🚀 Introduction to Ktor

📖 What is Ktor?

Ktor is a lightweight asynchronous web framework built with Kotlin. It allows you to easily create fast and efficient servers based on coroutines!

💡 Creating a Project

build.gradle.kts

plugins {
    kotlin("jvm") version "1.9.0"
    id("io.ktor.plugin") version "2.3.5"
}

dependencies {
    implementation("io.ktor:ktor-server-core:2.3.5")
    implementation("io.ktor:ktor-server-netty:2.3.5")
    implementation("ch.qos.logback:logback-classic:1.4.11")
}

🎯 First Server

Hello World

import io.ktor.server.application.*
import io.ktor.server.engine.*
import io.ktor.server.netty.*
import io.ktor.server.response.*
import io.ktor.server.routing.*

fun main() {
    embeddedServer(Netty, port = 8080) {
        routing {
            get("/") {
                call.respondText("Hello, Ktor!")
            }
        }
    }.start(wait = true)
}

After running, visit http://localhost:8080 in your browser!

Multiple Routes

fun main() {
    embeddedServer(Netty, port = 8080) {
        routing {
            get("/") {
                call.respondText("홈페이지")
            }

            get("/hello") {
                call.respondText("안녕하세요!")
            }

            get("/about") {
                call.respondText("소개 페이지")
            }
        }
    }.start(wait = true)
}

🎨 Routing

Path Parameters

fun Application.configureRouting() {
    routing {
        get("/user/{id}") {
            val id = call.parameters["id"]
            call.respondText("사용자 ID: $id")
        }

        get("/product/{category}/{id}") {
            val category = call.parameters["category"]
            val id = call.parameters["id"]
            call.respondText("카테고리: $category, 상품 ID: $id")
        }
    }
}

fun main() {
    embeddedServer(Netty, port = 8080) {
        configureRouting()
    }.start(wait = true)
}

// 접속: http://localhost:8080/user/123
// 출력: 사용자 ID: 123

Query Parameters

routing {
    get("/search") {
        val query = call.request.queryParameters["q"]
        val page = call.request.queryParameters["page"]

        call.respondText("검색어: $query, 페이지: $page")
    }
}

// 접속: http://localhost:8080/search?q=kotlin&page=1
// 출력: 검색어: kotlin, 페이지: 1

🔧 JSON Responses

Setting up ContentNegotiation

plugins {
    // build.gradle.kts에 추가
    implementation("io.ktor:ktor-server-content-negotiation:2.3.5")
    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.5")
}

JSON Responses

import io.ktor.serialization.kotlinx.json.*
import io.ktor.server.plugins.contentnegotiation.*
import kotlinx.serialization.Serializable

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

fun Application.module() {
    install(ContentNegotiation) {
        json()
    }

    routing {
        get("/user/{id}") {
            val id = call.parameters["id"]?.toInt() ?: 0
            val user = User(id, "홍길동", "hong@example.com")
            call.respond(user)
        }

        get("/users") {
            val users = listOf(
                User(1, "홍길동", "hong@example.com"),
                User(2, "김철수", "kim@example.com")
            )
            call.respond(users)
        }
    }
}

🎯 Practical Example

Simple API

@Serializable
data class Product(
    val id: Int,
    val name: String,
    val price: Double
)

fun Application.productAPI() {
    install(ContentNegotiation) {
        json()
    }

    val products = mutableListOf(
        Product(1, "노트북", 1500000.0),
        Product(2, "마우스", 30000.0),
        Product(3, "키보드", 80000.0)
    )

    routing {
        // 전체 상품 조회
        get("/products") {
            call.respond(products)
        }

        // 특정 상품 조회
        get("/products/{id}") {
            val id = call.parameters["id"]?.toInt()
            val product = products.find { it.id == id }

            if (product != null) {
                call.respond(product)
            } else {
                call.respondText("상품을 찾을 수 없습니다", status = HttpStatusCode.NotFound)
            }
        }
    }
}

fun main() {
    embeddedServer(Netty, port = 8080) {
        productAPI()
    }.start(wait = true)
}

Status Codes

import io.ktor.http.*

routing {
    get("/status/ok") {
        call.respondText("정상", status = HttpStatusCode.OK)
    }

    get("/status/created") {
        call.respondText("생성됨", status = HttpStatusCode.Created)
    }

    get("/status/not-found") {
        call.respondText("없음", status = HttpStatusCode.NotFound)
    }

    get("/status/error") {
        call.respondText("서버 오류", status = HttpStatusCode.InternalServerError)
    }
}

🔥 Middleware

Logging

import io.ktor.server.plugins.callloging.*

fun Application.module() {
    install(CallLogging)

    routing {
        get("/") {
            call.respondText("Hello!")
        }
    }
}

// 콘솔에 요청 로그 출력

CORS Configuration

import io.ktor.server.plugins.cors.routing.*

fun Application.module() {
    install(CORS) {
        anyHost()  // 모든 호스트 허용 (개발 환경)
        allowHeader(HttpHeaders.ContentType)
    }

    routing {
        get("/api/data") {
            call.respond(mapOf("message" to "CORS 설정됨"))
        }
    }
}

🛠️ Application Structure

Modularization

// routes/UserRoutes.kt
fun Route.userRoutes() {
    route("/users") {
        get {
            call.respond(listOf("User1", "User2"))
        }

        get("/{id}") {
            val id = call.parameters["id"]
            call.respondText("User $id")
        }
    }
}

// routes/ProductRoutes.kt
fun Route.productRoutes() {
    route("/products") {
        get {
            call.respond(listOf("Product1", "Product2"))
        }
    }
}

// Application.kt
fun Application.module() {
    install(ContentNegotiation) {
        json()
    }

    routing {
        userRoutes()
        productRoutes()
    }
}

🤔 Frequently Asked Questions

Q1. Ktor vs. Spring Boot?

A: Ktor is lightweight and coroutine-friendly!

// Ktor - 간결하고 가벼움
fun Application.module() {
    routing {
        get("/hello") {
            call.respondText("Hello!")
        }
    }
}

// Spring Boot - 더 많은 기능과 생태계
@RestController
class HelloController {
    @GetMapping("/hello")
    fun hello() = "Hello!"
}

Q2. How do I change the port?

A: Specify it when creating the server!

// 코드에서
embeddedServer(Netty, port = 3000) {
    // ...
}.start(wait = true)

// application.conf 파일
ktor {
    deployment {
        port = 3000
    }
}

Q3. How do I serve static files?

A: Use the Static plugin!

import io.ktor.server.http.content.*

routing {
    static("/static") {
        resources("static")
    }

    static("/files") {
        files("uploads")
    }
}

🎬 Conclusion

Get started with server development quickly using Ktor!

핵심 정리:
✅ 경량 비동기 프레임워크
✅ 코루틴 기반
✅ 간단한 라우팅
✅ JSON 직렬화 지원
✅ 모듈화 가능

Next Step: Build a complete API in REST API!