JavaWeb 使用Session实现一次性验证码功能

<form action="loginServlet" method="post">
 请输入验证码：<input type="text" name="code" />
 <img src="getCodeServlet" /><br />
 <button type="submit">提交</button>
</form>

@WebServlet("/getCodeServlet")
public class GetCodeServlet extends HttpServlet {
  //验证码的宽、高
  private static int WIDTH=80;
  private static int HEIGHT=25;
  //绘制背景
  private void drawBg(Graphics g){
    //rgb
    g.setColor(new Color(128, 128, 128));
    //绘制矩形。x,y,wigth,height
    g.fillRect(0,0,WIDTH,HEIGHT);
    //随机绘制100个干扰点
    Random random=new Random();
    for (int i=0;i<100;i++){
      //产生(0,1）上的小数，*WIDTH|HEIGHT，再取整也行
      int x=random.nextInt(WIDTH);
      int y=random.nextInt(HEIGHT);
      g.drawOval(x,y,1,1);
      //干扰点的颜色也可以随机，随机产生red，green，blue即可
      //g.setColor(new Color(red,green,blue));
    }
  }
  //绘制验证码
  private void drawCode(Graphics g,char[] code){
    g.setColor(Color.BLACK);
    //字体、样式（多个时竖线分隔）、字号
    g.setFont(new Font("serif",Font.ITALIC|Font.BOLD,18));
    //在不同位置绘制验证码字符，参数：要绘制的String、横、纵坐标。+""是为了char转String。
    g.drawString(code[0]+"",1,17);
    g.drawString(code[1]+"",16,15);
    g.drawString(code[2]+"",31,18);
    g.drawString(code[3]+"",46,16);
  }
  //随机产生4位验证码
  private char[] getCode(){
    String chars="0123456789QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm";
    char[] code=new char[4];
    Random random=new Random();
    for (int i=0;i<4;i++){
      //[0,62)
      int index= random.nextInt(62);
      code[i]=chars.charAt(index);
    }
    return code;
  }
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
    HttpSession session = request.getSession();
    ServletOutputStream sos = response.getOutputStream();
    response.setContentType("image/jpeg");
    //设置浏览器不缓存此图片
    response.setHeader("Pragma","No-cache");
    response.setHeader("Cache-Control","no-cache");
    response.setDateHeader("Expires",0);
    //创建内存图片
    BufferedImage bufferedImage = new BufferedImage(WIDTH, HEIGHT, TYPE_INT_RGB);
    Graphics g= bufferedImage.getGraphics();
    char[] code=getCode();
    //将验证码放到session域中。session对象要在提交响应之前获得
    session.setAttribute("code",new String(code));
    drawBg(g);
    drawCode(g,code);
    g.dispose();
    //将图片输出到浏览器
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ImageIO.write(bufferedImage,"JPEG",baos);
    baos.writeTo(sos);
    baos.close();
    sos.close();
  }
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    doPost(request,response);
  }
}

@WebServlet("/loginServlet")
public class LoginServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html;charset=utf-8");
    HttpSession session = request.getSession();
    String trueCode= (String) session.getAttribute("code");
    String code=request.getParameter("code");

    if (code.equals(trueCode)){
      response.getWriter().write("验证码正确");
    }
    else {
      response.getWriter().write("验证码错误");
    }
  }

  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request,response);
  }
}

//先转换为全大写|全小写，再判断
    trueCode=trueCode.toLowerCase();
    code=code.toLowerCase();
    //trueCode=trueCode.toUpperCase();
    //code=trueCode.toUpperCase();