json serialization in ktor

I'm trying to create a simple server that parses json and makes some logs.

package com.example

import com.example.plugins.*
import io.ktor.http.content.*
import io.ktor.serialization.*
import io.ktor.server.application.*
import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.server.request.*
import io.ktor.server.response.*
import io.ktor.server.routing.*
import org.slf4j.LoggerFactory

fun main(args: Array<String>) {
    io.ktor.server.netty.EngineMain.main(args)
}

fun Application.module() {
    configureSerialization()
    configureDatabases()
    configureMonitoring()
    configureHTTP()
    configureSecurity()
    configureRouting()
}

data class LoginData(val username: String, val password: String)

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

fun Application.configureRouting() {
    val logger = LoggerFactory.getLogger("Application")

    routing {
        post("/login") {
            logger.info("GET SOME RESPONSE")
            val loginData = call.receive<LoginData>()
            val username = loginData.username
            val password = loginData.password

            logger.info("GET SOME RESPONSE: $username")
            println("GET SOME RESPONSE: $username")

            if (username == "admin" && password == "admin123") {
                logger.info("Authorization successful for user: $username")
                call.respondText("Authorization successful!")
            } else {
                logger.info("Authorization failed for user: $username")
                call.respondText("Authorization failed!")
            }
        }
    }
}

build.gradle.kts:

val ktor_version: String by project
val kotlin_version: String by project
val logback_version: String by project

plugins {
    kotlin("jvm") version "1.9.24"
    id("io.ktor.plugin") version "2.3.11"
    id("org.jetbrains.kotlin.plugin.serialization") version "1.9.24"
}

group = "com.example"

version = "0.0.1"

application {
    mainClass.set("io.ktor.server.netty.EngineMain")

    val isDevelopment: Boolean = project.ext.has("development")
    applicationDefaultJvmArgs = listOf("-Dio.ktor.development=$isDevelopment")
}

repositories { mavenCentral() }

dependencies {
    implementation("io.ktor:ktor-server-core-jvm")
    implementation("io.ktor:ktor-serialization-kotlinx-json-jvm")
    implementation("io.ktor:ktor-server-content-negotiation-jvm")
    implementation("org.postgresql:postgresql:42.5.1")
    implementation("com.h2database:h2:2.1.214")
    implementation("io.ktor:ktor-server-metrics-jvm")
    implementation("io.ktor:ktor-server-caching-headers-jvm")
    implementation("io.ktor:ktor-server-auth-jvm")
    implementation("io.ktor:ktor-server-netty-jvm")
    implementation("ch.qos.logback:logback-classic:$logback_version")
    implementation("io.ktor:ktor-server-config-yaml:2.3.11")
    testImplementation("io.ktor:ktor-server-tests-jvm")
    testImplementation("org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version")
}

I get a simple error: "Unresolved reference: json"

It looks like I'm importing something wrong and I read the documentation, but it didn't help. Since I’m a newbie, I’m not entirely sure that I understand what plugins are for and how they differ from dependencies; from my experience with java, I haven’t encountered this, I would appreciate any help.

json is an extension function declared in the io.ktor.serialization.kotlinx.json package. You did not import that. You should add this import:

import io.ktor.serialization.kotlinx.json.json
// or
// import io.ktor.serialization.kotlinx.json.*

Also, LoginData should be marked @Serializable since you are doing call.receive<LoginData>().