Using Metadata Transformations to Integrate Class Extensions in an Existing Class Hierarchy

摘要

Class extensions provide a fine-grained mechanism to define incremental modifications to class-based systems when standard subclassing mechanisms are inappropriate. To control the impact of class extensions, the concept of classboxes has emerged that defines a new module system to restrict the visibility of class extensions to selected clients. However, the existing implementations of the classbox concept rely either on a "classbox-aware" virtual machine, an expensive runtime introspection of the method call stack to build the structure of a classbox, or both. In this paper we present an implementation technique that allows for the structure of a classbox to be constructed at compile-time by means of metadata transformations to rewire the inheritance graph of refined classes. These metadata transformations are language-neutral and more importantly preserve both the semantics of the classbox concept and the integrity of the underlying deployment units. As a result, metadata transformation provides a feasible approach to incorporate the classbox concept into programming environments that use a virtual execution system.