Golang setting HTTP response response headers and pits

1, set the order of Writeheader problem

Before encountering a problem, in a piece of code to set the Writeheader, and finally in the header to take the name of how can not be taken.

w.WriteHeader(201)w.Header().Set("Name", "my name is smallsoup")

When writing HTTP server with Golang, it is convenient to set the contents of the Header in HTTP response by W.header.set (K, v). However, it is important to note that some time not only to modify the response header, but also to modify the response statuscode. Modifying the response statuscode can be done by: W. Writeheader (code) to implement, for example:

w.WriteHeader(404)

If these two modifications are done together, the W.writeheader must be left behind after all W.header.set because the Set Header is invalid after W.writeheader.

and must be before W.write ([]byte ("HelloWorld")), otherwise http:multiple response will be reported. Writeheader calls because actually called W. Write will also call the Writeheader () method, and then set W.wroteheader to True, again Writeheader () will be judged Wroteheader, if True will error, and this call does not take effect.

You can see the following source code description Writeheader must be called before write.

func (w *response) WriteHeader(code int) { if w.conn.hijacked() {   w.conn.server.logf("http: response.WriteHeader on hijacked connection")   return }//第二次WriteHeader()进来满足if条件就报错直接return if w.wroteHeader {   w.conn.server.logf("http: multiple response.WriteHeader calls")   return }//第一次write()进来这里会将w.wroteHeader置为true w.wroteHeader = true w.status = code if w.calledHeader && w.cw.header == nil {   w.cw.header = w.handlerHeader.clone() } if cl := w.handlerHeader.get("Content-Length"); cl != "" {   v, err := strconv.ParseInt(cl, 10, 64)   if err == nil && v >= 0 {     w.contentLength = v   } else {     w.conn.server.logf("http: invalid Content-Length of %q", cl)     w.handlerHeader.Del("Content-Length")   } }}

2, go will be the key in the header to standardize processing

Go will normalize the key in the header, so be careful when you get the K,v value in the header of response.

Reader.go non-export method Canonicalmimeheaderkey There is such a paragraph, the header key will be normalized processing.

1) The istokentable array is defined in Reader.go, and if the key is longer than 127 or contains characters that are not in istokentable, the key is not processed.

2) Capitalize the first letter of the key, and the first letter of the character-after word.

Analysis of the following source code, you can explain the capitalization of key processing:

for i, c := range a {  // 规范化:首字母大写  // - 之后单子的首字母大写  // 如:(Host, User-Agent, If-Modified-Since).  if upper && 'a' <= c && c <= 'z' {    //大写转小写    c -= toLower  } else if !upper && 'A' <= c && c <= 'Z' {    //小写转大写    c += toLower  }  //重新给key数组赋值  a[i] = c  //设置大小写标志位  upper = c == '-' // for next time}

The correct way to call:

Server: Myserver.go

package mainimport ( "net/http")func main() { http.HandleFunc("/", func (w http.ResponseWriter, r *http.Request){   w.Header().Set("name", "my name is smallsoup")   w.WriteHeader(500)   w.Write([]byte("hello world\n")) }) http.ListenAndServe(":8080", nil)}

Client:

Myhttp.go:

package mainimport ( "fmt" "io/ioutil" "net/http")func main() { myHttpGet()}func myHttpGet() { rsp, err := http.Get("http://localhost:8080") if err != nil {   fmt.Println("myHttpGet error is ", err)   return } defer rsp.Body.Close() body, err := ioutil.ReadAll(rsp.Body) if err != nil {   fmt.Println("myHttpGet error is ", err)   return } fmt.Println("response statuscode is ", rsp.StatusCode,          "\nhead[name]=", rsp.Header["Name"],            "\nbody is ", string(body))}

1. Running the server

Go Run myserver.go

2. Running the client

Go Run myhttp.go

The output is as follows: StatusCode is the value we set for the 500,name also taken.

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