Generics with Spring RESTTemplate

Question

I have a class like that:

public class Wrapper<T> {

 private String message;
 private T data;

 public String getMessage() {
    return message;
 }

 public void setMessage(String message) {
    this.message = message;
 }

 public T getData() {
    return data;
 }

 public void setData(T data) {
    this.data = data;
 }

}

and I use resttemplate as follows:

...
Wrapper<Model> response = restTemplate.getForObject(URL, Wrapper.class, myMap);
Model model = response.getData();
...

However it throws a:

ClassCastException

I read that: Issue when trying to use Jackson in java but didn't help. There are some topics related to my problem etc.: https://jira.springsource.org/browse/SPR-7002 and https://jira.springsource.org/browse/SPR-7023

Any ideas?

PS: My error is that:

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to a.b.c.d.Model

I think resttemplate can not understand my generic variable and maybe it accepts it as an Object instead of generic T. So it becomes a LinkedHashMap. You can read from it here It says that when explaining from what it marshalls to:

JSON Type | Java Type

object | LinkedHashMap

I suppose solution for this is added in 3.2 [SPR-7023](https://jira.springsource.org/browse/SPR-7023) — raksja, Jun 14 '13 at 00:20
Did you ever find a solution for this? I'm using ParameterizedTypeReference and while it works when I pass in Map it doesn't work when I pass in my concrete class that represents `data` in your model. — danieljimenez, May 03 '14 at 21:34

score 170 · Accepted Answer · answered Jan 16 '14 at 09:44

170

ParameterizedTypeReference has been introduced in 3.2 M2 to workaround this issue.

Wrapper<Model> response = restClient.exchange(loginUrl, 
                          HttpMethod.GET, 
                          null, 
                          new ParameterizedTypeReference<Wrapper<Model>>() {}).getBody();

However, the postForObject/getForObject variant was not introduced.

answered Jan 16 '14 at 09:44

Manimaran Selvan

2,166
1
14
14

1

I tried another optioned for your guys,@alexanoid . Just first get the response as `String` ,then use Jackson to parse the string to generics object ,see : `String body = restTemplate.exchange(request,String.class).getBody(); ObjectMapper mapper=new ObjectMapper(); DataTablesOutput readValue = mapper.readValue(body, DataTablesOutput.class);` – Alter Hu Dec 19 '16 at 05:29
2

Why Model cannot here be also generic?, etc. like this: `Model` – zygimantus Jun 13 '17 at 12:40
1

To complement Alter's answer you can use: `mapper.readValue(body, new TypeReference>(){});` – cs94njw Oct 04 '19 at 08:51
1

My question also why we can't make Model also generic here ? – u91 Jul 14 '20 at 20:04

score 12 · Answer 2 · answered Jan 22 '14 at 16:53

12

The only thing I think you could do is creating a new class that extends Wrapper and uses model as a generic.

class WrapperWithModel extends Wrapper<Model>{};

WrapperWithModel response = restTemplate.getForObject(URL, WrapperWithModel.class);

It's not the best solution, but at least you won't have to unmarshall manually the response.

answered Jan 22 '14 at 16:53

Juan Luis

3,317
4
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23

I personally don't like this solution but right now I can't upgrade spring to use its new parameterizedtype object (https://jira.springsource.org/browse/SPR-7023), so this is good enough. thanks – Gustavo Matias Feb 24 '14 at 20:33
it must be static class – Adam Silenko Sep 20 '19 at 13:45

score 2 · Answer 3 · answered Jul 19 '12 at 13:17

2

Do not use generics with RestTemplate. Wrap request and response object with wrapper object that will hide the generics.

answered Jul 19 '12 at 13:17

Dima

1,774
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Generics with Spring RESTTemplate

3 Answers3

Linked

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