Can you use fragments in graphql server schema file? Could you please point me to an example
No. Fragments are defined for operations (queries) only. In other words, the client defines them ad-hoc, not the server in its schema. If what you're looking for is to reuse a bunch of definitions, I'm afraid you're constrained to implementing an interface, or just good ol' composition. Most implementations also support type extensions, graphql-java certainly does.