http_switch_over.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
)

func main() {
	server := &http.Server{
		Addr: ":9090",
	}
	// First handler
	server.Handler = createHandler("Hello, World!")

	// Starts webserver in background
	go func() {
		if err := server.ListenAndServe(); err != nil {
			log.Fatal(err)
		}
	}()
	fmt.Println("Started web server")

	// Our request
	req, err := http.NewRequest("GET", "http://127.0.0.1:9090/", nil)
	if err != nil {
		log.Fatal(err)
	}
	req.Close = true

	fmt.Println("Calling once")
	resp, err := lookup(req)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("Received response, %s\n", resp)

	server.Handler = createHandler("Foo, Bar!")
	fmt.Println("Switched server handler")
	resp, err = lookup(req)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("Received response, %s\n", resp)

	fmt.Println("All done! Exited")
}

func lookup(req *http.Request) ([]byte, error) {
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	return ioutil.ReadAll(resp.Body)
}

func createHandler(msg string) http.Handler {
	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte(msg))
	})
	return mux
}