JSP与Servlet(1) - 第一个servlet

什么是servlet ?

一个 servlet 就是 Java 编程语言中的一个类 , 是用 Java 编写的 Server 端程序 . 客户机提交 ” 请求 ” 到服务器 , 服务器中的 servlet 对请求进行处理 , 将处理的结果反馈回客户机 ( 即 ” 响应 ” 客户机的请求 ), 这就是 servlet 在 “ 请求 - 响应 ” 编程模型中扮演的角色 , 它被用来扩展服务器的性能。服务器上驻留着可以通过 “ 请求 - 响应 ” 编程模型来访问的应用程序。虽然 servlet 可以对任何类型的请求产生响应，但通常只用来扩展 Web 服务器中的应用程序。

编写并运行你的Servlet！
当一个servlet已经写好怎样来运行测试呢？下面通过一个例子详细的说说，这样会有一个感性的把握。我用的运行环境是Tomcat5.0
首先我们来写一个最简单的servlet: 命名为HelloServlet.java

package test;
import java.io. * ;
import javax.servlet. * ;
import javax.servlet.http. * ;

public class HelloServlet extends HttpServlet{
public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException{
response.setContentType( " text/html;charset=UTF-8 " );
response.setCharacterEncoding( " UTF-8 " );

PrintWriter out = response.getWriter();
out.println( " <HTML> " );
out.println( " <BODY> " );
out.println( " <p>Hello!这是我的第一个Java Servlet程序。</p> " );
out.println( " </BODY> " );
out.println( " </HTML> " );
}
}

把编译好的HelloServlet.class文件放到\Tomcat文件夹\webapps\ourappfiles\WEB-INF\classes\test的文件夹下。

接着编写我们的web.xml文件。

<? xml version="1.0" encoding="UTF-8" ?>
<! DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd" >
< web-app >
< servlet >
   < servlet-name > helloservlet </ servlet-name >
   < servlet-class > test.HelloServlet </ servlet-class >
</ servlet >
< servlet-mapping >
   < servlet-name > helloservlet </ servlet-name >
   < url-pattern > /Hello </ url-pattern >
</ servlet-mapping >
</ web-app >

把编写好的web.xml文件放到\Tomcat文件夹\webapps\ourappfiles\WEB-INF下。
好了，现在万事俱备，就差启动Tomcat运行我们的Servlet了。
在IE地址栏中输入http://localhost:8080/ourappfiles/Hello，就可以看到结果了。
以上就是运行Servlet的几个步骤。

通过这个例子可以看出纯粹的Servlet编程的缺点.所有的内容------甚至是基本的HTML格式------都必须由Servlet输出,你不得不在Servlet中放置大量的print语句来结束,唯一的目的就是向用户返回内容.另外,甚至最简单的HTML格式变化都必须在JAVA源代码中解决,这意味着非JAVA开发人员不能进行WEB站点的设计.

为了克服上述缺点, JavaServer Pages(JSP)应运而生. JSP表面上看是"嵌入了Java 的HTML", 而实际上JSP是"嵌入了HTML的Java Servlet ". 要明白这一点,需要了解一下JSP是如何响应请求的.

当浏览器向服务器请求一个JSP页面(A.JSP)时, 源页面(A.JSP)通过转换器(在TOMCAT中,这个转换器叫做Jasper)被转换成一个Java类(如:A_jsp.java),该类继承于org.apache.jasper.runtime.HttpJspBase基类，而HttpJspBase实现了HttpServlet接口。可见，JSP在运行前首先被编译为一个Servlet, 这个Servlet对请求进行处理,将结果内容发送回客户端.所以说, JSP是"嵌入了HTML的Java Servlet ", 其本质就是Servlet.

如果将上面的servlet程序中的doGet方法写成下面的样子:

public void doGet(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
        response.setContentType( " application/vnd.ms-excel " );
        PrintWriter out = response.getWriter();
        out.println( " \tQ1\tQ2\tQ3\tQ4\tTotal " );
        out.println( " Apples\t78\t87\t92\t29\t=SUM(B2:E2) " );
        out.println( " Oranges\t77\t86\t93\t30\t=SUM(B3:E3) " );
    }

运行后,就可以输出一个excel文件啦!

附: 对 Content-type的详细说明:

Content-type 的说明 <?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' ,
) ;

JSP与Servlet(1) - 第一个servlet

什么是servlet ?

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