Скачать или смотреть Solving HttpMessageNotWritableException with Spring Boot's MockMvc

Discover how to resolve `HttpMessageNotWritableException` errors in your Spring Boot test cases with MockMvc by properly organizing your packages.
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Understanding the Problem: HttpMessageNotWritableException in Spring Boot

If you're a developer working with Spring Boot, you've likely encountered the dreaded HttpMessageNotWritableException error when running controller tests using MockMvc. This commonly indicates that Spring is unable to convert your Java objects into JSON format (or any other format specified) for the response when the corresponding endpoint is hit.

In this post, we'll explore a specific case of this error, identify its cause, and provide a clear solution.

The Scenario

You have a REST controller set up in Spring Boot for handling requests related to a User entity. When you attempt to execute tests for this controller using MockMvc, you receive the following error message:

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

This indicates that Spring could not find a suitable message converter to serialize the User class into JSON format, despite the controller working fine when hit directly via Postman.

Analyzing the Code

Your test class looks something like this:

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

Potential Causes of the Issue

Incorrect Package Structure: One of the common reasons for encountering this exception is that the test class is not located in the same package as the main application class or the component being tested. Spring Boot auto-configures beans based on the package structure, so if your test class is misplaced, it may not correctly detect the necessary components.

Missing Dependencies: Another reason might be the absence or misconfiguration of the Jackson dependencies required for JSON serialization.

Possibly Applied Solutions

In this particular case, the issue stemmed from the package structure, causing Spring to be unable to find the UserController when executing the test. Let's delve into how to address this effectively.

The Solution: Correcting Package Structure

Step 1: Organize Your Packages

Ensure that your test class, UserControllerTest, is located in the same package or a sub-package of the UserController class. For instance, if your UserController is in com.example.myexample.controller, your test class should ideally be in com.example.myexample.controller.test.

Step 2: Verify Dependencies

While your issue was resolved by fixing the package structure, ensure that your project has the required Jackson dependencies set up in the pom.xml file if needed:

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

This will ensure that your application has the necessary tools for JSON serializations.

Step 3: Re-run Your Tests

After making the changes to package structure and confirming the necessary dependencies, re-run your tests. They should now pass without the HttpMessageNotWritableException.

Conclusion

By following these simple organization strategies, you can rapidly troubleshoot and resolve issues such as HttpMessageNotWritableException when working with MockMvc in your Spring Boot applications. Always remember to keep your testing classes close to the classes they are testing to ensure proper detection by Spring’s auto configuration.

Now, you can write efficient, error-free tests for your REST controllers, ensuring smooth and consistent development workflows!