Kotlinで高速にデータをシリアライズ！Kotlinx.serializationの使い方と実装例

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

plugins {
    kotlin("plugin.serialization") version "1.9.0"
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")
}

plugins {
    kotlin("multiplatform") version "1.9.0"
    kotlin("plugin.serialization") version "1.9.0"
}

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

val user = User("Alice", 25)
val jsonString = Json.encodeToString(user)
val decodedUser = Json.decodeFromString<User>(jsonString)

import kotlinx.serialization.Serializable

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

import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json

fun main() {
    val user = User("Alice", 25)
    val jsonString = Json.encodeToString(user)
    println(jsonString)  // {"name":"Alice","age":25}
}

import kotlinx.serialization.decodeFromString

fun main() {
    val jsonString = """{"name":"Alice","age":25}"""
    val user = Json.decodeFromString<User>(jsonString)
    println(user)  // User(name=Alice, age=25)
}

@Serializable
data class Company(val name: String, val employees: List<User>)

fun main() {
    val company = Company("TechCorp", listOf(User("Bob", 30), User("Eve", 28)))
    val jsonString = Json.encodeToString(company)
    println(jsonString)  // {"name":"TechCorp","employees":[{"name":"Bob","age":30},{"name":"Eve","age":28}]}
}

@Serializable
data class User(
    val name: String,
    val age: Int = 18  // デフォルト値を設定
)

fun main() {
    val jsonString = """{"name":"Alice"}"""  // ageが存在しない
    val user = Json.decodeFromString<User>(jsonString)
    println(user)  // User(name=Alice, age=18)
}

@Serializable
data class Address(val city: String, val zip: String)

@Serializable
data class User(val name: String, val address: Address)

fun main() {
    val user = User("Bob", Address("New York", "10001"))
    val jsonString = Json.encodeToString(user)
    println(jsonString)  // {"name":"Bob","address":{"city":"New York","zip":"10001"}}
}

@Serializable
data class Survey(val questions: List<String>)

fun main() {
    val survey = Survey(listOf("Q1", "Q2", "Q3"))
    val jsonString = Json.encodeToString(survey)
    println(jsonString)  // {"questions":["Q1","Q2","Q3"]}
}

@Serializable
data class Scores(val results: Map<String, Int>)

fun main() {
    val scores = Scores(mapOf("Alice" to 90, "Bob" to 85))
    val jsonString = Json.encodeToString(scores)
    println(jsonString)  // {"results":{"Alice":90,"Bob":85}}
}

@Serializable
sealed class Shape {
    @Serializable
    data class Circle(val radius: Double) : Shape()

    @Serializable
    data class Rectangle(val width: Double, val height: Double) : Shape()
}

fun main() {
    val shapes: List<Shape> = listOf(
        Shape.Circle(5.0),
        Shape.Rectangle(4.0, 6.0)
    )
    val jsonString = Json.encodeToString(shapes)
    println(jsonString)
}

import kotlinx.serialization.*
import kotlinx.serialization.descriptors.*
import kotlinx.serialization.encoding.*
import java.time.LocalDate
import java.time.format.DateTimeFormatter

object LocalDateSerializer : KSerializer<LocalDate> {
    private val formatter = DateTimeFormatter.ISO_LOCAL_DATE

    override val descriptor: SerialDescriptor =
        PrimitiveSerialDescriptor("LocalDate", PrimitiveKind.STRING)

    override fun serialize(encoder: Encoder, value: LocalDate) {
        encoder.encodeString(value.format(formatter))
    }

    override fun deserialize(decoder: Decoder): LocalDate {
        return LocalDate.parse(decoder.decodeString(), formatter)
    }
}

@Serializable
data class User(
    val name: String,
    @Serializable(with = LocalDateSerializer::class)
    val birthDate: LocalDate
)

fun main() {
    val user = User("Alice", LocalDate.of(1995, 8, 20))
    val jsonString = Json.encodeToString(user)
    println(jsonString)  // {"name":"Alice","birthDate":"1995-08-20"}

    val decodedUser = Json.decodeFromString<User>(jsonString)
    println(decodedUser)  // User(name=Alice, birthDate=1995-08-20)
}

val json = Json {
    serializersModule = SerializersModule {
        contextual(LocalDate::class, LocalDateSerializer)
    }
}

fun main() {
    val user = User("Bob", LocalDate.of(2000, 1, 1))
    val jsonString = json.encodeToString(user)
    println(jsonString)
}

val inputStream = File("data.json").inputStream()
val user = Json.decodeFromStream<User>(inputStream)

val json = Json {
    encodeDefaults = false  // デフォルト値はシリアライズしない
    ignoreUnknownKeys = true  // 不明なキーを無視
    isLenient = true  // 厳密な形式チェックを緩和
    explicitNulls = false  // nullは出力しない
}

@Serializable
data class User(val name: String, val age: Int)  // IntやStringは高速処理可能

val json = Json {
    classDiscriminator = "type"  // デフォルトの"@type"からカスタマイズ
}

suspend fun serializeAsync(user: User): String {
    return withContext(Dispatchers.IO) {
        Json.encodeToString(user)
    }
}

val module = SerializersModule {
    contextual(LocalDate::class, LocalDateSerializer)
}

val json = Json { serializersModule = module }

val json = Json {
    prettyPrint = true  // バッファを増やして見やすくする
    prettyPrintIndent = "  "  // インデント幅を広げる
}

@Serializable
data class User(
    val name: String,
    @Transient val password: String = ""  // シリアライズ対象外
)

SerializationException: Serializer for class 'User' is not found.  

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

UnknownFieldException: Field 'gender' is not found in class 'User'.  

val json = Json {
    ignoreUnknownKeys = true
}

SerializationException: Field 'age' is required but it was missing or null.  

@Serializable
data class User(
    val name: String,
    val age: Int = 0  // デフォルト値を設定
)

@Serializable
data class User(
    val name: String,
    val age: Int?
)

JsonDecodingException: Unexpected JSON token at offset 0: Expected '{', but got '['.  

val jsonString = """{"name":"Alice","age":25}"""  
val user = Json.decodeFromString<User>(jsonString)

Serializer for class 'LocalDate' is not found.  

@Serializable(with = LocalDateSerializer::class)
data class User(val name: String, val birthDate: LocalDate)

ClassCastException: Cannot cast 'String' to 'List<User>'.  

val userList = Json.decodeFromString<List<User>>(jsonString)

Enum constant 'ADMIN' does not exist in enum class 'Role'.  

@Serializable
enum class Role {
    @SerialName("ADMIN") Admin,
    @SerialName("USER") User
}