Represent null value as empty element in xml jaxb

Question

I need to display null value as empty element in jaxb. I am using moxy implementation of jaxb. I found this option

@XmlNullPolicy(emptyNodeRepresentsNull = true, nullRepresentationForXml = XmlMarshalNullRepresentation.EMPTY_NODE)

Is there any similar extension that can be applied at Class level (for all elements defined in it)

Answer 1

I would strongly recommend representing null with either the absence of the node or with the xsi:nil="true" attribute. This works best with schema validation (i.e. <age/> or <age></age> is not a valid element of type xsd:int. However if you can't here is how you can accomplish your use case:

STANDARD JAXB BEHAVIOUR

Using the standard APIs you can control whether null is represented as an absent node or with xsi:nil="true" with the @XmlElement annotation (see: http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html).

import javax.xml.bind.annotation.*;

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

    private String street;

    @XmlElement(nillable=true)
    private String city;

}

Below is the XML output if the values of both fields are null.

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <city xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</address>

---

MOXy - OVERRIDING THIS BEHAVIOUR PER CLASS

MOXy does not provide an annotation to specify the null policy for all the properties on a class. However you can leverage a DescriptorCustomizer via the @XmlCustomizer annotation and tweak the native MOXy mapping metadata to accomplish the same thing.

DescriptorCustomizer (AddressCustomizer)

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.mappings.DatabaseMapping;
import org.eclipse.persistence.oxm.mappings.XMLDirectMapping;
import org.eclipse.persistence.oxm.mappings.nullpolicy.XMLNullRepresentationType;

public class AddressCustomizer implements DescriptorCustomizer {

    @Override
    public void customize(ClassDescriptor descriptor) throws Exception {
        for(DatabaseMapping mapping : descriptor.getMappings()) {
            if(mapping.isAbstractDirectMapping()) {
                XMLDirectMapping xmlDirectMapping = (XMLDirectMapping) mapping;
                xmlDirectMapping.getNullPolicy().setMarshalNullRepresentation(XMLNullRepresentationType.EMPTY_NODE);
                xmlDirectMapping.getNullPolicy().setNullRepresentedByEmptyNode(true);
            }
        }
    }

}

DomainModel (Address)

import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlCustomizer(AddressCustomizer.class)
public class Address {

    private String street;

    @XmlElement(nillable=true)
    private String city;

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <street/>
   <city/>
</address>

---

MOXy - OVERRIDING THIS BEHAVIOUR FOR ALL CLASSES

If instead you want to override null handling for all of the mapped classes I would recommend using a SessionEventListener instead. If you prefer you could also use this approach to update the metadata for a single class.

SessionEventListener (NullPolicySessionEventListener)

import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.mappings.DatabaseMapping;
import org.eclipse.persistence.oxm.mappings.XMLDirectMapping;
import org.eclipse.persistence.oxm.mappings.nullpolicy.XMLNullRepresentationType;
import org.eclipse.persistence.sessions.*;

public class NullPolicySessionEventListener extends SessionEventAdapter {

    @Override
    public void preLogin(SessionEvent event) {
        Project project = event.getSession().getProject();
        for(ClassDescriptor descriptor : project.getOrderedDescriptors()) {
            for(DatabaseMapping mapping : descriptor.getMappings()) {
                if(mapping.isAbstractDirectMapping()) {
                    XMLDirectMapping xmlDirectMapping = (XMLDirectMapping) mapping;
                    xmlDirectMapping.getNullPolicy().setMarshalNullRepresentation(XMLNullRepresentationType.EMPTY_NODE);
                    xmlDirectMapping.getNullPolicy().setNullRepresentedByEmptyNode(true);
                }
            }
        }
     }

}

Demo Code

import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextProperties;
import org.eclipse.persistence.sessions.SessionEventListener;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>(1);
        SessionEventListener sessionEventListener = new NullPolicySessionEventListener();
        properties.put(JAXBContextProperties.SESSION_EVENT_LISTENER, sessionEventListener);
        JAXBContext jc = JAXBContext.newInstance(new Class[] {Address.class}, properties);

        Address address = new Address();

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

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <street/>
   <city/>
</address>

Answer 2

A "bad practice" workaround if you have only String fields in the class is to override the setter for the element like this:

public class Address {

  private String street;

  @XmlElement(name = "street")
  public void setStreet(String street){
    this.street = street;
    if (this.street == null){
      this.street = ""; // Not NULL, empty string like new String()!
    }
  }
}

This wont work with other types like date or number! But sometimes String is enought.

@Blaise Doughan's answer seems much better in long term if you can deal with it. :)