Existing distributed middleware technologies and Enterprise Application frameworks lack in support to service flexibility from both the developer's and user's point of view. In this paper we propose a JINI-based framework, namely PRINCEPS (Pluggable Reliable Infrastructure for Network Computing and Enhanced Properties of Service), which provides a distributed and dynamic environment for flexible service provision. We claim that the adoption of JINI makes it possible to dramatically improve service flexibility by federating services in a dynamic and self-healing networked community. The framework provides the clients with a (web-based) mechanism for selecting services according to functional requirements (i.e., the service interface) and non-functional requirements (the quality of service, i.e., reliability, performance). More implementations of the same service can coexist in the framework, each of one satisfying different non-functional requirements. PRINCEPS provides service substitutability at various levels. In fact, any kind of service can be plugged in PRINCEPS: CORBA-based services, socket-based services, and centralized services. PRINCEPS also supports service developers with its own mechanism to integrate existing services. Preliminary performance experiments and lessons learned from the PRINCEPS prototype are also discussed.