servlet给前端传递json参数时，前端js没有获取到

Question

前端js：

    $.ajax({
        type: "get",//请求类型
        url: path + "/jsp/user.do",//请求的url
        data: {method: "rolelist"},//请求参数
        dataType: "json",//ajax接口（请求url）返回的数据类型
        contentType:"application/json;charset=utf-8",
        async:false,
        success: function (data) {//data：返回数据（json对象）
            alert(data);
            if (data != null) {
                userRole.html("");
                var options = "<option value=\"0\">请选择</option>";
                for (var i = 0; i < data.length; i++) {
                    alert(data[i].id);
                    alert(data[i].roleName);
                    options += "<option value=\"" + data[i].id + "\">" + data[i].roleName + "</option>";
                }
                userRole.html(options);
            }
        },
        error: function (data) {//当访问时候，404，500 等非200的错误状态码
            validateTip(userRole.next(), {"color": "red"}, imgNo + " 获取用户角色列表error", false);
        }
    })

后端servlet：

public void rolelist(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException, SQLException {
        System.out.println("rolelist");
        RoleServiceImpl roleService = new RoleServiceImpl();
        List<Role> roleList = roleService.getRoleList();
        req.setAttribute("RoleList",roleList);

        Map<String, String> roleMap = new HashMap<String, String>();
        for (Role role : roleList) {
            roleMap.put(String.valueOf(role.getId()),role.getRoleName());
        }

        resp.setContentType("application/json");
        PrintWriter writer = resp.getWriter();
        writer.write(JSONArray.toJSONString(roleMap));
        writer.flush();//刷新
        writer.close();//关闭
        System.out.println(JSONArray.toJSONString(roleMap));//这能输出预想参数
        System.out.println(writer);
    }