如何使用Spring MVC构建简单的计数器

Question

我想基于与Spring MVC的会话构建一个简单的计数器，当我重新加载页面时该计数器增加一。下面是我到目前为止尝试过的代码：

@Controller
@SessionAttributes("counter")
public class DemoController {

    @RequestMapping(value="/", method=RequestMethod.GET)
    public ModelAndView home(@ModelAttribute Counter c) {

        System.out.println(c);
        c.setCounter(c.getCounter+1);
        ModelAndView mv = new ModelAndView();
        mv.addObject("counter", c);
        mv.setViewName("home");
        return mv;
    }
}

但是我总是得到0 ..

如果我使用HttpServletRequest和HttpSession，则在Spring MVC中，尝试访问会话属性时会出错。

    @RequestMapping(value="/", method=RequestMethod.GET)
    public ModelAndView home(HttpServletRequest req) {

        if (req.getSession().isNew()) {
            req.getSession().setAttribute("counter", new Integer(1));
        } else {
// java.lang.NullPointerException: null
            int counter = (int) req.getSession().getAttribute("counter");
            System.out.println(counter);
        }

谢谢

编辑：以下代码有效：

@RequestMapping(value="/", method=RequestMethod.GET)
public ModelAndView home(@CookieValue(value="counter", defaultValue="0") Long counter, HttpServletResponse res) {

    counter++;

    Cookie cookie = new Cookie("counter", counter.toString());
    res.addCookie(cookie);

    ModelAndView mv = new ModelAndView();
    mv.addObject("messaggio", "hellooo "+counter);
    mv.setViewName("home");
    return mv;
}