How can I customize serialization of a list of JAXB objects to JSON?

Question

I'm using Jersey to create a REST web service for a server component.

The JAXB-annotated object I want to serialize in a list looks like this:

@XmlRootElement(name = "distribution")
@XmlType(name = "tDistribution", propOrder = {
    "id", "name"
})
public class XMLDistribution {
    private String id;
    private String name;
    // no-args constructor, getters, setters, etc
}

I have a REST resource to retrieve one distribution which looks like this:

@Path("/distribution/{id: [1-9][0-9]*}")
public class RESTDistribution {
    @GET
    @Produces("application/json")
    public XMLDistribution retrieve(@PathParam("id") String id) {
        return retrieveDistribution(Long.parseLong(id));
    }
    // business logic (retrieveDistribution(long))
}

I also have a REST resource to retrieve a list of all distributions, which looks like this:

@Path("/distributions")
public class RESTDistributions {
    @GET
    @Produces("application/json")
    public List<XMLDistribution> retrieveAll() {
        return retrieveDistributions();
    }
    // business logic (retrieveDistributions())
}

I use a ContextResolver to customize JAXB serialization, which is currently configured like this:

@Provider
@Produces("application/json")
public class JAXBJSONContextResolver implements ContextResolver<JAXBContext> {
    private JAXBContext context;
    public JAXBJSONContextResolver() throws Exception {
        JSONConfiguration.MappedBuilder b = JSONConfiguration.mapped();
        b.nonStrings("id");
        b.rootUnwrapping(true);
        b.arrays("distribution");
        context = new JSONJAXBContext(b.build(), XMLDistribution.class);
    }
    @Override
    public JAXBContext getContext(Class<?> objectType) {
        return context;
    }
}

Both REST resources work, as well as the context resolver. This is an example of output for the first one:

// path: /distribution/1
{
  "id": 1,
  "name": "Example Distribution"
}

Which is exactly what I want. This is an example of output for the list:

// path: /distributions
{
  "distribution": [{
    "id": 1,
    "name": "Sample Distribution 1"
  }, {
    "id": 2,
    "name": "Sample Distribution 2"
  }]
}

Which is not quite what I want.

I don't understand why there is an enclosing distribution tag there. I wanted to remove it with .rootUnwrapping(true) in the context resolver, but apparently that only removes another enclosing tag. This is the output with .rootUnwrapping(false):

// path: /distribution/1
{
  "distribution": {
    "id": 1,
    "name": "Example Distribution"
  }
} // not ok
// path: /distributions
{
  "xMLDistributions": {
    "distribution": [{
      "id": 1,
      "name": "Sample Distribution 1"
    }, {
      "id": 2,
      "name": "Sample Distribution 2"
    }]
  }
}

I also had to configure .arrays("distribution") to always get a JSON array, even with only one element.

Ideally, I'd like to have this as an output:

// path: /distribution/1
{
  "id": 1,
  "name": "Example Distribution"
} // currently works
// path: /distributions
[{
  "id": 1,
  "name": "Sample Distribution 1"
}, {
  "id": 2,
  "name": "Sample Distribution 2"
}]

I tried to return a List<XMLDistribution>, a XMLDistributionList (wrapper around a list), a XMLDistribution[], but I couldn't find a way to get a simple JSON array of distributions in my required format.

I also tried the other notations returned by JSONConfiguration.natural(), JSONConfiguration.mappedJettison(), etc, and couldn't get anything resembling what I need.

Does anyone know if it is possible to configure JAXB to do this?

Answer 1

I found a solution: replace the JAXB JSON serializer with a better behaved JSON serializer like Jackson. The easy way is to use jackson-jaxrs, which has already done it for you. The class is JacksonJsonProvider. All you have to do is edit your project's web.xml so that Jersey (or another JAX-RS implementation) scans for it. Here's what you need to add:

<init-param>
  <param-name>com.sun.jersey.config.property.packages</param-name>
  <param-value>your.project.packages;org.codehaus.jackson.jaxrs</param-value>
</init-param>

And that's all there is to it. Jackson will be used for JSON serialization, and it works the way you expect for lists and arrays.

The longer way is to write your own custom MessageBodyWriter registered to produce "application/json". Here's an example:

@Provider
@Produces("application/json")
public class JsonMessageBodyWriter implements MessageBodyWriter {
    @Override
    public long getSize(Object obj, Class type, Type genericType,
            Annotation[] annotations, MediaType mediaType) {
        return -1;
    }

    @Override
    public boolean isWriteable(Class type, Type genericType,
            Annotation annotations[], MediaType mediaType) {
        return true;
    }

    @Override
    public void writeTo(Object target, Class type, Type genericType,
            Annotation[] annotations, MediaType mediaType,
            MultivaluedMap httpHeaders, OutputStream outputStream)
            throws IOException {        
        new ObjectMapper().writeValue(outputStream, target);
    }
}

You'll need to make sure your web.xml includes the package, as for the ready-made solution above.

Either way: voila! You'll see properly formed JSON.

You can download Jackson from here: http://jackson.codehaus.org/