Implementing aspects via callback in a virtual machine, where an aspect weaver weaves the aspect with a module to provide primary and crosscutting functionality in the runtime execution of the module. The virtual machine has multiple modules that are part of an application implementing separation of concerns. Each of the multiple modules has a distinct functionality portion and a common functionality portion, common to all modules. The common functionality is referenced by the virtual machine via callback for the multiple modules. Aspect code is loaded as a callback that is applied to the multiple modules by the virtual machine. Loading the aspect code as a callback can implement the aspect at any point in the modules, including conditional statements, calls to methods of objects or classes, loops, any point in an exception handler, or to monitor changes in variables.