Скачать или смотреть Fixing NullPointerException in Spring Boot Controller

Learn how to resolve the `NullPointerException` error occurring in your Spring Boot application's controller when handling a request. This guide walks you through identifying the issue and implementing a solution effectively.
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Fixing NullPointerException in Spring Boot Controller: A Step-by-Step Guide

When working with a Spring Boot application, developers may occasionally encounter errors that can disrupt the normal flow of data. One such common issue is the NullPointerException thrown within a controller when trying to save an object to the database. This can be particularly frustrating when the application continues to run without any indications of failure.

Recently, a developer faced a NullPointerException due to an issue with the JSON format in the request body, specifically with how the ToDoDto object was handled. Let’s explore this problem in detail and understand how to resolve it.

The Encountered Problem

The developer was trying to send a POST request to save a to-do item but received a 400 Bad Request response. The issue presented as follows:

The JSON input was structured correctly, but when the ToDoRequest was processed, the ToDoDto value was null.

The relevant pieces of code included a controller that received the request, a service that processed the data, and various data transfer objects (DTOs) that facilitated data handling.

Sample POST Request

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

The Code Snippet

This is the controller code that received the request:

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Upon examining the stack trace, it was confirmed that a NullPointerException occurred when the method attempted to process the ToDoDto object.

Solution to the Problem

After assessing the situation, we can identify the root cause: a mismatch between the JSON property name and the variable name in the ToDoRequest class. The JSON object used ToDoDto while the corresponding field in ToDoRequest was named toDoDto.

Step-by-Step Fix

Here’s how to fix the issue:

Annotate the Field in ToDoRequest: Ensure that the field in ToDoRequest is correctly linked to the JSON property. You can achieve this using the @ JsonProperty annotation from the Jackson library.

Modify the ToDoRequest class:
Change the definition of the toDoDto field as follows:

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

Updated ToDoRequest Class

Here’s the updated class definition after applying the fix:

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

By adding the @ JsonProperty annotation, we are informing the Jackson library that the JSON key ToDoDto should populate the toDoDto field in the ToDoRequest class.

Conclusion

With that small tweak, the NullPointerException caused by the null value of ToDoDto in the request body should be resolved. This example highlights the importance of aligning your JSON payload structure with the expectations of your Java objects.

For anyone working with Spring Boot and JSON, it's essential to ensure that your data-binding configuration correctly maps properties between DTOs and the incoming JSON format. With diligence, these kinds of issues can be quickly identified and remedied, paving the way for smoother development processes.

By following this guide, developers can tackle similar issues with confidence, improving the reliability of their applications while reducing debugging time.