Why does RestTemplate not bind response representa

问题:

I am using spring-data-rest to expose entities as (paged) rest resources. Everything works fine, but when I request data via RestTemplate, I get an useless HATEOAS JSON (which I didn't ask for). The JSON seems to be a PagedResources. I could live with that, but the JSON isn't converted into an object correctly. There is no content inside.

Repository:

@RepositoryRestResource(collectionResourceRel = "people", path = "people")
public interface PersonRepository extends PagingAndSortingRepository<Person, Long>
{
    List<Person> findByLastName(@Param("name") String name);
}

Client:

public List<Person> getPersons()
{
    RestTemplate rt = new RestTemplate();
    System.out.println(rt.getForObject(URL, PagedResources.class).getContent().size());
    System.out.println(rt.getForObject(URL, PagedResources.class).getLinks().size());
    System.out.println(rt.getForObject(URL, PagedResources.class).getMetadata().getTotalElements());
    return new ArrayList<Person>(rt.getForObject(URL, PagedResources.class).getContent()); // <-- empty
}

System.out:

0 // getContent().size()
4 // getLinks().size()
2 // getTotalElements()

curl:

C:\...>curl http://localhost:8080/spring-jsf-rest/rest/people
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/spring-jsf-rest/rest/people{?page,size,sort}",
      "templated" : true
    },
    "search" : {
      "href" : "http://localhost:8080/spring-jsf-rest/rest/people/search"
    }
  },
  "_embedded" : {
    "people" : [ {
      "firstName" : "John",
      "lastName" : "Rambo",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/spring-jsf-rest/rest/people/1"
        }
      }
    }, {
      "firstName" : "Chuck",
      "lastName" : "Norris",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/spring-jsf-rest/rest/people/2"
        }
      }
    } ]
  },
  "page" : {
    "size" : 20,
    "totalElements" : 2,
    "totalPages" : 1,
    "number" : 0
  }
}

It seems like _embedded is not mapped correctly to content?!

回答1:

As you've discovered correctly, PagedResources does not have an _embedded property, that's why you don't get the content property populated.

This dilemma can be solved in two different ways:

1. Providing a type that matches the representation in the first place. Thus, craft a custom class and either stick to the property names of the representation or customize it using Jackson annotations etc.

2. Set up a custom MappingJackson2HttpMessageConverter and customize the ObjectMapperto get the Jackson2HalModule configured that Spring HATEOAS ships out of the box.

   ObjectMapper mapper = new ObjectMapper();
   mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
   mapper.registerModule(new Jackson2HalModule());
   
   MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
   converter.setSupportedMediaTypes(MediaType.parseMediaTypes("application/hal+json"));
   converter.setObjectMapper(mapper);
   
   RestTemplate template = new RestTemplate(Collections.<HttpMessageConverter<?>> singletonList(converter));