Kotlin GraphQL Library | API Tools

Kotlin GraphQL Library: A Complete Guide to Working with API

GraphQL is a query language for APIs that allows clients to request only the data they actually need. In the Kotlin ecosystem, there are several powerful libraries for working with GraphQL. In this article, we will take a detailed look at three main libraries: graphql-kotlin, Apollo Kotlin, and KGraphQL, and also show examples of their use.

1. graphql-kotlin — Server Library from Expedia Group

graphql-kotlin is the most popular library for creating GraphQL servers in Kotlin. It fully supports all features of the GraphQL specification and integrates perfectly with Spring Boot, Ktor, and Micronaut frameworks.

Installation and Setup

To include the library, add the dependency to build.gradle.kts:

dependencies {    // Main library    implementation("com.expediagroup:graphql-kotlin-spring-server:6.5.0")        // For schema generation from annotations    implementation("com.expediagroup:graphql-kotlin-schema-generator:6.5.0")}

Creating a Simple GraphQL API

Let's define a data model and a resolver for queries:

// Data modeldata class User(val id: Int, val name: String, val email: String)

// Query resolverclass UserQuery : Query {    fun getUser(id: Int): User? {        return users.find { it.id == id }    }        companion object {        val users = listOf(            User(1, "Anna Petrova", "anna@example.com"),            User(2, "Ivan Sidorov", "ivan@example.com")        )    }}

// Mutation resolverclass UserMutation : Mutation {    fun createUser(name: String, email: String): User {        val newUser = User(users.size + 1, name, email)        users.add(newUser)        return newUser    }}

Now let's register the resolvers in the Spring Boot configuration:

@Configurationclass GraphQLConfig {    @Bean    fun userQuery(): UserQuery = UserQuery()        @Bean    fun userMutation(): UserMutation = UserMutation()}

API Queries

Example of a GraphQL query:

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

The response will contain only the requested fields:

{  "data": {    "getUser": {      "name": "Anna Petrova",      "email": "anna@example.com"    }  }}

2. Apollo Kotlin — Client Library for Android and JVM

Apollo Kotlin is a powerful GraphQL client designed specifically for Kotlin and Android. It provides type-safe queries and automatic code generation.

Including the Library

dependencies {    implementation("com.apollographql.apollo:apollo-runtime:3.8.0")    implementation("com.apollographql.apollo:apollo-normalized-cache-sqlite:3.8.0")}

Creating a Query

First, define the GraphQL query in the GetUserQuery.graphql file:

query GetUser($id: Int!) {  getUser(id: $id) {    id    name    email  }}

Then use the generated code in Kotlin:

val apolloClient = ApolloClient.builder()    .serverUrl("http://localhost:8080/graphql")    .build()

suspend fun fetchUser(userId: Int): User? {    val response = apolloClient        .query(GetUserQuery(userId))        .execute()        return response.data?.getUser?.let { user ->        User(            id = user.id,            name = user.name,            email = user.email        )    }}