Implementing a profiler to trace a program execution is non-trivial. One way to do this on running Java programs is through bytecode instrumentation. Nowadays, tools exist that ease the instrumentation process itself, but as far as we know, none offers an entirely dynamic implementation technique which is able to include the instrumentation of Java Runtime Environement (JRE) classes. In this paper we present the main principles of our technique, which performs such online bytecode instrumentation of both application and JRE classes, at basic block level.