How to consume Spring HATEOAS REST resources containing links to another resources?

Question

I have /studentCourses endpoint on the server (built with Spring Data REST) which returns the following content:

{
  "_embedded" : {
    "studentCourses" : [
      {
        "uid" : "5f23abe9-b24e-4e76-86b0-d539950a0a41",
        "registrationDate" : "7/23/2016",
        "_links" : {
          "self" : {
            "href" : "http://localhost:8080/studentCourses/5f23abe9-b24e-4e76-86b0-d539950a0a41"
          },
          "studentCourse" : {
            "href" : "http://localhost:8080/studentCourses/5f23abe9-b24e-4e76-86b0-d539950a0a41"
          },
          "course" : {
            "href" : "http://localhost:8080/studentCourses/5f23abe9-b24e-4e76-86b0-d539950a0a41/course"
          },
          "student" : {
            "href" : "http://localhost:8080/studentCourses/5f23abe9-b24e-4e76-86b0-d539950a0a41/student"
          }
        }
      },
      {
        ...
      },
      ...
    ]
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/studentCourses"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile/studentCourses"
    }
  },
  "page" : {
    ...
  }
}

And the following client code:

class StudentCourseDTO {

    String uuid;

    String registrationDate;

    StudentDTO student; // contains uuid, firstName, lastName, etc.

    CourseDTO course; // contains uuid, name, etc.

    // getters, setters

}

RestTemplate restTemplate() {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new Jackson2HalModule());
    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    MappingJackson2HttpMessageConverter messageConverter =
            new MappingJackson2HttpMessageConverter();
    messageConverter.setObjectMapper(objectMapper);
    messageConverter.setSupportedMediaTypes(Arrays.asList(MediaTypes.HAL_JSON));
    return new RestTemplate(Arrays.asList(messageConverter));
}

...

Collection<StudentCourseDTO> studentCourses = restTemplate().exchange(
        "http://localhost:8080/studentCourses",
        HttpMethod.GET, null,
        new ParameterizedTypeReference<PagedResources<StudentCourseDTO>>() {})
        .getBody().getContent();

The problem is that StudentCourseDTO.student and StudentCourseDTO.course are always null , but StudentCourseDTO.uuid and StudentCourseDTO.registrationDate are retrieved correctly from the server.
Anyone has an idea what I have missed?
I think there must be someway to tell RestTemplate to automatically follow the links in the returned content like student and course in the example above, but I haven't found a way to do this.

Answer 1

Just because there are links that does not mean they are automatically followed.

I would change the StudentCourseDTO to:

class StudentCourseDTO {

    String uuid;

    String registrationDate;

}

And then you would deserialize the response to a

PagedResources<Resource<StudentCourseDTO>> studentCourses = restTemplate().exchange(
        "http://localhost:8080/studentCourses",
        HttpMethod.GET, null,
        new ParameterizedTypeReference<PagedResources<Resource<StudentCourseDTO>>>() {})
        .getBody().getContent();

For each Resource<StudentCourseDTO> you can then follow the links for student and course , eg by using the RestTemplate to retrieve the resources.

Of course this gives you two additional calls per response item - but the only way to avoid this is to change the service to embed this information in the list resource.