Solving JSON Serialization Issues in Spring WebFlux Testing with WebTestClient

Описание: Discover how to effectively handle JSON serialization issues in your Spring WebFlux testing environment using `WebTestClient` instead of `MockMvc`.
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This video is based on the question https://stackoverflow.com/q/74351288/ asked by the user 'Addison Joseph' ( https://stackoverflow.com/u/8759986/ ) and on the answer https://stackoverflow.com/a/74351950/ provided by the user 'Lesiak' ( https://stackoverflow.com/u/1570854/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving JSON Serialization Issues in Spring WebFlux Testing

When developing with Spring WebFlux, you may encounter a common issue during testing: While the application returns a JSON object as expected when run, your integration tests tend to return the actual POJO (Plain Old Java Object) instead of a serialized JSON response. This can be particularly frustrating when you're trying to mock service responses in your tests. Let's explore how to troubleshoot and solve this issue effectively.

Understanding the Problem

In your case, you're testing a controller that provides an asynchronous response. When performing integration tests, you might see this:

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This indicates that the test is returning the actual object instead of a JSON response. Running the application in an environment like Postman behaves as expected and returns a correct JSON object, leading to confusion regarding the testing framework's behavior.

Solution Overview

To address the issue, it's recommended to switch from the traditional MockMvc approach to the more suitable WebTestClient for testing in a WebFlux environment. Here's a structured approach to implementing this change:

Step 1: Use WebFluxTest Instead of WebMvcTest

By replacing -WebMvcTest with -WebFluxTest, you optimize your testing configuration for the reactive model provided by Spring WebFlux. This ensures that your test context is correctly set up for asynchronous requests.

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Step 2: Use WebTestClient Instead of MockMvc

The WebTestClient is designed explicitly for testing WebFlux applications, providing a fluent API for performing requests and asserting responses. Replace MockMvc with WebTestClient in your test setup as follows:

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Step 3: Update Your Test Method

Now, modify your test case to utilize WebTestClient. The revised function should look like this:

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This code snippet demonstrates how to make a GET request to the controller endpoint and validate the JSON response returned.

Additional Considerations

Argument Matcher: You should also fine-tune the argument matcher in your mock to ensure better alignment with your controller's expected input. Instead of using any(), consider using eq() for the OtcBalanceRequest. This helps ensure that the request is properly deserialized and processed by the controller.

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This step enhances your test's reliability, guaranteeing that the correct request structure is being passed to your service.

Conclusion

Testing in Spring WebFlux requires an understanding of the reactive approach to application design. By utilizing WebTestClient instead of MockMvc, you align your testing strategy with the asynchronous behavior of your application, ensuring that you receive the expected JSON responses in your tests. This adjustment not only resolves your immediate issue but also improves the overall robustness of your testing process.

With these steps, you should see your tests passing, ultimately delivering a much more satisfying development experience. Happy coding!