Team lead for the TopLink/EclipseLink JAXB & SDO implementations, and the Oracle representative on those specifications. Blaise is a DZone MVB and is not an employee of DZone and has posted 44 posts at DZone. You can read more from them at their website. View Full User Profile

Binding to JSON & XML - Handling Null

04.24.2012
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In a previous post I demonstrated how EclipseLink MOXy can be leveraged to produce both XML and JSON representations of your domain model.  The same metadata is used for both representations and MOXy applies it to leverage the capabilities of the media type.  In this post I'll focus on how null is handled in each of these representations.

Domain Model

By default a JAXB (JSR-222) implementation will not include a mapped field/property with a null value in the output.  If you want the null value represented then you simply add the following annotation @XmlElement(nillable=true).

package blog.json.nillable;

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Customer {

    private String firstName;
    private String middleName;
    private String lastName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getMiddleName() {
        return middleName;
    }

    public void setMiddleName(String middleName) {
        this.middleName = middleName;
    }

    @XmlElement(nillable=true)
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

}

Demo

In the demo code below we will set both the middleName and lastName properties to null. Since we have mapped these properties differently in the domain model we will examine the output to see the impact of using @XmlElement(nillable=true).

package blog.json.nillable;

import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        Customer customer = new Customer();
        customer.setFirstName("Jane");
        customer.setMiddleName(null);
        customer.setLastName(null);
        
        JAXBContext jc = JAXBContext.newInstance(Customer.class);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        // Output XML
        marshaller.marshal(customer, System.out);

        // Output JSON
        marshaller.setProperty("eclipselink.media-type", "application/json");
        marshaller.marshal(customer, System.out);
    }

}

XML Output

By default JAXB implementations do not include null values in the output, so there is no element corresponding to the middleName property.  Since we annotated the lastName property with @XmlElement(nillable=true) and it had a null value, it is represented in the output.  In XML an element with a null value is represented by adding the xsi:nil="true" attribute.

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

JSON Output

Just like in the XML output, an entry for the middleName property does not appear in the JSON output. Since we annotated the lastName property with @XmlElement(nillable=true) and it had a null value, it is represented in the output.  In JSON a null value is represented with null

{
   "customer" : {
      "firstName" : "Jane",
      "lastName" : null
   }
}
Published at DZone with permission of Blaise Doughan, author and DZone MVB. (source)

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