There was an unexpected error(type=Bad Request,status=400).Required String parameter'username' isnot

异常信息：

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Tue Jan 28 23:32:38 CST 2020

There was an unexpected error (type=Bad Request, status=400).

Required String parameter 'username' is not present

错误代码：

   @PostMapping("/login")
    public String login(Model model , @RequestParam("username") String username,
                       @RequestParam("password") String password,
                       @RequestParam(value = "rememberme", defaultValue = "false")
                       boolean rememberme,HttpServletResponse response) {
        Map<String, Object> map = userService.login(username, password);
            if (map.containsKey("msg")) {
            model.addAttribute("msg",map.get("msg"));
            return "reglogin";
        } else {
            if (map.containsKey("ticket")) {
                Cookie cookie = new Cookie("ticket", map.get("ticket").toString());
                cookie.setPath("/");
                if (rememberme) {
                    cookie.setMaxAge(3600 * 24 * 5);
                }
                response.addCookie(cookie);
            }
            return "home";
        }

出错原因：

如果我们在方法中定义了@RequestParam变量，如果访问的URL中不带有相应的参数，就会抛出异常。而上面的代码里用的是post请求，所以出现了异常 。 不过Spring会尝试帮我们进行绑定，然而没有成功。但有的时候，参数确实不一定永远都存在，这是我们可以通过定义required属性：

@RequestParam(name="username",required=false)
//在参数不存在的情况下，可能希望变量有一个默认值
@RequestParam(name="username",required=false,defaultValue="0")

改正后的代码：

 @PostMapping("/login")
    public String login(Model model , @RequestParam(value = "username" , required = false) String username,
                       @RequestParam(value = "password" , required = false) String password,
                       @RequestParam(value = "rememberme", defaultValue = "false") boolean rememberme,
                       HttpServletResponse response) {
        Map<String, Object> map = userService.login(username, password);
            if (map.containsKey("msg")) {
            model.addAttribute("msg",map.get("msg"));
            return "reglogin";
        } else {
            if (map.containsKey("ticket")) {
                Cookie cookie = new Cookie("ticket", map.get("ticket").toString());
                cookie.setPath("/");
                if (rememberme) {
                    cookie.setMaxAge(3600 * 24 * 5);
                }
                response.addCookie(cookie);
            }
            return "home";
        }