How to force Spring HATEOAS resources to render an empty embedded array?

Question

I have the following controller method:

@RequestMapping(produces = MediaType.APPLICATION_JSON_VALUE, value = "session/{id}/exercises")
public ResponseEntity<Resources<Exercise>> exercises(@PathVariable("id") Long id) {

  Optional<Session> opt = sessionRepository.findWithExercises(id);
  Set<Exercise> exercises = Sets.newLinkedHashSet();

  if (opt.isPresent()) {
    exercises.addAll(opt.get().getExercises());
  }

  Link link = entityLinks.linkFor(Session.class)
                         .slash(id)
                         .slash(Constants.Rels.EXERCISES)
                         .withSelfRel();

  return ResponseEntity.ok(new Resources<>(exercises, link));
}

So basically I am trying to get the expose a Set<> of Exercise entities for a particular Session. When the exercises entity is empty however I get a JSON representation like this:

{
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/sessions/2/exercises"
        }
    }
}

So basically there is no embedded entity, while something like the following would be preferrable:

{
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/sessions/2/exercises"
        }
    }, 
    "_embedded": {
        "exercises": [] 
    }    
}

any idea how to enforce this?

Answer 1

The problem here is that without additional effort there's no way to find out that the empty collection is a collection for Exercise. Spring HATEOAS has a helper class to work around this though:

EmbeddedWrappers wrappers = new EmbeddedWrappers(false);
EmbeddedWrapper wrapper = wrappers.emptyCollectionOf(Exercise.class);
Resources<Object> resources = new Resources<>(Arrays.asList(wrapper));

An EmbeddedWrapper allows you to explicitly mark objects to be added to the Resource or Resources as embedded, potentially even manually defining the rel they should be exposed under. As you can see above the helper also allows you to add an empty collection of a given type to the _embedded clause.

Answer 2

One can use the PagedResourceAssembler::toEmptyResource() method. For example, the following works:

Page<EWebProduct> products = elasticSearchTemplate.queryForPage(query, EWebProduct.class);

if(!products.hasContent()){
            PagedResources pagedResources = pageAssembler.toEmptyResource(products, WebProductResource.class,baseLink);
            return new ResponseEntity<PagedResources<WebProductResource>>(pagedResources, HttpStatus.OK);
}

I'd bet it works with other ResourceAssemblers as well.