Ktor dependency injection with Koin

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 Ktor features
    install(Koin) {
        slf4jLogger()
        modules(helloAppModule)
    }

}

note

Koin Ktor plugin uses isolated Koin context. You won't be able to start Koin outside of Ktor

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.")
    }

    //...
}