graphql-java循环类型依赖性

Question

在构造相互依赖的类型时，我碰到了砖墙，下面是代码：

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();
    }

}

由于我得到了这个异常，所以不可能对这个类进行定位。

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)

显然，classType在createStudentType()引用它的点上还没有定音。我该如何解决这个问题？

Answer 1

GraphQLTypeReference确实是答案。这应该可以做到：

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

你试过使用new GraphQLTypeReference("ForwardType")吗？我说的是这个https://github.com/graphql-java/graphql-java#recursive-type-references