如题，以下是代码

public class OAuthServlet extends HttpServlet {    
   private static final long serialVersionUID = -1847238807216447030L;    
   public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding(
        request.setCharacterEncoding("utf-8");
        response.setCharacterEncoding("utf-8");       
 // 用户同意授权后，能获取到code
String code = request.getParameter("code");
        String state = request.getParameter("state");        
        // 用户同意授权
if (!"authdeny".equals(code)) {            
      // 获取网页授权access_token
WeixinOauth2Token weixinOauth2Token = AdvancedUtil.getOauth2AccessToken("wxe34a90ac7bxxcab85c", "1207d566090a8344xxxd6224c02c", code);            
// 网页授权接口访问凭证
String accessToken = weixinOauth2Token.getAccessToken();           
 // 用户标识
String openId = weixinOauth2Token.getOpenId();            
// 获取用户信息
SNSUserInfo snsUserInfo = AdvancedUtil.getSNSUserInfo(accessToken, openId);           
 // 设置要传递的参数
request.setAttribute("snsUserInfo", snsUserInfo);
            request.setAttribute("state", state);
        }        // 跳转到index.jsp
        request.getRequestDispatcher("index.jsp").forward(request, response);
    }
}
第一次可以获取到信息，但是第二次就code已被使用

怎么改才能让code不被重复使用，求具体代码