Building APIs with Spring for GraphQL

Spring for GraphQL simplifies the development of GraphQL APIs by integrating with Spring Boot. This guide demonstrates how to create a GraphQL API using Java and Kotlin, with examples for schema definition, query handling, and dependency injection.

🌟 Why Use GraphQL?

GraphQL allows clients to request specific data, reducing over-fetching and under-fetching compared to REST. It also supports strong typing and facilitates efficient data fetching, making it a popular choice for modern APIs.

🌟 Prerequisites

📋 Ensure you have the following:

• ☕ Java Development Kit (JDK) 17+
• 📦 Maven or Gradle installed
• 🔤 A Java IDE (e.g., IntelliJ IDEA, Eclipse)

🛠️ Step 1: Add Dependencies

Include the following dependencies in your project to enable Spring for GraphQL.

• Maven:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-graphql</artifactId>
</dependency>

• Gradle:

implementation 'org.springframework.boot:spring-boot-starter-graphql'

📋 Step 2: Define the GraphQL Schema

Create a schema file named schema.graphqls under the src/main/resources/graphql directory.

type Query {
  getUser(id: ID!): User
  getUsers: [User]
}

type User {
  id: ID!
  name: String!
  email: String!
}

📖 Step 3: Implement the Data Model and Services

Define the data model and service layer for handling queries.

• Java
• Kotlin

package com.example.demo.model;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private String id;
    private String name;
    private String email;
}

package com.example.demo.service;

import com.example.demo.model.User;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

@Service
public class UserService {

    private final Map<String, User> userData = Stream.of(
        new User("1", "Alice", "alice@example.com"),
        new User("2", "Bob", "bob@example.com")
    ).collect(Collectors.toMap(User::getId, user -> user));

    public User getUser(String id) {
        return userData.get(id);
    }

    public List<User> getUsers() {
        return List.copyOf(userData.values());
    }
}

package com.example.demo.model

data class User(
    val id: String,
    val name: String,
    val email: String
)

package com.example.demo.service

import com.example.demo.model.User
import org.springframework.stereotype.Service

@Service
class UserService {

    private val userData = mapOf(
        "1" to User("1", "Alice", "alice@example.com"),
        "2" to User("2", "Bob", "bob@example.com")
    )

    fun getUser(id: String): User? = userData[id]

    fun getUsers(): List<User> = userData.values.toList()
}

📘 Step 4: Implement GraphQL Controllers

Controllers handle GraphQL queries and mutations. Use the @Controller annotation in Spring for GraphQL.

• Java
• Kotlin

package com.example.demo.controller;

import com.example.demo.model.User;
import com.example.demo.service.UserService;
import lombok.RequiredArgsConstructor;
import org.springframework.graphql.data.method.annotation.QueryMapping;
import org.springframework.stereotype.Controller;

import java.util.List;

@Controller
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @QueryMapping
    public User getUser(String id) {
        return userService.getUser(id);
    }

    @QueryMapping
    public List<User> getUsers() {
        return userService.getUsers();
    }
}

package com.example.demo.controller

import com.example.demo.model.User
import com.example.demo.service.UserService
import org.springframework.graphql.data.method.annotation.QueryMapping
import org.springframework.stereotype.Controller

@Controller
class UserController(
    private val userService: UserService
) {

    @QueryMapping
    fun getUser(id: String): User? = userService.getUser(id)

    @QueryMapping
    fun getUsers(): List<User> = userService.getUsers()
}

▶️ Running the Application

Run the application using the following commands:

• Spring Boot (Java/Kotlin):

  ./mvnw spring-boot:run
  

Access the GraphQL Playground at http://localhost:8080/graphiql to test your API.

🧪 Testing the GraphQL API

Here are some example queries to test your API:

• Fetch a user by ID:

query {
  getUser(id: "1") {
    id
    name
    email
  }
}

• Fetch all users:

query {
  getUsers {
    id
    name
    email
  }
}

This guide demonstrates how to build powerful and flexible GraphQL APIs using Spring for GraphQL with Java and Kotlin. Leverage GraphQL’s advantages to make your project more efficient and user-friendly.