Java获取URL链接的文件类型

问题发生：

Java从网络批量读取图片并保存至本网站服务器后再插入文章中

今天转入一篇文章 http://news.qq.com/a/20170605/045860.htm 发现图片未能成功上传

查看源码发现：

因为没有文件类型所以在转存图片的时候出错了

谷歌百度了一下发现解决办法：

URLConnection class provides two static methods to help programs figure out the MIME type of some data; you can use these if the content type just isn't available or if you have reason to believe that the content type you're given isn't correct。

就是说URLConnection提供了两种方法可以猜测（根据实测结果，这个猜测是相当的准）数据的MIME类型。

第一个是：

public static String guessContentTypeFromName(String name)

这个方法根据URL文件部分的后缀名来判断类型，跟之前我的方法一样。这个不能解决上面那个问题。
第二个是：

public static String guessContentTypeFromStream(InputStream in)
这个方法是根据流的前面几个字节来判断类型，这个就不需要文件后缀名了，完全可以解决上面那个问题。

BufferedInputStream bis = null;
HttpURLConnection urlconnection = null;
URL url = null;
        url = new URL(strUrl);
    urlconnection = (HttpURLConnection) url.openConnection();
    urlconnection.connect();
bis = new BufferedInputStream(urlconnection.getInputStream());
    System.out.println("file type:"+HttpURLConnection.guessContentTypeFromStream(bis));

但生产中发现有很多类型不能正确的判断出来，如bmp

然后继续各种找，最后解决了...

再看一下无后缀名的判断..

HTTP Content-type 对照表

Content-Type，内容类型，一般是指网页中存在的Content-Type，用于定义网络文件的类型和网页的编码，决定浏览器将以什么形式、什么编码读取这个文件，比如用PHP输出图片文件、JSON数据、XML文件等非HTML内容时，就必须用header函数来指定Content-Type，才能达到输出一张图片或是其它指定内容类型的需求。

Application Type

文件扩展名	Content-Type(Mime-Type)	描述
.	application/x-
.*	application/octet-stream	二进制流，不知道下载文件类型
.pdf	application/pdf	PDF（Portable Document Format的简称，意为“便携式文件格式”）
.ai	application/postscript	PostScript（PS）是主要用于电子产业和桌面出版领域的一种页面描述语言和编程语言。
.xml	application/atom+xml	Atom feeds
.js	application/ecmascript	相当于application/javascript但是严格的处理规则
.edi	application/EDI-X12	EDI ANSI ASC X12数据
.edi	application/EDIFACT	EDI EDIFACT数据
.json	application/json	JSON（JavaScript Object Notation）
.js	application/javascript	ECMAScript/JavaScript（相当于application/ecmascript但是宽松的处理规则）
.ogg	application/ogg	Ogg, 视频文件格式
.rdf	application/rdf+xml	资源描述框架（Resource Description Framework，缩写 RDF），是万维网联盟（W3C）提出的一组标记语言的技术标准，以便更为丰富地描述和表达网络资源的内容与结构。
.xml	application/rss+xml	RSS（Really Simple Syndication,简易信息聚合）是一种消息来源格式规范，用以聚合经常发布更新数据的网站，例如博客文章、新闻、音频或视频的网摘
.xml	application/soap+xml	简单对象访问协议（SOAP，全写为Simple Object Access Protocol）是交换数据的一种协议规范，使用在计算机网络Web服务（web service）中，交换带结构信息。
.woff	application/font-woff	Web开放字体格式（Web Open Font Format，简称WOFF）是一种网页所采用的字体格式标准。（推荐使用；使用application/x-font-woff直到它变为官方标准）
.xhtml	application/xhtml+xml	可扩展超文本标记语言（eXtensible HyperText Markup Language，XHTML），是一种标记语言，表现方式与超文本标记语言（HTML）类似，不过语法上更加严格。
.xml	application/xml	可扩展标记语言（英语：eXtensible Markup Language，简称: XML），是一种标记语言。
.dtd	application/xml-dtd	XML文件的文件型别定义（Document Type Definition）可以看成一个或者多个XML文件的模板，在这里可以定义XML文件中的元素、元素的属性、元素的排列方式、元素包含的内容等等。
.xml	application/xop+xml	二进制优化封装协议(Xmlbinary Optimized Packaging)
.zip	application/zip	ZIP压缩文件
.gzip	application/gzip	Gzip是若干种文件压缩程序的简称，通常指GNU计划的实现，此处的gzip代表GNU zip。
.xls	application/x-xls	XLS 就是 Microsoft Excel 工作表，是一种非常常用的电子表格格式。
.001	application/x-001
.301	application/x-301
.906	application/x-906
.a11	application/x-a11
.awf	application/vnd.adobe.workflow
.bmp	application/x-bmp
.c4t	application/x-c4t
.cal	application/x-cals
.cdf	application/x-netcdf
.cel	application/x-cel
.cg4	application/x-g4
.cit	application/x-cit
.bot	application/x-bot
.c90	application/x-c90
.cat	application/vnd.ms-pki.seccat
.cdr	application/x-cdr
.cer	application/x-x509-ca-cert
.cgm	application/x-cgm
.cmx	application/x-cmx
.crl	application/pkix-crl
.csi	application/x-csi
.cut	application/x-cut
.dbm	application/x-dbm
.cmp	application/x-cmp
.cot	application/x-cot
.crt	application/x-x509-ca-cert
.dbf	application/x-dbf
.dbx	application/x-dbx
.dcx	application/x-dcx
.dgn	application/x-dgn
.dll	application/x-msdownload
.dot	application/msword
.der	application/x-x509-ca-cert
.dib	application/x-dib
.doc	application/msword
.drw	application/x-drw
.dwf	application/x-dwf
.dxb	application/x-dxb
.edn	application/vnd.adobe.edn
.dwg	application/x-dwg
.dxf	application/x-dxf
.emf	application/x-emf
.epi	application/x-epi
.eps	application/postscript
.exe	application/x-msdownload
.fdf	application/vnd.fdf
.eps	application/x-ps
.etd	application/x-ebx
.fif	application/fractals
.frm	application/x-frm
.gbr	application/x-gbr
.g4	application/x-g4
.gl2	application/x-gl2
.hgl	application/x-hgl
.hpg	application/x-hpgl
.hqx	application/mac-binhex40
.hta	application/hta
.gp4	application/x-gp4
.hmr	application/x-hmr
.hpl	application/x-hpl
.hrf	application/x-hrf
.icb	application/x-icb
.ico	application/x-ico
.ig4	application/x-g4
.iii	application/x-iphone
.ins	application/x-internet-signup
.iff	application/x-iff
.igs	application/x-igs
.img	application/x-img
.isp	application/x-internet-signup
.jpe	application/x-jpe
.js	application/x-javascript
.jpg	application/x-jpg
.lar	application/x-laplayer-reg
.latex	application/x-latex
.lbm	application/x-lbm
.ls	application/x-javascript
.ltr	application/x-ltr
.man	application/x-troff-man
.mdb	application/msaccess
.mac	application/x-mac
.mdb	application/x-mdb
.mfp	application/x-shockwave-flash
.mi	application/x-mi
.mil	application/x-mil
.mocha	application/x-javascript
.mpd	application/vnd.ms-project
.mpp	application/vnd.ms-project
.mpt	application/vnd.ms-project
.mpw	application/vnd.ms-project
.mpx	application/vnd.ms-project
.mxp	application/x-mmxp
.nrf	application/x-nrf
.out	application/x-out
.p12	application/x-pkcs12
.p7c	application/pkcs7-mime
.p7r	application/x-pkcs7-certreqresp
.pc5	application/x-pc5
.pcl	application/x-pcl
.pdx	application/vnd.adobe.pdx
.pgl	application/x-pgl
.pko	application/vnd.ms-pki.pko
.p10	application/pkcs10
.p7b	application/x-pkcs7-certificates
.p7m	application/pkcs7-mime
.p7s	application/pkcs7-signature
.pci	application/x-pci
.pcx	application/x-pcx
.pdf	application/pdf
.pfx	application/x-pkcs12
.pic	application/x-pic
.pl	application/x-perl
.plt	application/x-plt
.png	application/x-png
.ppa	application/vnd.ms-powerpoint
.pps	application/vnd.ms-powerpoint
.ppt	application/x-ppt
.prf	application/pics-rules
.prt	application/x-prt
.ps	application/postscript
.pwz	application/vnd.ms-powerpoint
.ra	audio/vnd.rn-realaudio	RealAudio是一种由RealNetworks发展的RealMedia多媒体音频文件格式，仅指RealPlayer中能够识别的音频文件，也可以理解为real格式的音频文件。
.ras	application/x-ras
.pot	application/vnd.ms-powerpoint
.ppm	application/x-ppm
.ppt	application/vnd.ms-powerpoint
.pr	application/x-pr
.prn	application/x-prn
.ps	application/x-ps
.ptn	application/x-ptn
.red	application/x-red
.rjs	application/vnd.rn-realsystem-rjs
.rlc	application/x-rlc
.rm	application/vnd.rn-realmedia
.rat	application/rat-file
.rec	application/vnd.rn-recording
.rgb	application/x-rgb
.rjt	application/vnd.rn-realsystem-rjt
.rle	application/x-rle
.rmf	application/vnd.adobe.rmf
.rmj	application/vnd.rn-realsystem-rmj
.rmp	application/vnd.rn-rn_music_package
.rmvb	application/vnd.rn-realmedia-vbr
.rnx	application/vnd.rn-realplayer
.rpm	audio/x-pn-realaudio-plugin
.rms	application/vnd.rn-realmedia-secure
.rmx	application/vnd.rn-realsystem-rmx
.rsml	application/vnd.rn-rsml
.rtf	application/msword
.rv	video/vnd.rn-realvideo
.sat	application/x-sat
.sdw	application/x-sdw
.slb	application/x-slb
.rtf	application/x-rtf
.sam	application/x-sam
.sdp	application/sdp
.sit	application/x-stuffit
.sld	application/x-sld
.smi	application/smil
.smk	application/x-smk
.smil	application/smil
.spc	application/x-pkcs7-certificates
.spl	application/futuresplash
.ssm	application/streamingmedia
.stl	application/vnd.ms-pki.stl
.sst	application/vnd.ms-pki.certstore
.tdf	application/x-tdf
.tga	application/x-tga
.sty	application/x-sty
.swf	application/x-shockwave-flash
.tg4	application/x-tg4
.tif	application/x-tif
.vdx	application/vnd.visio
.vpg	application/x-vpeg005
.vsd	application/x-vsd
.vst	application/vnd.visio
.vsw	application/vnd.visio
.vtx	application/vnd.visio
.torrent	application/x-bittorrent
.vda	application/x-vda
.vsd	application/vnd.visio
.vss	application/vnd.visio
.vst	application/x-vst
.vsx	application/vnd.visio
.wb1	application/x-wb1
.wb3	application/x-wb3
.wiz	application/msword
.wk4	application/x-wk4
.wks	application/x-wks
.wb2	application/x-wb2
.wk3	application/x-wk3
.wkq	application/x-wkq
.wmf	application/x-wmf
.wmd	application/x-ms-wmd
.wp6	application/x-wp6
.wpg	application/x-wpg
.wq1	application/x-wq1
.wri	application/x-wri
.ws	application/x-ws
.wmz	application/x-ms-wmz
.wpd	application/x-wpd
.wpl	application/vnd.ms-wpl
.wr1	application/x-wr1
.wrk	application/x-wrk
.ws2	application/x-ws
.xdp	application/vnd.adobe.xdp
.xfd	application/vnd.adobe.xfd
.xfdf	application/vnd.adobe.xfdf
.xls	application/vnd.ms-excel
.xwd	application/x-xwd
.sis	application/vnd.symbian.install
.x_t	application/x-x_t
.apk	application/vnd.android.package-archive
.x_b	application/x-x_b
.sisx	application/vnd.symbian.install
.ipa	application/vnd.iphone
.xap	application/x-silverlight-app
.xlw	application/x-xlw
.xpl	audio/scpls
.anv	application/x-anv
.uin	application/x-icq

Text Type

文件扩展名	Content-Type(Mime-Type)	描述
.323	text/h323
.biz	text/xml
.cml	text/xml
.asa	text/asa
.asp	text/asp
.css	text/css	层叠样式表（英语：Cascading Style Sheets，简写CSS），又称串样式列表、层次结构式样式表文件，一种用来为结构化文档（如HTML文档或XML应用）添加样式（字体、间距和颜色等）的计算机语言，由W3C定义和维护。
.csv	text/csv	逗号分隔值（Comma-Separated Values，CSV，有时也称为字符分隔值，因为分隔字符也可以不是逗号），其文件以纯文本形式存储表格数据（数字和文本）。
.dcd	text/xml
.dtd	text/xml
.ent	text/xml
.fo	text/xml
.htc	text/x-component
.html	text/html
.htx	text/html
.htm	text/html
.htt	text/webviewhtml
.jsp	text/html
.math	text/xml
.mml	text/xml
.mtx	text/xml
.plg	text/html
.rdf	text/xml
.rt	text/vnd.rn-realtext
.sol	text/plain
.spp	text/xml
.stm	text/html
.svg	text/xml
.tld	text/xml
.txt	text/plain	纯文字内容
.uls	text/iuls
.vml	text/xml
.tsd	text/xml
.vcf	text/x-vcard
.vxml	text/xml
.wml	text/vnd.wap.wml
.wsdl	text/xml
.wsc	text/scriptlet
.xdr	text/xml
.xql	text/xml
.xsd	text/xml
.xslt	text/xml
.xml	text/xml
.xq	text/xml
.xquery	text/xml
.xsl	text/xml
.xhtml	text/html
.odc	text/x-ms-odc
.r3t	text/vnd.rn-realtext3d
.sor	text/plain

Audio Type

文件扩展名	Content-Type(Mime-Type)	描述
.acp	audio/x-mei-aac
.aif	audio/aiff
.aiff	audio/aiff
.aifc	audio/aiff
.au	audio/basic
.la1	audio/x-liquid-file
.lavs	audio/x-liquid-secure
.lmsff	audio/x-la-lms
.m3u	audio/mpegurl
.midi	audio/mid
.mid	audio/mid
.mp2	audio/mp2
.mp3	audio/mp3
.mp4	audio/mp4	MP4，全称MPEG-4 Part 14，是一种使用MPEG-4的多媒体计算机文件格式，扩展名为.mp4，以存储数字音频及数字视频为主。
.mnd	audio/x-musicnet-download
.mp1	audio/mp1
.mns	audio/x-musicnet-stream
.mpga	audio/rn-mpeg
.pls	audio/scpls
.ram	audio/x-pn-realaudio
.rmi	audio/mid
.rmm	audio/x-pn-realaudio
.snd	audio/basic
.wav	audio/wav
.wax	audio/x-ms-wax
.wma	audio/x-ms-wma

Video Type

文件扩展名	Content-Type(Mime-Type)	描述
.asf	video/x-ms-asf
.asx	video/x-ms-asf
.avi	video/avi
.IVF	video/x-ivf
.m1v	video/x-mpeg
.m2v	video/x-mpeg
.m4e	video/mpeg4
.movie	video/x-sgi-movie
.mp2v	video/mpeg
.mp4	video/mpeg4	MP4，全称MPEG-4 Part 14，是一种使用MPEG-4的多媒体计算机文件格式，扩展名为.mp4，以存储数字音频及数字视频为主。
.mpa	video/x-mpg
.mpe	video/x-mpeg
.mpg	video/mpg
.mpeg	video/mpg
.mps	video/x-mpeg
.mpv	video/mpg
.mpv2	video/mpeg
.wm	video/x-ms-wm
.wmv	video/x-ms-wmv
.wmx	video/x-ms-wmx
.wvx	video/x-ms-wvx

Image Type

文件扩展名	Content-Type(Mime-Type)	描述
.tif	image/tiff	标签图像文件格式（Tagged Image File Format，简写为TIFF）是一种主要用来存储包括照片和艺术图在内的图像的文件格式。它最初由Aldus公司与微软公司一起为PostScript打印开发。
.fax	image/fax
.gif	image/gif	图像互换格式（GIF，Graphics Interchange Format）是一种位图图形文件格式，以8位色（即256种颜色）重现真彩色的图像。
.ico	image/x-icon
.jfif	image/jpeg
.jpe	image/jpeg
.jpeg	image/jpeg	JPEG是一种针对相片图像而广泛使用的一种有损压缩标准方法。
.jpg	image/jpeg
.net	image/pnetvue
.png	image/png	便携式网络图形（Portable Network Graphics，PNG）是一种无损压缩的位图图形格式，支持索引、灰度、RGB三种颜色方案以及Alpha通道等特性。
.rp	image/vnd.rn-realpix
.tif	image/tiff
.tiff	image/tiff
.wbmp	image/vnd.wap.wbmp

Message Type

文件扩展名	Content-Type(Mime-Type)	描述
.eml	message/rfc822
.mht	message/rfc822
.mhtml	message/rfc822
.nws	message/rfc822

Drawing Type

文件扩展名	Content-Type(Mime-Type)	描述
.907	drawing/907
.slk	drawing/x-slk
.top	drawing/x-top

Java Type

文件扩展名	Content-Type(Mime-Type)	描述
.class	java/*
.java	java/*

Other Type

文件扩展名	Content-Type(Mime-Type)	描述
.dwf	Model/vnd.dwf