main.go

package main

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

type Request struct {
	Name string `json:"name"`
}

type Response struct {
	Message string `json:"message"`
}

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		message := "Hello world!"

		ct := r.Header.Get("Content-Type")

		if ct == "application/json" {
			b, err := ioutil.ReadAll(r.Body)
			if err != nil {
				w.WriteHeader(500)
				return
			}

			var req Request

			if err := json.Unmarshal(b, &req); err != nil {
				w.WriteHeader(500)
				w.Write([]byte(err.Error()))
				return
			}

			if len(req.Name) > 0 {
				message = "Hello " + req.Name + "!"
			}

			if err := json.NewEncoder(w).Encode(Response{Message: message}); err != nil {
				w.WriteHeader(500)
				return
			}

			return
		}

		r.ParseForm()
		name := r.Form.Get("name")

		if len(name) > 0 {
			message = "Hello " + name + "!"
		}

		w.Write([]byte(message))
	})

	http.ListenAndServe(":8080", nil)
}