setup 404 NotFound handler on a gorilla mux router

Question

Here is my code about a small demonstration webserver written with the Go language and the gorilla mux package :

package main

import (
    "fmt"
    "github.com/gorilla/mux"
    "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    fmt.Fprintf(w, "Hi there, I love %s!", vars["username"])
}

func homeHandler(w http.ResponseWriter, r *http.Request) {
    if r.URL.Path != "/" {
        errorHandler(w, r, http.StatusNotFound)
        return
    }
    vars := mux.Vars(r)
    fmt.Fprintf(w, "Hi there, I love %s!", vars["username"])
}

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/help/{username}/", handler)
    http.Handle("/", r)
    http.ListenAndServe(":8080", nil)
}

But I don't find a way on how to implement a custom 404 page.

But I can't make a r.HandleFunc("/",...) but it will be too greedy.

Gene

For serving a custom error page yourself, see httprouter configuring NotFound. – Faultless

Answer 1

The Router exports a NotFoundHandler field which you can set to your custom handler.

r := mux.NewRouter()
r.NotFoundHandler = MyCustom404Handler

Demonism

Answer 2

Sometimes, you spend a lot of time building a stack of middleware that does many things like logging, sending metrics and so... And the default 404 handler just skips all the middlewares.

I was able to solve this issue by re-setting the default 404 handler like this:

router := mux.NewRouter()
router.Use(someMiddleware())
// Re-define the default NotFound handler
router.NotFoundHandler = router.NewRoute().HandlerFunc(http.NotFound).GetHandler()

Now, the 404 default handler is also going thru all the middlewares.

Gumbo

Cool solution. But you have to do this assigning either after declaring your real routes or by using BuildOnly() method to prevent this route from acting like a wildcard. – Sawyer

Works, but You should define this NotFoundHandler registered once all the handlers are registered, otherwise, it will override all the handlers – Palatable

Answer 3

Set the NotFoundHandler to a handler method that returns your custom 404 page.

Ekaterinodar

Answer 4

r := mux.NewRouter()

h := http.HandlerFunc(NotFound)
r.NotFoundHandler = h
    
func NotFound(w http.ResponseWriter, r *http.Request) {
        
}

Rotunda

Please add some explanation about what this does – Giuliana