injection.go

package main

import (
	"fmt"
	"io"
	"net/http"
)

type externalService interface {
	getData(r io.Reader) string
}

type externalSource struct{}

func (s externalSource) getData(r io.Reader) string {
	return "test"
}

type dependencies struct {
	externalService externalService
}

func main() {
	d := dependencies{
		externalService: externalSource{},
	}

	http.HandleFunc("/", d.HandleRequest)
	http.ListenAndServe(":8080", nil)
}

func (d *dependencies) HandleRequest(rw http.ResponseWriter, r *http.Request) {
	data := d.externalService.getData(r.Body)
	fmt.Println(data)
}