Dependency injection keeps component construction outside the component itself. In a Go HTTP service, handlers receive the smallest domain interface they need instead of creating or depending directly on storage. section 11.3.2
Narrowing the injected dependency provides practical design benefits:
- Makes allowed operations explicit through the type system. section 11.3.2
- Simplifies tests by requiring mocks for only the operations used. section 11.3.2
- Separates HTTP translation from domain storage and construction. section 11.3.2
Injecting an ArticleReader
type Handlers struct { articles domain.ArticleReader }\n\nfunc New(articles domain.ArticleReader) *Handlers {\n return &Handlers{articles: articles}\n}\n\nfunc main() {\n store, _ := storage.NewBoltStore("./articles.db", true)\n h := handlers.New(store)\n h.RegisterRoutes(mux)\n}\n\nThe concrete BoltStore is created at the application boundary, while handlers depend only on the read-only interface. section 11.3.2
Listing 11.4 HTTP handlers with an injected ArticleReader
package handlers
import (
"encoding/json"
"net/http"
"strconv"
"github.com/YOUR_USERNAME/go-news/domain"
)
type Handlers struct { #1
articles domain.ArticleReader
}
func New(articles domain.ArticleReader) *Handlers { #2
return &Handlers{
articles: articles,
}
}
func (h *Handlers) RegisterRoutes(mux *http.ServeMux) { #3
mux.HandleFunc("/articles", h.articlesHandler)
}
func (h *Handlers) articlesHandler(w http.ResponseWriter, r *http.Request) { #4
if r.Method != http.MethodGet {
http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
return
}
n := 10
if countStr := r.URL.Query().Get("count"); countStr != "" { #5
if count, err := strconv.Atoi(countStr); err == nil && count > 0 {
n = count
}
}
articles := h.articles.GetRecent(n)
w.Header().Set("Content-Type", "application/json") #6
if err := json.NewEncoder(w).Encode(articles); err != nil {
http.Error(w, "failed to encode response", http.StatusInternalServerError)
return
}
}
Inspect the complete handler implementation, including its injected ArticleReader and route registration. section 11.3.2
