Spring Data REST - Consume List of Entities, Java HATEOAS client

Question

I'm using Spring Data REST 2.5.1, Jackson 2.8.0, Spring Boot 1.3.6.

I'm trying to retrieve a simple list of entities from my Repository via RestTemplate. I can hit the end point in a browser, and get the expected HAL data. Retrieving a single Entity works fine as below. These are all using the default SDR endpoints (e.g. localhost:{port}/myEntity).

    ResponseEntity<Resource<MyEntity>> responseEntity =
    new RestTemplate()
        .exchange(
        uri + "/1",
            HttpMethod.GET,
            HttpEntity.EMPTY,
        new ParameterizedTypeReference<Resource<MyEntity>>() {}, port
        )

Or new RestTemplate().getForEntity(uri + "/1", MyEntity.class, port)

As a lot of SO questions seem to indicate, finding examples of retrieving a list is a problem. I've tried the ParameterizedTypeReference with Resources,Resource, MyEntity, an array, List. All with no luck.

        ResponseEntity<Resources<Resource<MyEntity>>> responseEntity =
            new RestTemplate()
                    .exchange(
                    uri,
                    HttpMethod.GET,
                    HttpEntity.EMPTY,
                    new ParameterizedTypeReference<Resources<Resource<MyEntity>>>() {}
            , port
            )

When called like above with pretty much any variety of Resources, Resource, List<MyEntity>, MyEntity, etc., the ResponseEntity is empty. Like:

<200 OK,Resources { content: [], links: [] },{Server=[Apache-Coyote/1.1], Content-Type=[application/json;charset=UTF-8], Transfer-Encoding=[chunked], Date=[...]}>

The string JSON looks like below in browser.

{
"_embedded" : {
"myEntities" : [ ... ]
},
"_links" : {
"self" : {
  "href" : "http://localhost:8080/myEntity"
},
"profile" : {
  "href" : "http://localhost:8080/profile/myEntity"
},
"search" : {
  "href" : "http://localhost:8080/myEntity/search"
}
},
"page" : {
  "size" : 20,
  "totalElements" : 10,
  "totalPages" : 1,
  "number" : 1
 }
}

Repository Definition:

@RepositoryRestResource(collectionResourceRel = "myEntities", path = "myEntity")
public interface MyEntityRepository extends PagingAndSortingRepository<MyEntity, Long>
, QueryDslPredicateExecutor<MyEntity>
, QuerydslBinderCustomizer<QMyEntity> { }

Any thoughts on what I am missing?

If you look at the structure of Resources you will notice that it's different from the structure of the JSON document. It's like trying to open your house with your car key. — a better oliver
Yeah, I get that the way I'm trying isn't working. I'm trying to find out what does work, not what doesn't. If SDR is creating this format, there has to be a Spring class that expects to consume it. I shouldn't have to roll my own for its standard format. — JudgingNotJudging
The basis for the document in your example is PagedResources. _embedded relies on EmbeddedWrapper. But there is no class that directly represents the document. The document is built dynamically because the _embedded property is dynamic. — a better oliver
I added the example repository definition. I get that it is a PagedResource because I'm extending the PagingAndSortingRepository, but is there no support for Java clients? I'm not doing anything specific to create an _embedded section. This is what SDR is giving me by default. Do I need to create a MyEntityResource? — JudgingNotJudging

JudgingNotJudging JudgingNotJudging · Accepted Answer · 2016-07-21T15:05:15

I solved this by doing a few things.

I had to update to spring-hateoas:0.20.0.RELEASE from 0.19.0. spring-hateoas:0.19.0 did not support jackson 2.7+ as specified here.

I updated my Client to call as below.

ObjectMapper mapper = builder.build()
MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();

messageConverter.setSupportedMediaTypes(MediaType.parseMediaTypes("application/hal+json"))
messageConverter.setObjectMapper(mapper)

ResponseEntity<PagedResources<MyEntity>> responseEntity =
        new RestTemplate(Arrays.asList(messageConverter))
                .exchange(
                uri,
                HttpMethod.GET,
                HttpEntity.EMPTY,
                new ParameterizedTypeReference<PagedResources<MyEntity>>() {}, port
        )

PagedResources now looks like this:

<200 OK,PagedResource { content: [{<List of MyEntities>}], metadata: Metadata { number: 0, total pages: 1, total elements: 10, size: 20 }, links: [<List of hateoas links for MyEntities>] },{Server=[Apache-Coyote/1.1], Content-Type=[application/hal+json;charset=UTF-8], Transfer-Encoding=[chunked], Date=[Thu, 21 Jul 2016 14:57:18 GMT]}>

@zeroflagL's comment got me looking more closely at the PagedResources implementation, which eventually led to the 'aha!' moment with this blog.

The relevant bit is that the default RestTemplate doesn't set the accept header to application/hal+json. Instead, the default is application/x-spring-data-compact+json;charset=UTF-8 which has not content and only links. This is why I was getting empty content for my Resources types. Explicitly setting the MediaType as above fixed the issue.

Spring Data REST - Consume List of Entities, Java HATEOAS client

2 Answers