How to Use Fileserver With Gorillas Subrouter

I’ve just spent much more time than I ever wanted to get this right, so here’s how I did it for future reference.

I have a function that returns an http.Handler, kind of like this:

func Handler(prefix string) http.Handler {
	r := mux.NewRouter().PathPrefix(prefix).Subrouter()
	r.HandleFunc("/foo/", fooHandler).Methods("GET")
	r.HandleFunc("/bar/", barHandler).Methods("GET")
	return r
}

The prefix could be something like “/api/” or “/ui” or whatever, so this http.Handler will serve /api/foo and /api/bar, for example. So far so good.

Now, I also want to serve some static files under the prefix’s “root” (again, something like “/api/”.) My initial thinking was something like this:

r.Handle("/", http.StripPrefix(prefix, http.Dir("./somedir")))

It works fine for anything under the root of “./somedir” but it failed with a 404 error for anything under different subdirectories. I found some answers online, but they never seemed to use a subrouter and thus didn’t work for me at all.

I finally figured it out:

r.PathPrefix("/").Handler(http.StripPrefix(prefix, http.FileServer(http.Dir("./somedir"))))

It wasn’t obvious to me at all.