Private API Access and Functional Mocking in Automated Unit Test Generation

Not all object oriented code is easily testable: Dependency objects might be difficult or even impossible to instantiate, and object-oriented encapsulation makes testing potentially simple code difficult if it cannot easily be accessed. When this happens, then developers can resort to mock objects that simulate the complex dependencies, or circumvent object-oriented encapsulation and access private APIs directly through the use of, for example, Java reflection. Can automated unit test generation benefit from these techniques as well? In this paper we investigate this question by extending the EvoSuite unit test generation tool with the ability to directly access private APIs and to create mock objects using the popular Mockito framework. However, care needs to be taken that this does not impact the usefulness of the generated tests: For example, a test accessing a private field could later fail if that field is renamed, even if that renaming is part of a semantics-preserving refactoring. Such a failure would not be revealing a true regression bug, but is a false positive, which wastes the developer's time for investigating and fixing the test. Our experiments on the SF110 and Defects4J benchmarks confirm the anticipated improvements in terms of code coverage and bug finding, but also confirm the existence of false positives. However, by ensuring the test generator only uses mocking and reflection if there is no other way to reach some part of the code, their number remains small.