# Kotlin on the Server and Android — Kotlin

Source: https://www.skillbyai.com/en/kotlin/e-platforms

> Ktor, Spring Boot and Jetpack Compose at a glance.

## Where Kotlin runs in production

On the server, **Ktor** is a JetBrains framework built around coroutines, with routing, plugins and serialization via kotlinx.serialization. **Spring Boot** has first-class Kotlin support, including coroutine-based controllers and a Kotlin bean DSL; many teams use it with existing Java ecosystems. On **Android**, Kotlin is the recommended language, and **Jetpack Compose** builds UIs with `@Composable` functions that describe UI from state; when state changes, Compose recomposes the affected parts. Android Jetpack libraries provide Kotlin-friendly APIs such as `viewModelScope` and Flow integration. APIs evolve quickly, so check the docs for your version.

## From code to shipped software

Kotlin powers servers, Android apps and shared multiplatform code, backed by testing and style tools.

![Four ideas: server and Android, Multiplatform, testing and style, a final checklist.](assets/figures/kotlin/section-8-map.svg) — Figure 8.1 — Shared Kotlin code feeding server, Android, iOS and web.

## A Ktor route and a Compose screen

Two sketches; dependencies and imports trimmed.

```kotlin
// Server: Ktor routing (inside an Application module)
fun Application.module() {
    routing {
        get("/health") {
            call.respondText("ok")
        }
        get("/users/{id}") {
            val id = call.parameters["id"]?.toLongOrNull()
                ?: return@get call.respond(HttpStatusCode.BadRequest)
            call.respond(userService.find(id))   // suspend call, serialized as JSON
        }
    }
}

// Android: Jetpack Compose
@Composable
fun Counter() {
    var count by remember { mutableStateOf(0) }
    Button(onClick = { count++ }) {
        Text("Clicked $count times")
    }
}
```

## One language, several workshops

Learning Kotlin is like learning one set of tools that fits several workshops: the server room, the mobile studio and the shared parts bench all use the same wrenches.

**Quiz:** What is Jetpack Compose?

- [ ] A build tool replacing Gradle
- [ ] A server framework from JetBrains
- [ ] A Kotlin linter
- [x] Android's declarative UI toolkit built on @Composable functions

*Answer:* Android's declarative UI toolkit built on @Composable functions. Compose describes UI as a function of state.