JSONからKotlinデータクラスへ：kotlinx.serialization、Gson、Moshiの完全ガイド

Why Use Kotlin Data Classes for JSON?

Kotlin data classes are purpose-built for holding structured data. Unlike Java POJOs, they eliminate hundreds of lines of boilerplate — equals(), hashCode(), toString(), and copy() are all generated by the compiler.

When deserializing JSON APIs, data classes give you compile-time type safety, IDE autocomplete, and Kotlin null safety — which prevents the null pointer exceptions that plague Java JSON code.

Basic Data Class from JSON (kotlinx.serialization)

The modern, Kotlin-first approach uses kotlinx.serialization. It is multiplatform, generates serializers at compile time (no reflection), and integrates with Kotlin null safety.

Given this JSON:

{
  "id": 42,
  "username": "alice",
  "email": "alice@example.com",
  "age": 30,
  "is_active": true,
  "bio": null
}

The Kotlin data class with kotlinx.serialization:

import kotlinx.serialization.Serializable
import kotlinx.serialization.SerialName

@Serializable
data class User(
    val id: Int,
    val username: String,
    val email: String,
    val age: Int,
    @SerialName("is_active")
    val isActive: Boolean,
    val bio: String? = null  // nullable + default value
)

// Deserialize
val user = Json.decodeFromString<User>(jsonString)

// Serialize back to JSON
val json = Json.encodeToString(user)

Add to build.gradle.kts:

plugins {
    kotlin("plugin.serialization") version "1.9.22"
}

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

Gson Alternative

Google\u0027s Gson is battle-tested on Android and requires zero code generation. Use it when you need a simple drop-in that works with existing Java libraries. Note that Gson uses reflection and does not understand Kotlin null safety — a null JSON field on a non-nullable Kotlin property causes a runtime crash.

import com.google.gson.annotations.SerializedName

// No @Serializable needed — Gson uses reflection
data class User(
    val id: Int,
    val username: String,
    val email: String,
    val age: Int,
    @SerializedName("is_active")
    val isActive: Boolean,
    val bio: String? = null
)

// Setup
val gson = GsonBuilder()
    .setLenient()
    .create()

// Deserialize
val user = gson.fromJson(jsonString, User::class.java)

// Serialize
val json = gson.toJson(user)

// build.gradle.kts
dependencies {
    implementation("com.google.code.gson:gson:2.10.1")
}

Moshi Alternative

Square\u0027s Moshi is designed with Kotlin in mind and supports both reflection and code generation (kapt/KSP). Unlike Gson, Moshi understands Kotlin null safety and throws JsonDataException for null mismatches instead of letting them pass silently.

import com.squareup.moshi.Json
import com.squareup.moshi.JsonClass

@JsonClass(generateAdapter = true)  // triggers KSP/kapt code generation
data class User(
    val id: Int,
    val username: String,
    val email: String,
    val age: Int,
    @Json(name = "is_active")
    val isActive: Boolean,
    val bio: String? = null
)

// Setup
val moshi = Moshi.Builder()
    .addLast(KotlinJsonAdapterFactory())
    .build()

val adapter = moshi.adapter(User::class.java)

// Deserialize
val user = adapter.fromJson(jsonString)

// Serialize
val json = adapter.toJson(user)

// build.gradle.kts
plugins {
    id("com.google.devtools.ksp") version "1.9.22-1.0.17"
}

dependencies {
    implementation("com.squareup.moshi:moshi:1.15.1")
    implementation("com.squareup.moshi:moshi-kotlin:1.15.1")
    ksp("com.squareup.moshi:moshi-kotlin-codegen:1.15.1")
}

Jackson with Kotlin

Jackson is the de-facto standard in Spring Boot projects. Add the jackson-module-kotlin to make it understand Kotlin data classes, nullable types, and default parameters.

import com.fasterxml.jackson.annotation.JsonProperty

// No special annotation needed if jackson-module-kotlin is registered
data class User(
    val id: Int,
    val username: String,
    val email: String,
    val age: Int,
    @JsonProperty("is_active")
    val isActive: Boolean,
    val bio: String? = null
)

// Setup with Kotlin module
val mapper = ObjectMapper().apply {
    registerModule(KotlinModule.Builder().build())
    configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
}

// Deserialize
val user = mapper.readValue<User>(jsonString)

// Serialize
val json = mapper.writeValueAsString(user)

// build.gradle.kts (Spring Boot adds these automatically)
dependencies {
    implementation("com.fasterxml.jackson.module:jackson-module-kotlin:2.16.1")
    implementation("com.fasterxml.jackson.core:jackson-databind:2.16.1")
}

Nested Objects and Lists

JSON with nested objects maps to separate data classes. Arrays map to List<T>.

// Input JSON
{
  "order_id": "ORD-001",
  "customer": {
    "id": 42,
    "name": "Alice"
  },
  "items": [
    { "sku": "ITEM-A", "quantity": 2, "price": 9.99 },
    { "sku": "ITEM-B", "quantity": 1, "price": 24.99 }
  ],
  "tags": ["priority", "gift"]
}

import kotlinx.serialization.Serializable
import kotlinx.serialization.SerialName

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

@Serializable
data class OrderItem(
    val sku: String,
    val quantity: Int,
    val price: Double
)

@Serializable
data class Order(
    @SerialName("order_id") val orderId: String,
    val customer: Customer,
    val items: List<OrderItem>,
    val tags: List<String> = emptyList()
)

// Parse entire hierarchy in one call
val order = Json.decodeFromString<Order>(jsonString)

// Access nested data
println(order.customer.name)       // Alice
println(order.items[0].sku)        // ITEM-A
println(order.items.sumOf { it.price * it.quantity })  // total cost

Nullable Fields and Default Values

Kotlin\u0027s type system distinguishes nullable (String?) from non-nullable (String) types. Map JSON null values to nullable types, and use default values for optional JSON keys that may be absent entirely.

@Serializable
data class UserProfile(
    val id: Int,                           // required, never null
    val username: String,                  // required, never null
    val bio: String? = null,               // JSON null or absent -> null
    val avatarUrl: String? = null,         // JSON null or absent -> null
    val followersCount: Int = 0,           // absent in JSON -> defaults to 0
    val isVerified: Boolean = false,       // absent in JSON -> defaults to false
    val roles: List<String> = emptyList(), // absent in JSON -> empty list
    val metadata: Map<String, String> = emptyMap()
)

// Lenient parsing — ignores extra keys, uses defaults for missing keys
val json = Json {
    ignoreUnknownKeys = true
    coerceInputValues = true  // coerces invalid values to defaults
}

// Null-safe access
val profile = json.decodeFromString<UserProfile>(jsonString)
val displayBio = profile.bio ?: "No bio provided"
profile.avatarUrl?.let { loadAvatar(it) }  // safe call

With ignoreUnknownKeys = true, the parser silently ignores JSON fields that have no matching property in the data class — essential for robust API clients that must handle evolving APIs.

Enum Handling

JSON string values that represent a finite set of options map naturally to Kotlin enums. Both kotlinx.serialization and Gson support enum serialization out of the box.

import kotlinx.serialization.SerialName

@Serializable
enum class OrderStatus {
    @SerialName("pending") PENDING,
    @SerialName("processing") PROCESSING,
    @SerialName("shipped") SHIPPED,
    @SerialName("delivered") DELIVERED,
    @SerialName("cancelled") CANCELLED,
}

@Serializable
data class Order(
    val id: String,
    val status: OrderStatus,  // maps JSON "shipped" to OrderStatus.SHIPPED
    val amount: Double
)

// Usage
val order = Json.decodeFromString<Order>("""
    {"id": "ORD-1", "status": "shipped", "amount": 49.99}
""")
println(order.status)  // SHIPPED

// Unknown enum values — use lenient mode or provide a fallback
val lenientJson = Json {
    ignoreUnknownKeys = true
    isLenient = true
}

// With Gson — enums serialize by name automatically
val gson = Gson()
val json = gson.toJson(OrderStatus.SHIPPED)  // "SHIPPED"

Date/Time with kotlinx.serialization

kotlinx-datetime provides multiplatform date/time classes. Use Instant for timestamps (ISO 8601), LocalDate for dates without time.

import kotlinx.datetime.Instant
import kotlinx.datetime.LocalDate
import kotlinx.datetime.serializers.InstantIso8601Serializer

@Serializable
data class Event(
    val id: Int,
    val title: String,
    // ISO 8601 string in JSON: "2026-02-27T10:30:00Z"
    @Serializable(with = InstantIso8601Serializer::class)
    val createdAt: Instant,
    // Date string in JSON: "2026-03-01"
    val eventDate: LocalDate,
)

// build.gradle.kts
// implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.5.0")

// For JVM-only (Gson/Jackson) with java.time
data class EventJvm(
    val id: Int,
    @JsonAdapter(InstantAdapter::class)  // Gson custom adapter
    val createdAt: java.time.Instant,
)

// Custom Gson TypeAdapter for Instant
class InstantAdapter : TypeAdapter<java.time.Instant>() {
    override fun write(out: JsonWriter, value: java.time.Instant) {
        out.value(value.toString())
    }
    override fun read(`in`: JsonReader): java.time.Instant {
        return java.time.Instant.parse(`in`.nextString())
    }
}

Sealed Classes for Union Types

Sealed classes model JSON payloads that have different shapes based on a discriminator field. kotlinx.serialization supports polymorphism through SerializersModule.

// JSON variants:
// {"type": "card", "last4": "1234", "brand": "Visa"}
// {"type": "paypal", "email": "user@example.com"}
// {"type": "bank", "account_number": "****5678"}

@Serializable
sealed class PaymentMethod {
    abstract val type: String
}

@Serializable
@SerialName("card")
data class CardPayment(
    override val type: String = "card",
    val last4: String,
    val brand: String
) : PaymentMethod()

@Serializable
@SerialName("paypal")
data class PayPalPayment(
    override val type: String = "paypal",
    val email: String
) : PaymentMethod()

@Serializable
@SerialName("bank")
data class BankPayment(
    override val type: String = "bank",
    @SerialName("account_number") val accountNumber: String
) : PaymentMethod()

// Configure polymorphic serialization
val json = Json {
    serializersModule = SerializersModule {
        polymorphic(PaymentMethod::class) {
            subclass(CardPayment::class)
            subclass(PayPalPayment::class)
            subclass(BankPayment::class)
        }
    }
}

// Parse — Kotlin when expression gives exhaustive handling
val payment = json.decodeFromString<PaymentMethod>(jsonString)
when (payment) {
    is CardPayment  -> println("Card: ${payment.brand} *${payment.last4}")
    is PayPalPayment -> println("PayPal: ${payment.email}")
    is BankPayment  -> println("Bank: ${payment.accountNumber}")
}

Coroutines + Ktor Client Integration

Ktor is the Kotlin-native HTTP client that pairs naturally with kotlinx.serialization. It handles JSON content negotiation, request/response serialization, and coroutine integration.

import io.ktor.client.*
import io.ktor.client.call.*
import io.ktor.client.engine.cio.*
import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.client.request.*
import io.ktor.serialization.kotlinx.json.*

@Serializable
data class GitHubUser(
    val login: String,
    val id: Int,
    val name: String?,
    @SerialName("public_repos") val publicRepos: Int,
    val bio: String? = null
)

// Create Ktor client with kotlinx.serialization
val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        json(Json {
            ignoreUnknownKeys = true
            isLenient = true
        })
    }
}

// Suspend function — call from coroutine scope
suspend fun fetchUser(username: String): GitHubUser {
    return client.get("https://api.github.com/users/$username") {
        header("Accept", "application/vnd.github.v3+json")
    }.body()
}

// Usage in coroutine
viewModelScope.launch {
    val user = fetchUser("octocat")
    println(user.name)         // The Octocat
    println(user.publicRepos)  // number of public repos
}

// POST with serialized body
suspend fun createUser(user: User): User {
    return client.post("https://api.example.com/users") {
        contentType(ContentType.Application.Json)
        setBody(user)
    }.body()
}

// build.gradle.kts
dependencies {
    implementation("io.ktor:ktor-client-core:2.3.9")
    implementation("io.ktor:ktor-client-cio:2.3.9")
    implementation("io.ktor:ktor-client-content-negotiation:2.3.9")
    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.9")
}

Android / Retrofit Integration

Retrofit is the dominant Android HTTP client. Combine it with either Gson (traditional) or kotlinx.serialization (modern) converter factories.

// Data classes (same structure works with both converters)
@Serializable
data class Post(
    val id: Int,
    val title: String,
    val body: String,
    @SerialName("user_id") val userId: Int
)

// Retrofit interface
interface PostsApi {
    @GET("posts")
    suspend fun getPosts(): List<Post>

    @GET("posts/{id}")
    suspend fun getPost(@Path("id") id: Int): Post

    @POST("posts")
    suspend fun createPost(@Body post: Post): Post
}

// Option A: kotlinx.serialization converter (recommended)
val retrofit = Retrofit.Builder()
    .baseUrl("https://jsonplaceholder.typicode.com/")
    .addConverterFactory(
        Json { ignoreUnknownKeys = true }
            .asConverterFactory("application/json".toMediaType())
    )
    .build()

// Option B: Gson converter (simpler setup)
val retrofit = Retrofit.Builder()
    .baseUrl("https://jsonplaceholder.typicode.com/")
    .addConverterFactory(GsonConverterFactory.create())
    .build()

val api = retrofit.create(PostsApi::class.java)

// Usage in ViewModel
viewModelScope.launch {
    try {
        val posts = api.getPosts()  // returns List<Post>
        _uiState.value = UiState.Success(posts)
    } catch (e: Exception) {
        _uiState.value = UiState.Error(e.message ?: "Unknown error")
    }
}

// build.gradle.kts (Android)
dependencies {
    // Retrofit
    implementation("com.squareup.retrofit2:retrofit:2.9.0")

    // Option A: kotlinx.serialization converter
    implementation("com.jakewharton.retrofit:retrofit2-kotlinx-serialization-converter:1.0.0")
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")

    // Option B: Gson converter
    implementation("com.squareup.retrofit2:converter-gson:2.9.0")

    // Coroutines for Android
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.7.3")
}

Tools for JSON to Kotlin Conversion

Several tools automate Kotlin data class generation from JSON — saving time and preventing typos:

The DevToolBox JSON to Kotlin converter supports all three major libraries — paste your JSON and instantly get annotated data classes with proper @SerialName, @SerializedName, or @Json(name) annotations for snake_case fields.

Common Pitfalls

Avoid these common mistakes when working with JSON and Kotlin data classes:

// Wrong
data class Address(val city: String)  // missing @Serializable

// Correct
@Serializable
data class Address(val city: String)

// Dangerous with Gson
data class User(val name: String)  // crash if JSON has "name": null

// Safe
data class User(val name: String? = null)

val json = Json { ignoreUnknownKeys = true }

val mapper = ObjectMapper().registerModule(KotlinModule.Builder().build())

@Entity
data class UserEntity(
    @PrimaryKey val id: Int,
    @Embedded val address: Address,  // flattened to columns
    @TypeConverters val tags: List<String>  // needs TypeConverter
)

// JSON: {"user_name": "alice"}
@Serializable
data class User(
    @SerialName("user_name") val userName: String  // correct mapping
)

Library Comparison Summary

Frequently Asked Questions