Problems consuming json+hal _embedded resources with spring RestTemplate

user_3795212 0
Viewed 1031

I have a simple use case where I would like to consume a resource collection that is represented with json+hal.

I use the spring RestTemplate and have configuired it to use the Jackson2HalModule. When I debug my code I find that the Response object does contain accurate metadata (e.g. number of pages and resources) and response headers but there is no content or links. I have looked at many articles and guides on the internet over the last day, and I feel that my custom rest template should be working for my use case based on my findings.

If anybody can shed any light on this I would be eternally grateful.

My code for my service is as follows:

@Service
public class EventServiceImpl extends BaseService implements EventService {

private static final String knownEntity = "59d786d642572853721728f6";

private static  String SERVICE_URL = "http://EVENTS-SERVER";

private static String EVENTS_PATH = "/events";


@Autowired
@LoadBalanced
protected RestTemplate restTemplate;

@Override
public ResponseEntity<PagedResources<Event>> fetchEventsList() {
    // acceptable media type
    List<MediaType> acceptableMediaTypes = Arrays.asList(HAL_JSON);

    // header
    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(acceptableMediaTypes);

    HttpEntity<String> entity = new HttpEntity<String>(null, headers);

    ResponseEntity<PagedResources<Event>> response = getRestTemplateWithHalMessageConverter()
            .exchange(SERVICE_URL + EVENTS_PATH, HttpMethod.GET, entity, new ParameterizedTypeReference<PagedResources<Event>>(){});
    return response;
}

public RestTemplate getRestTemplateWithHalMessageConverter() {
    List<HttpMessageConverter<?>> existingConverters = restTemplate.getMessageConverters();
    List<HttpMessageConverter<?>> newConverters = new ArrayList<>();
    newConverters.add(getHalMessageConverter());
    newConverters.addAll(existingConverters);
    restTemplate.setMessageConverters(newConverters);
    return restTemplate;
}

private HttpMessageConverter getHalMessageConverter() {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new Jackson2HalModule());
    MappingJackson2HttpMessageConverter halConverter = new TypeConstrainedMappingJackson2HttpMessageConverter(ResourceSupport.class);
    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    halConverter.setSupportedMediaTypes(Arrays.asList(HAL_JSON));
    halConverter.setObjectMapper(objectMapper);
    return halConverter;
}

And my simple model is:

public class Event {

private String name;
private String location;
private int capacity;

public String getName() {
    return name;
}

public String getLocation() {
    return location;
}

public int getCapacity() {
    return capacity;
}

}

For completeness, here is a samle of the shape of the hal+json I am attempting to consume:

{
"_embedded": {
    "events": [
        {
            "name": null,
            "location": null,
            "capacity": 0,
            "currentState": "CANCELLED",
            "_links": {
                "self": {
                    "href": "http://192.168.1.6:2221/events/59d786d642572853721728f6"
                },
                "event": {
                    "href": "http://192.168.1.6:2221/events/59d786d642572853721728f6"
                },
                "reinstate": {
                    "href": "http://192.168.1.6:2221/events/59d786d642572853721728f6/reinstate"
                },
                "reschedule": {
                    "href": "http://192.168.1.6:2221/events/59d786d642572853721728f6/reschedule"
                }
            }
        },
        {
            "name": null,
            "location": null,
            "capacity": 0,
            "currentState": "ADVERTISED",
            "_links": {
                "self": {
                    "href": "http://192.168.1.6:2221/events/59d7f14342572812ceca7fc6"
                },
                "event": {
                    "href": "http://192.168.1.6:2221/events/59d7f14342572812ceca7fc6"
                },
                "cancel": {
                    "href": "http://192.168.1.6:2221/events/59d7f14342572812ceca7fc6/cancel"
                },
                "reschedule": {
                    "href": "http://192.168.1.6:2221/events/59d7f14342572812ceca7fc6/reschedule"
                }
            }
        },
        {
            "name": null,
            "location": null,
            "capacity": 0,
            "currentState": "ADVERTISED",
            "_links": {
                "self": {
                    "href": "http://192.168.1.6:2221/events/59d7f14742572812ceca7fc7"
                },
                "event": {
                    "href": "http://192.168.1.6:2221/events/59d7f14742572812ceca7fc7"
                },
                "cancel": {
                    "href": "http://192.168.1.6:2221/events/59d7f14742572812ceca7fc7/cancel"
                },
                "reschedule": {
                    "href": "http://192.168.1.6:2221/events/59d7f14742572812ceca7fc7/reschedule"
                }
            }
        },
        {
            "name": null,
            "location": null,
            "capacity": 0,
            "currentState": "ADVERTISED",
            "_links": {
                "self": {
                    "href": "http://192.168.1.6:2221/events/59d7f14c42572812ceca7fc8"
                },
                "event": {
                    "href": "http://192.168.1.6:2221/events/59d7f14c42572812ceca7fc8"
                },
                "cancel": {
                    "href": "http://192.168.1.6:2221/events/59d7f14c42572812ceca7fc8/cancel"
                },
                "reschedule": {
                    "href": "http://192.168.1.6:2221/events/59d7f14c42572812ceca7fc8/reschedule"
                }
            }
        }
    ]
},
"_links": {
    "self": {
        "href": "http://192.168.1.6:2221/events{?page,size,sort}",
        "templated": true
    },
    "profile": {
        "href": "http://192.168.1.6:2221/profile/events"
    }
},
"page": {
    "size": 20,
    "totalElements": 4,
    "totalPages": 1,
    "number": 0
}

}

EDIT: I can consume an individual Event with no problems.

resttemplate spring-boot jackson rest java
1 Answers

I had a similar problem and I ended up using org.springframework.hateoas.Resources

For my example below, this object is located in org.springframework.hateoas:spring-hateoas:jar:0.25.2.RELEASE which is being pulled in from org.springframework.boot:spring-boot-starter-data-rest:jar:2.1.7.RELEASE, so there's a good chance its already in your classpath assuming you declare the spring-boot-starter-data-rest dependency.

Here's a simple example (using your info):

RestTemplate restTemplate = new RestTemplate();
Resources<Event> resourceEvents = restTemplate.getForObject("http://192.168.1.6:2221/events", Resources.class);
List<Event> events = new ArrayList<>(resourceEvents.getContent());

There are probably some gotchas the make this not so straight forward, but hopefully this provides a start to solving your problem.

user_31209820
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