I can't seem to find a strait forward answer online:

Let's say this is the pertinent part of my schema:

input CustomStringAttributeInput {
    name: String!
    value: String!
}

input CustomBooleanAttributeInput {
    name: String!
    value: Boolean!
}

input CustomAttributeInput @oneOf {
    boolean: CustomBooleanAttributeInput    
    string: CustomStringAttributeInput
}

When I attempt to use CustomAttributeInput in a query or mutation I get:
Exception in thread "main" graphql.kickstart.tools.SchemaError: Expected type 'CustomBooleanAttributeInput' to be a GraphQLInputType, but it wasn't! Was a type only permitted for object types incorrectly used as an input type, or vice-versa?

I've used graphql-java-tools long enough to know that error message may be incorrect. I seemed to have the right classes, with the right hierarchy and the correct getters and setters.

I will make a small standalone sample to share. But if anyone can answer whether or not graphql-java-tools (and graphql-java) support this directive in the first place, it would be very helpful.