messages from other API calls.

So, any ideas how to correctly map a root element which is a list of things other than Strings into a Java object using JAXB?

Cheers

UPDATE

The following solution was provided by Pierre outside of stackoverflow.

It is possible to get this to work with Jackson see the following code:

@XmlRootElement(name = "plans")
public class Plans extends RecurlyObjects<Plan> {
}

The important details to note is extends RecurlyObjects<Plan>. RecurlyObjects is essentially an ArrayList.

@JsonIgnoreProperties(ignoreUnknown = true)
public abstract class RecurlyObjects<T extends RecurlyObject> extends ArrayList<T> {
}

This now works fine.

Answer 1

By default you should annotate the property. I’ve simplified your mapping where possible.

package forum11748159;

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

@XmlRootElement
public class Plans extends RecurlyObject {

    private List<Plan> plans;

    @XmlElement(name = "plan")
    public List<Plan> getPlans() { return this.plans; }

    public void setPlans(final List<Plan> plans) { this.plans = plans; }

}

If you want to annotate the fields you need to add the @XmlAccessorType(XmlAccessType.FIELD) annotation to your class:

package forum11748159;

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

@XmlRootElement(name = "plans")
@XmlAccessorType(XmlAccessType.FIELD)
public class Plans extends RecurlyObject {

    @XmlElement(name = "plan")
    private List<Plan> plans;

    public List<Plan> getPlans() { return this.plans; }

    public void setPlans(final List<Plan> plans) { this.plans = plans; }

}

---

Note: I am using the Jackson fasterxml lib for doing the mappings.

Jackson offers some support for JAXB annotations, but as far as I know is not a JAXB (JSR-222) compliant implementation. It is not guaranteed to interpret JAXB annotations according to the specification.