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.

Am I correct?
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.

Have you tried this without `lombok` annotations? Perhaps they are somehow making GSON think that name is an object, not a string? — Marc J, Apr 16 '16 at 17:49

xiaoyaoworm · Answer 1 · 2016-04-20T21:52:20.067

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

maybe override toString is better class Size :) – marlonpya May 15 '20 at 17:42 — marlonpya, May 15 '20 at 17:42

BNK · Answer 2 · 2016-04-20T01:46:55.493

Please use the following:

build.gradle file:

dependencies {
    ...
    compile 'com.squareup.retrofit2:retrofit:2.0.1'
    compile 'com.squareup.retrofit2:converter-gson:2.0.1'
    compile 'com.google.code.gson:gson:2.6.2'
}

WebAPIService.java:

public interface WebAPIService {
    @GET("/json.txt") // I use a simple json file to get the JSON Array as yours
    Call<JsonArray> readJsonArray();
}

Size.java:

public class Size {
    @SerializedName("id")
    private int id;

    @SerializedName("name")
    private String name;

    @SerializedName("active")
    private boolean active;

    @SerializedName("created_At")
    private String createdAt;

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

MainActivity.java:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

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

    WebAPIService service = retrofit.create(WebAPIService.class);
    Call<JsonArray> jsonCall = service.readJsonArray();
    jsonCall.enqueue(new Callback<JsonArray>() {
        @Override
        public void onResponse(Call<JsonArray> call, Response<JsonArray> response) {
            String jsonString = response.body().toString();
            Log.i("onResponse", jsonString);
            Type listType = new TypeToken<List<Size>>() {}.getType();
            List<Size> yourList = new Gson().fromJson(jsonString, listType);
            Log.i("onResponse", yourList.toString());
        }

        @Override
        public void onFailure(Call<JsonArray> call, Throwable t) {
            Log.e("onFailure", t.toString());
        }
    });
}

Here is the debug screenshot:

UPDATE: You can also use the following option:

@GET("/json.txt")
Call<List<Size>> readList();

and

    Call<List<Size>> listCall1 = service.readList();
    listCall1.enqueue(new Callback<List<Size>>() {
        @Override
        public void onResponse(Call<List<Size>> call, Response<List<Size>> response) {
            for (Size size : response.body()){
                Log.i("onResponse", size.toString());
            }
        }

        @Override
        public void onFailure(Call<List<Size>> call, Throwable t) {
            Log.e("onFailure", t.toString());
        }
    });

score 5 · Accepted Answer · edited May 23 '17 at 12:03

The funny fact is... My code is perfectly fine. At least the one presented in the question above.

I've ended up removing one line from my Size model.

As I focused on the code itself (especially Retrofit's configuration) I've totally ignored imports.

It turned out - while implementing Size model when I've started typing String class for model's fields:

name
createdAt
updatedAt

IntelliJ IDEA's code completion suggested me

not java.lang.String
but com.sun.org.apache.xpath.internal.operations.String

what totally messed up Gson's deserialization.

When it comes to rewards...

I've decided to mark as valid my own answer. Why?

To ensure that, every of those developers, who will come with exactly same trouble as me - make sure you have valid imports.

Many thanks goes to gentlmen above for their great services.

As I have only one bounty I've decided reward xiaoyaoworm as his code better match my needs (I haven't written it in my question but the idea of writing such simple service - as I've presented in my question - is to hide from the end-user implementation details and not use JsonArray and such like in BNK response).

Update 1:

The only problem with xiaoyaoworm's answer is that, he suggested the Size model do not need any annotations what is totally wrong for the quoted JSON example.

For above case, exact two fields of the Size model needs annotations - created_at and updated_at.

I've even tested few versions of the converter-gson library (I saw xiaoyaoworm have used other than me) - it hasn't changed anything. Annotations were necessary.

Otherwise - again, many thanks!

I am also very confused about when I copy paste your code, nothing is wrong. Glad to see that you find this wrong import yourself. — xiaoyaoworm, Apr 20 '16 at 00:29
@xiaoyaoworm - Your response forces me to check my code few more times. I owe you a beer. ;) At the end - please take a look at my update. — Tomasz Dzieniak, Apr 20 '16 at 00:35
Now you don't owe me a beer. Your update force me to check more info about this SerializedName annotation. Now I know SerializedName can make name in model be different. However, in your example, created_at and updated_at are not in the json response, so that you have to use this annotation. Am I correct? — xiaoyaoworm, Apr 20 '16 at 03:10
because I use your json response to generate sample on my local server, I am missing these two parameter I think. Now I understand it clearly, hah, good question can make everyone learn a lot. Thank you also. — xiaoyaoworm, Apr 20 '16 at 03:14
Yes. My JSON contains also these fields (I do not mention them as I suppose they has nothing to the error - what at the end was true). Thanks. — Tomasz Dzieniak, Apr 20 '16 at 07:18

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

3 Answers3

Update 1:

Linked