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();