Kotlin Multiplatform
Kotlin Multiplatform FAQ
Answers from an engineer who builds and ships KMP for banks, insurers, and startups.
What is Kotlin Multiplatform (KMP)?
Kotlin Multiplatform is a JetBrains technology for sharing Kotlin code across Android, iOS, desktop, web, and the server. Business logic, API clients, data models, and validation live in one common module, and every platform runs that same code.
Isn't Kotlin just for Android?
No, that's a leftover impression from Kotlin's early days. Kotlin is a first-class JVM language and a modern alternative to Java. It runs backends on Spring Boot, Quarkus, Micronaut, and Ktor, and it's fully interoperable with Java, so you can adopt it file by file in an existing Gradle or Maven project.
How is KMP different from Flutter or React Native?
Flutter and React Native share the UI layer. Flutter draws its own UI, and React Native runs your app through a JavaScript runtime. KMP shares what sits underneath: networking, persistence, business rules. Each platform keeps its fully native UI, SwiftUI on iOS and Jetpack Compose on Android. And if you want shared UI as well, Compose Multiplatform adds that on top.
Is KMP production-ready? Who uses it?
Yes. KMP is stable and backed by JetBrains and Google. Netflix, McDonald's, Forbes, and Cash App ship it in production. Swiss banks and insurers do too. I've built KMP modules that run in the frankly 3a and Helsana+ apps.
Can I use KMP with my existing Java backend?
Yes. A shared KMP module drops into an existing Java project without touching the rest. Share data classes and validation between your backend and your clients, and a renamed field breaks the build instead of production.
How does Kotlin code run on iOS?
The Kotlin/Native compiler builds a regular iOS framework (an .xcframework). Swift imports it like any other dependency. There's no bridge and no embedded VM, just compiled native code. Tools like SKIE make the generated Swift API feel natural to iOS developers.
Can I share the user interface as well?
Yes, with Compose Multiplatform. You write the interface once in Kotlin and run it on Android, iOS, desktop (Windows, macOS, Linux), and web via WebAssembly, which is still in Beta but works well in my experience. It suits teams that want maximum code sharing. For others, native UI per platform on top of shared logic is the better fit, and KMP supports both.
Is KMP just for mobile, or a fullstack option?
A single Kotlin codebase can cover the JVM backend (Ktor, Spring Boot, Quarkus), the Android app, the iOS app, the web frontend, and a desktop client. Shared modules keep models and rules identical from the database to the screen.
What does adopting KMP look like in an existing project?
You don't rewrite anything. Start with one shared module, often the networking layer or a set of data models, consume it from the existing Android and iOS apps, and grow from there. From the first shared module on, that code is written and maintained once instead of twice.
What are the cost benefits of Kotlin Multiplatform?
Shared logic is written, tested, and fixed once instead of twice, and the Android and iOS apps can't disagree on business rules because both run the same code. Native UI and platform-specific code are still built per platform.