This paper presents an approach for dynamic reconfiguration of an embedded Java system in real-time. A class loader executed as a separate thread on a multithreaded microcontroller loads an updated class concurrent to the real-time application. At an uncritical time the application thread switches from the old to the newly updated class. A switching time of 246 clock cycles is determined by a worst-case execution time analysis and checked by measurements on the system. Although this approach is based on a realtime Java system it can be adapted to other languages without extensive additional costs.