利用servlet产生随机数,原理是获取Graphics对象进行绘图

public class ResonpeRandomImgDemo extends HttpServlet {

   int width=100;  

int height=30;

 public void doGet(HttpServletRequest request, HttpServletResponse response)    throws ServletException, IOException {

  response.setContentType("text/html;charset=utf-8");   

//获取BufferedImage对象，并设置画布的大小还有类型   

BufferedImage img=new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);   

//获取绘图对象   

Graphics2D g =(Graphics2D)img.getGraphics();   

//设置背景颜色   

setBackgroudColor(g);  

 //产生随机数   

getNum(g);   

//把图形写给浏览器   

response.setContentType("image/jpeg");   

ImageIO.write(img, "jpg", response.getOutputStream());  

}

   private void getNum(Graphics2D g) {   

String str="0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM";   

g.setColor(Color.BLUE);   

for (int i = 0; i < 4; i++) {    

//产生随机数，%30为了产生随机数-30-30之间,这个也是旋转角度    

int degee=new Random().nextInt()%30;    

//进行旋转    

g.rotate(degee*Math.PI/180 , 20+(i*12), 15);   

 g.setFont(new Font("宋体",Font.BOLD, 20));   

 g.drawString(str.charAt(new Random().nextInt(str.length()))+"", 20+(i*12), 15);    

//旋转回来    

g.rotate(-degee*Math.PI/180 , 20+(i*12), 15);   

}  

}

 private void setBackgroudColor(Graphics2D g) {  

 g.setBackground(Color.WHITE);  

 g.fillRect(0, 0, width, height);  

}

 public void doPost(HttpServletRequest request, HttpServletResponse response)    throws ServletException, IOException {

  doGet(request, response);  

}

}

//jsp页面

<script type="text/javascript">
  function changeImg(){
   var img= document.getElementById("img");
   img.src=img.src+"?";
  }
 </script>

验证码:<img alt="" src="/ServletTest/servlet/ResonpeRandomImgDemo" id="img" onclick="changeImg()" style="cursor: pointer;">///ServletTest/servlet/ResonpeRandomImgDemo获取servelt页面的信息