Making GsonRequest to accept empty list or null array

问题

I have a Json data to be pulled from a server. This data contains several objects and arrays.

The first model is as follows:

{
  "results": [
    {
      "id": "17",
      "name": "Accessories",
      "child": [
        {
          "id": "371",
          "name": "Belt"
        },
        {
          "id": "55",
          "name": "Derp"
        },
        ...
      ]
    }
  ]
}

However, some of the results array doesn't have child array. Instead, it have a String with an empty value.

{
  "results": [
    {
      "id": "19",
      "name": "Stuff",
      "child": ""
    }
  ]
}

When the code is executed, it returns this line:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING

This is how the Model looks like:

public class CategoryModel {
    @SerializedName("id")
    private String category_id;
    private String name;
    private ArrayList<CategoryChildModel> child;

    ...
}

And this is how I implement the GsonRequest (which using Volley as background asynctask):

private void loadCategory() {
    mRequestQueue = Volley.newRequestQueue(getActivity());

    String url = Constants.CATEGORIES_LIST;

    GsonRequest<CategoryContainer> myReq = new GsonRequest<CategoryContainer>(
            Request.Method.GET, url, CategoryContainer.class,
            createMyReqSuccessListener(), createMyReqErrorListener());

    mRequestQueue.add(myReq);
}

So, anyone knows how to make null object pass through GsonRequest?

回答1:

Actually your json response should return an empty array not a string for null cases. But if you don't have an option to change server's response then you may try to write a custom json deserializer:

class ChildDeserializer implements JsonDeserializer<ChildHolder> {
    @Override
    public ChildHolder deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        String currentValueOfChild = json.toString();
        Log.d("ChildDeserializer", "ChildDeserializer: child=" + currentValueOfChild);

        ChildHolder childHolder = null;
        if (json instanceof JsonArray) {
            Log.d("ChildDeserializer", "ChildDeserializer: We have an array for 'child'");

            Type listType = new TypeToken<List<Child>>() {}.getType();

            JsonArray jsonArray= json.getAsJsonArray();

            childHolder = new ChildHolder();
            childHolder.childList = context.deserialize(jsonArray, listType);
        }

        return childHolder;
    }
}

Your response java model should look like below:

class Response {
    List<Result> results;
}

class Result {
    private String id, name;
    private ChildHolder child;
}

class ChildHolder {
    private List<Child> childList;
}

class Child {
    private String id, name;
}

Apply deserializer while parsing json to java model:

String jsonTest1 = "{\"results\":[{\"id\":\"17\",\"name\":\"Accessories\",\"child\":[{\"id\":\"371\",\"name\":\"Belt\"},{\"id\":\"55\",\"name\":\"Derp\"}]}]}";
String jsonTest2 = "{\"results\":[{\"id\":\"19\",\"name\":\"Stuff\",\"child\":\"\"}]}";

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(ChildHolder.class, new ChildDeserializer());

Gson gson = gsonBuilder.create();
Response response1 = gson.fromJson(jsonTest1, Response.class);
Response response2 = gson.fromJson(jsonTest2, Response.class);

Also please read this link for further information.

回答2:

Another option for writing a custom serializer is to simply convert the empty string to an empty array in the JSON. Then your class can remain the way it is:

class ChildDeserializer implements JsonDeserializer<CategoryModel>
{
    @Override
    public ChildHolder deserialize(JsonElement json, Type typeOfT, 
                                   JsonDeserializationContext context) 
                                       throws JsonParseException {

        JsonObject obj = json.getAsObject();
        JsonElement e = obj.get("child");
        if (e.isJsonPrimitive()) // it's a String
        {
            obj.remove("child");
            obj.add("child", new JsonArray());
        }

        return new Gson().fromJson(obj, CategoryModel.class);
    }
}

来源：https://stackoverflow.com/questions/22142767/making-gsonrequest-to-accept-empty-list-or-null-array