AJAX中Get请求报错404的原因以及解决办法

function login(){
        //获取数据
        var username = $("#username").val();
        var password = $("#password").val();
        console.log(username)
        console.log(password)
        //校验参数
        if(isEmpty(username)){
            $("#sp").html("用户名不能为空");
            return;
        }

        if(isEmpty(password)){
            $("#sp").html("密码不能为空");
            return;
        }

        //发送请求
        $.ajax({
            type:'get',
            /*url:'../userlogin',*/
            url:'userlogin',
            data:{
                username:username,
                password:password
            },
            dataType:'json',
            success:function(data){
                if(data.code == 1){ //操作成功
                    //跳转
                    console.log(data);
                    alert("登录成功")
                    window.location.href = "../index.jsp";
                }else{ //操作失败
                    console.log(data);
                    $("#sp").html(data.msg);
                }
            }
        });

    }

    //非空校验
    function isEmpty(str){
        if(str == undefined || str.trim() == ""){
            return true;
        }
        return false;
    }

response.setContentType("application/json;charset=UTF-8");

        //创建一个返回值对象
        ResultInfo resultInfo = new ResultInfo();
        resultInfo.setCode(1);

        //接收参数
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        System.out.println(username+"----"+password);

        //参数校验
        if(username == null || "".equals(username.trim())){
            resultInfo.setCode(0);
            resultInfo.setMsg("账号不存在");
        }
        if(password == null || "".equals(password.trim())){
            resultInfo.setCode(0);
            resultInfo.setMsg("账号不存在");
        }

        if(!"zhangsan".equals(username)){
            resultInfo.setCode(0);
            resultInfo.setMsg("用户不存在");

        }
        if("zhangsan".equals(username)&&!"123456".equals(password)){

            resultInfo.setCode(0);
            resultInfo.setMsg("密码有误");
        }


        //将数据转换成json格式的数据
        String ri = JSON.toJSONString(resultInfo);

        //写出json数据
        response.getWriter().write(ri);

@WebServlet("/userlogin")

        //发送请求
        $.ajax({
            type:'get',
            url:'../userlogin',
            /*url:'userlogin',*/
            data:{
                username:username,
                password:password
            },
            dataType:'json',