How to Extract Nested JSON Objects from API Responses using Gson and Retrofit?

Extracting Nested JSON Objects Using Gson and Retrofit

In modern applications, it's common to encounter API responses with nested JSON structures. This can pose challenges when you want to work with specific data fields within the nested hierarchy. This guide will demonstrate how to create a custom Gson deserializer to extract the desired content from nested JSON objects using Retrofit.

Problem Statement

Consider an API response with the following structure:

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

You have POJOs with status and reason fields, but the data you need resides within the nested content object.

Solution

To extract the nested content, you'll create a custom deserializer that wraps the actual POJO deserializer. Here's how it works:

1. Create a Content POJO: Define a POJO to represent the nested content data.
2. Implement a Deserializer: Create a deserializer that extracts the content field:

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

    // Deserialize it using a new Gson instance to avoid recursion
    return new Gson().fromJson(content, Content.class);
  }
}

3. Register the Deserializer: Configure Gson with your custom deserializer:

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

4. Use Retrofit with Custom Converter: Create a Retrofit instance using the modified Gson:

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

Now, you can deserialize your API responses and access the nested content directly as Content objects using Retrofit.

Generic Deserializer for Multiple Types

If you have multiple types of messages sharing a "content" field, you can create a generic deserializer as follows:

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

    // Deserialize it using a new Gson instance to prevent recursion
    return new Gson().fromJson(content, type);
  }
}

Register this deserializer for each of your content types. Retrofit will automatically use the appropriate deserializer based on the type of the API response.

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