今天解决了一个设置下载文件名为中文的问题:直接在Content-Disposition中设置中文会导致乱码。按照网上的办法(Content-Disposition + UTF-8)就搞定了。不过为了能搞清楚问题的关键所在,我还是去看了下官方文档,了解了下Content-Disposition的字段与意义。使用Content-Disposition可以设置文件名,但是要设置中文就需要进行编码,而RFC 822规定Message只能为ASCII,这就是问题所在。
Content-Disposition的定义
Hypertext Transfer Protocol – HTTP/1.1中的描述Content-Disposition is not part of the HTTP standard, but since it is widely implemented, we are documenting its use and risks for implementors.
The Content-Disposition response-header field has been proposed as a means for the origin server to suggest a default filename if the user requests that the content is saved to a file. This usage is derived from the definition of Content-Disposition in RFC 1806.
RFC 1806中的描述
the Content-Disposition header field is defined as follows:
disposition := "Content-Disposition" ":" disposition-type *(";" disposition-parm) disposition-type := "inline" / "attachment" / extension-token ; values are not case-sensitive disposition-parm := filename-parm / parameter filename-parm := "filename" "=" value;‘extension-token’, ‘parameter’ and`’value’ are defined according to [RFC 822] and [RFC 1521].
首先要注意的是disposition-type。由上面给出的是Content-Disposition header 字段我们可以知道,Content-Disposition有两种type,即inline和attachment。根据文档中的介绍可知,inline类型会自动显示附件内容,比如显示一个图片;而attachment不会自动显示,在邮件中可能会显示为一个带图标的附件,在浏览器中可能会提示下载。
其次就是disposition-parm。主要作用就是提供一个建议的文件名(filename-parm),客户端(浏览器、邮件系统)在尽可能的情况下会以该文件名去保存文件。尽可能的意思是存在不一样的情况,比如文件名非法、存在同名文件,这些情况下客户端会采取一些措施,比如修改文件名。
最后看看filename-parm的value。这个value就是文件名(本文的目的是给value设置一个中文,主要的坑就是这里)。
通过上面的介绍,要给前端发送一个文件,并且定义文件的名字为中文,可以在发送文件之前返回这样一个HTTP Response Header :
Content-Disposition: attachment; filename=文件.txt需要注意的是,RFC 822( Standard for ARPA Internet Text Messages)规定了文本消息只能为ASCII,因此这个Content-Disposition是非法的。RFC 1521(Multipurpose Internet Mail Extensions)基于前者对编码方式进行了拓展,使用了4种机制:
MIME-Version header
Content-Type header Content-Transfer-Encoding header Content-ID and Content-Description header 可惜这些与给HTTP Response Header中设置中文没啥关系。后来google了一下,通过stackoverflow找到了一个叫 RFC 5987 - Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters 的文档,顿时拨开迷雾见青天: By default, message header field parameters in HTTP ([RFC2616]) messages cannot carry characters outside the ISO-8859-1 character set. RFC 2231 defines an encoding mechanism for use in MIME headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.文中的Guidelines for Usage in HTTP Header Field Definitions给出了一个通用表达式:
foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param title-param = "title" LWSP "=" LWSP value / "title*" LWSP "=" LWSP ext-value ext-value = charset "'" [ language ] "'" value-chars charset = "UTF-8" / "ISO-8859-1" / mime-charset value-chars = *( pct-encoded / attr-char )将前面非法的Content-Disposition转化过来就是:
Content-Disposition : attachment; filename* = UTF-8''%E6%96%87%E4%BB%B6.txt这里对“文件.txt”进行了编码:先进行UTF-8编码,再进行pct-encoded编码。其实就是URL_ENCODE的过程。。。
Node中的用法
除了使用filename*=UTF-8”+value外,还需要对中文进行编码。在node中的写法:let name = urlencode("文件.txt", "utf-8");res.setHeader("Content-Disposition", "attachment; filename* = UTF-8''"+name);亲测在Chrome、Edge、IE 11 下有效。