How to Deserialize JSON with Nested Objects in Kotlin

Learn how to correctly deserialize a JSON array into Kotlin data classes using Jackson and RestTemplate, tackling common issues like HttpMessageNotReadableException.
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Understanding JSON Deserialization in Kotlin

When working with JSON data in Kotlin, particularly when dealing with nested objects in arrays, it can be a bit tricky. A common predicament developers face is the inability to deserialize JSON correctly into Kotlin data classes.

In this post, we will discuss a specific scenario where a JSON response consists of a list item that includes another object. We'll break down an example scenario and provide a clear solution to overcome this issue.

The Problem Statement

Imagine you are making a request to a REST API, and you receive the following JSON response:

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This JSON structure is an array containing one Object with a single property source. Now, when you attempt to deserialize this JSON into a Kotlin class like this:

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You may encounter an error:

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This error occurs because the deserializer is expecting a JSON object, but it is receiving a JSON array instead.

The Solution

Recognize the Structure

The main point to understand here is that the entire response is a JSON array, not a single JSON object. In Kotlin, due to this array structure, you cannot directly map it to a class like AbcdResponse.

Step 1: Modify Your Data Classes

Instead of attempting to map the JSON directly to AbcdResponse, you should create a Kotlin data class that matches the JSON array structure. In this case, you can change your approach by using a list of DaDataAddress.

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Step 2: Update Your Deserialization Code

Now, update your post call to reflect this change. Instead of trying to deserialize it to AbcdResponse, you would do it like this:

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Key Takeaways

Understand JSON Structure: Be aware if you're dealing with an array or an object to avoid deserialization issues.

Use Lists for Arrays: If the JSON response is an array, always map it to a Kotlin list of type List<T> where T matches the object within the array.

Utilize Kotlin Data Classes: Use Kotlin data classes to represent the JSON objects effectively.

Conclusion

Deserializing JSON correctly can sometimes be challenging, especially when dealing with nested structures. Understanding the structure of the JSON you are working with allows you to create the proper data classes and use them effectively.

By adapting your Kotlin data classes and method calls based on the JSON structure, you can avoid common exceptions, such as HttpMessageNotReadableException. With these tips, you should find yourself better equipped to handle JSON data in Kotlin.