HTTP Handlers

In this lesson, explore http handlers through one small Go program, and connect its output to the language rules behind it.

An http.Handler has a ServeHTTP(ResponseWriter, *Request) method. http.HandlerFunc adapts ordinary functions to that interface.

Handlers can be composed, tested and supplied to middleware. Avoid storing per-request data in unsynchronized global variables.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

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

type Greeting struct{ Text string }

func (g Greeting) ServeHTTP(w http.ResponseWriter, r *http.Request) { fmt.Fprint(w, g.Text) }
func main() {
	rec := httptest.NewRecorder()
	Greeting{"Hello"}.ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
	fmt.Println(rec.Body.String())
}

Output

Hello

The Greeting type implements ServeHTTP, so it is an HTTP handler. The recorder captures Hello.

• A handler method can have a pointer receiver if it needs controlled mutable state.
• Write an HTTP response once; do not try to change its status after the body starts.

Create a handler type that returns a configurable service name.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced HTTP Handlers in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.