Custom JSON deserializer using Gson

Question

I have a problem with parsing a JSON response using Gson.

JSON string:

response: [
  2, {
    owner_id: 23972237,
    album_id: 25487692,
    title: 'album not new'
  }, {
    owner_id: 23972237,
    album_id: 25486631,
    title: 'фыв'
  }
]

I have these 2 classes:

public class VkAudioAlbumsResponse {
    public ArrayList<VkAudioAlbum> response;
    public VkError error;
}

public class VkAudioAlbum {
    public int owner_id;
    public int album_id;
    public String title;
}

But I have an Exception when parse this using Gson. I know this is because response array first element is not an object, but integer.

So the question is, can I solve it somehow?

Answer 1

You have to write a custom deserializer. I'd do something like this:

First you need to include a new class, further than the 2 you already have:

public class Response {
    public VkAudioAlbumsResponse response;
}

And then you need a custom deserializer, something similar to this:

private class VkAudioAlbumsResponseDeserializer 
    implements JsonDeserializer<VkAudioAlbumsResponse> {

  @Override
  public VkAudioAlbumsResponse deserialize(JsonElement json, Type type,
        JsonDeserializationContext context) throws JsonParseException {

    JsonArray jArray = (JsonArray) json;

    int firstInteger = jArray.get(0); //ignore the 1st int

    VkAudioAlbumsResponse vkAudioAlbumsResponse = new VkAudioAlbumsResponse();

    for (int i=1; i<jArray.size(); i++) {
      JsonObject jObject = (JsonObject) jArray.get(i);
      //assuming you have the suitable constructor...
      VkAudioAlbum album = new VkAudioAlbum(jObject.get("owner_id").getAsInt(), 
                                            jObject.get("album_id").getAsInt(), 
                                            jObject.get("title").getAsString());
      vkAudioAlbumsResponse.getResponse().add(album);
    }

    return vkAudioAlbumsResponse;
  }  
}

Then you have to deserialize your JSON like:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(VkAudioAlbumsResponse.class, new VkAudioAlbumsResponseDeserializer());
Gson gson = gsonBuilder.create();
Response response = gson.fromJson(jsonString, Response.class);

With this approach, when Gson tries to deserialize the JSON into Response class, it finds that there is an attribute response in that class that matches the name in the JSON response, so it continues parsing.

Then it realises that this attribute is of type VkAudioAlbumsResponse, so it uses the custom deserializer you have created to parse it, which process the remaining portion of the JSON response and returns an object of VkAudioAlbumsResponse.

Note: The code into the deserializer is quite straightforward, so I guess you'll have no problem to understand it... For further info see Gson API Javadoc