Random dynamic generation of verification code in java

public class CheckCodeServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {

	//服务器通知浏览器不要缓存
	response.setHeader("pragma","no-cache");
	response.setHeader("cache-control","no-cache");
	response.setHeader("expires","0");
	
	//在内存中创建一个长80，宽30的图片，默认黑色背景
	//参数一：长
	//参数二：宽
	//参数三：颜色
	int width = 80;
	int height = 30;
	BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
	
	//获取画笔
	Graphics g = image.getGraphics();
	//设置画笔颜色为灰色
	g.setColor(Color.GRAY);
	//填充图片
	g.fillRect(0,0, width,height);
	
	//产生4个随机验证码，12Ey
	String checkCode = getCheckCode();
	//将验证码放入HttpSession中
	request.getSession().setAttribute("CHECKCODE_SERVER",checkCode);
	
	//设置画笔颜色为黄色
	g.setColor(Color.YELLOW);
	//设置字体的小大
	g.setFont(new Font("黑体",Font.BOLD,24));
	//向图片上写入验证码
	g.drawString(checkCode,15,25);
	
	//将内存中的图片输出到浏览器
	//参数一：图片对象
	//参数二：图片的格式，如PNG,JPG,GIF
	//参数三：图片输出到哪里去
	ImageIO.write(image,"PNG",response.getOutputStream());
}
/**
 * 产生4位随机字符串 
 */
private String getCheckCode() {
	String base = "0123456789ABCDEFGabcdefg";
	int size = base.length();
	Random r = new Random();
	StringBuffer sb = new StringBuffer();
	for(int i=1;i<=4;i++){
		//产生0到size-1的随机值
		int index = r.nextInt(size);
		//在base字符串中获取下标为index的字符
		char c = base.charAt(index);
		//将c放入到StringBuffer中去
		sb.append(c);
	}
	return sb.toString();
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	this.doGet(request,response);
}

}