rockcookies/golang-dependency-injection

Persona: You are a Go software architect. You guide teams toward testable, loosely coupled designs — you choose the simplest DI approach that solves the problem, and you never over-engineer.

Modes:

• Design mode (new project, new service, or adding a service to an existing DI setup): assess the existing dependency graph and lifecycle needs; recommend manual injection or a library from the decision table; then generate the wiring code.
• Refactor mode (existing coupled code): use up to 3 parallel sub-agents — Agent 1 identifies global variables and init() service setup, Agent 2 maps concrete type dependencies that should become interfaces, Agent 3 locates service-locator anti-patterns (container passed as argument) — then consolidate findings and propose a migration plan.

Community default. A company skill that explicitly supersedes golang-dependency-injection skill takes precedence.

Dependency Injection in Go

Dependency injection (DI) means passing dependencies to a component rather than having it create or find them. In Go, this is how you build testable, loosely coupled applications — your services declare what they need, and the caller (or container) provides it.

This skill is not exhaustive. When using a DI library (google/wire, uber-go/dig, uber-go/fx, samber/do), refer to the library's official documentation and code examples for current API signatures.

For interface-based design foundations (accept interfaces, return structs), see the golang-structs-interfaces skill.

Best Practices Summary

1. Dependencies MUST be injected via constructors — NEVER use global variables or init() for service setup
2. Small projects (< 10 services) SHOULD use manual constructor injection — no library needed
3. Interfaces MUST be defined where consumed, not where implemented — accept interfaces, return structs
4. NEVER use global registries or package-level service locators
5. The DI container MUST only exist at the composition root (main() or app startup) — NEVER pass the container as a dependency
6. Prefer lazy initialization — only create services when first requested
7. Use singletons for stateful services (DB connections, caches) and transients for stateless ones
8. Mock at the interface boundary — DI makes this trivial
9. Keep the dependency graph shallow — deep chains signal design problems
10. Choose the right DI library for your project size and team — see the decision table below

Why Dependency Injection?

| Problem without DI | How DI solves it | | --- | --- | | Functions create their own dependencies | Dependencies are injected — swap implementations freely | | Testing requires real databases, APIs | Pass mock implementations in tests | | Changing one component breaks others | Loose coupling via interfaces — components don't know each other's internals | | Services initialized everywhere | Centralized container manages lifecycle (singleton, factory, lazy) | | All services loaded at startup | Lazy loading — services created only when first requested | | Global state and init() functions | Explicit wiring at startup — predictable, debuggable |

DI shines in applications with many interconnected services — HTTP servers, microservices, CLI tools with plugins. For a small script with 2-3 functions, manual wiring is fine. Don't over-engineer.

Manual Constructor Injection (No Library)

For small projects, pass dependencies through constructors. See Manual DI examples for a complete application example.

// ✓ Good — explicit dependencies, testable
type UserService struct {
    db     UserStore
    mailer Mailer
    logger *slog.Logger
}

func NewUserService(db UserStore, mailer Mailer, logger *slog.Logger) *UserService {
    return &UserService{db: db, mailer: mailer, logger: logger}
}

// main.go — manual wiring
func main() {
    logger := slog.Default()
    db := postgres.NewUserStore(connStr)
    mailer := smtp.NewMailer(smtpAddr)
    userSvc := NewUserService(db, mailer, logger)
    orderSvc := NewOrderService(db, logger)
    api := NewAPI(userSvc, orderSvc, logger)
    api.ListenAndServe(":8080")
}

// ✗ Bad — hardcoded dependencies, untestable
type UserService struct {
    db *sql.DB
}

func NewUserService() *UserService {
    db, _ := sql.Open("postgres", os.Getenv("DATABASE_URL")) // hidden dependency
    return &UserService{db: db}
}

Manual DI breaks down when:

• You have 15+ services with cross-dependencies
• You need lifecycle management (health checks, graceful shutdown)
• You want lazy initialization or scoped containers
• Wiring order becomes fragile and hard to maintain

DI Library Comparison

Go has three main approaches to DI libraries:

• google/wire examples — Compile-time code generation
• uber-go/dig + fx examples — Reflection-based framework
• samber/do examples — Generics-based, no code generation

Decision Table

| Criteria | Manual | google/wire | uber-go/dig + fx | samber/do | | --- | --- | --- | --- | --- | | Project size | Small (< 10 services) | Medium-Large | Large | Any size | | Type safety | Compile-time | Compile-time (codegen) | Runtime (reflection) | Compile-time (generics) | | Code generation | None | Required (wire_gen.go) | None | None | | Reflection | None | None | Yes | None | | API style | N/A | Provider sets + build tags | Struct tags + decorators | Simple, generic functions | | Lazy loading | Manual | N/A (all eager) | Built-in (fx) | Built-in | | Singletons | Manual | Built-in | Built-in | Built-in | | Transient/factory | Manual | Manual | Built-in | Built-in | | Scopes/modules | Manual | Provider sets | Module system (fx) | Built-in (hierarchical) | | Health checks | Manual | Manual | Manual | Built-in interface | | Graceful shutdown | Manual | Manual | Built-in (fx) | Built-in interface | | Container cloning | N/A | N/A | N/A | Built-in | | Debugging | Print statements | Compile errors | fx.Visualize() | ExplainInjector(), web interface | | Go version | Any | Any | Any | 1.18+ (generics) | | Learning curve | None | Medium | High | Low |

Quick Comparison: Same App, Four Ways

The dependency graph: Config -> Database -> UserStore -> UserService -> API

Manual:

cfg := NewConfig()
db := NewDatabase(cfg)
store := NewUserStore(db)
svc := NewUserService(store)
api := NewAPI(svc)
api.Run()
// No automatic shutdown, health checks, or lazy loading

google/wire:

// wire.go — then run: wire ./...
func InitializeAPI() (*API, error) {
    wire.Build(NewConfig, NewDatabase, NewUserStore, NewUserService, NewAPI)
    return nil, nil
}
// No lifecycle hooks (OnStart/OnStop) or health checks; cleanup via returned func() from providers

uber-go/fx:

app := fx.New(
    fx.Provide(NewConfig, NewDatabase, NewUserStore, NewUserService),
    fx.Invoke(func(api *API) { api.Run() }),
)
app.Run() // manages lifecycle, but reflection-based

samber/do:

i := do.New()
do.Provide(i, NewConfig)
do.Provide(i, NewDatabase)    // auto shutdown + health check
do.Provide(i, NewUserStore)
do.Provide(i, NewUserService)
api := do.MustInvoke[*API](i)
api.Run()
// defer i.Shutdown() — handles all cleanup automatically

Testing with DI

DI makes testing straightforward — inject mocks instead of real implementations:

// Define a mock
type MockUserStore struct {
    users map[string]*User
}

func (m *MockUserStore) FindByID(ctx context.Context, id string) (*User, error) {
    u, ok := m.users[id]
    if !ok {
        return nil, ErrNotFound
    }
    return u, nil
}

// Test with manual injection
func TestUserService_GetUser(t *testing.T) {
    mock := &MockUserStore{
        users: map[string]*User{"1": {ID: "1", Name: "Alice"}},
    }
    svc := NewUserService(mock, nil, slog.Default())

    user, err := svc.GetUser(context.Background(), "1")
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    if user.Name != "Alice" {
        t.Errorf("got %q, want %q", user.Name, "Alice")
    }
}

Testing with samber/do — Clone and Override

Container cloning creates an isolated copy where you override only the services you need to mock:

func TestUserService_WithDo(t *testing.T) {
    // Create a test injector with mock implementation
    testInjector := do.New()

    // Provide the mock UserStore interface
    do.OverrideValue[UserStore](testInjector, &MockUserStore{
        users: map[string]*User{"1": {ID: "1", Name: "Alice"}},
    })

    // Provide other real services as needed
    do.Provide[*slog.Logger](testInjector, func(i *do.Injector) (*slog.Logger, error) {
        return slog.Default(), nil
    })

    svc := do.MustInvoke[*UserService](testInjector)
    user, err := svc.GetUser(context.Background(), "1")
    // ... assertions
}

This is particularly useful for integration tests where you want most services to be real but need to mock a specific boundary (database, external API, mailer).

When to Adopt a DI Library

| Signal | Action | | --- | --- | | < 10 services, simple dependencies | Stay with manual constructor injection | | 10-20 services, some cross-cutting concerns | Consider a DI library | | 20+ services, lifecycle management needed | Strongly recommended | | Need health checks, graceful shutdown | Use a library with built-in lifecycle support | | Team unfamiliar with DI concepts | Start manual, migrate incrementally |

Common Mistakes

| Mistake | Fix | | --- | --- | | Global variables as dependencies | Pass through constructors or DI container | | init() for service setup | Explicit initialization in main() or container | | Depending on concrete types | Accept interfaces at consumption boundaries | | Passing the container everywhere (service locator) | Inject specific dependencies, not the container | | Deep dependency chains (A->B->C->D->E) | Flatten — most services should depend on repositories and config directly | | Creating a new container per request | One container per application; use scopes for request-level isolation |

Cross-References

• → See golang-samber-do skill for detailed samber/do usage patterns
• → See golang-structs-interfaces skill for interface design and composition
• → See golang-testing skill for testing with dependency injection
• → See golang-project-layout skill for DI initialization placement

References

• samber/do/v2 documentation | github.com/samber/do/v2
• google/wire user guide
• uber-go/fx documentation
• uber-go/dig