RestTemplate: returning a List of Entities

Question

There is a RestFull method that return a List of Menu objects

public ResponseEntity<List<Menu>> getMenus() {
..
}

But I don't know how to get them from the RestTemplate, getting the class from ResponseEntity>

ResponseEntity<List<Menu>> response = restTemplate
                  .exchange("http://127.0.0.1:8080/elcor/api/users/1/menus", HttpMethod.GET, entity,  ResponseEntity<List<Menu>>.getClass());

Statolatry

Answer 1

Try use ParameterizedTypeReference

ResponseEntity<List<Menu>> response = restTemplate
       .exchange("URI", HttpMethod.GET, entity,  new ParameterizedTypeReference<List<Menu>>() {
 });

Goodden

what is entity in above method ? – Sherer

Late to the party, but someone else might find this useful: entity is the request object (that is being sent in the body as payload). See below my full answear – Stulin

As the type is implied by the response variable the type doesn't need to be specified on the right side of the assignment: new ParameterizedTypeReference<> is sufficient – Marshmallow

Answer 2

Try this, It worked for me to getting List of Objects with RestTemplate in Spring

    RestTemplate restTemplate = new RestTemplate();
    try {
        ResponseEntity<List<Claim>> claimResponse = restTemplate.exchange(
                uri, 
                HttpMethod.GET,
                null,
                new ParameterizedTypeReference<List<Claim>>() {});
        if(claimResponse != null && claimResponse.hasBody()){
            claims = claimResponse.getBody();
        }
    } catch (RestClientException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } 

Draft

Answer 3

I've been trying to come up with some generic approach to encapsulate such functionality and use it across my current project. Finally I found out how to do this.

public static <T> List<T> getForList(RestTemplate restTemplate, String url, Class<T> cls, Object... uriVariables) {
    ObjectMapper mapper = new ObjectMapper();
    CollectionType type = mapper.getTypeFactory().constructCollectionType(List.class, cls);
    return restTemplate.<List<T>>exchange(url, HttpMethod.GET, null,
                ParameterizedTypeReference.forType(type), uriVariables)
        .getBody();
}

Cy

Answer 4

You can simply use an array

ResponseEntity<Menu[]> response = restTemplate.exchange("URI", HttpMethod.GET, entity, Menu[].class);

Kuroshio

Answer 5

Somebody else might be confused, so here is a more detailed answer:

private ResponseEntity<List<ResponseDto>> sendRequest(List<RequestDto> requestList) {
    String URL = "http://localhost:8080/endpoint"; // my endpoint

    HttpHeaders headers = new HttpHeaders(); //here you set any headers you need
    headers.setContentType(MediaType.APPLICATION_JSON);

    HttpEntity<List<RequestDto>> request = new HttpEntity<>(requestList, headers); // create the entity object you need to pass as a request object. In my case, it's an array of objects
    return restTemplate.exchange(URL, HttpMethod.POST, request, new ParameterizedTypeReference<List<ResponseDto>>() {
    });
}

In the last line, as the restTemplate.postForEntity(...) does not allow for Collections of elements as a response, we use the exchange(...) method. Its parameters are:

• URL -> String object, the endpoint for the request
• method -> HttpMethod object (GET, POST, PUT, PATCH etc)
• requestEntity -> HttpEntity object, the one that is being created above (mine was called request). It has both the headers for the request and the payload/body
• responseType -> it's the 4th argument, a ParameterizedTypeReference abstract class. This one is used when the returned type is a Collection, not a simple array or just an object. When the endpoint returns a collection of objects, you cannot do something like Collection<ResponseDto>.class because it's impossible

Another workaround, if you don't need the ResponseEntity object itself, but only the result object, you can do something like

ResponseEntity<ResponseDto[]> responseEntity = restTemplate.postForEntity(URL, request, ResponseDto[].class); // convert the response body to a simple array, not Collection
return Arrays.asList(responseEntity.getBody()); // convert to Collection if needed

Stulin