Java 验证码 不显示图片

大家好，我用了一个网上的产生登录验证码的类。已经部署好了，但是运行的时候图片没有产生，是一个红叉。我把这个类贴出来，大家帮看看，浪费大家时间了，谢谢！import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.awt.*; import java.awt.image.*; import javax.imageio.ImageIO;  public class CheckCodeServlet extends HttpServlet {        private static int WIDTH = 60;        private static int HEIGHT = 20;        public void doGet(HttpServletRequest request,HttpServletResponse response)                      throws ServletException,IOException        {                          HttpSession session = request.getSession();               response.setContentType("image/jpeg");               ServletOutputStream sos = response.getOutputStream();                // 设置浏览器不要缓存此图片               response.setHeader("Pragma","No-cache");               response.setHeader("Cache-Control","no-cache");               response.setDateHeader("Expires", 0);                             // 创建内存图象并获得其图形上下文               BufferedImage image =                      new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);               Graphics g = image.getGraphics();                             // 产生随机的认证码               char [] rands = generateCheckCode();                             // 产生图像               drawBackground(g);               drawRands(g,rands);                             // 结束图像 的绘制 过程， 完成图像               g.dispose();                             
                  // 将图像输出到客户端               ByteArrayOutputStream bos = new ByteArrayOutputStream();               ImageIO.write(image, "JPEG", bos);               byte [] buf = bos.toByteArray();               response.setContentLength(buf.length);               // 下面的语句也可写成： bos.writeTo(sos);               sos.write(buf);               bos.close();               sos.close();                // 将当前验证码存入到 Session 中               session.setAttribute("check_code",new String(rands));               // 直接使用下面的代码将有问题， Session 对象必须在提交响应前获得               //request.getSession().setAttribute("check_code",new String(rands));        }               private char [] generateCheckCode()        {               // 定义验证码的字符表               String chars = "0123456789abcdefghijklmnopqrstuvwxyz";               char [] rands = new char[4];               for(int i=0; i<4; i++)               {                      int rand = (int)(Math.random() * 36);                      rands[i] = chars.charAt(rand);               }               return rands;        }               private void drawRands(Graphics g , char [] rands)        {               g.setColor(Color.BLACK);               g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));               // 在不同的高度上输出验证码的每个字符                        g.drawString("" + rands[0],1,17);               g.drawString("" + rands[1],16,15);               g.drawString("" + rands[2],31,18);               g.drawString("" + rands[3],46,16);               System.out.println(rands);        }               private void drawBackground(Graphics g)        {              // 画背景               g.setColor(new Color(0xDCDCDC));               g.fillRect(0, 0, WIDTH, HEIGHT);               // 随机产生 120 个干扰点               for(int i=0; i<120; i++)               {                      int x = (int)(Math.random() * WIDTH);                      int y = (int)(Math.random() * HEIGHT);                      int red = (int)(Math.random() * 255);                      int green = (int)(Math.random() * 255);                      int blue = (int)(Math.random() * 255);                      g.setColor(new Color(red,green,blue));                             g.drawOval(x,y,1,0);               }        } }