After updating graphql-java-tools to 6.1.0 i have encountered a stackoverflow exception on startup. I think that the problem was introduced in this commit: 10ee43f and happens because of the interaction between determineInputType and createInputObject.
Example schema:

input A { id: ID! b: B } input B { id: ID! a: A } 

According to the spec this should be allowed (graphql/graphql-spec#445, graphql/graphql-spec#189), which leads me to believe that this is a bug.