When flatten = true, Jackson2HashMapper serializes the List and deserializes it out of order

[ainydexiaohai]

Hi,
When I tried to serialize an object containing the List attribute, I deserialized it to obtain a disordered List.

1.Problem

Test.class

Jackson2HashMapper jackson2HashMapper = new Jackson2HashMapper(true);
List<Integer> list = IntStream.range(0, 20).boxed().collect(Collectors.toList());
User user = new User();
user.setUserAgeList(list);
System.out.println("before serialize, user=" + user);
Map<String, Object> map = jackson2HashMapper.toHash(user);
User user1 = (User)jackson2HashMapper.fromHash(map);
System.out.println("after serialize, user=" + user1);

User.class

@Data
public class User {
    private List<Integer> userAgeList;
}

result:

before serialize, user=User(userAgeList=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
after serialize, user=User(userAgeList=[0, 1, 18, 3, 4, 2, 19, 7, 8, 5, 6, 9, 12, 13, 10, 11, 14, 15, 16, 17])

2.Reason

org.springframework.data.redis.hash.Jackson2HashMapper#flattenMap use HashMap to store elements.

private Map<String, Object> flattenMap(Iterator<Entry<String, JsonNode>> source) {
	Map<String, Object> resultMap = new HashMap();
	this.doFlatten("", source, resultMap);
	return resultMap;
}

3.Solution

Modify the storage set to LinkedHashMap.

private Map<String, Object> flattenMap(Iterator<Entry<String, JsonNode>> source) {
	Map<String, Object> resultMap = new LinkedHashMap();
	this.doFlatten("", source, resultMap);
	return resultMap;
}

Thanks in advance!

[jxblum]

@ainydexiaohai - Did you actually test your solution(?), because when I tried it, it did NOT work, and upon reviewing the applicable code, the resultMap in the Jackson2HashMapper.flattenMap(..) method is not what needs to be ordered in this case.

Please do not post solutions 1) especially, if you have not tested the solution (don't guess), 2) you are not reasonably confident you are correct in this case, and 3) see #1.

I did uncover the underlying problem and have already crafted a fix, which will appear in the next 2.7.x (2.7.12 service release and the final 2.7.x release), 3.0.x (3.0.6 service release) and the newly minted 3.1 GA release, all occuring on Friday, May 12th, 2023 (see Spring release calendar).

[ainydexiaohai]

@jxblum Without much understanding, why didn't it work? After I rewrote flattenMap, the order after serialization and deserialization remained the same.

1、overwrite

public class MyJackson2HashMapper extends Jackson2HashMapper {
    public MyJackson2HashMapper(boolean flatten) {
        super(flatten);
    }

    @Override
    public Map<String, Object> toHash(Object source) {
        boolean flatten = false;
        ObjectMapper typingMapper = null;
        ObjectMapper untypedMapper = null;
        try {
            Field flattenField = getClass().getSuperclass().getDeclaredField("flatten");
            flattenField.setAccessible(true);
            flatten = (Boolean) flattenField.get(this);

            Field typingMapperField = getClass().getSuperclass().getDeclaredField("typingMapper");
            typingMapperField.setAccessible(true);
            typingMapper = (ObjectMapper) typingMapperField.get(this);

            Field untypedMapperField = getClass().getSuperclass().getDeclaredField("untypedMapper");
            untypedMapperField.setAccessible(true);
            untypedMapper = (ObjectMapper) untypedMapperField.get(this);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }

        JsonNode tree = typingMapper.valueToTree(source);
        return flatten ? flattenMap(tree.fields()) : untypedMapper.convertValue(tree, Map.class);
    }

    private Map<String, Object> flattenMap(Iterator<Map.Entry<String, JsonNode>> source) {
        // HashMap -> LinkedHashMap
        Map<String, Object> resultMap = new LinkedHashMap<>();
        try {
            Method doFlattenMethod = getClass().getSuperclass().getDeclaredMethod("doFlatten", String.class, Iterator.class, Map.class);
            doFlattenMethod.setAccessible(true);
            doFlattenMethod.invoke(this, "", source, resultMap);
        } catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException e) {
            e.printStackTrace();
        }
        return resultMap;
    }
}

2、Test

    @Test
    public void test1() {
        Jackson2HashMapper jackson2HashMapper = new Jackson2HashMapper(true);
        List<Integer> list = IntStream.range(0, 20).boxed().collect(Collectors.toList());
        User user = new User();
        user.setUserAgeList(list);
       // before serialize, user=User(userAgeList=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
        System.out.println("before serialize, user=" + user);
        Map<String, Object> map = jackson2HashMapper.toHash(user);
        User user1 = (User)jackson2HashMapper.fromHash(map);
        // after serialize, user=User(userAgeList=[0, 1, 18, 3, 4, 2, 19, 7, 8, 5, 6, 9, 12, 13, 10, 11, 14, 15, 16, 17])
        System.out.println("after serialize, user=" + user1);
    }

    @Test
    public void test2() {
        MyJackson2HashMapper jackson2HashMapper = new MyJackson2HashMapper(true);
        List<Integer> list = IntStream.range(0, 20).boxed().collect(Collectors.toList());
        User user = new User();
        user.setUserAgeList(list);
       // before serialize, user=User(userAgeList=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
        System.out.println("before serialize, user=" + user);
        Map<String, Object> map = jackson2HashMapper.toHash(user);
        User user1 = (User)jackson2HashMapper.fromHash(map);
        // after serialize, user=User(userAgeList=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
        System.out.println("after serialize, user=" + user1);
    }