Manage HTTP Cookie in Go with Cookie Jar

Go’s standard HTTP library provides an interface to manage the storage of cookies in HTTP requests. If we provide a jar to http.Client, the jar is used to insert relevant cookies into every request. The jar is also updated with the cookie values of every response. If the jar is nil, cookies are sent only when they are explicitly set on the request.

Consider this example. Even though the server sets a cookie header on the response, the HTTP client will simply ignore it because the cookie jar is not set.

package main

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

func main() {
	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if cookie, err := r.Cookie("Cookie"); err != nil {
			fmt.Println("You don't have a cookie. Here's one for you. Don't forget to keep it.")
			http.SetCookie(w, &http.Cookie{Name: "Cookie", Value: "Peanut Butter"})
		} else {
			fmt.Printf("You have %s cookie!\n", cookie.Value)
		}
	}))
	defer ts.Close()

	client := &http.Client{}

	if _, err := client.Get(ts.URL); err != nil {
		panic(err)
	}

	if _, err := client.Get(ts.URL); err != nil {
		panic(err)
	}
}

Output:

You don't have a cookie. Here's one for you. Don't forget to keep it.
You don't have a cookie. Here's one for you. Don't forget to keep it.

The net/http/cookiejar package provides in-memory CookieJar implementation. We can use that as the jar on the http.Client.

package main

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

	"golang.org/x/net/publicsuffix"
)

func main() {
	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if cookie, err := r.Cookie("Cookie"); err != nil {
			fmt.Println("You don't have a cookie. Here's one for you. Don't forget to keep it.")
			http.SetCookie(w, &http.Cookie{Name: "Cookie", Value: "Peanut Butter"})
		} else {
			fmt.Printf("You have %s cookie!\n", cookie.Value)
		}
	}))
	defer ts.Close()

	jar, err := cookiejar.New(&cookiejar.Options{PublicSuffixList: publicsuffix.List})
	if err != nil {
		panic(err)
	}

	client := &http.Client{
		Jar: jar,
	}

	if _, err = client.Get(ts.URL); err != nil {
		panic(err)
	}

	if _, err = client.Get(ts.URL); err != nil {
		panic(err)
	}
}

Now, the code produces the following output:

You don't have a cookie. Here's one for you. Don't forget to keep it.
You have Peanut Butter cookie!

Because the net/http/cookiejar package stores cookies in memory, the cookies will be destroyed once the program exits. If you want to persist the cookies, you can build your own cookie jar by implementing the http.CookieJar interface.