jackson用法

@GetMapping("findUserByUsername.action")
    public Object findUserByUsername(String username) throws JsonProcessingException {
        List<User> list=userService.findUserByUsername(username);
        HashMap<String,Object> hashMap=new HashMap<>();
        //引入mapper，jackson都是以他为核心
        ObjectMapper mapper = new ObjectMapper();
        if (list.size()!=0){
            hashMap.put("end","Ok");
            hashMap.put("data",list.get(0));
        }else {
            hashMap.put("end","err");
            hashMap.put("data",null);
        }
        return mapper.writeValueAsString(hashMap);
    }