net/http可以模仿，扩展或更改其中的大多数行为。尽管这
http.Client是实现HTTP客户端语义的具体类型，但其所有字段都可以导出并可以自定义。
Client.Transport特别是，可以替换该字段以使客户端执行任何操作，从使用自定义协议（例如ftp：//或file：//）到直接连接到本地处理程序（不生成HTTP协议字节或通过网络发送任何内容）
）。
客户端的功能，例如
http.Get，所有利用导出的
http.DefaultClient包变量（你可以修改），因此，利用这些方便的功能代码并
没有
，例如，必须改变到在定制客户端变量调用方法。请注意，虽然在公共可用的库中修改全局行为是不合理的，但在应用程序和测试（包括库测试）中进行修改非常有用。
http://play.golang.org/p/afljO086iB包含一个自定义
http.RoundTripper脚本，该自定义脚本重写了请求URL，以便将其路由到本地托管的
httptest.Server；另一个示例将请求直接传递给
http.Handler，以及自定义
http.ResponseWriter实现，为了创建一个
http.Response。第二种方法不像第一种方法那样努力（它没有填写Response值中的许多字段），但是效率更高，并且应该足够兼容以与大多数处理程序和客户端调用程序一起使用。
上面链接的代码也包括在下面：
package mainimport ( "fmt" "io" "log" "net/http" "net/http/httptest" "net/url" "os" "path" "strings")func Handler(w http.ResponseWriter, r *http.Request) { fmt.Fprintf(w, "hello %sn", path.base(r.URL.Path))}func main() { s := httptest.NewServer(http.HandlerFunc(Handler)) u, err := url.Parse(s.URL) if err != nil { log.Fatalln("failed to parse httptest.Server URL:", err) } http.DefaultClient.Transport = RewriteTransport{URL: u} resp, err := http.Get("https://google.com/path-one") if err != nil { log.Fatalln("failed to send first request:", err) } fmt.Println("[First Response]") resp.Write(os.Stdout) fmt.Print("n", strings.Repeat("-", 80), "nn") http.DefaultClient.Transport = HandlerTransport{http.HandlerFunc(Handler)} resp, err = http.Get("https://google.com/path-two") if err != nil { log.Fatalln("failed to send second request:", err) } fmt.Println("[Second Response]") resp.Write(os.Stdout)}// RewriteTransport is an http.RoundTripper that rewrites requests// using the provided URL's Scheme and Host, and its Path as a prefix.// The Opaque field is untouched.// If Transport is nil, http.DefaultTransport is usedtype RewriteTransport struct { Transport http.RoundTripper URL *url.URL}func (t RewriteTransport) RoundTrip(req *http.Request) (*http.Response, error) { // note that url.URL.ResolveReference doesn't work here // since t.u is an absolute url req.URL.Scheme = t.URL.Scheme req.URL.Host = t.URL.Host req.URL.Path = path.Join(t.URL.Path, req.URL.Path) rt := t.Transport if rt == nil { rt = http.DefaultTransport } return rt.RoundTrip(req)}type HandlerTransport struct{ h http.Handler }func (t HandlerTransport) RoundTrip(req *http.Request) (*http.Response, error) { r, w := io.Pipe() resp := &http.Response{ Proto: "HTTP/1.1", ProtoMajor: 1, ProtoMinor: 1, Header: make(http.Header), Body: r, Request: req, } ready := make(chan struct{}) prw := &pipeResponseWriter{r, w, resp, ready} go func() { defer w.Close() t.h.ServeHTTP(prw, req) }() <-ready return resp, nil}type pipeResponseWriter struct { r *io.PipeReader w *io.PipeWriter resp *http.Response ready chan<- struct{}}func (w *pipeResponseWriter) Header() http.Header { return w.resp.Header}func (w *pipeResponseWriter) Write(p []byte) (int, error) { if w.ready != nil { w.WriteHeader(http.StatusOK) } return w.w.Write(p)}func (w *pipeResponseWriter) WriteHeader(status int) { if w.ready == nil { // already called return } w.resp.StatusCode = status w.resp.Status = fmt.Sprintf("%d %s", status, http.StatusText(status)) close(w.ready) w.ready = nil}
