package main import (     "fmt"     "io"     "net/http"     "log"     "os" ) // 获取文件大小的接口 type Size interface {     Size() int64 } // 获取文件信息的接口 type Stat interface {     Stat() (os.FileInfo, error) } // hello world, the web server func HelloServer(w http.ResponseWriter, r *http.Request) {     if "POST" == r.Method {         file, _, err := r.FormFile("userfile")         if err != nil {             http.Error(w, err.Error(), 500)             return         }         if statInterface, ok := file.(Stat); ok {             fileInfo, _ := statInterface.Stat()             fmt.Fprintf(w, "上传文件的大小为: %d", fileInfo.Size())         }         if sizeInterface, ok := file.(Size); ok {             fmt.Fprintf(w, "上传文件的大小为: %d", sizeInterface.Size())         }         return     }     // 上传页面     w.Header().Add("Content-Type", "text/html")     w.WriteHeader(200)     html := ` <form enctype="multipart/form-data" action="/hello" method="POST">     Send this file: <input name="userfile" type="file" />     <input type="submit" value="Send File" /> </form> `     io.WriteString(w, html) } func main() {     http.HandleFunc("/hello", HelloServer)     err := http.ListenAndServe(":12345", nil)     if err != nil {         log.Fatal("ListenAndServe: ", err)     } }