jsp验证码问题

1.ImageServlet类的实现：package com.mobile.control;import java.awt.Color;import java.awt.Font;import java.awt.Graphics;import java.awt.image.BufferedImage;import java.io.IOException;import java.util.Random;import javax.imageio.ImageIO;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ImageServlet extends HttpServlet {       public void doGet(HttpServletRequest request, HttpServletResponse response)                     throws ServletException, IOException {              this.doPost(request, response);       }       // 生成数字和字母的验证码       public void doPost(HttpServletRequest request, HttpServletResponse response)                     throws ServletException, IOException {              BufferedImage img = new BufferedImage(68, 22,                            BufferedImage.TYPE_INT_RGB);              // 得到该图片的绘图对象              Graphics g = img.getGraphics();              Random r = new Random();              Color c = new Color(200, 150, 255);              g.setColor(c);              // 填充整个图片的颜色              g.fillRect(0, 0, 68, 22);              // 向图片中输出数字和字母              StringBuffer sb = new StringBuffer();              char[] ch = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();              int index, len = ch.length;              for (int i = 0; i < 4; i ++) {                     index = r.nextInt(len);                     g.setColor(new Color(r.nextInt(88), r.nextInt(188), r.nextInt(255)));                     g.setFont(new Font("Arial", Font.BOLD | Font.ITALIC, 22));// 输出的字体和大小                                       g.drawString("" + ch[index], (i * 15) + 3, 18);//写什么数字，在图片的什么位置画                     sb.append(ch[index]);              }              request.getSession().setAttribute("piccode", sb.toString());              ImageIO.write(img, "JPG", response.getOutputStream());       }     }2.web.xml中的配置       <servlet>              <servlet-name>ImageServlet</servlet-name>              <servlet-class>com.mobile.control.ImageServlet</servlet-class>       </servlet>       <servlet-mapping>              <servlet-name>ImageServlet</servlet-name>              <url-pattern>/servlet/ImageServlet</url-pattern>       </servlet-mapping>
3.页面源程序<%@ page contentType="text/html; charset=utf-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>       <head>              <title>系统登录</title>              <meta http-equiv="pragma" content="no-cache">              <meta http-equiv="cache-control" content="no-cache">              <meta http-equiv="expires" content="0">              <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">              <meta http-equiv="description" content="This is my page">                                         <link href="<%=request.getContextPath()%>/jsp/style/stylet.css"                     rel="stylesheet" type="text/css">                 <mce:script type="text/javascript"><!--                     // 重载验证码                     function reloadVerifyCode(){                            var timenow = new Date().getTime();                                                    document.getElementById("safecode").src="<%=request.getContextPath()%>/servlet/ImageServlet?d="+timenow;                     }                            // --></mce:script>       </head>              <body>              <div id="box1">                     <div id="tab_box">                            <form action="<%=request.getContextPath() %>/servlet/LoginServlet" method="post" name="login">                                   <span class="denglu_font_title">欢迎登录</span>                                   <br />                                   <br />                                   <label for="userid" class="yonghuming_font">                                          用户：                                   </label>                                   <input name="userid" type="text" id="userid" accesskey="n"                                          tabindex="1"                                          style="font-family: '宋体'; font-size: 12px; color: #575757" />                                   <br />                                   <br />                                   <label for="passwd" class="yonghuming_font">                                          密码：                                   </label>                                   <input name="passwd" type="password" id="passwd" tabindex="2"                                          style="font-family: '宋体'; font-size: 12px; color: #575757" />                                   <br />                                   <br />                                                                                  <label for="verifyCode" class="yonghuming_font">                                          验证码：                                   </label>                                   <input type="text" id="verifyCode" name="verifyCode" size="6" />                                   <img alt="验证码" id="safecode" src="<%=request.getContextPath()%>/servlet/ImageServlet">                                   <a href="javascript:reloadVerifyCode();" mce_href="javascript:reloadVerifyCode();">看不清楚</a>                                   <input name="denglu_tijiao" type="submit" id="denglu_tijiao"                                          tabindex="3" value="确认" />                                   <input name="denglu_chongzi" type="reset" id="denglu_reset"                                          tabindex="4" value="重置" />                                   <input type="hidden" name="flag" value="logon">                            </form>                            <font color='red'></font>                     </div>              </div>       </body></html>问题:这是随机生成验证的 不过里面有两句话不太懂第一句就是在  ImageServlet 类中的  ImageIO.write(img, "JPG", response.getOutputStream()); 是什么意思第二句话就在  页面源程序中的  javascript中  javascript是点 看不清时 切换另一张图片
document.getElementById("safecode").src="<%=request.getContextPath()%>/servlet/ImageServlet?d="+timenow;为什么要在  request.getContextPath()%>/servlet/ImageServlet 后面中加个d 而这个  d 在 ImageServlet 类中 又没用到 少了d  javascript又出错  而且可以换别的字母代替 所以不知道加这个d是干嘛用的

解决方案 »

  1.   

    顺便问一下 在doget中为什么调用dopost HttpServlet 老师没教我们  我看了有点晕
      

  2.   

    验证码可以用一个jcaptcha是做验证码的，你可以看看他的实现maven id为 com.octo.captcha:jcaptcha网址http://jcaptcha.sourceforge.net/
      

  3.   

    第一句就是在  ImageServlet 类中的  ImageIO.write(img, "JPG", response.getOutputStream()); 是什么意思 响应生成JPG格式的文件第二句话就在  页面源程序中的  javascript中  javascript是点 看不清时 切换另一张图片 
    document.getElementById("safecode").src=" <%=request.getContextPath()%>/servlet/ImageServlet?d="+timenow; 为什么要在  request.getContextPath()%>/servlet/ImageServlet 后面中加个d 而这个  d 在 ImageServlet 类中 又没用到 少了d  javascript又出错  而且可以换别的字母代替 所以不知道加这个d是干嘛用的没有用的，你试一下将 d 改为 xx ,aa ,都可以的
      

  4.   

    第一句就是在  ImageServlet 类中的  ImageIO.write(img, "JPG", response.getOutputStream()); 是什么意思 响应生成JPG格式的文件,相当于javascript中的document.write(),就是把图片打印出来
    第二句话就在  页面源程序中的  javascript中  javascript是点 看不清时 切换另一张图片 
    document.getElementById("safecode").src=" <%=request.getContextPath()%>/servlet/ImageServlet?d="+timenow; 为什么要在  request.getContextPath()%>/servlet/ImageServlet 后面中加个d 而这个  d 在 ImageServlet 类中 又没用到 少了d  javascript又出错  而且可以换别的字母代替 所以不知道加这个d是干嘛用的 应该是为了缓存设计的，对了<img />来说，相同的src，图片是不会刷新的，为了使图片刷新，所以要换一个不一样的src，大概应该是这样的