A Library for Dependency Injection in Go

Why this library?

• It is simpler than uber/fx, yet still powerful
• Static typing & without using reflection
• Easy to use and easy to replace objects for unit & integration testing (by using Override, one can replace any part of the default dependency graph)
• Easy to read & navigate with IDEs
• Safe to use in multiple goroutines
• Very easy-to-read stacktraces when something wrong happens
• Provide default wiring through the Register method, or no default wiring through RegisterEmpty

This library does NOT actually use the Service Locator pattern

Limitations

• Runtime wiring of objects
• Deep stack calls
• Override functions only work outside of the anonymous functions in the Register calls

Installtion

go get github.com/QuangTung97/svloc@v0.5.0

Examples

Assume RepoImpl implements interface Repo:

package main

type Repo interface {
	GetUser() string
}

type RepoImpl struct {
	name string
}

func NewRepo(name string) *RepoImpl {
	return &RepoImpl{
		name: name,
	}
}

func (r *RepoImpl) GetUser() string {
	return r.name
}

Assume Service with method Hello:

package main

import (
	"fmt"
)

type Service struct {
	rp Repo
}

func NewService(repo Repo) *Service {
	return &Service{
		rp: repo,
	}
}

func (s *Service) Hello() {
	fmt.Println("Hello User:", s.rp.GetUser())
}

We can create Locator[T] objects:

package main

import (
	"github.com/QuangTung97/svloc"
)

var usernameLoc = svloc.RegisterEmpty[string]()

var repoLoc = svloc.Register[Repo](func(unv *svloc.Universe) Repo {
	return NewRepo(
		usernameLoc.Get(unv),
	)
})

var serviceLoc = svloc.Register[*Service](func(unv *svloc.Universe) *Service {
	return NewService(repoLoc.Get(unv))
})

The 3 newly created objects: usernameLoc, repoLoc, serviceLoc are all immutable objects and safe to use concurrently.

The svloc.PreventRegistering will not allow Register functions to be called after that point. Usually at the start of the main() function.

To use in main(), first creates a new Universe. Then call MustOverride() on usernameLoc to provide the username string. And then call the serviceLoc.Get() with that Universe, All of the wiring will happen automatically:

package main

func main() {
	svloc.PreventRegistering()

	unv := svloc.NewUniverse()
	usernameLoc.MustOverride(unv, "user01")

	svc := serviceLoc.Get(unv)
	svc.Hello()
}

Full example:

package main

import (
	"fmt"
	"github.com/QuangTung97/svloc"
)

type Repo interface {
	GetUser() string
}

type RepoImpl struct {
	name string
}

func NewRepo(name string) *RepoImpl {
	return &RepoImpl{
		name: name,
	}
}

func (r *RepoImpl) GetUser() string {
	return r.name
}

type Service struct {
	rp Repo
}

func NewService(repo Repo) *Service {
	return &Service{
		rp: repo,
	}
}

func (s *Service) Hello() {
	fmt.Println("Hello User:", s.rp.GetUser())
}

var usernameLoc = svloc.RegisterEmpty[string]()

var repoLoc = svloc.Register[Repo](func(unv *svloc.Universe) Repo {
	return NewRepo(
		usernameLoc.Get(unv),
	)
})

var serviceLoc = svloc.Register[*Service](func(unv *svloc.Universe) *Service {
	return NewService(repoLoc.Get(unv))
})

func main() {
	svloc.PreventRegistering()

	unv := svloc.NewUniverse()
	usernameLoc.MustOverride(unv, "user01")

	svc := serviceLoc.Get(unv)
	svc.Hello()
}

Using OnShutdown and Shutdown:

package main

var repoLoc = svloc.Register[Repo](func(unv *svloc.Universe) Repo {
	unv.OnShutdown(func() {
		fmt.Println("Shutdown Repo")
	})

	return NewRepo(
		usernameLoc.Get(unv),
	)
})

var serviceLoc = svloc.Register[*Service](func(unv *svloc.Universe) *Service {
	unv.OnShutdown(func() {
		fmt.Println("Shutdown Service")
	})
	return NewService(repoLoc.Get(unv))
})

func main() {
	svloc.PreventRegistering()

	unv := svloc.NewUniverse()
	defer unv.Shutdown()

	usernameLoc.MustOverride(unv, "user01")

	svc := serviceLoc.Get(unv)
	svc.Hello()
}