How do I serialize a Kotlin data class with kotlinx.serialization?

Annotate the data class with @Serializable, then call Json.encodeToString(obj). For a list: Json.encodeToString(ListSerializer(MyClass.serializer()), list). For pretty output: Json { prettyPrint = true }.encodeToString(obj). kotlinx.serialization is the official Kotlin library and works on all platforms including KMP.

How do I send a Kotlin data class as JSON in a Retrofit request?

Add the converter to your Retrofit builder (.addConverterFactory(GsonConverterFactory.create()) or MoshiConverterFactory), then annotate the suspend function parameter with @Body: @POST('users') suspend fun createUser(@Body user: User): Response . Retrofit serializes the body automatically using the configured converter.

Convert Kotlin to JSON — Generate Mock Data & Snippets

Kotlin to JSON — 3 modes 100% client-side

Kotlin data class input

Instances

Formatting

Ready

Gson vs Moshi vs kotlinx.serialization

The three Kotlin JSON libraries and their one-liner serialization.

Library	Serialize	Best for
Gson	`Gson().toJson(obj)`	Legacy Android projects · zero annotations needed · Retrofit GsonConverterFactory
Moshi	`moshi.adapter(T::class.java).toJson(obj)`	Modern Android · Kotlin-first · null-safe · Retrofit MoshiConverterFactory
kotlinx	`Json.encodeToString(obj)`	KMP · Ktor · official Kotlin library · all targets (Android, iOS, Web, Desktop)

Kotlin parsed in your
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The mock JSON parser reads Kotlin data class property declarations — val/var with type annotations. It handles nullable types (Type?), List<T> generics and nested class references. The snippets cover the three main Kotlin JSON libraries with complete Gradle dependency coordinates and all common serialization scenarios.

3 libraries covered

Gson, Moshi and kotlinx.serialization — complete Gradle dependencies, imports and real-world usage patterns for each.

Retrofit + Ktor patterns

Complete Retrofit service interface with @Body annotation and Ktor client with ContentNegotiation plugin — the two main HTTP clients for Kotlin.

KMP-ready

The kotlinx.serialization and Ktor patterns work on all Kotlin Multiplatform targets — Android, iOS, Desktop and Web.

47 tools, always free

No file size limits, no watermarks, no account. Funded by non-intrusive display advertising only.

Frequently asked questions

How do I convert a Kotlin data class to JSON?

With Gson: val json = Gson().toJson(myObject). With kotlinx.serialization: annotate with @Serializable then call Json.encodeToString(myObject). With Moshi: build a Moshi instance and get a JsonAdapter. Select the Snippets mode for complete code with Gradle dependencies.

Which Kotlin JSON library should I use?

For new Android projects: Moshi with codegen adapters — Kotlin-first, null-safe, no reflection. For Kotlin Multiplatform: kotlinx.serialization — the only option that works on all targets. For legacy Android: Gson — zero configuration, works without annotations. All three integrate with Retrofit.

Is the Kotlin to JSON converter free?

Yes, completely free. No file size limits, no account required. JSONshift is funded by non-intrusive display advertising.

Convert Kotlin to JSONMock data, snippetsor framework guide.

Kotlin parsed in yourbrowser. No IDE needed.

Convert Kotlin to JSON
Mock data, snippets
or framework guide.

Kotlin parsed in your
browser. No IDE needed.