How to parse list of JSON objects surrounded by [] using Retrofit and GSON?

Question

I've created a simple REST endpoint:

http://<server_address>:3000/sizes

This URL returns a very simple response containing a json array as follows:

[
  { "id": 1, "name": "Small", "active": true },
  { "id": 2, "name": "Medium", "active": true },
  { "id": 3, "name": "Large", "active": true }
]

Now, I'm trying to consume this response using Retrofit 2 with GSON.

I've added a model:

@lombok.AllArgsConstructor
@lombok.EqualsAndHashCode
@lombok.ToString
public class Size {
    private int id;
    private String name;
    private boolean active;

    @SerializedName("created_at")
    private String createdAt;

    @SerializedName("updated_at")
    private String updatedAt;
}

And service:

public interface Service {
    @GET("sizes")
    Call<List<Size>> loadSizes();
}

I've instantiated a Retrofit:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("http://<server_address>:3000")
    .addConverterFactory(GsonConverterFactory.create())
    .build();

And my service:

Service service = retrofit.create(Service.class);

Now, trying to call the data:

service.loadSizes().enqueue(new Callback<List<Size>>() {
    @Override
    public void onResponse(Call<List<Size>> call, Response<List<Size>> response) {
        for(Size size: response.body()) {
            System.out.println(size.toString());
        }
    }

    @Override
    public void onFailure(Call<List<Size>> call, Throwable t) {
        System.out.println(t.getMessage());
    }
});

What ends up with an exception:

java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 18 path $[0].name

I suppose the error is caused by that, the REST API returns a response which is an array nor object.

1. Am I correct?
2. What is the easiest way to make this code to work?

REST service cannot be modified, so the response must stay as is.

Also, deserialization of the above json using pure GSON might be done by:

Type sizesType = new TypeToken<List<Size>>(){}.getType();
List<Size> size = new Gson().fromJson(json, sizesType);

But I have no idea how to make Retrofit 2 to use this.

Thanks in advance.

Answer 1

Recently I just finish one project related to retrofit2. Based on my source, I copy all your stuff into my project to give a try, making some minor change, it works well on my side.

In your build.gradle, add those:

 compile 'com.squareup.retrofit2:retrofit:2.0.1'
 compile 'com.google.code.gson:gson:2.6.2'
 compile 'com.squareup.okhttp3:okhttp:3.1.2'
 compile 'com.squareup.retrofit2:converter-gson:2.0.1'
 compile 'com.squareup.okhttp3:logging-interceptor:3.2.0'

Model: (UPDATE: Follow tommus's case, createdAt and updatedAt is now shown in his json response sample, these two values needs annotation due to the name in the model is different from json respone)

public class Size {
    private int id;
    private String name;
    private boolean active;

    @SerializedName("created_at")
    private String createdAt;

    @SerializedName("updated_at")
    private String updatedAt;
}

Service: (Exactly same as what you have)

public interface service {
    @GET("sizes")
    Call<List<Size>> loadSizes();    
}

RestClient: (I add log here, so that you can see all the request info and response info, be careful not using Localhost but your server ip address in the URL )

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.xiaoyaoworm.prolificlibrary.test.Service;

import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RestClient {

    private static Service service;

    public static Service getClient() {
        if (service == null) {
            Gson gson = new GsonBuilder()
                    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
                    .create();

            // Add logging into retrofit 2.0
            HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
            logging.setLevel(HttpLoggingInterceptor.Level.BODY);
            OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
            httpClient.interceptors().add(logging);

            Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl("http://YOURSERVERIPNOTLOCALHOST:3000/")
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .client(httpClient.build()).build();

            service = retrofit.create(Service.class);
        }
        return service;
    }
}

On your activity, add this function to run your code: (exactly same as what you have done. The response will be your list of size)

   private void loadSize() {
        Service serviceAPI = RestClient.getClient();
        Call<List<Size>> loadSizeCall = serviceAPI.loadSizes();
        loadSizeCall.enqueue(new Callback<List<Size>>() {
            @Override
            public void onResponse(Call<List<Size>> call, Response<List<Size>> response) {
                for(Size size: response.body()) {
                    System.out.println(size.toString());
                }
            }

            @Override
            public void onFailure(Call<List<Size>> call, Throwable t) {
                System.out.println(t.getMessage());
            }
        });
    }

Running this you will see the info you want to print out:

Here is my github repo which I use retrofit2.0 to make simple GET POST PUT DELETE work. You can use this as reference. My Github retrofit2.0 repo