http.Request r.FormValue不返回任何内容/地图[]

I have the following Go code:

package main

import (
  "encoding/json"
  "fmt"
  "github.com/gorilla/mux"
  "github.com/gorilla/handlers"
  "log"
  "net/http"
  "io/ioutil"
)

type rLog struct {
  Method string
  URI string
  FormParam string
}

func commonMiddleware(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Add("Content-Type", "application/json")
    formBs, err := ioutil.ReadAll(r.Body)
    if err != nil {
      log.Fatalf("Failed to decode postFormByteSlice: %v", err)
    }
    rl := rLog{Method: r.Method, URI: r.RequestURI, FormParam: string(formBs)}
    log.Printf("%+v", rl)
    next.ServeHTTP(w, r)
  })
}

func main() {
  port := ":3000"
  var router = mux.NewRouter()
  router.Use(commonMiddleware)
  router.HandleFunc("/m/{msg}", handleMessage).Methods("GET")
  router.HandleFunc("/n/", handleNumber).Methods("POST")

  headersOk := handlers.AllowedHeaders([]string{"Authorization"})
  originsOk := handlers.AllowedOrigins([]string{"*"})
  methodsOk := handlers.AllowedMethods([]string{"GET", "POST", "OPTIONS"})

  fmt.Printf("Server is running at http://localhost%s
", port)
  log.Fatal(http.ListenAndServe(port, handlers.CORS(originsOk, headersOk, methodsOk)(router)))
}

func handleMessage(w http.ResponseWriter, r *http.Request) {
  vars := mux.Vars(r)
  message := vars["msg"]
  response := map[string]string{"message": message}
  json.NewEncoder(w).Encode(response)
}

func handleNumber(w http.ResponseWriter, r *http.Request) {
  log.Println(r.FormValue("name")) // this returns nothing
  response := map[string]string{"name": "1"} // dummy response
  json.NewEncoder(w).Encode(response)
}

What I try to do here is very simple.

1. I want to log all POST form data (it's done inside the commonMiddleware)
2. Access the form data inside the handleNumber i.e. name (and many more later) and do some logic with it.

The problem that I have right now is, the log.Println(r.FormValue("name")) returns nothing and I really wonder why that happened.

I've tried adding r.ParseForm() before log.Println(r.FormValue("name")). But it didn't work.

And, when I add log.Println(r.Form) line, it returns map[].

What did I missed here?

You truing to read r.Body twice, first in commonMiddleware with ioutil.ReadAll(r.Body) and then in handleNumber with r.ParseForm(). You can't. It's io.Reader, you can't read it two times. You can instead, for example, r.ParseForm() in middlewear and then use parsed form data. In middlewear r.PosrForm.Encode() to log, and in handler r.FormValue() or r.Form.Get() to extract. I think something like this should do

func commonMiddleware(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Add("Content-Type", "application/json")
    err:=r.ParseForm() //parse in middleware, data will be contained in r.PostForm
    if err != nil {
      log.Fatalf("Failed to decode postFormByteSlice: %v", err)
    }
    rl := rLog{Method: r.Method, URI: r.RequestURI, FormParam: r.PostForm.Encode()} //url.Values.Encode() stringifys form data
    log.Printf("%+v", rl)
    next.ServeHTTP(w, r)
  })
}

func main() {
  port := ":3000"
  var router = mux.NewRouter()
  router.Use(commonMiddleware)
  router.HandleFunc("/m/{msg}", handleMessage).Methods("GET")
  router.HandleFunc("/n/", handleNumber).Methods("POST")
}

func handleNumber(w http.ResponseWriter, r *http.Request) {
  log.Println(r.PostForm.Get("name")) // or just r.Form.Get("name") or r.FormValue("name")
  response := map[string]string{"name": "1"} // dummy response
  json.NewEncoder(w).Encode(response)
}