验证码实现（servlet）

代码实现（记得添加依赖servlet-api.jar）

public class ImageServlet extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		//浏览器1s刷新一次
		resp.setHeader("refresh", "1");
		//内存中创建一张图片
		BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
		Graphics2D g = (Graphics2D)image.getGraphics();//笔
		//设置图片的背景颜色
		g.setColor(Color.white);
		g.fillRect(0, 0, 80, 20);
		//给图片写数据
		g.setColor(Color.BLUE);
		g.setFont(new Font(null, Font.BOLD, 20));
		g.drawString(makeNum(), 0, 20);
		
		//告诉浏览器，这个请求用图片的方式打开
		resp.setContentType("image/jpeg");
		//网站存在缓存，不让浏览器缓存
		resp.setDateHeader("expires", -1);
		resp.setHeader("Cache-Control", "no-cache");
		resp.setHeader("Progma", "no-cache");
		
		//把图片写给浏览器
		ImageIO.write(image, "jpg", resp.getOutputStream());
	}
	
	//生成随机数，永远保证七位，for是为了保证不足七位的时候用0补足。
	private String makeNum(){
		Random random = new Random();
		String num = random.nextInt(9999999)+"";
		StringBuffer sb = new StringBuffer();
		for(int i = 0; i < 7 - num.length(); i ++){
			sb.append("0");
		}
		num = sb.toString() + num;
		return num;
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doGet(req, resp);
	}
}

在web.xml中配置

 <servlet>
  	<servlet-name>ImageServlet</servlet-name>
  	<servlet-class>com.servlet.ImageServlet</servlet-class>
  </servlet>
  <servlet-mapping>
  	<servlet-name>ImageServlet</servlet-name>
  	<url-pattern>/img</url-pattern>
  </servlet-mapping>

测试访问