Library developers are often unaware of how their library is used exactly in practice. When a library developer changes the internals of a library, this may unintentionally affect or even break the working of the library users' code. While it is possible to detect when a syntactic breaking change occurs, it is not as easy to detect semantic breaking changes, where the implicit contract of a functionality changes, sometimes unbeknownst to the library developer. Because library users rarely test the behaviour they expect of the library, neither the library developer nor the library user will be aware of the new behaviour. As a library developer, you want to be able to see how a change in your library will affect your users before a new version of the library is deployed. More specifically, you want to gain insight into how users use the library, and want to see if and how changes affect users. This will allow you to determine whether the new version of the library is backwards compatible. Finally, after deploying the breaking changes, you want to notify the affected users of the changes and of a solution to the issue. Schaapi, a tool for early detection of breaking changes based on API usages, addresses these needs. It mines public repositories for projects using a given library, analyses their usage of the API of that library, and generates tests that capture this behaviour. Finally, it offers a continuous integration service that automatically executes these tests against new versions of the library and warns developers of any potentially breaking changes in functionality. The tool has also been validated against real-world data to demonstrate its performance in realistic usage scenarios and to answer a selection of related research questions.

The most common reason for Continuous Integration (CI) build failures is failing tests. When a build fails, a developer often has to scroll through hundreds to thousands of log lines to find which test is failing and why. Finding the issue is a tedious process that relies on a developer's experience and increases the cost of software testing. Providing CI build test results with additional context in the developer's local development environment could help solve failing tests more quickly. We propose TestAxis, a test result inspection tool that brings CI test results to the Integrated Development Environment (IDE) offering an experience similar to running a test locally. Moreover, it surfaces additional information that is too expensive to collect in local development, for example, a unique view of the code under test that was changed leading up to the build failure. We implement TestAxis as a plugin for IntelliJ and conduct a user study to evaluate its usefulness and performance benefits. The participants solve programming assignments evaluating the three main features: the test results overview, the test code editor, and the changed code under test display. We show that TestAxis helps developers fix failing tests 13.4% to 30.4% faster. The participants found the features of TestAxis useful and would incorporate it in their development workflow to save time. With TestAxis we set an important step towards removing the need to manually inspect build logs and bringing CI build results to the IDE, ultimately saving developers time.