Since version 1.1 of the Java Development Kit, the Java reflective facilities have been successively extended. However, they still prove to be limited. A number of systems (e.g. MetaXa, Guaraná, Kava, Javassist) have addressed this limitation by providing reflective extensions of Java with richer MetaObject Protocols (MOPs). All these extensions provide a particular, monolithic, infrastructure that reflects the commitment of the designer to particular trade-offs between efficiency, portability, expressiveness and flexibility. Unfortunately, these trade-offs are not satisfactory for all applications, since different applications may have different needs. This calls for breaking down the building of a reflective extension into different components that can be specialized in order to fit specific needs. We qualify such a reflective extension as open. In this paper, we present Reflex, a prototype open reflective extension of Java. Reflex can be seen both as a reflective extension of Java and as a first step towards a framework for building such extensions.