Sessionでユーザーログインを完了し、フォームの重複コミットを防止します.

 //     
   String base = "\u7684\u4e00\u4e86\u662f\u6211\u4e0d\u5728\u4eba\u4eec\u6709\u6765\u4ed6\u8fd9\u4e0a\u7740";
        // 4  
  Color c = Color.BLUE;
  g.setColor(c);
  Font font = new Font("  ",Font.ITALIC, 20);
        g.setFont(font);
        
        StringBuffer sb = newStringBuffer();
        //    
        for(int i=0;i<4;i++){
         int location = newRandom().nextInt(base.length()-1);
      char chr = base.charAt(location);
      sb.append(chr+"");
        }
        checkCode=sb.toString();
        Graphics2D gd = (Graphics2D) g;
        gd.rotate(0.05);
        //        
        g.drawString(checkCode, 10, 20);

 }

 

 

 

public static  voidoutImage(OutputStream os) throws IOException{
  //    
  BufferedImage image = new BufferedImage(WIDTH, HEIGHT, IMAGETYPE);
  //    
  Graphics g = image.getGraphics();
  //        
  setBackground(g);
  //       
  setBorder(g);
  //        
  setRandomLine(g);
  // 7、       
  setFont(g);
  // 8、        
  ImageIO.write(image, "gif", os);
 }


//       

      response.setIntHeader("expires", 0);

      response.setHeader("Cache-Control", "no-cache");

      response.setHeader("Pragma", "no-cache");

      

      //             

      response.setHeader("Content-Type", "image/jpeg");