I have a problem to implement custom handler for Map through MyBatis in my Spring Boot example.

Here is my request object shown below

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class QueryRequest implements Serializable {

    private List<String> years;
    private List<String> months;
    private List<String> region;
    private List<String> office;

}

Here is the response object shown below

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class TaskCountResponse implements Serializable {

    private String region;
    private String office;
    private Map<String, Integer> monthlyCounts;

}

Here is the dao shown below

@Repository
public interface QueryTaskDao {

    List<TaskCountResponse> getTaskStatusCounts(QueryRequest queryRequest);
}

Here is the custom handler for Map<String,Integer> shown below

@Slf4j
public class MapTypeHandler extends BaseTypeHandler<Map<String, Integer>> {

    private static final ObjectMapper objectMapper = new ObjectMapper();

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, Map<String, Integer> parameter, JdbcType jdbcType) throws SQLException {
        try {
            String json = objectMapper.writeValueAsString(parameter);
            log.info("MapTypeHandler | setNonNullParameter | json : " + json);
            ps.setString(i, json);
        } catch (IOException e) {
            throw new SQLException("Error converting Map to JSON", e);
        }
    }

    @Override
    public Map<String, Integer> getNullableResult(ResultSet rs, String columnName) throws SQLException {
        try {
            String json = rs.getString(columnName);
            log.info("MapTypeHandler | getNullableResult(ResultSet rs, String columnName) | json : " + json);
            if (json != null) {
                return objectMapper.readValue(json, new TypeReference<Map<String, Integer>>() {});
            }
            return null;
        } catch (IOException e) {
            throw new SQLException("Error converting JSON to Map", e);
        }
    }

    @Override
    public Map<String, Integer> getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        try {
            String json = rs.getString(columnIndex);
            log.info("MapTypeHandler | getNullableResult(ResultSet rs, int columnIndex) | json : " + json);
            if (json != null) {
                return objectMapper.readValue(json, new TypeReference<Map<String, Integer>>() {});
            }
            return null;
        } catch (IOException e) {
            throw new SQLException("Error converting JSON to Map", e);
        }
    }

    @Override
    public Map<String, Integer> getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        try {
            String json = cs.getString(columnIndex);
            log.info("MapTypeHandler | getNullableResult(CallableStatement cs, int columnIndex) | json : " + json);
            if (json != null) {
                return objectMapper.readValue(json, new TypeReference<Map<String, Integer>>() {});
            }
            return null;
        } catch (IOException e) {
            throw new SQLException("Error converting JSON to Map", e);
        }
    }
}

Here is the XML part of mybatis

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.dao.QueryTaskDao">
    <cache/>

    <!-- Result Map to map query results to TaskStatusCountResponse -->
    <resultMap id="taskStatusCountResultMap" type="com.example.model.TaskStatusCountResponse">
        <result property="region" column="region_name"/>
        <result property="office" column="office_name"/>
        <!-- Handling dynamic columns for monthlyCounts -->
        <result property="monthlyCounts" column="monthlyCounts" javaType="java.util.Map" typeHandler="com.example.utils.MapTypeHandler"/>
    </resultMap>

    <!-- SQL fragment for dynamic filtering -->
    <sql id="queryTaskStatusCondition">
        <where>
            <!-- Filter by years -->
            <if test="years != null and !years.isEmpty()">
                AND SUBSTRING(tt.task_finish_time, 1, 4) IN
                <foreach collection="years" item="year" open="(" close=")" separator=",">
                    #{year}
                </foreach>
            </if>

            <!-- Filter by months -->
            <if test="months != null and !months.isEmpty()">
                AND SUBSTRING(tt.task_finish_time, 6, 2) IN
                <foreach collection="months" item="month" open="(" close=")" separator=",">
                    #{month}
                </foreach>
            </if>

            <!-- Filter by region -->
            <if test="region != null and !region.isEmpty()">
                AND tt.region_name IN
                <foreach collection="region" item="region" open="(" close=")" separator=",">
                    #{region}
                </foreach>
            </if>

            <!-- Filter by office -->
            <if test="office != null and !office.isEmpty()">
                AND tt.office_name IN
                <foreach collection="office" item="office" open="(" close=")" separator=",">
                    #{office}
                </foreach>
            </if>
        </where>
    </sql>

    <!-- Main SQL query to get Task Status Counts -->
    <select id="getTaskStatusCounts" resultMap="taskStatusCountResultMap" parameterType="com.example.model.QueryRequest">
        SELECT
        tt.region_name,
        tt.office_name,

        <!-- Dynamically create counts for each year-month combination -->
        <trim prefix="" suffix="" suffixOverrides=",">
            <foreach collection="years" item="year" separator=",">
                <foreach collection="months" item="month" separator=",">
                    COUNT(CASE WHEN SUBSTRING(tt.task_finish_time, 1, 7) = CONCAT(#{year}, '-', #{month}) THEN 1 END) AS month_${year}_${month}
                </foreach>
            </foreach>
        </trim>

        FROM
        task_list tt
        <include refid="queryTaskStatusCondition"/>
        GROUP BY
        tt.region_name,
        tt.office_name
        ORDER BY
        tt.region_name_en ASC;
    </select>
</mapper>

When I send a request to http://localhost:8048/statistic/getTaskCountsbyTime

{
    "years": ["2022", "2023"],
    "months": ["01", "02", "03"],
    "region": ["A Region", "B Region"],
    "office": ["A Region Office", "B Region Office"]
}

I get this response

[
    {
        "region": "A Region",
        "office": "A Region Office",
        "monthlyCounts": null
    },
    {
        "region": "B Region",
        "office": "B Region Office",
        "monthlyCounts": null
    }
]

The response should look something like this (with actual counts in place of null):

[
    {
        "region": "A Region",
        "office": "A Region Office",
        "monthlyCounts": {
            "month_2022_01": 10,
            "month_2022_02": 15,
            "month_2022_03": 12,
            "month_2023_01": 5,
            "month_2023_02": 7,
            "month_2023_03": 6
        }
    },
    {
        "region": "B Region",
        "office": "B Region Office",
        "monthlyCounts": {
            "month_2022_01": 8,
            "month_2022_02": 14,
            "month_2022_03": 13,
            "month_2023_01": 4,
            "month_2023_02": 6,
            "month_2023_03": 9
        }
    }
]

I get null of monthlyCounts .Where is the problem in xml mapper or mapper type handler?
Can you revise it to fix ?