graphql-java 循环类型依赖答案

【问题标题】：graphql-java cyclic types dependenciesgraphql-java 循环类型依赖
【发布时间】：2016-11-09 11:37:43
【问题描述】：

我在尝试构建相互依赖的类型时碰了壁，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();
    }

}

不可能实例化这个类，因为我得到了这个异常

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() 引用它时还没有实例化。我该如何解决这个问题？

【问题讨论】：

标签： java graphql graphql-java

【解决方案1】：

GraphQLTypeReference 确实是答案。应该这样做：

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();
    }

}

【讨论】：

【解决方案2】：

您是否尝试使用new GraphQLTypeReference("ForwardType")？我说的是这个https://github.com/graphql-java/graphql-java#recursive-type-references

【讨论】：