Hello HTTP

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Goal

Run a Go program that serves the text hello at http://localhost:8080/hello. After this page you will know how to start an HTTP server in Go without any third-party library.

Prerequisites

• Functions

What an HTTP handler is

Go's standard library has everything you need to answer web requests. The package is called net/http.

A handler is a function that takes two things: a ResponseWriter (what you write the reply into) and a *Request (what the caller sent you). You write your reply into the writer and Go ships it back to the caller.

A mux is a routing table — it maps a URL path to a handler. The default mux ships with Go, so you don't have to build one.

For everything net/http can do, see the official package docs. The two pieces we need now are HandleFunc (register a handler) and ListenAndServe (start the server).

Code

Create a file called main.go:

package main

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

func hello(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "hello")
}

func main() {
	http.HandleFunc("/hello", hello)
	fmt.Println("listening on :8080")
	log.Fatal(http.ListenAndServe(":8080", nil))
}

Run it:

go run main.go

You should see:

listening on :8080

In a second terminal:

curl http://localhost:8080/hello

Output:

hello

Stop the server with Ctrl-C.

The second argument to ListenAndServe is the mux. Passing nil tells Go to use the default mux, which is what http.HandleFunc registered against.

For frontend developers

If you've used Express in Node, this is the same idea: register a handler against a path, start a listener on a port. Go's standard library plays the role Express does — no install required.

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