How to Successfully Deserialize Json from String with Rest Client and Object Mapper in Java

Learn how to handle tricky JSON responses in Java 17 with Spring Boot, particularly when dealing with double serialization issues using Rest Client and Object Mapper.
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Understanding the Challenge of Deserializing JSON Strings in Java

When building a REST API in Java, particularly with frameworks like Spring Boot, you often need to interact with external APIs. A common scenario is encountering unexpected data formats, such as receiving a JSON string instead of a standard JSON object. This is the problem faced by many developers, including one dealing with responses from an external API that returns JSON as a string. Let's break down how to handle this issue effectively.

The Problem Statement

Imagine you're making API calls from your Java application to an external service. You expect a clean JSON object, but instead, the external API is returning a JSON string like this:

[[See Video to Reveal this Text or Code Snippet]]

This is problematic because most Java JSON deserialization libraries (like Jackson, which is commonly used with Spring Boot) expect actual JSON objects instead of string representations of them. The challenge, therefore, is to convert such JSON strings back into usable Java objects.

Understanding Double Serialization

The underlying issue in this case is what we can refer to as double serialization. This occurs when your data is serialized (converted to JSON format) more than once. The external API first creates a JSON representation, which is then serialized into a string format that includes the quotes around the JSON object itself. The result is a string that is not directly usable for deserialization.

The Solution: Two Rounds of Deserialization

To tackle this problem, you need to perform two rounds of deserialization. Here's how it works:

Deserialize the String response into a Java String: The first step is to read the entire JSON string as a raw Java String. This is essential because your original JSON is encapsulated in quotes and is considered a string type.

Deserialize the Java String into the desired Java object: Once you have the raw JSON string, you perform a second round of deserialization to convert it into the appropriate Java class object.

Step-by-Step Implementation

Here’s a code example to illustrate this:

[[See Video to Reveal this Text or Code Snippet]]

Breaking Down the Code

ObjectMapper: This is a core component in the Jackson library that is responsible for converting Java objects to JSON and vice versa.

Mapping to ApiResponse: Finally, you pass the intermediate string representation to another readValue call to convert it into the ApiResponse object that you initially intended to work with.

Final Thoughts

Handling unexpected formats in API responses can be frustrating, but with a structured approach to deserialization, you can effectively manage these scenarios. By understanding the double serialization problem and applying the two-step deserialization process, you can effortlessly convert problematic JSON strings back into usable Java objects. This pragmatic approach ensures that your applications remain robust and flexible in the face of irregularities in external API responses.

If you find yourself facing similar challenges, remember this two-step method, and you'll be well-equipped to seamlessly integrate with various REST APIs.