Jersey Jackson Custom Response with Generics in JSON and XML

Question

I've been trying for a while to come up with a way to return a custom response object with a data attribute that is either an object or a list. Here are some simplified relevant classes. I'm using Jersey v2.27 and Jackson v2.9+

@XmlRootElement(name = "response")
public class ResponseEnvelope<T> {
  private Metadata metadata;
  private T data;

  public ResponseEnvelope(Metadata metadata, T data) {
      this.metadata = metadata;
      this.data = data;
  }
}

@XmlRootElement
public class Person {
  private String name;
  private int age;

  public Person(int age, String name) {
      this.age = age;
      this.name = name;
  }
}

@XmlRootElement
public class ListWrapper<T> {
  private List<T> list;

  public ListWrapper(List<T> list) {this.list = list;}
}

@Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
public class PersonResource {
    private List<Person> getPeople() {
        return Arrays.asList(new Person(20, "Monty Python"), new Person(25, "Brian");
    }

    @GET
    public Response getAllPeople() {
       ResponseEnvelope<ListWrapper<Person> response = new ResponseEnvelope<ListWrapper<Person>>(new Metadata(), new ListWrapper(getPeople());
       return Response.status(200).entity(response).build();
    }

    @GET
    @Path("{id}")
    public Response getExampleById(@PathParam("id") String id) {
        ResponseEnvelope<Person> response = new ResponseEnvelope<Person>(new Metadata(), getPeople().get(0));
       return Response.status(200).entity(response).build();
    }

}

@Provider
@Produces(MediaType.APPLICATION_XML)
public class CustomResolver implements ContextResolver<JAXBContext> {
  private JAXBContext context;

  public CustomResolver() {
    try {
      this.context = JAXBContext.newInstance(
        ArrayList.class,
        Person.class,
        ListWrapper.class,
        ResponseEnvelope.class
      );
    }
    catch (JAXBException e) {
      throw new RuntimeException(e);
    }
  }

  @Override
  public JAXBContext getContext(Class<?> type) {
    return (type.equals(ResponseEnvelope.class)) ? context : null;
  }
}

@ApplicationPath("")
public class ServerConfig extends ResourceConfig {
  public ServerConfig() {
    this.packages(true,"com.example");
    this.register(JacksonFeature.class);
  }
}

I've tried things from several tutorials/questions, such as from here, here, here, etc. (I've tried a lot...)

I can't seem to get a unified response between the two formats. When I try to use a List directly as T data, JAXB complains about not having context for ArrayList (when I do put it in there, it doesn't output the elements themselves).

When I use the GenericEntity class directly in the response, such as with

 GenericEntity<List<Person>> list = new GenericEntity<List<Person>>(people){};
 return Response.status(200).entity(list).build();`

The output is correct for both json and xml. However if I use this in my T data field, I get a JAXB exception that it can't find my resource class (it seems you can't use GenericEntity in a generic class?).

If I use a generic ListWrapper class to put my collections in, I'm able to create valid xml and json, and using various annotations (@XmlAnyElement(lax = true) or @XmlElements({@XmlElement(name = "person", type = Person.class)}) I can get the XML formatted properly, but the json includes the extra ListWrapper<T> list property (i.e. {"data": {"list": []}}, instead of {"data": []}. I've tried using @JsonUnwrapped for this, and using @JsonSerialize with a custom serializer, but I can't seem to flatten it out.

A desired sample response for a response for a unique Person id

{
    "metadata": {
        // some content
    },
    "data": {
        "age": 20,
        "name": "Monty Python"
    }
}

and XML

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<response>
    <data>
        <age>20</age>
        <name>Monty Python</name>
    </data>
    <metadata>
        <!-- some content -->
    </metadata>
</response>

and for a list of people:

{
    "metadata": {
        // some content
    },
    "data": [
       {
            "age": 20,
            "name": "Monty Python"
       },
       {
            "age": 25,
            "name": "Brian"
       }
    ]
}

and XML

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<response>
    <data>
        <person>
            <age>20</age>
            <name>Monty Python</name>
        </person>
        <person>
            <age>25</age>
            <name>Brian</name>
        </person>
    </data>
    <metadata>
        <!-- some content -->
    </metadata>
</response>

Answer 1

The answer was to use the ListWrapper<T> class, and to annotate the getter with @JsonValue, along with the @XmlAnyElement(lax = true). These create the properly formatted JSON and XML responses. The class should look like this

import java.util.List;

import javax.xml.bind.annotation.XmlAnyElement;
import javax.xml.bind.annotation.XmlRootElement;

import com.fasterxml.jackson.annotation.JsonValue;

@XmlRootElement(name = "data")
public class ListWrapper<T> {
  private List<T> list;

  public ListWrapper() {}

  public ListWrapper(List<T> list) {
    this.list = list;
  }

  /**
   * @return the list
   */
  @XmlAnyElement(lax = true)
  @JsonValue
  public List<T> getList() {
    return list;
  }

  /**
   * @param list
   *               the list to set
   */
  public void setList(List<T> list) {
    this.list = list;
  }
}