import org.apache.ibatis.annotations.Select;
import ru.mark.yourmood.domain.UserDomain;

public interface UserMapper {
    @Select("SELECT id, avatar FROM users WHERE id = #{id}")
    public UserDomain getUserById(int id);
}

import java.io.Serializable;
import com.fasterxml.jackson.databind.ObjectMapper;

public class UserDomain implements Serializable {
    private int id;
    private String avatar;
 
    public Map getAvatar() throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        Map readValue = mapper.readValue(avatar, Map.class);
        return readValue;
    }
 
    public void setAvatar(String avatar) {
        this.avatar = avatar;
    }
}

@Results({
     @Result(property = "avatar", column = "avatar", javaType = String.class, typeHandler = ExampleTypeHandler.class) 
})
@Select("SELECT id, avatar FROM users WHERE id IN(1,3)")
public List<UserDomain> getUsers();

@MappedJdbcTypes(JdbcType.VARCHAR)
public class ExampleTypeHandler extends BaseTypeHandler<HashMap> {
    // Не статические объекты
    private final ObjectMapper mapper = new ObjectMapper();

    @Override
    public HashMap getNullableResult(ResultSet rs, String columnName) throws SQLException {

        HashMap value = null;

        try {
            value = mapper.readValue(rs.getString(columnName), HashMap.class);
        } catch (IOException ex) {

        }
        return value;
    }
}

public HashMap getAvatar() {
    return avatar;
}

public void setAvatar(HashMap avatar) {
    this.avatar = avatar;
}

public class Jackson {
    
    private static final ObjectMapper mapper = new ObjectMapper();

    static {
        mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
    }

    public static ObjectMapper mapper() {
        return mapper;
    }

}