HTTP Request multiplexer

DefaultServerMux: Is a default implementation of the ServeMux.

Quoting the docs:

ServeMux is an HTTP request multiplexer. It matches the URL of each incoming request against a list of registered patterns and calls the handler for the pattern that most closely matches the URL.

Let provide an example of what the above means in the context of our hello world:

http.HandleFunc("/", handler)

That line instructs the DefaultServerMux to "map" any HTTP request with to call the handler function.

A careful reader will notice that the handler function is just a function and it is not a struct implementing the http.Handler interface.

http.HandleFunc is a convenience function that allows you to pass a function instead of a struct.

http.HandleFunc("/", handler)

func handler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("Hello world"))
}

is equivalent to:

hn := HelloHandler{}
http.Handle("/", hn)

type HelloHandler struct {
}

func (h HelloHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello world"))
}

Let's see some examples of different patterns that can be used.

package main

import (
	"encoding/json"
	"net/http"
)

func main() {
	http.HandleFunc("/hello", handler)
	http.HandleFunc("GET /say-bye", handler2)
	http.HandleFunc("GET /say/{word}", handler3)
	http.HandleFunc("GET /json/{word}", handler4)
	err := http.ListenAndServe("127.0.0.1:8080", nil)
	if err != nil {
		panic(err)
	}
}

func handler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("Hello world"))
}

func handler2(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("Bye"))
}

func handler3(w http.ResponseWriter, r *http.Request) {
	word := r.PathValue("word")
	w.Write([]byte(word))
}

func handler4(w http.ResponseWriter, r *http.Request) {
	word := r.PathValue("word")

	w.Header().Set("Content-Type", "application/json")

	ans := map[string]string{
		"word": word,
	}

	json.NewEncoder(w).Encode(ans)
}

Start the server:

go run ./example-muxpatterns

curl -i http://localhost:8080/hello
curl -XPOST -i http://localhost:8080/hello
curl -XGET -i http://localhost:8080/say-bye`
curl -XPOST -i http://localhost:8080/say-bye
curl -XGET -i http://localhost:8080/say/this-works

See in the documentation how patterns work

Also notice the response of curl -XPOST -i http://localhost:8080/say-bye:

HTTP/1.1 405 Method Not Allowed
Allow: GET, HEAD
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Sun, 04 May 2025 05:44:41 GMT
Content-Length: 19

Method Not Allowed

We are not going to dive into more details at the moment.

The important part is to understand that Go gives you the ability to register a method that is executed based on the pattern. The pattern can contain a Method a Path and that the path can contain a wildcard (see the handler3 example).