jpa查询List对象并根据对象的属性归档放到map中

List<MeetingMember> meetingMemberList = MeetingMember.find("meeting.id in ?1", meetingList.stream().map(meeting -> meeting.id).collect(Collectors.toList())).fetch();
Map<String, List<MeetingMember>> map = new HashMap<>();
meetingMemberList.forEach(meetingMember -> {
    String id = meetingMember.meeting.id;
    List<MeetingMember> list = null;
    if (map.get(id) == null) {
        list = new ArrayList<>();

    } else {
        list = map.get(id);
    }
    list.add(meetingMember);
    map.put(id, list);
});