CoreJSON

JSON to Kotlin

This JSON to Kotlin tool generates concise data classes with support for Jackson, kotlinx.serialization and Klaxon plus a custom package name, with nullable (?) types inferred accurately. Generation runs locally in your browser, suited to both Android and server-side development.

Open in the workbench Runs locally · nothing is uploaded

How to use it

  1. 1

    Paste your JSON

    Paste API JSON; nested objects produce related data classes and collections become List<T>.

  2. 2

    Choose framework and package

    Pick Jackson (Spring ecosystem), kotlinx.serialization (official / KMP) or Klaxon, and enter a package name.

  3. 3

    Copy the Kotlin code

    Data classes with accurate nullable annotations are generated; copy them into your project.

  4. 4

    Wire into your project

    Add the matching serialisation plugin or dependency, then deserialise the response.

Key features

Three serialisation options

Jackson annotations, kotlinx.serialization with @Serializable, or the lightweight Klaxon — switch as needed.

Accurate null-safety

Null fields and multi-sample analysis determine which fields get a ?, reducing NPEs and non-null assertions.

Concise data class output

Primary-constructor properties with defaults; equals/hashCode/toString/copy come for free from Kotlin.

Configurable package

The package declaration follows your project convention so multi-module Android projects can drop it straight in.

Default values for nullable fields

Nullable fields default to null so deserialisation does not throw when a key is missing.

Local and private

Generation runs in your browser and the data is not uploaded.

Common errors & troubleshooting

  • ! Android with Retrofit commonly uses kotlinx.serialization or Moshi; if you choose Jackson annotations here you can tweak them by hand.
  • ! A Spring Boot Kotlin backend can use Jackson directly by adding jackson-module-kotlin.
  • ! A pure data class (types only) carries no framework annotations and can also be deserialised with Gson.
  • ! Review the required semantics against the API docs, since defaults are inferred from sample presence.

Frequently asked questions

Why do some field types have a question mark?

When a field is null or missing in the JSON it is treated as nullable (String?). Kotlin null-safety requires an explicit marker, so the compiler forces a null check when the value is used.

How do I set up kotlinx.serialization?

After choosing the kotlinx framework, classes carry @Serializable and fields carry @SerialName. The project must enable the kotlin("plugin.serialization") plugin and add the kotlinx-serialization-json dependency.

How are field default values decided?

Nullable fields default to null so that deserialisation does not throw on a missing key; non-null fields are inferred from sample presence, and you should verify the required semantics against your API docs.

Is my data uploaded?

No. Generation runs locally in your browser.

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