public class ValidateServlet extends HttpServlet {
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  HttpSession session = request.getSession();
  // 设置响应格式
  response.setContentType("image/jpeg");
  // 定义一个BufferedImage对象 其构造函数属性参照API
  BufferedImage image = new BufferedImage(60, 20,
    BufferedImage.SCALE_SMOOTH);
  // 根据上述对象产生一个Graphics2D上下文
  Graphics2D graphics = image.createGraphics();
  // 将上下文颜色设置为白色
  graphics.setColor(Color.WHITE);
  graphics.fillRect(0, 0, 100, 20);
  // 设置书写字体
  graphics.setFont(new Font("微软雅黑", Font.CENTER_BASELINE, 16));
  // randomString方法见后面，负责产生由4个字符组成的字符串
  String randomStr = randomString(4);
  for (int i = 0; i < randomStr.length(); i++) {
   // 随机设置上下文颜色
   graphics.setColor(new Color(new Random().nextInt(50), new Random()
     .nextInt(50), new Random().nextInt(50)));
   // 按字符把其画出来
   graphics.drawString(randomStr.substring(i, i + 1), 15 * i, 20);
   // 产生干扰线
   graphics.drawLine(5, 5, 5, 5);
  }
  /*将验证码保存在Session中备用*/
  session.setAttribute("ValidateCode", randomStr);
  // 禁止缓冲代码，没有这段的话则不能进行更新显示，删除效果请自行测试
  response.setHeader("Prama", "no-cache");
  response.setHeader("Coche-Control", "no-cache");
  response.setDateHeader("Expires", 0);
  // 写入到客户端
  ImageIO.write(image, "jpeg", response.getOutputStream());
 }

 public String randomString(int number) {
  // 字符串的内容组成
  String str = "1234567890";
  char[] chars = str.toCharArray();
  int length = chars.length;
  StringBuilder sb = new StringBuilder();
  Random random = new Random();
  // 随机生成number个字符
  for (int i = 0; i < number; i++) {
   // 把单个字符转换成字符串然后追加到已经产生的字符串中
   sb.append(new String(new Character(chars[random.nextInt(length)])
     .toString()));
  }
  return sb.toString();
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }
}
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 <servlet-mapping>
    <servlet-name>ValidateServlet</servlet-name>
    <url-pattern>/servlet/ValidateServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>

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请输入验证码<input size=4><img src="servlet/ValidateServlet">