golang-dependency-injection

Use manual constructor injection: pass dependencies as parameters to constructors like NewUserService(db, mailer, logger) and wire everything explicitly in main(). Initialize infrastructure first, then repositories, services, and transport. This is the recommended approach for projects with fewer than 10 services.

Choose based on project needs: google/wire gives compile-time safety via code generation but no lifecycle management; uber-go/fx adds lifecycle hooks but uses reflection; samber/do offers compile-time generics safety with built-in health checks and shutdown. Small projects should skip libraries entirely.

No, passing the container or injector into services is the service locator anti-pattern. The container must only exist at the composition root in main(). Inject the specific dependencies each service needs through its constructor instead.

Define interfaces in the consuming package, not the implementation package. Follow the principle of accepting interfaces and returning structs: the implementation returns a concrete struct pointer, and the consumer declares the small interface it needs locally.

Manual DI breaks down around 15-20+ services when wiring order becomes fragile, lifecycle management must be hand-coded, and there is no lazy initialization. At that scale, adopt a library with built-in lifecycle support such as samber/do or uber-go/fx.

Create a mock implementing the dependency interface and inject it through the constructor in your test, avoiding real databases. With samber/do, you can clone the container and override only the services you need to mock for integration tests.