How Can I Customize Retrofit\'s GSON Converter to Extract Nested JSON Objects?

Customizing Retrofit GSON Converter to Retrieve Nested JSON Objects

When consuming APIs that embed your desired data within nested JSON structures, leveraging GSON with Retrofit can be challenging. This question explores a solution to extract the specific content object from a nested API response.

The JSON response in question follows a pattern where the data you need is placed within a "content" field. The goal is to create a GSON converter that directly retrieves this content, disregarding the outer layers.

Solution: Creating a Custom Deserializer

The solution revolves around creating a custom deserializer that focuses on extracting the embedded object. The deserializer overrides the "deserialize" method and performs the following steps:

1. Retrieves the "content" element from the parsed JSON.
2. Utilizes a fresh Gson instance to deserialize it (preventing recursion).
3. Returns the resulting object, typically the desired data structure.

Registering the Deserializer and Creating Gson

To make the custom deserializer available to Retrofit, it needs to be registered using GsonBuilder. Register a separate instance for each type you encounter with nested JSON. The resulting Gson instance can then be used in the Retrofit client.

Example Implementation

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

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

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

By adopting this approach, your Retrofit client can seamlessly deserialize your desired data objects, even when they are deeply nested within the API response.

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