How does http.Handler demonstrate real-world interface design in Go?

http.Handler is one of Go's most widely used interfaces. It models exactly one behaviour — handling an HTTP request — with a single method. The entire net/http server is built around this interface, making it extensible, testable, and composable through middleware.

// The entire http.Handler interface:
type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

// Implement it on a struct
type HelloHandler struct{ Greeting string }
func (h HelloHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "%s, %s!", h.Greeting, r.URL.Query().Get("name"))
}
http.Handle("/hello", HelloHandler{Greeting: "Hello"})

// http.HandlerFunc — adapter: converts a function to a Handler
func greetFunc(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintln(w, "Hi!")
}
http.Handle("/greet", http.HandlerFunc(greetFunc)) // function cast to HandlerFunc

// Middleware pattern — wraps a Handler with cross-cutting concerns
func loggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        next.ServeHTTP(w, r)   // call the inner handler
        log.Printf("%s %s %v", r.Method, r.URL.Path, time.Since(start))
    })
}
func authMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if r.Header.Get("X-Token") == "" {
            http.Error(w, "unauthorized", 401)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Chain middleware
handler := loggingMiddleware(authMiddleware(HelloHandler{Greeting: "Hello"}))
http.Handle("/secure", handler)

http.HandlerFunc is a named function type that satisfies http.Handler. It is a classic Go pattern: define a named type on a function signature, then implement the interface on that type. This converts any compatible function into an interface implementor — zero overhead, no wrapper struct needed.

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