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.")
    }

    //...
}