Convert JSON String to generic object in JAVA (with GSON)

Question

I have an Api that returns JSON. The response is in some format that can fit into an object called ApiResult and contains a Context <T> and an int Code.

ApiResult is declared in a generic way, e.g. ApiResult<SomeObject>

I would like to know how to get GSON to convert the incoming JSON String to ApiResult<T>

So far I have:

Type apiResultType = new TypeToken<ApiResult<T>>() { }.getType();
ApiResult<T> result = gson.fromJson(json, apiResultType);

But this still returns converts the Context to a LinkedHashMap instead (which I assume its what GSON falls back to)

Answer 1

You have to know what T is going to be. The incoming JSON is fundamentally just text. GSON has no idea what object you want it to become. If there's something in that JSON that you can clue off of to create your T instance, you can do something like this:

public static class MyJsonAdapter<X> implements JsonDeserializer<ApiResult<X>>
{
    public ApiResult<X> deserialize( JsonElement jsonElement, Type type, JsonDeserializationContext context )
      throws JsonParseException
    {
      String className = jsonElement.getAsJsonObject().get( "_class" ).getAsString();
      try
      {
        X myThing = context.deserialize( jsonElement, Class.forName( className ) );
        return new ApiResult<>(myThing);
      }
      catch ( ClassNotFoundException e )
      {
        throw new RuntimeException( e );
      }
    }
}

I'm using a field "_class" to decide what my X needs to be and instantiating it via reflection (similar to PomPom's example). You probably don't have such an obvious field, but there has to be some way for you to look at the JsonElement and decide based on what's itn it what type of X it should be.

This code is a hacked version of something similar I did with GSON a while back, see line 184+ at: https://github.com/chriskessel/MyHex/blob/master/src/kessel/hex/domain/GameItem.java