Java JSON Mapping of List size 1

Question

I am mapping an object to JSON and have a problem with one variable called Parameters of type List <SimilarityParameter> .

SimilarityParameter looks like this:

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

    private String name;
    private String type;

    public SimilarityParameter() {

    }

    public SimilarityParameter(String name, String type) {
        this.name = name;
        this.type = type;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }
}

When this List is large everything is OK. The JSON looks like this

{
  "parameters":[
    {
      "name":"threshold",
      "type":"Double"
    },
    {
      "name":"numberOfResults",
      "type":"Integer"
    }
  ]
}

This is OK because after JSON encoding on the client side I have an array of Parameters.

The problem is when List is of size 1. Mapper transfers it logically to:

{
  "parameters":{
    "name":"numberOfResults",
    "type":"Integer"
  }
}

When the client decodes this JSON gets gets an array containing name and type . It causes inconsistencies on the client side.

I would like to map List of size 1 to this:

{
  "parameters":[
    {
      "name":"threshold",
      "type":"Double"
    }
  ]
}

So after encoding array containing one parameter.

This is how the response looks:

@XmlRootElement(name = "availableSimilarities")
@XmlAccessorType(XmlAccessType.FIELD)
public class SimilarityInfoResult {

    private String similarityName;
    private List <SimilarityParameter> parameters;

    public SimilarityInfoResult() {

    }

    public SimilarityInfoResult(String similarityName, List<SimilarityParameter> parameters) {
        this.similarityName = similarityName;
        this.parameters = parameters; 
    }

    public String getName() {
        return similarityName;
    }

    public void setName(String similarityName) {
        this.similarityName = similarityName;
    }

    public List<SimilarityParameter> getParameters() {
        return parameters;
    }

    public void setParameters(List<SimilarityParameter> parameters) {
        this.parameters = parameters;
    }


}

Is it possible to achieve this?

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

The problem is when List is of size 1. Mapper transfers it logically to:

The JAXB specification itself does not cover JSON-binding so sometimes implementations are used with libraries like Jettison which converts XML events to/from JSON and problems occur such as collections of size 1 being represented incorrectly. This is because Jettison (and similar) libraries can only detect collections when they see an element appear more than once.

• http://blog.bdoughan.com/2011/04/jaxb-and-json-via-jettison.html

---

Is it possible to achieve this?

The EclipseLink MOXy implementation of JAXB offers native JSON binding meaning that items like collections of size 1 will be properly converted to JSON.

jaxb.properties

To specify MOXy as your JAXB provider you need to include a file called jaxb.properites in the same package as your domain model with the following entry:

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

Demo

import java.util.*;
import javax.xml.bind.*;
import javax.xml.transform.stream.StreamSource;

import org.eclipse.persistence.jaxb.JAXBContextProperties;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>(2);
        properties.put(JAXBContextProperties.MEDIA_TYPE, "application/json");
        properties.put(JAXBContextProperties.JSON_INCLUDE_ROOT, false);
        JAXBContext jc = JAXBContext.newInstance(new Class[] {SimilarityInfoResult.class}, properties);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        StreamSource json = new StreamSource("src/forum15316288/input.json");
        SimilarityInfoResult result = unmarshaller.unmarshal(json, SimilarityInfoResult.class).getValue();

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

}

input.json/Output

{
   "parameters" : [ {
      "name" : "threshold",
      "type" : "Double"
   } ]
}

For More Information

• http://blog.bdoughan.com/2011/08/json-binding-with-eclipselink-moxy.html