Safety in Flexible Dynamic Linking

Dynamic linking lets programs use the most recent versions of classes without re-compilation. In Java and .NET, bytecode specifies which classes should be dynamically linked. This information represents the compiler’s knowledge of the compilation environment, but the execution environment might be different. For example, the execution environment on a mobile phone might provide fewer, simpler classes than on a desktop PC. As bytecode cannot adapt to its execution environment, component reuse is restricted and development costs are increased. We suggest flexible dynamic linking that binds bytecode as late as possible to the classes available in an execution environment. Rather than specifying classes, bytecode contains type variables that are substituted by classes as late as execution. We present a non-deterministic model that treats substitution as a linking step, and interleaves linking with execution. We examine how linking builds a well-formed program and prove soundness for the laziest substitution strategy, representing the most general execution.