How Can I Efficiently Extract Nested JSON Data Using Retrofit and Gson?

Get Nested JSON Object with GSON Using Retrofit

Problem Overview:

When consuming an API with JSON responses structured like:

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

where the desired POJO data is nested within the "content" field, Retrofit by default deserializes the entire JSON response into a POJO with "status" and "reason" fields, but not the actual data of interest.

Customizing Deserialization:

To extract the nested content field automatically, you can create a custom deserializer using Gson:

Content POJO:

Define a POJO class representing the nested data, such as:

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

Deserializer:

Implement a JsonDeserializer that extracts the "content" element from the JSON response and deserializes it into the Content POJO:

class MyDeserializer implements JsonDeserializer<Content> {
    @Override
    public Content deserialize(...) {
        JsonElement content = je.getAsJsonObject().get("content");
        return new Gson().fromJson(content, Content.class);
    }
}

Gson Configuration:

Create a Gson instance with GsonBuilder, registering the custom deserializer:

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

Enhanced Deserializer for Multiple Types:

If you have multiple types of messages with "content" fields, you can generalize the deserializer as follows:

class MyDeserializer<T> implements JsonDeserializer<T> {
    @Override
    public T deserialize(...) {
        JsonElement content = je.getAsJsonObject().get("content");
        return new Gson().fromJson(content, type);
    }
}

Register an instance of this deserializer for each of your content types.

Retrofit Integration:

Pass the customized Gson instance to the Retrofit builder:

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

With this setup, Retrofit will now automatically extract the "content" field and parse it into the appropriate POJO, eliminating the need for manual field extraction.

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