Get nested JSON object with GSON using retrofit

Question

I'm consuming an API from my android app, and all the JSON responses are like this:

{
    'status': 'OK',
    'reason': 'Everything was fine',
    'content': {
         < some data here >
}

The problem is that all my POJOs have a status, reason fields, and inside the content field is the real POJO I want.

Is there any way to create a custom converter of Gson to extract always the content field, so retrofit returns the appropiate POJO?

Answer 1

You would write a custom deserializer that returns the embedded object.

Let's say your JSON is:

{
    "status":"OK",
    "reason":"some reason",
    "content" : 
    {
        "foo": 123,
        "bar": "some value"
    }
}

You'd then have a Content POJO:

class Content
{
    public int foo;
    public String bar;
}

Then you write a deserializer:

class MyDeserializer implements JsonDeserializer<Content>
{
    @Override
    public Content deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
        throws JsonParseException
    {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("content");

        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        return new Gson().fromJson(content, Content.class);

    }
}

Now if you construct a Gson with GsonBuilder and register the deserializer:

Gson gson = 
    new GsonBuilder()
        .registerTypeAdapter(Content.class, new MyDeserializer())
        .create();

You can deserialize your JSON straight to your Content:

Content c = gson.fromJson(myJson, Content.class);

Edit to add from comments:

If you have different types of messages but they all have the "content" field, you can make the Deserializer generic by doing:

class MyDeserializer<T> implements JsonDeserializer<T>
{
    @Override
    public T deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
        throws JsonParseException
    {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("content");

        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        return new Gson().fromJson(content, type);

    }
}

You just have to register an instance for each of your types:

Gson gson = 
    new GsonBuilder()
        .registerTypeAdapter(Content.class, new MyDeserializer<Content>())
        .registerTypeAdapter(DiffContent.class, new MyDeserializer<DiffContent>())
        .create();

When you call .fromJson() the type is carried into the deserializer, so it should then work for all your types.

And finally when creating a Retrofit instance:

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(url)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();