JAXB unmarshalling with java generics

Question

I am trying to unmarshall an XML document from a legacy system using JAXB. I have an xml structure as follows :

<response>
    <id>000000</id>
    <results>
        <result>
<!-- Request specific xml content -->
            <year>2003</year>
            <title>Lorem Ipsum</title>
            <items>
                <item>I1</item>
                <item>I2</item>
            </items>
        </result>
        <result>
            <year>2007</year>
            <title>Dolor sit amet</title>
            <items>
                <item>K1</item>
                <item>K2</item>
            </items>
        </result>
    </results>
</response>

The tags inside the part specified by <result> tag will change depending on my request. Since the content may change I decided to use generics for result items and I have prepared my java beans with annotations as following:

// imports here
@XmlRootElement(name="response")
@XmlAccessorType(XmlAccessType.FIELD)
public class XResponse<T>{
    private String id;

    @XmlElementWrapper(name="results")
    @XmlElement(name="result")
    private List<T> results;

// setters and getters
}

...

@XmlRootElement(name="result")
@XmlAccessorType(XmlAccessType.FIELD)
public class X1Result{
    private String year;
    private String title;
    @XmlElementWrapper(name="items")
    @XmlElement(name="item")
    private List<String> items;

// setters and getters
}
...

I tried unmarshalling the xml document via the code below:

JAXBContext context = JAXBContext.newInstance(XResponse.class, X1Result.class);
Unmarshaller um = context.createUnmarshaller();
XResponse<X1Result> response = (XResponse<X1Result>) um.unmarshal( xmlContent );

List<X1Result> results = unmarshal.getResults();
for (X1Result object : results) {
    System.out.println(object.getClass());
}

I have a problem during the unmarshalling that it can't cast the list items into X1Result class. Instead it uses org.apache.xerces.dom.ElementNSImpl.

What should I do to make JAXB Unmarshaller use X1Result class?

Thanks in advance

Answer 1

I think you should use inheritance instead of generics. Given an XML like this:

<?xml version="1.0" encoding="UTF-8"?>
<response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <id>000000</id>
    <results>
        <result xsi:type="X1Result">
            <year>2003</year>
            <title>Lorem Ipsum</title>
            <items>
                <item>I1</item>
                <item>I2</item>
            </items>
        </result>
        <result xsi:type="X1Result">
            <year>2007</year>
            <title>Dolor sit amet</title>
            <items>
                <item>K1</item>
                <item>K2</item>
            </items>
        </result>
    </results>
</response>

You can dynamically bind your <result> entries. You have a top-level type:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "XResult")
@XmlSeeAlso({
    X1Result.class
})public abstract class XResult {

}

And you have implementing classes:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "X1Result")
public class X1Result extends XResult {
    @XmlElement(name = "year")
    private String year;

    @XmlElement(name = "title")
    private String title;

    @XmlElementWrapper(name = "items")
    @XmlElement(name = "item")
    private List<String> items;
    ...
}

Use the top-level type in your XResponse class:

@XmlRootElement(name = "response")
@XmlAccessorType(XmlAccessType.FIELD)
public class XResponse {
    @XmlElement(name = "id")
    private String id;

    @XmlElementWrapper(name = "results")
    @XmlElement(name = "result")
    private List<XResult> results;
    ...
}

And you can unmarshall using the top-level type:

context = JAXBContext.newInstance(XResponse.class, XResult.class);
Unmarshaller unmarshaller = context.createUnmarshaller();
XResponse response = (XResponse) unmarshaller.unmarshal(new File("testfile.xml"));

List<XResult> results = response.getResults();
for (XResult object : results) {
    System.out.println(object.getClass());
}