Background: Mocking is a common technique for isolating test dependencies, yet duplicated mock setup code-what we call “mock clones”-can increase test maintenance overhead and reduce clarity. While code clone detection is a mature research area, the prevalence, characteristics, and refactoring of mock clones remain insufficiently understood.Aims: This study investigates the prevalence, detection challenges, and practical elimination of mock clones in real-world Java test suites, aiming to identify systematic patterns and assess the feasibility and value of mock clone refactoring.Method: We analyzed 698 mock clone instances across six open-source Java projects. We evaluated the effectiveness of existing code clone detection tools in capturing mock clones, manually refactored a large subset of clones to assess behavior preservation, and measured the structural impact of mock clone elimination.Results: Mock clones are prevalent, with frequently mocked classes often exhibiting extensive duplication. Existing clone detection tools failed to reliably detect mock clones due to scattered, reordered, and semantically varied mocking logic. Manual refactoring successfully eliminated $60 \%$ of identified mock clones while preserving test behavior, resulting in up to $64 \%$ reduction in mock objects and up to $61 \%$ reduction in mocking-related LOC. Targeting only the top $\mathbf{5 - 1 0}$ high-impact clones still achieved substantial simplification.Conclusions: Mock clone elimination is both feasible and highly valuable for improving test code maintainability. Systematic refactoring strategies can address common patterns, and prioritizing high-impact clones enables efficient gains with minimal effort. These findings motivate the need for mock-aware detection and automated refactoring tools to unlock broader maintainability improvements in testing practices.

Abstract Mocking frameworks provide convenient APIs, which create mock objects, manipulate their behavior, and verify their execution, for the purpose of isolating test dependencies in unit testing. This study contributes an in-depth empirical study of whether and how mocking frameworks are used in Apache projects. The key findings and insights of this study include: First, mocking frameworks are widely used in 66% of Apache Java projects, with Mockito, EasyMock, and PowerMock being the top three most popular frameworks. Larger-scale and more recent projects tend to observe a stronger need to use mocking frameworks. This underscores the importance of mocking in practice and related future research. Second, mocking is overall practiced quite selectively in software projects—not all test files use mocking, nor all dependencies of a test target are mocked. It calls for more future research to gain a more systematic understanding of when and what to mock to provide formal guidance to practitioners. On top of this, the intensity of mocking in different projects shows different trends in the projects’ evolution history—implying the compound effects of various factors, such as the pace of a project’s growth, the available resources, time pressure, and priority, etc. This points to an important future research direction in facilitating best mocking practices in software evolution. Furthermore, we revealed the most frequently used APIs in the three most popular frameworks, organized based on the function types. The top five APIs in each functional type of the three mocking frameworks usually take the majority (78% to 100%) of usage in Apache projects. This indicates that developers can focus on these APIs to quickly learn the common usage of these mocking frameworks. We further investigated informal methods of mocking, which do not rely on any mocking framework. These informal mocking methods point to potential sub-optimal mocking practices that could be improved, as well as limitations of existing mocking frameworks. Finally, we conducted a developer survey to collect additional insights regarding the above analysis based on their experience, which complements our analysis based on repository mining. Overall, this study offers practitioners profound empirical knowledge of how mocking frameworks are used in practice and sheds light on future research directions to enhancing mocking in practice.