Go语言学习之mime包(the way to go)

生命不止，继续 go go go !!!

很久之前，写过一篇文章来介绍Windows下如何获取文件的MIME：
《windows客户端开发–获取windows系统中文件的MIME》

其中，用到了叫做名为FindMimeFromData的Windows api.

所以今天介绍一下golang中为我们提供的mime package。

MIME

MIME是英文Multipurpose Internet Mail Extensions的缩写：

MIME is an Internet standard that extends the format of email to support:
Text in character sets other than ASCII
Non-text attachments: audio,video,images,application programs etc.
Message bodies with multiple parts
Header information in non-ASCII character sets

Package mime

func AddExtensionType

func AddExtensionType(ext,typ string) error

AddExtensionType sets the MIME type associated with the extension ext to typ. The extension should begin with a leading dot,as in “.html”.
函数将扩展名和mimetype建立偶联；扩展名应以点号开始，例如”.html”。

mime.AddExtensionType(".svg","image/svg+xml")
 mime.AddExtensionType( ".m3u8","application/x-mpegURL" );
 mime.AddExtensionType( ".ts","video/MP2T" );

func FormatMediaType

func FormatMediaType(t string,param map[string]string) string

FormatMediaType serializes mediatype t and the parameters param as a media type conforming to RFC 2045 and RFC 2616. The type and parameter names are written in lower-case. When any of the arguments result in a standard violation then FormatMediaType returns the empty string.
函数根据RFC 2045和 RFC 2616的规定将媒体类型t和参数param连接为一个mime媒体类型，类型和参数都采用小写字母。任一个参数不合法都会返回空字符串。

s := mime.FormatMediaType("image/svg+xml",map[string]string{"svg": "\u0001"})
    fmt.Printf("%#v",s)

func ParseMediaType

func ParseMediaType(v string) (mediatype string,params map[string]string,err error)

ParseMediaType parses a media type value and any optional parameters,per RFC 1521. Media types are the values in Content-Type and Content-disposition headers (RFC 2183). On success,ParseMediaType returns the media type converted to lowercase and trimmed of white space and a non-nil map. The returned map,params,maps from the lowercase attribute to the attribute value with its case preserved.
函数根据RFC 1521解析一个媒体类型值以及可能的参数。媒体类型值一般应为Content-Type和Conten-disposition头域的值（参见RFC 2183）。成功的调用会返回小写字母、去空格的媒体类型和一个非空的map。返回的map映射小写字母的属性和对应的属性值。

package main

 import (
         "fmt"
         "mime"
         "os"
 )

 func main() {

         if len(os.Args) != 2 {
                 fmt.Printf("Usage : %s filename \n",os.Args[0])
                 os.Exit(1)
         }

         mType,parameters,err := mime.ParseMediaType(os.Args[1])

         if err != nil {
                 fmt.Println(err)
                 os.Exit(1)
         }

         fmt.Println("Media type : ",mType)

         for param := range parameters {
                 fmt.Printf("%v = %v\n\n",param,parameters[param])
         }
 }

func TypeByExtension

func TypeByExtension(ext string) string

TypeByExtension returns the MIME type associated with the file extension ext. The extension ext should begin with a leading dot,as in “.html”. When ext has no associated type,TypeByExtension returns “”.
函数返回与扩展名偶联的MIME类型。扩展名应以点号开始，如”.html”。如果扩展名未偶联类型，函数会返回”“。
内建的偶联表很小，但在unix系统会从本地系统的一或多个mime.types文件（参加下表）进行增补。

MIME应用

获取文件的mime：
这里使用的path包的Ext方法，获取路径字符串中的文件扩展名。

package main

import (
    "fmt"
    "mime"
    "path"
)

func main() {
    filepath := "./1.png"
    mimetype := mime.TypeByExtension(path.Ext(filepath))
    fmt.Println(mimetype)

    filepath = "./2.txt"
    mimetype = mime.TypeByExtension(path.Ext(filepath))
    fmt.Println(mimetype)

    filepath = "./3.html"
    mimetype = mime.TypeByExtension(path.Ext(filepath))
    fmt.Println(mimetype)
}

输出：
image/png
text/plain; charset=utf-8
text/html; charset=utf-8

设置Content-Type

func SetContentTypeFromExtension(w http.ResponseWriter,extension string) {
    mime := mime.TypeByExtension(extension)
    if mime != "" {
        w.Header().Set("Content-Type",mime)
    }
}

解析Content-Type

mediatype,err := mime.ParseMediaType(contentType)
fmt.Printf("mediatype=%v,params=%v %v,err=%v\n",mediatype,len(params),err)