Testing HTTP Handlers

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

net/http/httptest provides an in-memory response recorder and request constructor. Tests can assert status, headers and bodies without opening a real port.

Test successful and invalid cases; for integration behavior, use an httptest.Server when network-level behavior matters.

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

handler_test.go

package main
import (
    "net/http"
    "net/http/httptest"
    "testing"
)
func TestHello(t *testing.T) {
    rec := httptest.NewRecorder()
    http.HandlerFunc(hello).ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
    if rec.Code != 200 || rec.Body.String() != "hello" { t.Fatalf("unexpected response: %d %q", rec.Code, rec.Body.String()) }
}

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"
)

func hello(w http.ResponseWriter, r *http.Request) { fmt.Fprint(w, "hello") }
func main() {
	rec := httptest.NewRecorder()
	http.HandlerFunc(hello).ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
	fmt.Println("Pass:", rec.Code == 200 && rec.Body.String() == "hello")
}

Output

Pass: true

The recorder captures status and body, and the sample prints Pass: true. Put the same expectations in a TestHello function for a real go test run.

• Handler tests do not exercise TLS, proxies, and deployment configuration.
• Check content types as well as bodies for JSON endpoints.

Add tests for both a successful GET and a rejected method.

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

You have practiced Testing 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.