golang-dependency-injection

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 samber/cc-skills-golang@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 samber/cc-skills-golang@golang-structs-interfaces skill.

Best Practices Summary

• Dependencies MUST be injected via constructors — NEVER use global variables or init() for service setup

• Small projects (< 10 services) SHOULD use manual constructor injection — no library needed

• Interfaces MUST be defined where consumed, not where implemented — accept interfaces, return structs

• NEVER use global registries or package-level service locators

• The DI container MUST only exist at the composition root (main() or app startup) — NEVER pass the container as a dependency

• Prefer lazy initialization — only create services when first requested

• Use singletons for stateful services (DB connections, caches) and transients for stateless ones

• Mock at the interface boundary — DI makes this trivial

• Keep the dependency graph shallow — deep chains signal design problems

• 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 shutdown or health check support

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.Override[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 samber/cc-skills-golang@golang-samber-do skill for detailed samber/do usage patterns

• → See samber/cc-skills-golang@golang-structs-interfaces skill for interface design and composition

• → See samber/cc-skills-golang@golang-testing skill for testing with dependency injection

• → See samber/cc-skills-golang@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

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