It seems like a valid goal to want to implement (extend) a Server - there are only a few implementations in grpc-java and I can imagine quite a few alternatives that I wouldn't expect the grpc-java community to want to support. But if you try it you find that the reflection service doesn't work (and therefore I assume potentially other services) because it accesses the current Server via the Server.SERVER_CONTEXT_KEY. If you don't have access to that key you can't set up the context for the reflection service. There is an escape hatch via InternalServer.SERVER_CONTEXT_KEY but that is clearly documented as "not for use outside grpc-java" and annotated with @Internal.