0Pricing
GraphQL APIs with Spring Boot · 课时

实现接口与联合类型

利用接口和联合类型创建灵活且支持多态的模式设计。

实现接口与联合类型 是 CoddyKit 上的免费 GraphQL APIs with Spring Boot 课时。 这是第 2 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 GraphQL APIs with Spring Boot 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 GraphQL APIs with Spring Boot 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Flexible Schemas with Polymorphism

GraphQL allows you to design highly flexible APIs. Sometimes, you need a field that can return different types of objects, or you want to define a shared set of fields that multiple types must implement. This is where polymorphism comes in.

We'll explore two key concepts for achieving this: Interfaces and Union Types.

Understanding GraphQL Interfaces

Imagine you have different types of media, like a Book and a Movie. Both might have a title and a releaseYear. Instead of duplicating these fields, you can define an Interface.

An interface is a contract that specifies a set of fields that any type implementing it must include. It's like a blueprint for common functionality.

Defining an Interface in SDL

In GraphQL's Schema Definition Language (SDL), an interface is defined using the interface keyword. Here, we define a Media interface that both Book and Movie will share.

interface Media {
  id: ID!
  title: String!
  releaseYear: Int
}

Types Implementing an Interface

Now, let's make our Book and Movie types implement the Media interface. They must include all fields defined by Media, and can add their own unique fields. Notice the implements keyword.

type Book implements Media {
  id: ID!
  title: String!
  releaseYear: Int
  author: String
  pages: Int
}

type Movie implements Media {
  id: ID!
  title: String!
  releaseYear: Int
  director: String
  durationMinutes: Int
}

Resolvers for Interfaces

When you query a field that returns an interface type, Spring GraphQL automatically resolves the concrete type based on the data. You usually create Java data classes that implement a common Java interface, and Spring GraphQL handles the rest. Try running this example:

package com.coddykit.graphql;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.graphql.data.method.annotation.QueryMapping;
import org.springframework.stereotype.Controller;

import java.util.List;
import java.util.ArrayList;

@SpringBootApplication
public class GraphqlApp {
    public static void main(String[] args) {
        SpringApplication.run(GraphqlApp.class, args);
    }
}

interface Media {
    String getId();
    String getTitle();
    Integer getReleaseYear();
}

record Book(String id, String title, Integer releaseYear, String author, Integer pages) implements Media {}
record Movie(String id, String title, Integer releaseYear, String director, Integer durationMinutes) implements Media {}

@Controller
class MediaController {

    @QueryMapping
    public List<Media> allMedia() {
        List<Media> mediaList = new ArrayList<>();
        mediaList.add(new Book("b1", "The Hitchhiker's Guide", 1979, "Douglas Adams", 193));
        mediaList.add(new Movie("m1", "Inception", 2010, "Christopher Nolan", 148));
        return mediaList;
    }
}

Exploring GraphQL Union Types

While interfaces ensure types share common fields, Union Types allow a field to return one of several distinct types, without requiring them to share any common fields. Think of it as an 'either-or' situation. For example, a search result might be a Product OR a User.

Schema Definition for Unions

In SDL, union types are defined using the union keyword, followed by the types it can represent, separated by a vertical bar |. Here's a SearchResult union that can be either a Book or a Movie.

union SearchResult = Book | Movie

Resolvers for Union Types

When a field returns a union type, Spring GraphQL needs to know which concrete type it is. For Java, you simply return an instance of one of the types defined in the union. Spring GraphQL automatically adds the __typename field for clients to distinguish. Try running this example:

package com.coddykit.graphql;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.graphql.data.method.annotation.Argument;
import org.springframework.graphql.data.method.annotation.QueryMapping;
import org.springframework.stereotype.Controller;

@SpringBootApplication
public class GraphqlApp {
    public static void main(String[] args) {
        SpringApplication.run(GraphqlApp.class, args);
    }
}

record Book(String id, String title, Integer releaseYear, String author, Integer pages) {}
record Movie(String id, String title, Integer releaseYear, String director, Integer durationMinutes) {}

@Controller
class SearchController {

    @QueryMapping
    public Object search(@Argument String query) {
        if (query.equalsIgnoreCase("inception")) {
            return new Movie("m1", "Inception", 2010, "Christopher Nolan", 148);
        } else if (query.equalsIgnoreCase("hitchhiker")) {
            return new Book("b1", "The Hitchhiker's Guide", 1979, "Douglas Adams", 193);
        }
        return null;
    }
}

Choosing Between Interfaces and Unions

Both interfaces and unions offer polymorphism, but they serve different purposes:

  • Interfaces: Use when multiple types share a common set of fields and behavior. They enforce a contract.
  • Union Types: Use when a field can return one of several distinct types that do not necessarily share any common fields. It's about 'either A or B or C'.

Interfaces vs. Unions Quiz

You are designing a GraphQL API. You need a field that can return either a Photo object or a Video object. These two types have completely different fields and do not share any common properties. Which GraphQL schema feature is best suited for this scenario?

Recap: Flexible Schemas

You've learned how GraphQL Interfaces and Union Types enable flexible and polymorphic schema designs. Interfaces define a contract for common fields, while Union Types allow a field to return one of several distinct types. These tools are crucial for building robust and adaptable GraphQL APIs.

Next, you'll explore Input Types to streamline mutation arguments.

常见问题解答

「实现接口与联合类型」课时是免费的吗？

是的 — 「实现接口与联合类型」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 GraphQL APIs with Spring Boot 课程的其余内容，请升级到 CoddyKit PRO。 GraphQL APIs with Spring Boot 课程共包含 4 节课。

「实现接口与联合类型」这节课中我会学到什么？

利用接口和联合类型创建灵活且支持多态的模式设计。 你通过在浏览器中直接运行的动手代码来练习 GraphQL APIs with Spring Boot，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 GraphQL APIs with Spring Boot 需要有经验吗？

无需任何先前经验。CoddyKit 上的 GraphQL APIs with Spring Boot 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。

「实现接口与联合类型」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 GraphQL APIs with Spring Boot 课中编写并运行代码吗？

能。每节 GraphQL APIs with Spring Boot 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

  1. 建模嵌套对象与关系
  2. 实现接口与联合类型
  3. 利用输入类型实现变更
  4. 枚举与自定义标量类型
← 返回 GraphQL APIs with Spring Boot