httpctx.go

package main

import (
	"context"
	"fmt"
	"net/http"
	"net/url"
)

type contextKey int

const (
	proxyURLKey contextKey = 0
)

func proxy(req *http.Request) (*url.URL, error) {
	iproxy := req.Context().Value(proxyURLKey)
	if iproxy == nil {
		fmt.Printf("no proxy found\n")
		return nil, nil
	}
	proxyURL := iproxy.(*url.URL)
	fmt.Printf("proxy found:%s\n", proxyURL.String())
	return proxyURL, nil
}

func main() {
	transport := &http.Transport{
		Proxy: proxy,
	}
	client := http.Client{
		Transport: transport,
	}
	{
		proxyURL, _ := url.Parse("http://127.0.0.1:8000")
		ctx := context.WithValue(context.Background(), proxyURLKey, proxyURL)
		requestWithProxy, err := http.NewRequestWithContext(ctx, "GET", "http://www.baidu.com", nil)
		if err != nil {
			panic(err)
		}
		client.Do(requestWithProxy)
	}

	{
		requestWithoutProxy, err := http.NewRequest("GET", "http://www.baidu.com", nil)
		if err != nil {
			panic(err)
		}
		client.Do(requestWithoutProxy)
	}
}