Retrofit Gson 定制 json 通用转换器

Question

I've been working with Retrofit on a couple of my projects before but now I want to do something slightly different. I'm calling an api that wraps my response in a structure similar to this:

{ // only for demo purposes. Probably errors and data will never be populated together
  "body": { 
    "errors": {
      "username": [
        "Username is too short",
        "Username already exists"
      ]
    },
    "data": {
      "message": "User created."
    }
  }
}

I'm trying to convert all that to a generic class which will wrap that response for me. What I have in mind is something like

public class ApiResponse<T> {
    private T data;

    private Map<String, List<String>> errors;

    public ApiResponse(T data, Map<String, List<String>> errors) {
        this.data = data;
        this.errors = errors;
    }
}

Where T can be any class.

I tried implementing a JsonDeserializer<ApiResponse<T>> based on some examples I found around the internet but I can't wrap my head around how to make it work as much automatically as possible and let Retrofit and Gson do the heavy lifting My Converter class is as follows:

public class ApiResponseDeserializer<T> implements JsonDeserializer<ApiResponse<T>> {

    private Class clazz;

    public ApiResponseDeserializer(Class clazz) {
        this.clazz = clazz;
    }

    @Override
    public ApiResponse deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        final JsonObject jsonObject = json.getAsJsonObject();

        final JsonObject body = jsonObject.getAsJsonObject("body");

        final JsonObject errors = body.getAsJsonObject("errors");
        final JsonObject data = body.getAsJsonObject("data");

        Map<String, List<String>> parsedErrors = new HashMap<>();
        for(String key : errors.keySet()) {
            List<String> errorsList = new ArrayList<>();

            JsonArray value = errors.getAsJsonArray(key);
            Iterator<JsonElement> valuesIterator = value.iterator();
            while(valuesIterator.hasNext()) {
                String error = valuesIterator.next().getAsString();

                errorsList.add(error);
            }

            parsedErrors.put(key, errorsList);
        }

        T parsedData = context.deserialize(data, clazz);

        return new ApiResponse<T>(parsedData, parsedErrors);
    }
}

and then when building my retrofit client

public static Retrofit getClient() {

        if (okHttpClient == null) {
            initOkHttp();
        }

        Gson gson = new GsonBuilder()
                .registerTypeAdapter(ApiResponse.class, new ApiResponseDeserializer<>(......) // PROBLEM
                .create();

        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(Const.API_BASE_URL)
                    .client(okHttpClient)
                    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .build();
        }

        return retrofit;
    }

But I feel like it's not generic enough to be able to convert my classes automatically. And also I have no idea how should I hint Gson what type my data .

My endpoints are defined as follows:

@POST("users/signup")
Single<ApiResponse<RegisterResponseData>> register(@Body RegisterRequest request);

But how do I make a generic Retrofit instance with a generic Gson type adapter that knows how to convert my response to a ApiResponse<RegisterResponseData> ? And knows that the data property from the response should be converted to an object of type RegisterResponseData ...

Answer 1

When you specify return type in Retrofit's client it's passed to Retrofit's converter as Type and then Gson receives that type which will be your ApiResponse<RegisterResponseData> . From that point Gson will understand that data is of type RegisterResponseData and will produce your model object.

Just try it without your ApiResponseDeserializer and you'll see it's working.

Edit: Answering your additional question in comments:

If you want to skip your "body" object in json you can write your wrapper object like this:

public class ApiResponse<T> {

   @SerializedName("body")
   private ApiResponseBody<T> body;

   public ApiResponse() {
   }

   public ApiResponse(ApiData<T> body) { 
       this.body = body;
   }
}

public class ApiResponseBody<T> {

   @SerializedName("data")
   private T data;

   @SerializedName("errors")
   private Map<String, List<String>> errors;

   public ApiResponseBody() {
   }

   public ApiResponseBody(T data, Map<String, List<String>> errors) {
       this.data = data;
       this.errors = errors;
   }
}

And use it in usual way

@POST("users/signup")
Single<ApiResponse<RegisterResponseData>> register(@Body RegisterRequest request);