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演示：点击评论栏目的用户名文本框 即可显示 验证码http://www.javawind.net/5b016f381a9e3f16011a9ebe0b570002.jhtml

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package org.javawind.servlet;

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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

 public class SysCode extends javax.servlet.http.HttpServlet implements javax.servlet.Servlet {
 private static final long serialVersionUID = -5981700888850353189L;
 public SysCode() {
  super();
 }    

 @SuppressWarnings("static-access")
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  // TODO Auto-generated method stub
     response.setContentType("image/jpeg"); //必须设置ContentType为image/jpeg
     response.setHeader("Pragma","No-cache");
     response.setHeader("Cache-Control","no-cache");
     response.setDateHeader("Expires",0);
     //设置图片的长宽  验证码长度
     int width=60, height=20,len=4;
     String base = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
     int length = base.length();
     //创建内存图像
     BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
     //获取图形上下文
     Graphics g = image.getGraphics();
     //创建随机类的实例
     Random random = new Random();
     //设定图像背景色(因为是做背景，所以偏淡)
     g.setColor(getRandColor(random,200,250));
     g.fillRect(0, 0, width, height);
     //备选字体
     String[] fontTypes = {"tahoma","Atlantic Inline","fantasy","Times New Roman","Georgia","Arial", "Helvetica", "sans-serif","System"};
     int fontTypesLength = fontTypes.length;
     //在图片背景上增加噪点
     g.setColor(getRandColor(random,160,200));
     g.setFont(new Font("Times New Roman",Font.PLAIN,12));
     for (int i=0;i<6;i++)
     {
       g.drawString("!@#$%^,.;'[javawind.net]/<&*()>:5277",0,5*(i+2));
     }
    
     String sRand="",pStr="";
     for (int i=0;i<len;i++)
     {
       int start = random.nextInt(length);
       String rand=base.substring(start,start+1);
       sRand+=rand;      
       //设置字体的颜色
       g.setColor(getRandColor(random,10,150));
       //设置字体
       g.setFont(new Font(fontTypes[random.nextInt(fontTypesLength)],Font.BOLD,16));
       //将随机验证码画到图片上
       //g.drawString(rand,15*i,18);
       pStr = sRand.substring(i,i+1);
       if(i==0){
        g.drawString(pStr,2,14);
       }
       if(i==1){
        g.drawString(pStr,15,16);
       }
       if(i==2){
        g.drawString(pStr,30,15);
       }
       if(i==3){
        g.drawString(pStr,45,13);
       } 
     }
     
     //将认证码存入session
     request.getSession().setAttribute("SystemCode",sRand);
    
     g.dispose();
     //输出图象到页面
     ImageIO.write(image,"JPEG",response.getOutputStream());
 } 

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  // TODO Auto-generated method stub
 }
 
 protected Color getRandColor(Random random,int fc,int bc)
 {
  if(fc>255) fc=255;
  if(bc>255) bc=255;
  int r=fc+random.nextInt(bc-fc);
  int g=fc+random.nextInt(bc-fc);
  int b=fc+random.nextInt(bc-fc);
  return new Color(r,g,b);
 }
}

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这个数据保存的是 区分大小写的，如果不要区分大小写，这里改一下：

//将认证码存入session 
sRand＝sRand.toLowerCase(); // 把验证码转换成小写保存
request.getSession().setAttribute("SystemCode",sRand);

然后 获取表单验证码的时候 也要 toLowerCase() 再比较