ClassCastException: RestTemplate returning List<LinkedHashMap> instead of List<MymodelClass>

Question

I'm trying to access getter methods on my MyModelClass but my code is returning List<LinkedHashMap> instead of List<MyModelClass>. This is my code.

List<MyModelClass> myModelClass = 
   (List<MyModelClass>) restTemplate.postForObject(url,mvm,List.class);

System.out.println("Response= " +  myModelClass);

I tried to print the response and I got the JSON Response that I'm expecting. but when I tried to run this code.

System.out.println("Response= " +  myModelClass.get(0).getMessage());

It will produce this error.

java.lang.ClassCastException: 
java.util.LinkedHashMap cannot be cast to com.XXX.XXX.MyModelClass 

It is a mismatch. Can someone help me to get rid with this error? thanks.

MyModelClass

public class MyModelClass{
        
    /**
     * 
     */
    @JsonProperty("id")
    public String id;
    
    @JsonProperty("type")
    public String type;
    
    @JsonProperty("user")
    public String user;
    
    @JsonProperty("message")
    public String message;

    //getters

Error for

MyModelClass[] myModelClass= restTemplate.postForObject(url,mvm, myModelClass[].class);

org.springframework.http.converter.HttpMessageNotReadableException:
Could not read JSON: 
Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

JSON Response Structure

    [{"key1":"value1","key2":"value2","parameters":{"key1":"value1","key2":"value2","key3":"value3","key4":"value4","key5":"value5"}},
{"key12":"value12","key22":"value22","parameters":{"key12":"value12","key22":"value22","key32":"value32","key42":"value42","key52":"value52"}}]

If there is any suggestion on how to map this kind of JSON Response in RestTemplate,It would help a lot. thanks

Answer 1

With the following method call

List<MyModelClass> myModelClass=(List<MyModelClass>) restTemplate.postForObject(url,mvm,List.class);

All Jackson knows is that you want a List, but doesn't have any restriction on the type. By default Jackson deserializes a JSON object into a LinkedHashMap, so that's why you are getting the ClassCastException.

If your returned JSON is an array, one way to get it is to use an array

MyModelClass[] myModelClasses = restTemplate.postForObject(url,mvm, MyModelClass[].class);

You can always add the elements from that array to a List.

I can't remember since what version, but RestTemplate#exchange now has an overload that accepts a ParameterizedTypeReference argument. The ParameterizedTypeReference is the type token hack for suggesting a parameterized type as the target for deserialization.

You can refactor the code above to use exchange instead of postForObject, and use ParameterizedTypeReference to get a List<MyModelClass>. For example

ParameterizedTypeReference<List<MyModelClass>> typeRef = new ParameterizedTypeReference<List<MyModelClass>>() {
};
ResponseEntity<List<MyModelClass>> responseEntity = restTemplate.exchange(url, HttpMethod.POST, new HttpEntity<>(mvm), typeRef);
List<MyModelClass> myModelClasses = responseEntity.getBody();