JAXB empty element unmarshalling

Question

The problem is in the following :

I get the soap response with empty element inside (e.g. ... <someDate /> ... ) and as a result exception is being throwed when JAXB wants to parse this element instead to set the appropriate field with null value.

How to configure JAXB to treat empty elements as null ? Can we do this with JAXB only (not using some third-party workarounds)

Base Problem

Empty String is not a valid value for the xsd:date type. To be valid with the XML schema an optional element should be represented as an absent node.,

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Why the Base Problem is Impacting You

All JAXB implementations will recognize that empty String is not a valid value for xsd:date. They do this by reporting it to an instance of ValidationEventHandler. You can see this yourself by doing the following:

    Unmarshaller unmarshaller = jc.createUnmarshaller();
    unmarshaller.setEventHandler(new ValidationEventHandler() {

        @Override
        public boolean handleEvent(ValidationEvent event) {
            System.out.println(event);
            return true;
        }
    });

The implementation of JAX-WS you are using, leverages EclipseLink MOXy as the JAXB provider. And in the version you are using MOXy will by default throw an exception when a ValidationEvent of severity ERROR is encountered instead of FATAL_ERROR like the reference implementation. This has since been fixed in the following bug:

• http://bugs.eclipse.org/369994

---

Work Around

If you are using the JAXB APIs directly you could simply override the default ValidationEventHandler. In a JAX-WS environment a XmlAdapter can be used to provide custom conversion logic. We will leverage an XmlAdapter to override how the conversion to/from Date is handled.

XmlAdapter (DateAdapter)

import java.text.SimpleDateFormat;
import java.util.Date;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class DateAdapter extends XmlAdapter<String, Date>{

    private SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");

    @Override
    public Date unmarshal(String v) throws Exception {
        if(v.length() == 0) {
            return null;
        }
        return dateFormat.parse(v);
    }

    @Override
    public String marshal(Date v) throws Exception {
        if(null == v) {
            return null;
        }
        return dateFormat.format(v);
    }

}

Java Model (Root)

The XmlAdapter is referenced using the @XmlJavaTypeAdapter annotation. If you wish this XmlAdapter to apply to all instances of Date you can register it at the package level (see: http://blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html).

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

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

    @XmlSchemaType(name = "date")
    @XmlJavaTypeAdapter(value=DateAdapter.class, type=Date.class)
    private Date abc;

    @XmlSchemaType(name="date")
    @XmlJavaTypeAdapter(value=DateAdapter.class, type=Date.class)
    private Date qwe;

}

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Demo Code

Below is a standalone example you can run to see that everything works.

jaxb.properties

In a standalone example to use MOXy as your JAXB provider you need to include a file called jaxb.propeties in the same package as your domain model with the following entry (see: http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html).

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

input.xml

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <abc></abc>
    <qwe>2013-09-05</qwe>
</root>

Demo

import java.io.File;
import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum18617998/input.xml");
        Root root = (Root) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(root, System.out);
    }

}

Output

Note that in the marshalled XML the Date field that was null was marshalled as an absent element (see: http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html).

<?xml version="1.0" encoding="UTF-8"?>
<root>
   <qwe>2013-09-05</qwe>
</root>