Setting up a larger application using dependency injection always requires us developers to set up our application’s object graph.

Programming in Go there is a library that eases this task for us, offering a reflect based injector to set up the graph in a few steps, named facebookgo-inject.

In the following article I’d like to demonstrate dependency injection using this library for a small sample application.

facebookgo inject tutorial 1024x740
Figure 1. Go dependency injection with facebookgo

Dependency Injection Example

First of all we need to fetch the dependency for facebookgo-inject (thanks @bborbe for showing me this library! :))

$ go get github.com/facebookgo/inject

This is our sample application – as we can see, our BookService gets the dependencies for Database and Validator injected.

package main

import (
	"log"

	"github.com/facebookgo/inject"
)

type Book struct {
	title string
}

type BookService struct {
	database  Database  `inject:""`
	validator Validator `inject:""`
}

func (s *BookService) create(book Book) {
	log.Printf("creating new book entity: %v\n", book)
	if s.validator.validate(book) {
		s.database.save(book)
	}
}

type Database struct {
}

func (db *Database) save(entity interface{}) {
	log.Printf("saving entity to database: %v", entity)
}

type Validator struct {
}

func (v *Validator) validate(entity interface{}) bool {
	log.Printf("validating entity: %v", entity)
	return true
}

func main() {
	log.Println("starting injection example")
	var graph inject.Graph
	var service BookService
	graph.Provide(
		&inject.Object{Value: &service},
		&inject.Object{Value: Database{}},
		&inject.Object{Value: Validator{}}
	)
	graph.Populate()

	log.Println("listing known objects..")
	for _, obj := range graph.Objects() {
		log.Printf("object known: %v\n", obj)
	}

	service.create(Book{title: "Some book"})
}

Running the Example

We may now run our sample application in the command line like this:

$ go run example.go
2016/11/09 16:18:21 starting injection example
2016/11/09 16:18:21 listing known objects..
2016/11/09 16:18:21 object known: *main.BookService
2016/11/09 16:18:21 creating new book entity: {Some book}
2016/11/09 16:18:21 validating entity: {Some book}
2016/11/09 16:18:21 saving entity to database: {Some book}

Tutorial Sources

Please feel free to download the tutorial sources from my GitHub repository, fork it there or clone it using Git:

git clone https://github.com/hascode/facebookgo-inject-sample.git

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