How do I output null elements and attributes when outputting JSON using EclipseLink MOXy

In my XML I only output an element if it has a value, but the requirement for the JSON is to output the value even if not set

i.e Xml

<alias-list>
<alias sort-name="Afghan">Afghany</alias>
</alias-list>

I would like JSON to output other elements of the alias element even though not set

i.e

   "aliases" : [ {
      sort-name : "Afghan"
      begin-date : null
      end-date : null
      value : "Afghany"
   } ]

but all I can do is have a method that I use when ouputting json to set these null elements to empty string, this gives me

   "aliases" : [ {
      sort-name : "Afghan"
      begin-date : ""
      end-date : ""
      value : "Afghany"
   } ]

but that is not what I want

Update with attempt to use Denises answer

I've ran into a few problems with the proposed solution, first here is the full Alias class I think that would help (note the classes autogenned with JAXB, I came across this before ElipseLink and because my Xml output is okay Im not particulary keen to change this)

//
// This file was generated by the JavaTM Architecture for XML Binding(JAXB) Reference Implementation, vhudson-jaxb-ri-2.1-792 
// See <a href="http://java.sun.com/xml/jaxb">http://java.sun.com/xml/jaxb</a> 
// Any modifications to this file will be lost upon recompilation of the source schema. 
// Generated on: 2013.06.04 at 02:00:21 PM BST 
//


package org.musicbrainz.mmd2;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSchemaType;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.XmlValue;


/**
 * <p>Java class for anonymous complex type.
 * 
 * <p>The following schema fragment specifies the expected content contained within this class.
 * 
 * <pre>
 * &lt;complexType>
 *   &lt;complexContent>
 *     &lt;restriction base="{http://www.w3.org/2001/XMLSchema}anyType">
 *       &lt;attribute name="locale" type="{http://musicbrainz.org/ns/mmd-2.0#}def_iso-3166-2-code" />
 *       &lt;attribute name="sort-name" type="{http://www.w3.org/2001/XMLSchema}anySimpleType" />
 *       &lt;attribute name="type" type="{http://www.w3.org/2001/XMLSchema}anySimpleType" />
 *       &lt;attribute name="primary" type="{http://www.w3.org/2001/XMLSchema}anySimpleType" />
 *       &lt;attribute name="begin-date" type="{http://musicbrainz.org/ns/mmd-2.0#}def_incomplete-date" />
 *       &lt;attribute name="end-date" type="{http://musicbrainz.org/ns/mmd-2.0#}def_incomplete-date" />
 *     &lt;/restriction>
 *   &lt;/complexContent>
 * &lt;/complexType>
 * </pre>
 * 
 * 
 */
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
    "content"
})
@XmlRootElement(name = "alias")
public class Alias {

    @XmlValue
    protected String content;
    @XmlAttribute
    protected String locale;
    @XmlAttribute(name = "sort-name")
    @XmlSchemaType(name = "anySimpleType")
    protected String sortName;
    @XmlAttribute
    @XmlSchemaType(name = "anySimpleType")
    protected String type;
    @XmlAttribute
    @XmlSchemaType(name = "anySimpleType")
    protected String primary;
    @XmlAttribute(name = "begin-date")
    protected String beginDate;
    @XmlAttribute(name = "end-date")
    protected String endDate;

    /**
     * Gets the value of the content property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getContent() {
        return content;
    }

    /**
     * Sets the value of the content property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setContent(String value) {
        this.content = value;
    }

    /**
     * Gets the value of the locale property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getLocale() {
        return locale;
    }

    /**
     * Sets the value of the locale property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setLocale(String value) {
        this.locale = value;
    }

    /**
     * Gets the value of the sortName property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getSortName() {
        return sortName;
    }

    /**
     * Sets the value of the sortName property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setSortName(String value) {
        this.sortName = value;
    }

    /**
     * Gets the value of the type property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getType() {
        return type;
    }

    /**
     * Sets the value of the type property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setType(String value) {
        this.type = value;
    }

    /**
     * Gets the value of the primary property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getPrimary() {
        return primary;
    }

    /**
     * Sets the value of the primary property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setPrimary(String value) {
        this.primary = value;
    }

    /**
     * Gets the value of the beginDate property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getBeginDate() {
        return beginDate;
    }

    /**
     * Sets the value of the beginDate property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setBeginDate(String value) {
        this.beginDate = value;
    }

    /**
     * Gets the value of the endDate property.
     * 
     * @return
     *     possible object is
     *     {@link String }
     *     
     */
    public String getEndDate() {
        return endDate;
    }

    /**
     * Sets the value of the endDate property.
     * 
     * @param value
     *     allowed object is
     *     {@link String }
     *     
     */
    public void setEndDate(String value) {
        this.endDate = value;
    }

}

When I try the json binding solution I get

Exception Description: The property or field beginDate on the class org.musicbrainz.mmd2.Alias is required to be included in the propOrder element of the XmlType annotation.
 - with linked exception:
[Exception [EclipseLink-50013] (Eclipse Persistence Services - 2.5.0.v20130507-3faac2b): org.eclipse.persistence.exceptions.JAXBException
Exception Description: The property or field beginDate on the class org.musicbrainz.mmd2.Alias is required to be included in the propOrder element of the XmlType annotation.]
    at org.eclipse.persistence.jaxb.JAXBContext$TypeMappingInfoInput.createContextState(JAXBContext.java:1021)
    at org.eclipse.persistence.jaxb.JAXBContext.<init>(JAXBContext.java:174)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:165)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:152)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:112)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:102)
    at org.musicbrainz.search.servlet.mmd2.ResultsWriter.initJsonContext(ResultsWriter.java:100)
    ... 25 more

So i then edited Alias.java to add beginDate to propOrder, but this gave this error. Which is true - content is annotated with @XmlValue but I dont understand why that is a problem.

Exception Description: The property or field beginDate must be an attribute because another field or property is annotated with XmlValue.]
    at org.eclipse.persistence.jaxb.JAXBContext$TypeMappingInfoInput.createContextState(JAXBContext.java:1021)
    at org.eclipse.persistence.jaxb.JAXBContext.<init>(JAXBContext.java:174)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:165)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:152)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:112)
    at org.eclipse.persistence.jaxb.JAXBContextFactory.createContext(JAXBContextFactory.java:102)
    at org.musicbrainz.search.servlet.mmd2.ResultsWriter.initJsonContext(ResultsWriter.java:100)
    ... 30 more

Unfortunately I cannot use the @XmlElement option because beginDate is an attribute not an element and already has a @XmlAttribute annotation, and this does not seem to support nillable.

Solution

There is a nillable option on the @XmlElement annotation you can use to handle this. However if this is set then you will get what you want in JSON but the XML will add an xsi:nil attribute when the value is null. If you would like different behavior between JSON and XML you can use a bindings file instead of adding the annotation on your object. You will then create 2 JAXBContexts with different bindings files (or one with a bindings file and one without) to specify the different behavior.

With the updated information about Alias.java you will additionally want to clear out the prop-order (or you may list all elements and order as you like, I've just cleared it out in the oxm.xml example below). Also, since content has an @XmlValue annotation in the oxm.xml for the JSON binding it can be changed to be handled instead as a regular element (and name it value or whatever you want to call it).

Annotation Example

public class Alias{

  @XmlElement(name="begin-date", nillable= true)
  public String beginDate;
}

Bindings File Example (oxm.xml)

<xml-bindings xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm" package-name="mypackage.test">
    <java-types>
       <java-type name="Alias">
          <xml-type prop-order=""/>
          <java-attributes>
             <xml-element java-attribute="beginDate" name="begin-date" nillable="true"/>
             <xml-element java-attribute="content" name="value"/>
          </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

To create the JAXBContext with a bindings file you do the following

Map<String, Object> props = new HashMap<String, Object>();
StreamSource ss = new StreamSource(new File("pathtobindings/oxm.xml"));
props.put(JAXBContextProperties.OXM_METADATA_SOURCE, ss);
JAXBContext contextWithBindings = JAXBContext.newInstance(myClasses, props);