Jersey - JSON response empty if polymorphism used

Question

I created a very simple RESTful servise using Jersey following this guide. I adjusted the generated sample code to have a polymorphic structure where Animal is the base class and Sheep and Goat inherit from it:

@XmlRootElement
public abstract class Animal {
    public Animal() {}
}

@XmlRootElement
public class Sheep extends Animal {
    public String fur;
    public Sheep() {this.fur = "curly";} 
}

@XmlRootElement
public class Goat extends Animal {
    public String color;
    public Goat() {this.color = "yellow";}
}

and following resource class which produces the response:

@Path("animal")
public class MyResource {

    @GET
    @Path("{animal}")
    @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
    public Animal getIt(@PathParam("animal") String animal) {
        return (animal.equals("sheep")) ? new Sheep() : new Goat();
    }
}

the project dependencies in my pom.xml are:

<dependencies>
    <dependency>
        <groupId>org.glassfish.jersey.containers</groupId>
        <artifactId>jersey-container-servlet-core</artifactId>
    </dependency>
    <dependency>
        <groupId>org.glassfish.jersey.media</groupId>
        <artifactId>jersey-media-moxy</artifactId>
    </dependency>
</dependencies>

When I run the application in Eclipse on Server (using Tomcat v8.0 if that matters) and try to request the XML response, e.g. in a browser, the output is correct like this:

<sheep>
    <fur>curly</fur>
</sheep>

or if I request a goat:

<goat>
    <color>yellow</color>
</goat>

but once I request the JSON response, the output is an empty JSON document: {} I use curl to get the JSON media type like this:

curl -H "Accept: application/json" http://localhost:8080/simple-service-webapp/api/animal/goat

What do I need to configure or change to get a JSON response representing the correct object? I'm new to this topic, I hope the question is clear. Thank you.

PS: If the return type of the method getIt is set to e.g. Sheep and Sheep object is returned, the JSON response is correct, so I suppose I need to map the inheritance somewhere somehow so it recognizes the returned type, but I can not find out what to do.

Answer 1

Seem this requires the use of @XmlSeeAlso

@XmlRootElement
@XmlSeeAlso({Sheep.class, Goat.class})
public abstract class Animal {
    public Animal() {}
}

C:\>curl -H "Accept:application/json" http://localhost:8080/api/animal/sheep
{"type":"sheep","fur":"curly"}