How to Map JSON to an Object Containing a List of Objects in Spring Boot

Discover how to effectively map JSON data to complex objects in Spring Boot, including resolving common errors like null pointer exceptions.
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How to Map JSON to an Object Containing a List of Objects in Spring Boot

When working with Spring Boot, you may encounter challenges when mapping JSON to Java objects, particularly for complex objects containing lists. One common issue is a NullPointerException, which typically arises from incorrect JSON structures. In this guide, we will delve into the problem and provide a comprehensive solution to successfully map JSON to your Java classes.

The Problem

Suppose you are building a movie review website and you have the following model classes: Movie and Actor. Here's a brief overview of their structure:

Movie class: Represents a movie, containing fields like movieId, name, releaseDate, director, cast (a list of Actor objects), and more.

Actor class: Represents an actor, with details such as actorId, name, image, dateOfBirth, and other properties.

Here's an example of the JSON you are trying to pass through the API:

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

Error Encountered

As observed from the logs, when the JSON is processed, the cast field remains null which is not the expected behavior. The JSON structure contains the cast field twice, one of which is explicitly set to null. This leads to the NullPointerException during the mapping process as Spring Boot cannot correctly map the list of Actor objects.

The Solution

Step-By-Step Fix

To resolve this issue and ensure that your JSON maps correctly to the Movie object, follow these steps:

Remove Duplicate Fields: Ensure that the cast field is defined only once in your JSON. Your JSON structure should look like the following after correction:

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

Review Data Types: Ensure that all the attributes in your JSON correspond correctly to the expected data types in your Java classes. For example, ensure that dates are in proper String formats (yyyy-MM-dd) that can be parsed into LocalDate in Java.

Testing the API: Once you've made these corrections, use tools like Swagger or Postman to test the updated JSON structure. You should observe that the cast field now correctly receives a list of Actor objects.

Conclusion

By maintaining a clean and accurate JSON structure, you can avoid common mapping issues in Spring Boot. Always ensure that your JSON has no duplicate keys and matches the expected data types of your Java models.

If you still encounter issues, checking your server logs and debugging step-by-step can help isolate the problem. Happy coding!