Viscount: A Direct Method Call Coverage Tool for Java

Writing unit tests against implementation detail in production code, often embodied in non-public methods, is considered bad practice in formal and gray literature. This is because it leads to fragile tests that break easily when underlying implementation details change. For this reason, tests that focus on behavior are encouraged. One way to achieve this is to test units exclusively through their public API. However, our recent developer survey shows that this advice is not always followed in practice. Moreover, code coverage tools do not provide a way to determine which methods were called directly from tests, meaning there is no easy way to identify whether units make calls to non-public methods, other than through manual examination. To address this problem, we developed Viscount, a tool that can determine direct method call coverage for Java tests written in JUnit. Viscount reports the percentage of methods invoked directly from tests, according to their visibility - i.e., public or non-public (protected, package-private, or private). This can help developers and researchers identify tests that potentially need to be refactored or rewritten. In this paper, we describe Viscount's overall architecture, its core features, and how to use it. Viscount is also publicly available on GitHub: https://github.com/unittesting-nonpublic/viscount. A demo video of Viscount is available at: https://youtu.be/ZUyRtiUnbsU.