Kotlin Multiplatform Serialization — Complete Guide

// Not applying the serialization plugin
data class User(val id: String, val name: String) // No @Serializable

// Using Java serialization
class User : Serializable { // JVM-only!
    // ...
}

// build.gradle.kts
plugins {
    kotlin("plugin.serialization")
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
}

// commonMain
import kotlinx.serialization.*
import kotlinx.serialization.json.*

@Serializable
data class User(
    val id: String,
    val name: String,
    @SerialName("email_address")
    val email: String,
    @Serializable(with = DateSerializer::class)
    val createdAt: Long
)

@Serializable
data class ApiResponse<T>(
    val success: Boolean,
    val data: T // Polymorphic — requires context serializer
)

@Serializable
sealed class Status {
    @Serializable
    @SerialName("active")
    data class Active(val since: Long) : Status()

    @Serializable
    @SerialName("inactive")
    data class Inactive(val reason: String) : Status()
}

// Custom serializer
object DateSerializer : KSerializer<Long> {
    override val descriptor = PrimitiveSerialDescriptor("Date", PrimitiveKind.LONG)

    override fun serialize(encoder: Encoder, value: Long) {
        encoder.encodeLong(value)
    }

    override fun deserialize(decoder: Decoder): Long {
        return decoder.decodeLong()
    }
}

// Usage
val json = Json {
    ignoreUnknownKeys = true
    prettyPrint = true
    encodeDefaults = true
    classDiscriminator = "type" // For sealed class discrimination
}

fun serialize() {
    val user = User("1", "Alice", "alice@example.com", System.currentTimeMillis())
    val jsonString = json.encodeToString(user)

    val decoded = json.decodeFromString<User>(jsonString)

    // Polymorphic
    val status: Status = Status.Active(System.currentTimeMillis())
    val statusJson = json.encodeToString(status)
    val statusDecoded = json.decodeFromString<Status>(statusJson)
}