/golang-dependency-injection

name: golang-dependency-injection
description: "Comprehensive guide for dependency injection (DI) in Golang. Covers why DI matters (testability, loose coupling, separation of concerns, lifecycle management), manual constructor injection, and DI library comparison (google/wire, uber-go/dig, uber-go/fx, samber/do). Use this skill when designing service architecture, setting up dependency injection, refactoring tightly coupled code, managing singletons or service factories, or when the user asks about inversion of control, service containers, or wiring dependencies in Go. For a specific DI library, → See `samber/cc-skills-golang@golang-google-wire`, `samber/cc-skills-golang@golang-uber-dig`, `samber/cc-skills-golang@golang-uber-fx`, or `samber/cc-skills-golang@golang-samber-do` skills."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code or similar AI coding agents, and for projects using Golang.
metadata:
  author: samber
  version: "1.2.2"
  openclaw:
    emoji: "🔌"
    homepage: https://github.com/samber/cc-skills-golang
    requires:
      bins:
        - go
    install: []
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent WebFetch mcp__context7__resolve-library-id mcp__context7__query-docs AskUserQuestion

**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.

**Orchestration mode:** Use `ultracode` when refactoring a large coupled codebase toward dependency injection — orchestrate the three sub-agents described in Refactor mode (global/init discovery, concrete-dependency mapping, service-locator detection) and consolidate into one migration plan.

**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

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](./references/manual-di.md) 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.Get