Parcelize

Active contributors: Dmitriy Novozhilov, Kirill Rakhman, Rafal Galczak

plugins/parcelize/ is a small but heavily used Android-specific plugin that synthesizes the Parcelable implementation for Kotlin classes annotated with @Parcelize. Without it, Android developers have to hand-write tedious writeToParcel/createFromParcel boilerplate; with it, a single annotation suffices.

Purpose

Generate, at compile time, the methods and fields that Android's Parcelable interface requires (writeToParcel, describeContents, the CREATOR static field) for any class annotated @Parcelize. Make this work over the full Kotlin type system: nullable types, data classes, sealed hierarchies, parametric types, collections, and custom Parceler<T> implementations.

Directory layout

plugins/parcelize/
├── parcelize-compiler/         The compiler plugin
├── parcelize-runtime/          Annotations and runtime helpers (@Parcelize, Parceler)
└── parcelize-tests/            Tests

Key abstractions

Concept	Role
@Parcelize	Marks a class for Parcelable code generation.
Parceler<T>	A user-supplied custom serializer for a type.
@WriteWith<P>	Annotation that instructs the plugin to use a specific Parceler<T> for a property.

How it works

graph LR
    Class["@Parcelize<br/>data class Foo(val x: Int, val y: String?)"] --> FirExt["Parcelize FIR ext"]
    FirExt -->|declares synthetic Parcelable members| FirSyn["FIR with declarations"]
    Fir2Ir --> IR["IR"]
    IR --> IrExt["Parcelize IR generator"]
    IrExt -->|generates writeToParcel,<br/>describeContents, CREATOR| IR2["IR"]
    IR2 --> JVMBackend["JVM backend"]
    JVMBackend --> Cls[".class files"]

The plugin runs in both the FIR extension and IR generator phases:

• The FIR extension declares the synthetic Parcelable members so the frontend treats the class as if it implemented Parcelable already.
• The IR generator emits the bodies, walking each property and emitting the appropriate Parcel.write*/Parcel.read* calls based on the property type.

JVM-only

Parcelable is an Android-specific (i.e., JVM-on-Android) interface. The plugin is configured to be a no-op on non-JVM targets, but in practice it's only used in projects that target Android.

KGP integration

libraries/tools/kotlin-gradle-plugin/.../parcelize/ (or id("kotlin-parcelize")) wires the plugin into Gradle:

plugins {
    id("kotlin-parcelize")
}

Tests

plugins/parcelize/parcelize-tests/ runs golden-file IR tests plus end-to-end tests that exercise the generated Parcelable implementation against the Android framework's expectations.

Integration points

• Consumed by: Android Kotlin projects.
• Co-traveler: the Android Gradle plugin (which detects kotlin-parcelize and includes the runtime jar appropriately).

Where to start

For new property-type support (e.g., a new platform type that should be parcelable out of the box): the IR generator in plugins/parcelize/parcelize-compiler/.

For new annotation features (e.g., extending @Parceler): both the FIR extension and the IR generator.

See plugins/index.md for the broader plugin landscape.