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Dagger graph, bindings, qualifiers, and scopes

Dagger generates the object-creation code a manual container would contain and validates the graph at compile time.

Prefer constructor injection​

class OrdersRepositoryImpl @Inject constructor(
    private val local: OrdersDao,
    private val remote: OrdersApi,
) : OrdersRepository

@Module
interface OrdersModule {
    @Binds fun bindOrdersRepository(impl: OrdersRepositoryImpl): OrdersRepository
}

Use @Binds for an implementation already constructible by Dagger. Use @Provides for third-party builders, conditional construction, or factory methods you do not own.

@Provides
fun provideOrdersApi(client: OkHttpClient): OrdersApi =
    Retrofit.Builder().client(client).baseUrl(BASE_URL).build().create()

Qualifiers distinguish meaning​

@Qualifier annotation class Authenticated
@Qualifier annotation class Public

class ApiClients @Inject constructor(
    @Authenticated val privateClient: OkHttpClient,
    @Public val publicClient: OkHttpClient,
)

Qualify both binding and injection site. Prefer semantic names; @Named("client2") is typo-prone and does not explain policy.

Scope means one per component instance​

@Singleton does not mean one JVM object independent of graph. It means one instance per singleton-scoped component instance. Tests can build another component and get another object.

Scope stateful, synchronized, or measured-expensive objects whose lifetime matches the component. Unscoped objects are created as needed and are often correct.

❌ Singleton repository retains Activity callback.

✅ Application-scoped repository depends only on application-safe dependencies and exposes observable state.

Components and subcomponents​

Parent bindings are visible to a child; child bindings are not visible upward. A login subcomponent can use app network/database bindings while owning login-flow objects. Its scope must be different from ancestor scope.

Component dependencies expose selected parent APIs rather than inheriting the graph; useful across Gradle/dynamic feature boundaries but more explicit wiring is required.

Graph errors teach contracts​

  • missing binding: graph cannot construct a required type
  • duplicate binding: two unqualified providers claim same key
  • scope mismatch: shorter/longer lifetime is inconsistent
  • dependency cycle: creation requires itself

Do not silence errors with broad modules or global providers; fix ownership/identity.

Sources​

Interview exercises​

  1. Choose @Inject, @Binds, or @Provides for five dependencies.
  2. Explain why two component instances yield two “singletons.”
  3. Fix duplicate authenticated/public client bindings.