Code:
/ / send an asynchronous request for registration

    $.ajax({
        type:"post",
        url :"/user/register",
        data:$("form").serialize(),
        dataType:"json",
        success:function(data){
            alert(data);
            if(data == "success"){
                alert("");
                window.location.href="/";//
            }else if(data == "fail"){
                alert(":"+data);
            }

        },
        error:function(data){
            alert("error:"+data);
            if(data == "error")
            alert(""+data);
            window.location.href="/";
        }
    });

@ RequestMapping (value = "/ user/register", method = RequestMethod.POST)

public ModelMap register(User user){
    ModelMap map = new ModelMap();
    try {
        System.err.print(user.toString());
        int result = userService.register(user);
        if (result > 0) {
            //
            map.put("res","success");
        }else {
            map.put("res","fail");
        }
        return map;
    }catch (Exception e){
        e.printStackTrace();
        map.put("data","error");
        return map;
    }
}

and window.location.href= "/"; this line of code is not executed because it does not jump

the database inserts data and does not throw an exception, so why error?