If you compile NiFi you can retrieve a swagger definition from:
./nifi/nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-web/nifi-web-api/target/swagger-ui/swagger.json
You could then combine this with your preferred language codegen to produce a full Rest API client.
You can also use the one I maintain in Python at https://github.com/Chaffelson/nipyapi
I have already written wrapper functions to retrieve processor information.