The Subtle Pitfall of @RequiredArgsConstructor: A Lesson from Integration Testing2

Photo by Jeane Liu on Unsplash

Introduction:

Recently, I encountered a frustrating issue in our Spring Boot application test suite. One specific test kept failing with a 500 Internal Server Error while similar tests were succeeding. After hours of troubleshooting, the culprit turned out to be a single missing keyword — final. Let me walk you through the debugging journey and explain why this tiny oversight had such a significant impact.

The Problem:

I had a test case shouldSuccessfullyCallMyService() that consistently failed with a SERVER_ERROR, while similar tests like shouldSuccessfullyCallYourService() worked perfectly fine. Both tests used MockWebServer to simulate backend service responses, but only one was failing.

void shouldSuccessfullyCallMyService() {
    final String myServiceUrl = MY_SERVICE.replace("{resourceId}", RESOURCE_ID);
    final MyServiceResponse expectedMySertviceResponse = getStatus200MyServiceResponse();

    mockServiceServer.enqueue(
            new MockResponse()
                    .setResponseCode(200)
                    .setBody(convertObjectToJsonString(expectedMyServiceResponse))
                    .addHeader(CONTENT_TYPE, APPLICATION_JSON_VALUE));

    webTestClient.get().uri(myServiceUrl)
            // Headers and assertions omitted for brevity
            .expectStatus()
            .is2xxSuccessful();
}

The Investigation:

My first instinct was to check the differences between the passing and failing tests:

1. HTTP Method: The failing test used GET while successful tests used POST
2. Response Structure: Perhaps issues in the response model?
3. Mock Server Configuration: Different configurations between tests?

I added debug logs to see what was happening:

System.out.println("Mock server URL: " + mockServiceServer.url(myServiceUrl));
System.out.println("Response body: " + convertObjectToJsonString(expectedMyServiceResponse));

The logs showed the mock server was correctly configured, and the response body looked good. The most puzzling aspect was that the test was failing with a 500 error, suggesting an exception in our controller or service layer.

The Breakthrough:

After ruling out issues with the test configuration, I shifted focus to comparing the controllers handling these endpoints. The YourServiceController handled the successful endpoints, while MyServiceControllerhandled the failing endpoint.

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Looking at both controllers side by side revealed the subtle but critical difference:

// YourController.java - working correctly
@RequiredArgsConstructor
public class YourController implements YourApi {
    private final YourService yourService;
    // Other services...
}

// MyController.java - failing
@RequiredArgsConstructor
public class MyController implements MyApi {
    private MyService myService; // Missing 'final' keyword!
    // ...
}

The final keyword was missing from the service field declaration in MyController!

Why This Matters:

This tiny omission had massive consequences because of how Lombok’s @RequiredArgsConstructor works:

1. @RequiredArgsConstructor generates a constructor only for fields marked as final
2. Spring uses this constructor for dependency injection
3. Without final, no constructor parameter is generated for the service
4. Spring can’t inject the dependency, leaving the service as null
5. When the controller tries to use the service, it throws a NullPointerException
6. The exception bubbles up as a 500 Internal Server Error

The Fix:

@RequiredArgsConstructor
public class MyController implements MyApi {
    private final MyService myService; // adding 'final' keyword!
    // ...
}

After adding the final keyword, Spring could properly inject the dependency, and the test passed successfully.

Lessons Learned:

1. Be consistent with field declarations: When using Lombok’s @RequiredArgsConstructor, always mark dependencies as final.
2. Understand your annotations: Know exactly how annotations like @RequiredArgsConstructor behave.
3. Look beyond test configuration: When tests fail with server errors, examine your application code carefully.

A similar discussion can be found in https://medium.com/@luketong/the-subtle-pitfall-of-requiredargsconstructor-a-lesson-from-integration-testing-38f8b93f1d79

Conclusion:

This debugging journey highlights how a single keyword can make or break a Spring application. It also demonstrates why understanding the frameworks and libraries we use is crucial — what seemed like a complex issue with mock servers or HTTP methods was actually a fundamental misunderstanding of how Lombok and Spring work together.

Next time you encounter a 500 error in your Spring Boot tests, remember to check if your dependencies are properly declared and configured for injection. Sometimes the smallest details make all the difference!