Jackson Json Object mapper : how to map an array?

Question

I am trying to map an array from a backend api call. How can I map this data knowing that :

the following classes will be used to hold the json array data :

    @Data
    private static class UserListBean {
        private List<UserBean> userList;
    }

    @Data
    private static class UserBean {
        private String id;
        private String userName;
        private String password;
    }

the json will have the following format (the following example just have one item in it) :

[
   {
      "id":1,
      "userName":"bob",
      "password":"403437d5c3f70b1329f37a9ecce02adbbf3e986"
   }
]

I am using Jackson and I have tried the following so far but it keeps sending me back an exception

        final ObjectMapper mapper = new ObjectMapper();
        mapper.configure(Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        final ObjectReader reader = mapper.reader(UserListBean.class);
        GeoHubAccountListBean accounts = null;

        try {
            accounts = reader.readValue(jsonString);
        } catch (final IOException ex) {
            log.error("Cannot convert JSON into a list of users", ex);
        }

Here the final ObjectReader reader = mapper.reader(UserListBean.class); throws an exception

Can not deserialize instance of com.xxx.XXX$UserListBean out of START_ARRAY token

Any idea ?

thanks

Answer 1

Well, if you are trying to deserialize json to an object of type UserListBean, then you need to deserialize a JSONObject (Java Objects tend to map to JSONObjects).

Therefore, your outer most json construct should be an object. Your outer most json construct is a JSONArray.

Your UserListBean has a single field, which is a List<UserBean>. So your top level json construct (which is a JSONObject) should contain a single field with the name 'userList' with a value that is a JSONArray (Java Collections tend to map to JSONArrays).

I think this is the actual json you are looking for:

{
  "userList":[
     {
       "id":1,
       "userName":"bob",
       "password":"403437d5c3f70b1329f37a9ecce02adbbf3e986"
     }
  ]
}

If you have no control over the json coming in, then you probably want to ditch the parent object UserListBean and deal directly with the List<UserBean>, as that would work with the json you have provided.

Answer 2

Try something like the following:

public static <T> T mapFromJson(String json, Class<T> clazz) throws JsonParseException, JsonMappingException, IOException {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new JavaTimeModule());
    objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    return objectMapper.readValue(json, clazz);
}

public static void main(String[] args) throws IOException {
    HeaVO HeaVO=new HearVO();
    HeaVO.setId("Id");
    List<HeaVO> listVO=new ArrayList<HeaVO>();
    listVO.add(HeaVO);

    List<HeaVO> listHeaVORes=Arrays.asList(mapFromJson(mapToJson(listHeaVO), HeaVO[].class));
    System.out.println(((HeaVO)listVORes.get(0)).getId());
}