How to Fix Null Pointer Exception in Spring Boot @RestController Using Constructor Injection

Discover how to resolve the Null Pointer Exception in a Spring Boot `-RestController` when utilizing constructor injection. Learn what changes to make in your code to ensure proper functionality.
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Solving Null Pointer Exception in Spring Boot -RestController with Constructor Injection

When developing Spring Boot applications, encountering a Null Pointer Exception can be frustrating, especially if you believe your configuration is correctly set up, just like in your previous projects. In this post, we’ll unravel the common issues that lead to null exceptions when using constructor injection in a -RestController and provide a clear solution to ensure a smooth application flow.

The Problem: Null Pointer Exception at Startup

The application starts without issues, but upon making a request to the -RestController, a Null Pointer Exception is thrown, indicating that the -Service being injected is null. Here’s what the error log might typically look like:

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

The controller is expected to handle requests that involve fetching data based on a country code, city, and optional IP address. However, because of this null exception, the application never processes the request fully.

Common Reasons for Null Pointer Exception

Incorrect Access Modifiers: If you inadvertently set the access modifier of your method (in this case, getDistance) to private, Spring will not be able to access it to fulfill requests.

Missing -Autowired Annotation: It’s crucial to ensure Spring is aware of your beans in the context. Without the -Autowired annotation, dependency injection will fail.

Spring Bean Issues: If the Spring framework has not managed an instance of the service or repository, it would lead to null references.

The Solution: Making the GetMapping Method Public

Step 1: Change the Access Modifier

The primary fix in this case is straightforward. You need to change the access modifier of the getDistance method in your DistanceController class from private to public:

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

Step 2: Use -Autowired for Injection (if needed)

You might also consider using the -Autowired annotation on the constructor parameters to ensure that Spring correctly injects your dependencies:

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

Step 3: Verify Other Bean Configurations

Ensure that all your services and repositories are correctly annotated with -Service or -Repository respectively. For instance, your ICountriesRepository should look like this:

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

Also, confirm your custom configurations like ModelMapper are set up correctly:

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

Conclusion

By adjusting your method's access modifier from private to public, you resolve the null pointer exception caused by Spring’s inability to call the method when handling requests.

If you find further issues even after these changes, carefully review your other injected services and ensure they are being instantiated correctly in the Spring context. Adhering to these best practices not only helps mitigate issues but also strengthens your understanding of dependency injection in Spring Boot.

Thank you for reading! Have you encountered similar problems while working on your Spring Boot projects? Share your experiences in the comments below!