When using Mockito to mock exceptions, several common pitfalls can make your tests less effective or more difficult to maintain. Here are some of these pitfalls and how to avoid them:
1. Over-Mocking**
One of the most significant pitfalls is over-mocking. This occurs when you mock too many dependencies, leading to tests that are fragile and hard to maintain. Over-mocking can cause tests to focus more on the mocks than on the actual behavior of the system under test. To avoid this, focus on mocking only the collaborators that have non-trivial behavior or those whose interactions you want to verify. Use real instances for simple collaborations[3].2. Invalid Exceptions**
Mockito ensures that the exception being thrown is valid for the specific stubbed method. If you try to throw an exception that is not in the method's checked exceptions list, Mockito will complain. For example, if a method does not declare `IOException`, attempting to mock it to throw an `IOException` will result in an error[1]. Always ensure that the mocked exception is valid for the method.3. Using `thenThrow()` with Void Methods**
`when().thenThrow()` cannot be used with void methods because there is no return value to mock. Instead, use `doThrow()` for void methods. Mixing these methods incorrectly can lead to errors[2].4. Lack of Specificity**
When mocking exceptions, it's crucial to be specific about which exceptions you are testing. Avoid over-mocking by focusing on exceptions relevant to the test scenario. This helps keep tests focused and maintainable[2][8].5. Incorrect Handling of Executions**
Ensure that the application code is prepared to handle exceptions correctly, even when mocked during tests. This includes testing the logic for handling exceptions to ensure it behaves as expected[2].6. Use of Meaningful Error Messages**
When mocking exceptions, include meaningful messages to help diagnose issues if tests fail. This practice enhances the clarity and effectiveness of your tests[2].7. Testing Exception Handling Logic**
Make sure that the code handles exceptions as expected. This involves verifying that the application behaves correctly when exceptions are thrown, which is crucial for building resilient applications[2].8. Resetting and Clearing Mocks**
Mockito provides features like `reset()` and `clearInvocations()` for managing mocks. However, these should be used sparingly. Resetting a mock should be done only in rare cases, such as when a mock is injected by a container and cannot be recreated for each test. Clearing invocations can be useful if recreating a mock would lead to significant overhead[1].By being aware of these pitfalls and following best practices, you can effectively use Mockito to mock exceptions and ensure your tests are robust and maintainable.
Citations:[1] https://www.toptal.com/java/a-guide-to-everyday-mockito
[2] https://www.browserstack.com/guide/mockito-throw-exception
[3] https://javanexus.com/blog/avoiding-mockito-testing-pitfalls
[4] https://stackoverflow.com/questions/9186604/mockito-exception-when-requires-an-argument-which-has-to-be-a-method-call-on
[5] https://stackoverflow.com/questions/22540108/best-practices-with-mockito
[6] https://stackoverflow.com/questions/52512548/mock-method-to-do-not-throw-an-exception/52514628
[7] https://contributors.scala-lang.org/t/mockito-testing-nulls-and-exceptions/3915
[8] https://www.janbasktraining.com/community/java/throw-checked-exceptions-from-mocks-with-mockito