Mocking frameworks are dedicated to creating, manipulating, and verifying the execution of “faked” objects in unit testing. This helps developers to overcome the challenge of high inter-dependencies among software units. Despite the various benefits offered by existing mocking frameworks, developers often create a subclass of the dependent class and mock its behavior through method overriding. However, this requires tedious implementation and compromises the design quality of unit tests. We contribute a refactoring tool as an Eclipse Plugin, named JMocker, to automatically identify and replace the usage of inheritance by using Mockito—a well received mocking framework for Java projects. We evaluate JMocker on four open source projects and successfully refactored 214 cases in total. The evaluation results show that our framework is efficient, applicable to different projects, and preserves test behaviors. According to the feedback of six real-life developers, JMocker improves the design quality of test cases. JMocker is available at https://github.com/wx930910/JMocker. The tool demo can be found at https://youtu.be/HFoA2ZKCoxM.