实现一个Web服务器，用户在浏览器中输入文件的路径，然后服务器通过网页把相应文件显示给用户。
    web.go：

    1. package main
    3. import (
    4.     "net/http"
    6.     "os"
    8.     ".../filelisting"
    10.     "log"
    11. )
    13. type appHandler func(writer http.ResponseWriter, request *http.Request) error
    15. func errWrapper(handler appHandler) func(http.ResponseWriter, *http.Request) {
    16.     return func(writer http.ResponseWriter, request *http.Request) {
    17.         defer func() {
    18.             if r := recover(); r != nil {
    19.                 log.Printf("Panic: %v", r)
    20.                 http.Error(writer,
    21.                        http.StatusText(http.StatusInternalServerError),
    22.                        http.StatusInternalServerError)
    23.             }
    24.         }()
    26.         err := handler(writer, request)
    28.         if err != nil {
    29.             log.Printf("Error occurred " + "handling request: %s", err.Error())
    31.             if userErr, ok := err.(userError); ok {
    32.                 http.Error(writer, userErr.Message(), http.StatusBadRequest)
    33.                 return
    34.             }
    36.             code := http.StatusOK
    37.             switch {
    38.             case os.IsNotExist(err): // "文件不存在"错误 404
    39.                 code = http.StatusNotFound
    40.             case os.IsPermission(err): // "没有权限"错误 403
    41.                 code = http.StatusForbidden
    42.             default: // 其他错误(未知错误) 500
    43.                 code = http.StatusInternalServerError
    44.             }
    45.             http.Error(writer, http.StatusText(code), code)
    46.         }
    47.     }
    48. }
    50. type userError interface {
    51.     error
    52.     Message() string
    53. }
    55. func main() {
    56.     // 为了避免把程序的内部错误暴露给用户，需要对HandleFileList函数进行包装
    57.     http.HandleFunc("/list/", errWrapper(filelisting.HandleFileList))
    59.     err := http.ListenAndServe(":8888", nil)
    60.     if err != nil {
    61.         panic(err)
    62.     }
    63. }

    handler.go：

    package filelisting

import (
    "io/ioutil"
    "net/http"
    "os"
)

const prefix = "/list/"

type userError string

func (e userError) Error() string {
    return e.Message()
}

func (e userError) Message() string {
    return string(e)
}

func HandleFileList(writer http.ResponseWriter, request *http.Request) error {
    if strings.Index(request.URL.Path, prefix) != 0 {
        return userError("path must start " + "with " + prefix)
    }
    path := request.URL.Path[len(prefix):]
    file, err := os.Open(path)
    if err != nil {
        return err
    }
    defer file.Close()

    all, err := ioutil.ReadAll(file)
    if err != nil {
        return err
    }

    writer.Write(all)
    return nil
}

    用户访问服务器示例：
    ①成功返回
    image.png
    ②文件不存在
    image.png

    用error还是用panic?
    尽量不要用panic，尽量用error。
    能够预料到的错误都用error；预料不到的错误才用panic。