Resolving MismatchedInputException in Spring Boot with Jackson

Learn how to correctly deserialize JSON responses in Spring Boot, addressing the `MismatchedInputException` when mapping JSON data to Java objects using Jackson.
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Resolving MismatchedInputException in Spring Boot with Jackson: A Step-by-Step Guide

When working with external APIs in a Spring Boot application, you may encounter various issues while deserializing JSON responses into Java objects. One common error is the MismatchedInputException from the Jackson library, indicating that the expected object structure in Java does not match the JSON structure received.

In this guide, we will dissect a specific case of this error and guide you on how to correctly map your JSON response to Java classes, ensuring a seamless integration between JSON data and your Spring Boot application.

Understanding the Problem

Suppose you are trying to call an external API and retrieve data in a JSON format similar to the following:

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

You aim to save the above response into a HistoricalApiResponse DTO (Data Transfer Object). However, upon attempting the deserialization, you encounter an error message indicating a mismatch between the JSON tokens and your Java structure.

Analyzing the JSON Response Structure

The JSON response contains:

A key personsData, which is an object.

Inside personsData, the keys are dates (e.g., "2023-02-27"), and their values are lists of Person objects.

This means your Java DTO structure must reflect this nesting and the relationship between the keys (dates) and values (lists of persons).

Proposed Solution

To correctly deserialize the JSON response, you need to modify your Java classes as follows:

Step 1: Create the Correct Java Structure

Your HistoricalApiResponse should be structured to match the JSON format. Here’s how you can define your Java classes:

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

Step 2: Add Correct Field Annotations

Use -JsonProperty to ensure Jackson maps the JSON field updated_at to the Java field updatedAt.

Ensure that your field names correspond to the fields in your JSON.

Step 3: Testing the Implementation

To test this implementation, you can use the following code snippet:

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

Step 4: Run and Validate

Make sure to replace the json string in the test with the complete JSON structure from your API response.

Run the test to verify that the deserialization occurs as expected without throwing any exceptions.

Conclusion

By structuring your Java classes to match the JSON response format and carefully using Jackson annotations, you can successfully avoid the notorious MismatchedInputException. This approach not only ensures that your application can effectively handle external API calls but also enhances the maintainability of your code.

By following these steps, you should have a robust framework for handling JSON deserialization in your Spring Boot application.

For further reading, consider exploring more about Jackson annotations and customizing the serialization/deserialization process for complex objects.

Feel free to reach out if you face any more issues!