How Can I Customize the 404 Error Handling for a Go Static File Server?

Customizing File Not Found Handling in Go Static File Server

When serving static files in Go using http.FileServer(), unfound files typically return a 404 status code. To customize this behavior and serve a specific page instead, we need to wrap the http.FileServer() handler.

Creating a Custom HTTP Handler Wrapper

We create a custom http.ResponseWriter wrapper (NotFoundRedirectRespWr) to intercept the status code returned by the file server handler. When the status is 404, we prevent the response from being sent and redirect the request to a specified page (in this case, /index.html).

<code class="go">type NotFoundRedirectRespWr struct {
    http.ResponseWriter // We embed http.ResponseWriter
    status              int
}

func (w *NotFoundRedirectRespWr) WriteHeader(status int) {
    w.status = status // Store the status for our own use
    if status != http.StatusNotFound {
        w.ResponseWriter.WriteHeader(status)
    }
}

func (w *NotFoundRedirectRespWr) Write(p []byte) (int, error) {
    if w.status != http.StatusNotFound {
        return w.ResponseWriter.Write(p)
    }
    return len(p), nil // Lie that we successfully written it
}</code>

Wrapping the File Server Handler

Next, we wrap the http.FileServer() handler using our custom wrapHandler function. This function adds our custom response writer to the handler chain. If the response status is 404, we redirect to /index.html.

<code class="go">func wrapHandler(h http.Handler) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        nfrw := &amp;NotFoundRedirectRespWr{ResponseWriter: w}
        h.ServeHTTP(nfrw, r)
        if nfrw.status == 404 {
            log.Printf("Redirecting %s to index.html.", r.RequestURI)
            http.Redirect(w, r, "/index.html", http.StatusFound)
        }
    }
}</code>

Usage

To use our custom handler, we replace the original http.FileServer() handler with our wrapped handler in our main function:

<code class="go">func main() {
    fs := wrapHandler(http.FileServer(http.Dir(".")))
    http.HandleFunc("/", fs)
    panic(http.ListenAndServe(":8080", nil))
}</code>

Now, any unfound files will trigger our custom handler and redirect to /index.html. This provides a more user-friendly experience for single-page web applications.

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