Gson deserialize JSON array with multiple object types

Question

I have some odd JSON like:

[
  {
    "type":"0",
    "value":"my string"
  },
  {
    "type":"1",
    "value":42
  },
  {
    "type":"2",
    "value": {
    }
  }
]

Based on some field, the object in the array is a certain type. Using Gson, my thought is to have a TypeAdapterFactory that sends delegate adapters for those certain types to a TypeAdapter, but I'm hung up on understanding a good way of reading that "type" field to know which type to create. In the TypeAdapter,

Object read(JsonReader in) throws IOException {
  String type = in.nextString();
  switch (type) {
    // delegate to creating certain types.
  }
}

would assume the "type" field comes first in my JSON. Is there a decent way to remove that assumption?

Andrew Kelly · Accepted Answer · 2016-04-23 12:30:07Z

Here is some code I wrote to handle an array of NewsFeedArticle and NewsFeedAd items in Json. Both items implement a marker interface NewsFeedItem to allow me to easily check if the TypeAdater should be used for a particular field.

public class NewsFeedItemTypeAdapterFactory implements TypeAdapterFactory {

    @Override
    @SuppressWarnings("unchecked")
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        if (!NewsFeedItem.class.isAssignableFrom(type.getRawType())) {
            return null;
        }

        TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
        TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter = gson.getDelegateAdapter(this, TypeToken.get(NewsFeedArticle.class));
        TypeAdapter<NewsFeedAd> newsFeedAdAdapter = gson.getDelegateAdapter(this, TypeToken.get(NewsFeedAd.class));

        return (TypeAdapter<T>) new NewsFeedItemTypeAdapter(jsonElementAdapter, newsFeedArticleAdapter, newsFeedAdAdapter).nullSafe();
    }

    private static class NewsFeedItemTypeAdapter extends TypeAdapter<NewsFeedItem> {

        private final TypeAdapter<JsonElement> jsonElementAdapter;
        private final TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter;
        private final TypeAdapter<NewsFeedAd> newsFeedAdAdapter;

        NewsFeedItemTypeAdapter(TypeAdapter<JsonElement> jsonElementAdapter,
                TypeAdapter<NewsFeedArticle> newsFeedArticleAdapter,
                TypeAdapter<NewsFeedAd> newsFeedAdAdapter) {
            this.jsonElementAdapter = jsonElementAdapter;
            this.newsFeedArticleAdapter = newsFeedArticleAdapter;
            this.newsFeedAdAdapter = newsFeedAdAdapter;
        }

        @Override
        public void write(JsonWriter out, NewsFeedItem value) throws IOException {
            if (value.getClass().isAssignableFrom(NewsFeedArticle.class)) {
                newsFeedArticleAdapter.write(out, (NewsFeedArticle) value);

            } else if (value.getClass().isAssignableFrom(NewsFeedAd.class)) {
                newsFeedAdAdapter.write(out, (NewsFeedAd) value);

            }

        }

        @Override
        public NewsFeedItem read(JsonReader in) throws IOException {
            JsonObject objectJson = jsonElementAdapter.read(in).getAsJsonObject();

            if (objectJson.has("Title")) {
                return newsFeedArticleAdapter.fromJsonTree(objectJson);

            } else if (objectJson.has("CampaignName")) {
                return newsFeedAdAdapter.fromJsonTree(objectJson);
            }

            return null;
        }
    }
}

You can then register this with Gson using the following code.

return new GsonBuilder()
        .registerTypeAdapterFactory(new NewsFeedItemTypeAdapterFactory())
        .create();

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