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Dependency Injection in Ktor

The koin-ktor module is dedicated to bring dependency injection for Ktor.

Install Koin Plugin

To start a Koin container in Ktor, just install the Koin plugin like follow:

fun Application.main() {
// Install Koin
install(Koin) {
slf4jLogger()
modules(helloAppModule)
}

}

Inject in Ktor

Koin inject() and get() functions are available from Application,Route,Routing classes:

fun Application.main() {

// inject HelloService
val service by inject<HelloService>()

routing {
get("/hello") {
call.respondText(service.sayHello())
}
}
}

Resolve from Ktor Request Scope (since 3.5.0)

You can declare components to live within Ktor request scope timeline. For this, you just need to declare your component inside a scope<ScopeRequest> section. Given a ScopeComponent class to instantiate on RequestScope, let's declare it:

scope<RequestScope>{
scopedOf(::ScopeComponent)
}

And from your http call, just call call.scope.get() to resolve the right dependency:

routing {
get("/hello") {
val component = call.scope.get<ScopeComponent>()
// ...
}
}
note
For each new request, the scope will be recreated. This creates and drop scope instances, for each request

Run Koin from an external Ktor Module

For a Ktor module, you can load specific Koin modules. Just declare them with koin { } function:

fun Application.module2() {

koin {
// load koin modules
modules(appModule2)
}

}

Ktor Events

You can listen to KTor Koin events:

fun Application.main() {
// ...

// Install Ktor features
environment.monitor.subscribe(KoinApplicationStarted) {
log.info("Koin started.")
}

environment.monitor.subscribe(KoinApplicationStopPreparing) {
log.info("Koin stopping...")
}

environment.monitor.subscribe(KoinApplicationStopped) {
log.info("Koin stopped.")
}

//...
}