Safe Runtime Downcasts With Ownership Types

The possibility of aliasing between objects constitutes one of the primary challenges in understanding and reasoning about correctness of object-oriented programs. Ownership types provide a principled way of specifying statically enforcable restrictions on object aliasing. Ownership types have been used to aid program understanding and evolution, verify absence of data races and deadlocks in multithreaded programs, and verify absence of memory errors in programs with explicit deallocation. This paper describes an efficient technique for supporting safe runtime downcasts with ownership types. This technique uses the type passing approach, but avoids the associated significant space overhead by storing runtime ownership information only for objects that are potentially involved in downcasts. Moreover, this technique does not use any interprocedural analysis, so it preserves the separate compilation model of Java. We implemented our technique in the context of Safe Concurrent Java, which is an extension to Java that uses ownership types to guarantee the absence of data races and deadlocks in well-typed programs. Our approach is JVM-compatible: our implementation translates programs to bytecodes that can be run on regular JVMs.