graphql-java cyclic types dependencies

Question

i've hit a brick wall while trying to construct types that depend on each other, here is the code:

import graphql.schema.GraphQLObjectType;
import static graphql.schema.GraphQLObjectType.newObject;

import static graphql.Scalars.*;
import graphql.schema.GraphQLFieldDefinition;
import graphql.schema.GraphQLList;

import static graphql.schema.GraphQLFieldDefinition.newFieldDefinition;

public class GraphQLTypes {

    private GraphQLObjectType studentType;
    private GraphQLObjectType classType;

    public GraphQLTypes() {

       createStudentType();
        createClassType();
    }

    void createStudentType() {
        studentType = newObject().name("Student")
                .field(newFieldDefinition().name("name").type(GraphQLString).build())
            .field(newFieldDefinition().name("currentClass").type(classType).build())
            .build();
    }

    void createClassType() {
        classType = newObject().name("Class")
            .field(newFieldDefinition().name("name").type(GraphQLString).build())
            .field(newFieldDefinition().name("students").type(new GraphQLList(studentType)).build())
            .build();
    }

}

its impossible to intantiate this class, as i get this exception

Caused by: graphql.AssertException: type can't be null
at graphql.Assert.assertNotNull(Assert.java:10)
at graphql.schema.GraphQLFieldDefinition.<init>(GraphQLFieldDefinition.java:23)
at graphql.schema.GraphQLFieldDefinition$Builder.build(GraphQLFieldDefinition.java:152)
at graphql_types.GraphQLTypes.createStudentType(GraphQLTypes.java:26)
at graphql_types.GraphQLTypes.<init>(GraphQLTypes.java:19)

obviously classType is not yet intantiated at the point createStudentType() is referencing it. How to i get around this problem?

Answer 1

GraphQLTypeReference is the answer indeed. This should do it:

import graphql.schema.GraphQLList;
import graphql.schema.GraphQLObjectType;
import graphql.schema.GraphQLTypeReference;

import static graphql.Scalars.GraphQLString;
import static graphql.schema.GraphQLFieldDefinition.newFieldDefinition;
import static graphql.schema.GraphQLObjectType.newObject;

public class GraphQLTypes {

     private GraphQLObjectType studentType;
     private GraphQLObjectType classType;

    public GraphQLTypes() {
        createStudentType();
        createClassType();
    }

    void createStudentType() {
        studentType = newObject().name("Student")
                .field(newFieldDefinition().name("name").type(GraphQLString).build())
                .field(newFieldDefinition().name("currentClass").type(new GraphQLTypeReference("Class")).build())
                .build();
    }

    void createClassType() {
        classType = newObject().name("Class")
                .field(newFieldDefinition().name("name").type(GraphQLString).build())
                .field(newFieldDefinition().name("students").type(new GraphQLList(studentType)).build())
                .build();
    }

}

Answer 2

Did you try to use new GraphQLTypeReference("ForwardType")? I'm talking about this one https://github.com/graphql-java/graphql-java#recursive-type-references