Issue When Consume Rest Service with RestTemplate in Desktop App

Question

I have a problem when Consume Rest Service with RestTemplate in Desktop App whereas the problem doesn't appear when i use in Web app.

This Is the Debugging logs

15:30:40.448 [main] DEBUG o.s.web.client.RestTemplate - Reading [java.util.List] as "application/json" using [org.springframework.http.converter.json.MappingJacksonHttpMessageConverter@98adae2]
15:30:40.452 [main] DEBUG httpclient.wire.content - << "[{"name":"Indonesia","id":1},{"name":"AlaySia","id":2},{"name":"Autraliya","id":3}]"

Exception in thread "main" java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.mgm.domain.Country

And this is the Code that i use.

 String url = "http://localhost:8080/mgm/country";
    List<MediaType> mediaTypes = new ArrayList<MediaType>();
    mediaTypes.add(MediaType.APPLICATION_JSON);
    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(mediaTypes);
    HttpEntity<Country> httpEntity = new HttpEntity<Country>(null, headers);
    try {
        ResponseEntity<List> responseEntity = restTemplate.exchange(url, HttpMethod.GET, httpEntity, List.class);
        List<Country> countries = responseEntity.getBody();
        System.out.println(countries.get(0).getName());

    } catch (RestClientException exception) {
        exception.printStackTrace();
    }

Code above doesn't give errors when i place it in web app. I use Spring Rest MVC to Provide JSON and Consume it with RestTemplate.

I think there is a problem when Jackson Convert java.util.LinkedHashMap to Country . it Seems that countries.get(0) actually has LinkedHashMap type not Country and problem will appeared when i invoke one of Country methode like .getName()

Solution

Try using an array instead:

String url = "http://localhost:8080/mgm/country";
List<MediaType> mediaTypes = new ArrayList<MediaType>();
mediaTypes.add(MediaType.APPLICATION_JSON);
HttpHeaders headers = new HttpHeaders();
headers.setAccept(mediaTypes);
HttpEntity<Country> httpEntity = new HttpEntity<Country>(null, headers);
try {
    ResponseEntity<Country[]> responseEntity = restTemplate.exchange(url, HttpMethod.GET, httpEntity, Country[].class);
    Country[] countries = responseEntity.getBody();
    System.out.println(countries[0].getName());

} catch (RestClientException exception) {
    exception.printStackTrace();
}

Comments

[SingleShot]

+1. Having the same issue, after some research on this I discovered your solution, came here to answer it, and see you beat me to it :-) The reason apparently is that you can't pass List<Country>.class to the exchange() method, so when it unmarshalls it has no way to know the type (Country) the List is typed to. Arrays can be passed as Class objects, so the type information can be conveyed to the underlying unmarshaller.

[Tobi Nonymous]

yes, this is due to type erasure effects in the generics mechanism of Java. More found here: stackoverflow.com/questions/339699/… just adding my 2 cents