servlet中的方法实现代码：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;


import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;






public class IdentityServlet extends HttpServlet {
 
 
 public static final char[] CHARS={‘2‘,‘3‘,‘4‘,‘5‘,‘6‘,‘7‘,‘8‘,‘9‘,‘0‘,
                        ‘A‘,‘Q‘,‘W‘,‘E‘,‘R‘,‘T‘,‘G‘,‘D‘,‘S‘,‘W‘,‘G‘,‘H‘};//随机字符的字典
 public static Random random = new Random();//随机数
 
 public static String getRandomString(){
  //字符的缓存
  StringBuffer buf= new StringBuffer();
  for(int i=0;i<6;i++){//循环 六次
   buf.append(CHARS[random.nextInt(CHARS.length)]);
  }
  return buf.toString();
 }
 
 public static Color getRandomColor(){
  return new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));
 }
 
 public static Color getReverseColor(Color c){
  return new Color(255-c.getRed(),255-c.getGreen(),255-c.getBlue());
 }
 
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {


  response.setContentType("image/jpeg");//设置输出的类型
  String randomString =getRandomString();//得到返回的字符集
  request.getSession(true).setAttribute("randomString", randomString);
  int with=100;int hight=30;//生成图片的大小
  Color color=getRandomColor();//用于背景色
  Color reverse =getReverseColor(color);//用于前景色
  BufferedImage bi = new BufferedImage(with, hight, BufferedImage.TYPE_INT_RGB);//创建一个彩色的图片
  Graphics2D g = bi.createGraphics();//获取到绘图对象
  g.setFont(new Font(Font.SANS_SERIF,Font.BOLD,16));//设置字体
  g.setColor(color);//设置颜色
  g.fillRect(0, 0, with, hight);//绘制背景
  g.setColor(reverse);//设置颜色
  g.drawString(randomString, 18, 20);//绘制随机字符
  for(int i=0,n=random.nextInt(100);i<n;i++){//画最多100个噪音点
   g.drawRect(random.nextInt(with), random.nextInt(hight), 1, 1);//随机噪音点
  }
     ServletOutputStream  out =response.getOutputStream();
                                                         //转换图片格式
     JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
     encoder.encode(bi);//对图片进行编码
     out.flush();//输出
    
  }

<script type="text/javascript">
         function reloadImage(){
            document.getElementByIdx_x_x("btn").disabled=false;
            document.getElementByIdx_x_x("Identity").src=http://www.mamicode.com/‘servlet/IdentityServlet?ts=‘+new Date().getTime();>
效果如下：