什么是servlet ?

一個 servlet 就是 Java 編 程 語 言中的一個 類 , 是用 Java 編 寫的 Server 端程序 . 客戶機提交 ” 請求 ” 到服務器 , 服務器中的 servlet 對請求進行處理 , 將處理的結(jié)果反饋回客戶機 ( 即 ” 響應 ” 客戶機的請求 ), 這就是 servlet 在 “ 請 求 - 響 應 ” 編 程模型 中扮演的角色 , 它被用來 擴 展服 務 器的性能 。服 務 器 上 駐 留著可以通 過 “ 請 求 - 響 應 ” 編 程模型來 訪問 的 應 用程序。 雖 然 servlet 可以 對 任何 類 型的 請 求 產(chǎn) 生響 應 ，但通常只用來 擴 展 Web 服 務 器 中 的 應 用程序 。

把編譯好的HelloServlet.class文件放到\Tomcat文件夾\webapps\ourappfiles\WEB-INF\classes\test的文件夾下。

接著編寫我們的web.xml文件。

把編寫好的web.xml文件放到\Tomcat文件夾\webapps\ourappfiles\WEB-INF下。
好了，現(xiàn)在萬事俱備，就差啟動Tomcat運行我們的Servlet了。
在IE地址欄中輸入http://localhost:8080/ourappfiles/Hello，就可以看到結(jié)果了。
以上就是運行Servlet的幾個步驟。

通過這個例子可以看出純粹的Servlet編程的缺點.所有的內(nèi)容------甚至是基本的HTML格式------都必須由Servlet輸出,你不得不在Servlet中放置大量的print語句來結(jié)束,唯一的目的就是向用戶返回內(nèi)容.另外,甚至最簡單的HTML格式變化都必須在JAVA源代碼中解決,這意味著非JAVA開發(fā)人員不能進行WEB站點的設計.

為了克服上述缺點, JavaServer Pages(JSP)應運而生. JSP表面上看是"嵌入了Java 的HTML", 而實際上JSP是"嵌入了HTML的Java Servlet ". 要明白這一點,需要了解一下JSP是如何響應請求的.

當瀏覽器向服務器請求一個JSP頁面(A.JSP)時, 源頁面(A.JSP)通過轉(zhuǎn)換器(在TOMCAT中,這個轉(zhuǎn)換器叫做Jasper)被轉(zhuǎn)換成一個Java類(如:A_jsp.java),該類繼承于org.apache.jasper.runtime.HttpJspBase基類，而HttpJspBase實現(xiàn)了HttpServlet接口。可見，JSP在運行前首先被編譯為一個Servlet, 這個Servlet對請求進行處理,將結(jié)果內(nèi)容發(fā)送回客戶端.所以說, JSP是"嵌入了HTML的Java Servlet ", 其本質(zhì)就是Servlet.

如果將上面的servlet程序中的doGet方法寫成下面的樣子:

Content-type?的說明?<?php

$mimetypes?=?array(
????'ez'????????=>?'application/andrew-inset',
????'hqx'????????=>?'application/mac-binhex40',
????'cpt'????????=>?'application/mac-compactpro',
????'doc'????????=>?'application/msword',
????'bin'????????=>?'application/octet-stream',
????'dms'????????=>?'application/octet-stream',
????'lha'????????=>?'application/octet-stream',
????'lzh'????????=>?'application/octet-stream',
????'exe'????????=>?'application/octet-stream',
????'class'????????=>?'application/octet-stream',
????'so'????????=>?'application/octet-stream',
????'dll'????????=>?'application/octet-stream',
????'oda'????????=>?'application/oda',
????'pdf'????????=>?'application/pdf',
????'ai'????????=>?'application/postscript',
????'eps'????????=>?'application/postscript',
????'ps'????????=>?'application/postscript',
????'smi'????????=>?'application/smil',
????'smil'????????=>?'application/smil',
????'mif'????????=>?'application/vnd.mif',
????'xls'????????=>?'application/vnd.ms-excel',
????'ppt'????????=>?'application/vnd.ms-powerpoint',
????'wbxml'????????=>?'application/vnd.wap.wbxml',
????'wmlc'????????=>?'application/vnd.wap.wmlc',
????'wmlsc'????????=>?'application/vnd.wap.wmlscriptc',
????'bcpio'????????=>?'application/x-bcpio',
????'vcd'????????=>?'application/x-cdlink',
????'pgn'????????=>?'application/x-chess-pgn',
????'cpio'????????=>?'application/x-cpio',
????'csh'????????=>?'application/x-csh',
????'dcr'????????=>?'application/x-director',
????'dir'????????=>?'application/x-director',
????'dxr'????????=>?'application/x-director',
????'dvi'????????=>?'application/x-dvi',
????'spl'????????=>?'application/x-futuresplash',
????'gtar'????????=>?'application/x-gtar',
????'hdf'????????=>?'application/x-hdf',
????'js'????????=>?'application/x-javascript',
????'skp'????????=>?'application/x-koan',
????'skd'????????=>?'application/x-koan',
????'skt'????????=>?'application/x-koan',
????'skm'????????=>?'application/x-koan',
????'latex'????????=>?'application/x-latex',
????'nc'????????=>?'application/x-netcdf',
????'cdf'????????=>?'application/x-netcdf',
????'sh'????????=>?'application/x-sh',
????'shar'????????=>?'application/x-shar',
????'swf'????????=>?'application/x-shockwave-flash',
????'sit'????????=>?'application/x-stuffit',
????'sv4cpio'????=>?'application/x-sv4cpio',
????'sv4crc'????=>?'application/x-sv4crc',
????'tar'????????=>?'application/x-tar',
????'tcl'????????=>?'application/x-tcl',
????'tex'????????=>?'application/x-tex',
????'texinfo'????=>?'application/x-texinfo',
????'texi'????????=>?'application/x-texinfo',
????'t'????????????=>?'application/x-troff',
????'tr'????????=>?'application/x-troff',
????'roff'????????=>?'application/x-troff',
????'man'????????=>?'application/x-troff-man',
????'me'????????=>?'application/x-troff-me',
????'ms'????????=>?'application/x-troff-ms',
????'ustar'????????=>?'application/x-ustar',
????'src'????????=>?'application/x-wais-source',
????'xhtml'????????=>?'application/xhtml+xml',
????'xht'????????=>?'application/xhtml+xml',
????'zip'????????=>?'application/zip',
????'au'????????=>?'audio/basic',
????'snd'????????=>?'audio/basic',
????'mid'????????=>?'audio/midi',
????'midi'????????=>?'audio/midi',
????'kar'????????=>?'audio/midi',
????'mpga'????????=>?'audio/mpeg',
????'mp2'????????=>?'audio/mpeg',
????'mp3'????????=>?'audio/mpeg',
????'aif'????????=>?'audio/x-aiff',
????'aiff'????????=>?'audio/x-aiff',
????'aifc'????????=>?'audio/x-aiff',
????'m3u'????????=>?'audio/x-mpegurl',
????'ram'????????=>?'audio/x-pn-realaudio',
????'rm'????????=>?'audio/x-pn-realaudio',
????'rpm'????????=>?'audio/x-pn-realaudio-plugin',
????'ra'????????=>?'audio/x-realaudio',
????'wav'????????=>?'audio/x-wav',
????'pdb'????????=>?'chemical/x-pdb',
????'xyz'????????=>?'chemical/x-xyz',
????'bmp'????????=>?'image/bmp',
????'gif'????????=>?'image/gif',
????'ief'????????=>?'image/ief',
????'jpeg'????????=>?'image/jpeg',
????'jpg'????????=>?'image/jpeg',
????'jpe'????????=>?'image/jpeg',
????'png'????????=>?'image/png',
????'tiff'????????=>?'image/tiff',
????'tif'????????=>?'image/tiff',
????'djvu'????????=>?'image/vnd.djvu',
????'djv'????????=>?'image/vnd.djvu',
????'wbmp'????????=>?'image/vnd.wap.wbmp',
????'ras'????????=>?'image/x-cmu-raster',
????'pnm'????????=>?'image/x-portable-anymap',
????'pbm'????????=>?'image/x-portable-bitmap',
????'pgm'????????=>?'image/x-portable-graymap',
????'ppm'????????=>?'image/x-portable-pixmap',
????'rgb'????????=>?'image/x-rgb',
????'xbm'????????=>?'image/x-xbitmap',
????'xpm'????????=>?'image/x-xpixmap',
????'xwd'????????=>?'image/x-xwindowdump',
????'igs'????????=>?'model/iges',
????'iges'????????=>?'model/iges',
????'msh'????????=>?'model/mesh',
????'mesh'????????=>?'model/mesh',
????'silo'????????=>?'model/mesh',
????'wrl'????????=>?'model/vrml',
????'vrml'????????=>?'model/vrml',
????'css'????????=>?'text/css',
????'html'????????=>?'text/html',
????'htm'????????=>?'text/html',
????'asc'????????=>?'text/plain',
????'txt'????????=>?'text/plain',
????'rtx'????????=>?'text/richtext',
????'rtf'????????=>?'text/rtf',
????'sgml'????????=>?'text/sgml',
????'sgm'????????=>?'text/sgml',
????'tsv'????????=>?'text/tab-separated-values',
????'wml'????????=>?'text/vnd.wap.wml',
????'wmls'????????=>?'text/vnd.wap.wmlscript',
????'etx'????????=>?'text/x-setext',
????'xsl'????????=>?'text/xml',
????'xml'????????=>?'text/xml',
????'mpeg'????????=>?'video/mpeg',
????'mpg'????????=>?'video/mpeg',
????'mpe'????????=>?'video/mpeg',
????'qt'????????=>?'video/quicktime',
????'mov'????????=>?'video/quicktime',
????'mxu'????????=>?'video/vnd.mpegurl',
????'avi'????????=>?'video/x-msvideo',
????'movie'????????=>?'video/x-sgi-movie',
????'ice'????????=>?'x-conference/x-cooltalk',
);?