Definitions

Definitions declare how Koin creates and manages your dependencies. This guide covers all definition types using both DSL and Annotations.

Definition Types

Type	DSL	Annotation	Lifecycle	Use Case
Singleton	single()	@Singleton	One instance for app lifetime	Services, repositories, databases
Factory	factory()	@Factory	New instance each time	Presenters, use cases, stateful objects
Scoped	scoped()	@Scoped	One instance per scope	Activity-bound, session-bound objects
ViewModel	viewModel()	@KoinViewModel	Android ViewModel lifecycle	ViewModels

Declaring Definitions

Compiler Plugin DSL (Recommended)

import org.koin.plugin.module.dsl.*

val appModule = module {
    // Singleton
    single<Database>()
    single<UserRepository>()

    // Factory - new instance each time
    factory<UserPresenter>()

    // ViewModel
    viewModel<UserViewModel>()
}

Annotations

@Singleton  // or @Single
class Database

@Singleton
class UserRepository(private val database: Database)

@Factory
class UserPresenter(private val repository: UserRepository)

@KoinViewModel
class UserViewModel(private val repository: UserRepository) : ViewModel()

Classic DSL

val appModule = module {
    // With constructor reference (autowiring)
    singleOf(::Database)
    singleOf(::UserRepository)
    factoryOf(::UserPresenter)
    viewModelOf(::UserViewModel)

    // With lambda (manual wiring)
    single { Database() }
    single { UserRepository(get()) }
    factory { UserPresenter(get()) }
    viewModel { UserViewModel(get()) }
}

Definition Comparison

Concept	Compiler Plugin DSL	Classic DSL	Annotation
Singleton	single<MyClass>()	singleOf(::MyClass)	@Singleton / @Single
Factory	factory<MyClass>()	factoryOf(::MyClass)	@Factory
Scoped	scoped<MyClass>()	scopedOf(::MyClass)	@Scoped
ViewModel	viewModel<MyVM>()	viewModelOf(::MyVM)	@KoinViewModel
Worker	worker<MyWorker>()	workerOf(::MyWorker)	@KoinWorker

info

The compiler plugin is analysing your class and function parameters, to generate the right call to Koin with get() function you don't have to write anymore.

Single (Singleton)

Creates one instance that's reused throughout the app:

// DSL
single<DatabaseHelper>()

// Annotation
@Singleton
class DatabaseHelper

Both create the same result - a single instance shared across all consumers.

Factory

Creates a new instance each time:

// DSL
factory<UserPresenter>()

// Annotation
@Factory
class UserPresenter(private val repository: UserRepository)

Scoped

Creates one instance per scope:

// DSL
scope<MyActivity> {
    scoped<ActivityPresenter>()
}

// Annotation
@Scoped(MyActivityScope::class)
class ActivityPresenter

ViewModel

Android ViewModel with proper lifecycle:

// DSL
viewModel<UserViewModel>()

// Annotation
@KoinViewModel
class UserViewModel(private val repository: UserRepository) : ViewModel()

Interface Binding

Compiler Plugin DSL

single<UserRepositoryImpl>() bind UserRepository::class

// Multiple bindings
single<MyServiceImpl>() binds arrayOf(ServiceA::class, ServiceB::class)

Classic DSL

singleOf(::UserRepositoryImpl) bind UserRepository::class

// Or with lambda
single<UserRepository> { UserRepositoryImpl(get()) }

Annotations

Interface binding is automatic when your class implements an interface:

@Singleton
class UserRepositoryImpl(
    private val database: Database
) : UserRepository  // Automatically binds to UserRepository

For explicit binding:

@Singleton
@Binds(UserRepository::class)
class UserRepositoryImpl : UserRepository

Qualifiers (Named Definitions)

When you have multiple definitions of the same type. See also Injection with Qualifiers for retrieval.

Compiler Plugin DSL

With Compiler Plugin DSL, you need to annotate with @Named to use string qualifier (like you were previously using named())

@Named("local")
class LocalDatabase : Database

@Named("remote")
class RemoteDatabase : Database

class UserRepository(
    @Named("local") private val localDb: Database,
    @Named("remote") private val remoteDb: Database
)

single<LocalDatabase>()
single<RemoteDatabase>()
single<UserRepository>()

// Usage
val localDb: Database = get(named("local"))

Classic DSL

single<Database>(named("local")) { LocalDatabase() }
single<Database>(named("remote")) { RemoteDatabase() }

// Usage
val localDb: Database = get(named("local"))

Annotations

@Singleton
@Named("local")
class LocalDatabase : Database

@Singleton
@Named("remote")
class RemoteDatabase : Database

// In a consumer
@Singleton
class UserRepository(
    @Named("local") private val localDb: Database,
    @Named("remote") private val remoteDb: Database
)

Injected Parameters

Pass parameters at injection time:

Compiler Plugin DSL

Use the @InjectedParam to indicate that a parameter will be served by injected parameters.

class UserPresenter(
    @InjectedParam userId : String,
    repository : UserRepository
)

factory<UserPresenter>()

Classic DSL

class UserPresenter(
    userId : String,
    repository : UserRepository
)

factory { params ->
    UserPresenter(
        userId = params.get(),
        repository = get()
    )
}

Annotations

@Factory
class UserPresenter(
    @InjectedParam val userId: String,
    val repository: UserRepository  // Auto-injected
)

// Usage
val presenter: UserPresenter = get { parametersOf("user123") }

Optional Dependencies

Compiler Plugin DSL

class MyService(
    val required: RequiredDep,
    val optional: OptionalDep?  // Resolved with getOrNull()
)

single<MyService>()

Classic DSL

single {
    MyService(
        required = get(),
        optional = getOrNull()
    )
}

Annotations

Nullable parameters are handled automatically:

@Singleton
class MyService(
    val required: RequiredDep,
    val optional: OptionalDep?  // Resolved with getOrNull()
)

Lazy Injection

Defer instance creation:

Compiler Plugin DSL

class MyService(
    val lazyDep: Lazy<HeavyDependency>  // Deferred creation
)

single<MyService>()

Classic DSL

single {
    MyService(
        lazyDep = inject()  // Lazy<Dependency>
    )
}

Annotations

@Singleton
class MyService(
    val lazyDep: Lazy<HeavyDependency>  // Deferred creation
)

Properties

Inject configuration values:

Compiler Plugin DSL

class ApiClient(
    @Property("api_url") val url: String,
    @Property("api_key") val key: String
)

single<ApiClient>()

Classic DSL

single {
    ApiClient(
        url = getProperty("api_url"),
        key = getProperty("api_key", "default")
    )
}

Annotations

@Singleton
class ApiClient(
    @Property("api_url") val url: String,
    @Property("api_key") val key: String
)

Callbacks

onClose Callback

Execute code when instance is released:

single {
    Database()
} onClose {
    it?.close()  // Called when Koin stops or scope closes
}

createdAtStart

Create instance eagerly at startup:

// Compiler Plugin DSL
single<ConfigManager>() withOptions {
    createdAtStart()
}

// Classic DSL
single(createdAtStart = true) {
    ConfigManager()
}

Definition Override

Default: Last Wins

val prodModule = module {
    single<ApiService> { ProductionApi() }
}

val testModule = module {
    single<ApiService> { MockApi() }  // Overrides production
}

startKoin {
    modules(prodModule, testModule)
}

Explicit Override

In strict mode, mark overrides explicitly:

val testModule = module {
    single<ApiService> { MockApi() }.override()
}

startKoin {
    allowOverride(false)
    modules(prodModule, testModule)
}

Best Practices

1. Prefer Constructor Injection - Makes code testable without Koin
2. Use single for stateless services - Repositories, clients, helpers
3. Use factory for stateful objects - Presenters, use cases with state
4. Use scoped for lifecycle-bound objects - Activity, Fragment, Session
5. Minimize qualifiers - Use different interfaces instead when possible
6. Bind to interfaces - Depend on abstractions, not implementations
7. Use create(::builder) for external libraries - Safer dependency resolution

Next Steps

• Injection - Retrieve dependencies
• Qualifiers - Named and typed qualifiers
• Advanced Patterns - Collections, decorators, external libraries
• Scopes - Manage lifecycle