[FIXED] How to handle a response of a service returning either a single object or an array of these?

Issue

I am trying to invoke a third-party API through REST call in Spring. Currently, I'm using postForObject. I am converting the request class to string and calling the post for object. The response is taken as string and then converted it into the class. I have defined the class with below parameters

Class responseDto {
 private Arraylist < Response > response;
 getResponse();
 setResponse();
}

Response {
  String code;
  String trid;
  Getters();
  Setters();
}

I am using Jackson dependency to serialize and deserialize. This class is working fine for the below response:

{
"response":[
   {
     "code":"100",
     "trid":"123"
   }
 ]
}

However, in error scenario, the request returns a JSON class with the same name 'response' as given below

{
 "response":{
  "code":"700",
  "trid":"123"
 }
}

The deserialize fails for the class I defined with some JSON mapping exception:

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

How can I resolve this issue in Java and Spring?

Solution

SOLUTION 1: Using @JsonFormat ( > 2.6 version)

Just annotate your field with @JsonFormat as

import java.util.List;
import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonFormat.Feature;

public class ResponseDto {

    @JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
    private List<Response> response;

    public List<Response> getResponse() {
        return response;
    }

    public void setResponse(List<Response> response) {
        this.response = response;
    }
}

SOLUTION 2: Setting DeserializationFeature

public static void main(String[] args) throws IOException {

    ObjectMapper mapper = new ObjectMapper();

    // global setting, can be overridden using @JsonFormat in beans
    // when using @JsonFormat on fields, then this is not needed 
    mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

    ResponseDto dto =  mapper.readValue(stringResponse, ResponseDto.class);
}

Now response node in json containing single object, single object array, multiple object array will be successfully parsed as list of Response object.

Answered By - Pratapi Hemant Patel
Answer Checked By - Terry (JavaFixing Volunteer)