How to handle a list of resources with ResponseEntity

Question

I have an external REST API that returns a list of client objects as json. For example:

GET externalapi.io/api/clients

[ {
    "id" : "b15asgdc-6efd-4a2d-re08-2q3r3bae",
    "clientName" : "test1",
    ...
  },
  {
    "id" : "ryec-6efd-4a2d-aa08-29a9drjrteae",
    "clientName" : "test2",
    ...
  },
  ...
]

My API calls this external service using Spring's RestTemplate. The goal is to get the response and parse it into client Java objects. I've tried the following:

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

ResponseEntity<String> responseEntityJson = resttemplate.exchange(path, HttpMethod.GET, entity, String.class);

Type listType = new TypeToken<ArrayList<ClientResource>>(){}.getType();

ArrayList<ClientResource> responseResources = gson.fromJson(responseEntityJson.getBody(), listType);

My trouble is with this line:

ResponseEntity<String> responseEntityJson = resttemplate.exchange(path, HttpMethod.GET, entity, String.class);

My hope was that I could get the body as a JSON string, and use Google's GSON to parse it into objects, but this does not seem to work. I've seen suggestions to use

ResponseEntity<ClientResource> responseEntityJson = resttemplate.exchange(path, HttpMethod.GET, entity, ClientResource.class);

but, I don't think this works for a list of resources. I can set ResponseEntity<List<ClientResource>> but .exchange() doesn't easily accept a type like that. How can I handle a list of resources with ResponseEntity?

First error that led me to this was `Could not extract response: no suitable HttpMessageConverter found for response type [class java.lang.String] and content type [application/json] `. I've an http message converter for JSON, but my question is about how to handle a list of resources. — KJ0797, Feb 23 '23 at 03:04

score 1 · Accepted Answer · edited Feb 23 '23 at 13:05

1

Try this please, maybe it works for you.

HttpHeaders headers = new HttpHeaders();
// set headers if required

HttpEntity<?> entity = new HttpEntity<>(headers);

String url = "https://externalapi.io/api/clients";

ParameterizedTypeReference<List<ClientResource>> responseType = new ParameterizedTypeReference<List<ClientResource>>() {};
ResponseEntity<List<ClientResource>> response = restTemplate.exchange(url, HttpMethod.GET, entity, responseType);

List<ClientResource> clientResources = response.getBody();

edited Feb 23 '23 at 13:05

Mark Rotteveel

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answered Feb 22 '23 at 23:32

Madan

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4

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Feb 24 '23 at 16:28

score 1 · Answer 2 · answered Feb 23 '23 at 08:04

Madan's code should work. Next to using the Gson library from Google you can also use ObjectMapper from the Jackson library.

See the following:

HttpHeaders headers = new HttpHeaders();
headers.set("Accept", MediaType.APPLICATION_JSON_VALUE);
HttpEntity<Object> entity = new HttpEntity<Object>(null, headers);

ResponseEntity<String> response = resttemplate.exchange(
"https://externalapi.io/api/clients", HttpMethod.GET, entity, String.class);

ObjectMapper objectMapper = new ObjectMapper();
List<ClientResource> clients = objectMapper.readValue(
response.getBody(), new TypeReference<List<ClientResource>>() {});

How to handle a list of resources with ResponseEntity

2 Answers2