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

Answer 1

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.

Answer 2

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.

Answer 3

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