getWriter() has already been called for this response异常的一些问题

今天在项目中遇到一个问题，一个接口请求，返回了getWriter() has already been called for this response异常信息。

代码

 @RequestMapping("/lan")
    @FiledFilter(includes = {"userId"},encode = false)
    public Map test(HttpServletRequest request, HttpServletResponse response) throws IOException {
        List<User> userList = new ArrayList<>();
        User user = new User();
        user.setUserId(1);
        user.setUserName("mck");
        user.setPassword("123");
        userList.add(user);
        Map<String,Object> result = new HashMap<>();
        result.put("code","200");
        result.put("msg","成功");
        result.put("user",userList);
        response.getWriter().print(JSON.toJSONString(result));
      /*  ServletOutputStream outputStream = response.getOutputStream();
        outputStream.write(JSON.toJSONString(result).getBytes());*/
        //int g = 0/0;
        return result;
    }

百度了之后发现了问题，当一个请求中已经手动调用response的getWriter方法获取到PrintWriter之后，如果还存在返回值，servlet在执行返回的时候发现这个东西bei调用过，就会觉得是逻辑混乱，就会抛出异常了

解决方法很简单，要么没有返回值，要么不手动调用就可以了

还发现一种比较好玩的事情，如果非要两者都存在的情况怎么办呢！见代码

   @RequestMapping("/lan")
    @FiledFilter(includes = {"userId"},encode = false)
    public Map test(HttpServletRequest request, HttpServletResponse response) throws IOException {
        List<User> userList = new ArrayList<>();
        User user = new User();
        user.setUserId(1);
        user.setUserName("mck");
        user.setPassword("123");
        userList.add(user);
        Map<String,Object> result = new HashMap<>();
        result.put("code","200");
        result.put("msg","成功");
        result.put("user",userList);
        //response.getWriter().print(JSON.toJSONString(result));
        ServletOutputStream outputStream = response.getOutputStream();
        outputStream.write(JSON.toJSONString(result).getBytes());
        //int g = 0/0;
        return result;
    }

使用outputStream就可以了，但是要注意一个问题，这样写的结果是什么呢！！！

结果就是输出流中的内容和返回值都出现在客户端了。。。