proxy.go

package main

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

/*

Examples:

$ go run proxy.go
proxy: <nil>
proxy: <nil>

$ HTTP_PROXY=localhost:1234 go run proxy.go
proxy: http://localhost:1234
proxy: http://localhost:1234

 */
func main() {
	// The default transport many libraries use internally indirectly
	// via http.DefaultClient. Notice that the Proxy field is stateful
	// and guarded by a sync.Once:
	// https://golang.org/src/net/http/transport.go
	transport := http.DefaultTransport.(*http.Transport)

	parsedURL, _:= url.Parse("http://example.com")

	// The first request to flow through DefaultTransport will cause
	// the Proxy field to be set based on the environment at the current
	// execution point.
	request := &http.Request{URL: parsedURL}
	proxiedURL, _ := transport.Proxy(request)
	fmt.Printf("proxy: %s\n", proxiedURL)

	// Setting the environment now has no effect on the Proxy function
	// already assigned to DefaultTransport.
	os.Setenv("HTTP_PROXY", "localhost")
	request = &http.Request{URL: parsedURL}
	proxiedURL, _ = transport.Proxy(request)
	fmt.Printf("proxy: %s\n", proxiedURL)
}