Traditional test oracles are defined on the outputs of test executions, and cannot assert internal states of executions. Traditional assertions are common to all test execution, and are usually more difficult to construct than on oracle for one test input. In this paper we propose the concept of inner oracles, which are assertions on internal states that are specific to one test input. We first motivate the necessity of inner oracles, and then show that it can be implemented easily using the available programming mechanisms. Next, we report two initial empirical studies on inner oracles, showing that inner oracles have a significant impact on both the fault-detection capability of tests and the performance of test suite reduction. Finally, we highlight the implications of inner oracles on several research and practical problems.

Software testing plays an important role in ensuring software quality by running a program with test suites. Mutation testing is designed to evaluate whether a test suite is adequate in detecting faults. Due to the expensive cost of mutation testing, selective mutation testing was proposed to select a subset of mutants whose effectiveness is similar to the whole set of generated mutants. Although selective mutation testing has been widely investigated in recent years, many people still doubt whether it can suit well for large programs. To study the scalability of selective mutation testing, we systematically explore how the program size impacts selective mutation testing through four projects (including 12 versions all together). Based on the empirical study, for programs smaller than 16 KLOC, selective mutation testing has surprisingly good scalability. In particular, for a program whose number of lines of executable code is E, the number of mutants used in selective mutation testing is proportional to Ec, where c is a constant whose value is between 0.05 and 0.25.