Extensible typesafe systems, such as Java, rely critically on a large and complex software base for their overall protection and integrity, and are therefore difficult to test and verify. Traditional testing techniques, such as manual test generation and formal verification, are too time consuming, expensive, and imprecise, or work only on abstract models of the implementation and are too simplistic. Consequently, commercial virtual machines deployed so far have exhibited numerous bugs and security holes.>In this paper, we discuss our experience with using production grammars in testing large, complex and safety-critical software systems. Specifically, we describe lava, a domain specific language we have developed for specifying production grammars, and relate our experience with using lava to generate effective test suites for the Java virtual machine. We demonstrate the effectiveness of production grammars in generating complex test cases that can, whencombined with comparative and variant testing techniques, achieve high code and value coverage. We also describe an extension to production grammars that enables concurrent generation of certificates for test cases. A certificate is a behavioral description that specifies the intended outcome of the generated test case, and therefore acts as an oracle by which the correctness of the tested system can be evaluated in isolation. We report the results of applying these testing techniques to commercial Java implementations. We conclude that the use of production grammars in combination with other automated testing techniques is a powerful and effective method for testing software systems, and is enabled by a special purpose language for specifying extended production grammars.
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