The pervasive computing vision of the future communication infrastructure is that we will be everywhere connected with invisible devices, which may consume and provide computational resources over the network. Resources are often abstracted as autonomous services, which must be "continually" evolved at run time to satisfy changing requirements. This paper discusses the design, development and evaluation of the Microevolution of Pervasive Services framework, which aims at supporting the run-time small-scale evolution of services through code mobility. Indeed, the framework provides (i) a dispatching mechanism to migrate Java code throughout the pervasive networking environment, and (ii) a microevolution manager that exploits Java reflection mechanisms to optimize code transmission.