Currently (2.2.0) graphql-java does not appear to support integer-values for ID fields, either in query requests or in setting responses from data fetchers.

With the following schema query definition

final GraphQLObjectType queryType = newObject()
        .name("QueryType")
        .field(newFieldDefinition()
            .name("foo")
            .type(Foo)
            .argument(newArgument()
                .name("id")
                .description("The id of the foo")
                .type(new GraphQLNonNull(GraphQLID)))
            .dataFetcher(getFetcherForResource("foo")))
            .build(); 

The query

{ foo(id:"1") { id bar } }

works ok, but the query

{ foo(id:1) { id bar } }

fails with

{"errors":[{"validationErrorType":"WrongType","message":"Validation error of type WrongType: argument value IntValue{value=1} has wrong type","locations":[{"line":1,"column":10}],"errorType":"ValidationError"}],"data":null}

According to the GraphQL spec:

When expected as an input type, any string (such as "4") or integer (such as 4) input value should be coerced to ID as appropriate for the ID formats a given GraphQL server expects. Any other input value, including float input values (such as 4.0), must raise a query error indicating an incorrect type.

Also, if attempt to populate my response object to this query with a numeric value, it appears in the JSON result as null rather than a stringified form of that value. Again I believe the GraphQL spec states that servers should coerce results as appropriate before serializing ID values to JSON strings.