Collections not embedded with HAL compatible vendor specific media type

[arhohuttunen]

I have been searching for information about vendor specific media types that are HAL compatible, and found several issues related to this. Apparently it is possible to easily add custom media types.

However, I'm unable to make returned collections to be embedded.

Taking the CustomHyperMediaWebMvcTest as an example, and adding a collection:

@GetMapping("/employees")
public CollectionModel<Employee> findOne() {
    List<EntityModel<Employee>> employees = new ArrayList();
    employees.add(new EntityModel(new Employee("Frodo Baggins", "ring bearer"),
            linkTo(methodOn(EmployeeController.class).findOne()).withSelfRel()));
    return new CollectionModel(employees);
}

This will return:

{
  "links": [],
  "content": [
    {
      "name": "Frodo Baggins",
      "role": "ring bearer",
      "links": [
        {
          "rel": "self",
          "href": "http://localhost/employees"
        }
      ]
    }
  ]
}

Making the controller endpoint explicitly produce application/frodo+json has no effect either. However, making the endpoint produce application/hal+json works as expected:

{
  "_embedded": {
    "employees": [
      {
        "name": "Frodo Baggins",
        "role": "ring bearer",
        "_links": {
          "self": {
            "href": "http://localhost/employees"
          }
        }
      }
    ]
  }
}

Is there something fundamental I am missing about the configuration, or is this an actual bug?

[odrotbohm]

What's the client sending as Accept header? You should definitely see this working when it explicitly requests application/hal+json.

[arhohuttunen]

The client is sending application/frodo+json as the Accept header. Basically it's the same as CustomHyperMediaWebMvcTest but with a collection.

I also tried adding Jackson2HalModule into CustomHyperMediaType, which should add the serializers for HAL if I understood correctly, but it didn't seem to help.

Client explicitly requesting for application/hal+json works, but that is not the intent.

edit: I'm actually running this now from CustomHyperMediaWebMvcTest with this result.

[gregturn]

Can you post your custom media type?

[arhohuttunen]

public class CustomHypermediaType implements HypermediaMappingInformation {
    public static final MediaType FRODO_MEDIATYPE = MediaType.parseMediaType("application/frodo+json");

    @Override
    public List<MediaType> getMediaTypes() {
        return Collections.singletonList(FRODO_MEDIATYPE);
    }

    @Override
    public Module getJacksonModule() {
        return new Jackson2HalModule();
    }
}

[gregturn]

And that class is registered as an @Bean somewhere inside an @Configuration class?

[gregturn]

The behavior you describe smells like what happens when you request a mediatype that isn't registered.

Spring Framework and it's default converters will bypass HAL and end up with a default handler that responds to application/*+json, which matches applicatio/frodo+json. It then invokes Jackon's "default" rendering, producing what you initially pasted.

Hence, it implies your CustomHypermediaType isn't getting registered in the application context.

[arhohuttunen]

Sure, the bean is registered right here in https://github.com/spring-projects/spring-hateoas/blob/master/src/test/java/org/springframework/hateoas/config/CustomHypermediaWebMvcTest.java.

I am not running this from my own code. I am running the test code from this repository, with the modifications described above.

[gregturn]

Okay, I missed that you were tinkering inside our code.

Digging in, I saw what you saw. Which intrigued me. I tried Jackson2UberModule, and Jackson2CollectionJsonModule, and those two worked perfectly. I looked for other test cases that used Jackson2HalModule, and unearthed what was missing.

Basically, you need this:

public class CustomHypermediaType implements HypermediaMappingInformation {

	public static final MediaType FRODO_MEDIATYPE = MediaType.parseMediaType("application/frodo+json");

	/**
	 * {@link MediaType}s this hypermedia can handle.
	 */
	@Override
	public List<MediaType> getMediaTypes() {
		return Collections.singletonList(FRODO_MEDIATYPE);
	}

	/**
	 * Copy the incoming {@link ObjectMapper} and change it's output format along with disabling failure on unknown
	 * properties.
	 */
	@Override
	public ObjectMapper configureObjectMapper(ObjectMapper mapper) {

		mapper.registerModule(getJacksonModule());
		mapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(new EvoInflectorLinkRelationProvider(),
				CurieProvider.NONE, MessageResolver.DEFAULTS_ONLY));

		mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
		mapper.enable(SerializationFeature.INDENT_OUTPUT);

		return mapper;
	}

	@Override
	public Module getJacksonModule() {
		return new Jackson2HalModule();
	}
}

That is the proper way to register HAL support. Once you do that, of course, it breaks the existing test case, because webmvc-frodo.json isn't ACTUAL HAL. It's just unwrapped Jackson.

BTW, the proper way to declare that aggregate root is:

@GetMapping("/employees")
public CollectionModel<EntityModel<Employee>> findMany() {

    List<EntityModel<Employee>> employees = new ArrayList<>();
    employees.add(new EntityModel(new Employee("Frodo Baggins", "ring bearer"),
            linkTo(methodOn(EmployeeController.class).findOne()).withSelfRel()));
    return new CollectionModel(employees, linkTo(methodOn(EmployeeController.class).findMany()).withSelfRel());
}

Cool?

[arhohuttunen]

Ah, I was basically missing the part with the handler instantiator. I think I need to take a closer look at those handler instantiators, never played with them before.

I actually wrote it a bit differently in the fork to write a separate test case for webmvc-vnd-hal.json, since webmvc-frodo.json is not really HAL. After making the change I can make the test pass.

Thanks for taking the time to check it.

[arhohuttunen]

Seems like I ran into another problem with @Relation(collectionRelation = "workers") being ignored when using the custom media type. I think I need to dig deeper into implementation of the existing media types to see how these things should be configured.

[odrotbohm]

For reference (originally commented on #1259):

While I think the route via a custom media type registration is a decent workaround, it's not what it was designed for initially and going down that route shouldn't be necessary if all you want to do is register an media type name alias to application/hal+json. As this thread proves, users would have to know all the delicate details of how to properly set up mappers and their moving parts. That's a bit much of the framework falling into one's face for a very simple task to achieve.

The concrete media types a media type implementation is registered under is already obtained via HypermediaMappingInformation.getMediaTypes(). Our HAL support's implementation HalMediaTypeConfiguration currently exposes a hard coded application/hal+json, but could also inspect the HalConfiguration (a bean customizable by the user) it already has available to pick up other media type names. User code could then just list the additional name(s) on that configuration object, and our infrastructure setup would pick that up and make sure the HttpMessageConverter that we register is triggered for that defined alias.

WDYT?