一、Message表结构设计

• from_id: 发送方用户id
• to_id: 接受方用户id
• conversation_id: 标识一个会话，不区分发送方和接收方，用户id小的在前，大的在后

二、数据访问层

• 私信列表
  • 查询当前用户的会话列表，每个会话只显示一条最新的私信。
  • 支持分页显示。
• 私信详情
  • 查询某个会话所包含的私信。
  • 支持分页显示。

MessageMapper.java

@Mapper
public interface MessageMapper {

    // 查询当前用户的会话列表,针对每个会话只返回一条最新的私信.
    List<Message> selectConversations(int userId, int offset, int limit);

    // 查询当前用户的会话数量.
    int selectConversationCount(int userId);

    // 查询某个会话所包含的私信列表.
    List<Message> selectLetters(String conversationId, int offset, int limit);

    // 查询某个会话所包含的私信数量.
    int selectLetterCount(String conversationId);

    // 查询未读私信的数量
    int selectLetterUnreadCount(int userId, String conversationId);

}

message-mapper.xml

<?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.nowcoder.community.dao.MessageMapper">

    <sql id="selectFields">
        id, from_id, to_id, conversation_id, content, status, create_time
    </sql>

    <sql id="insertFields">
        from_id, to_id, conversation_id, content, status, create_time
    </sql>

    <select id="selectConversations" resultType="Message">
        select <include refid="selectFields"></include>
        from message
        where id in (
            select max(id) from message
            where status != 2
            and from_id != 1
            and (from_id = #{userId} or to_id = #{userId})
            group by conversation_id
        )
        order by id desc
        limit #{offset}, #{limit}
    </select>

    <select id="selectConversationCount" resultType="int">
        select count(m.maxid) from (
            select max(id) as maxid from message
            where status != 2
            and from_id != 1
            and (from_id = #{userId} or to_id = #{userId})
            group by conversation_id
        ) as m
    </select>

    <select id="selectLetters" resultType="Message">
        select <include refid="selectFields"></include>
        from message
        where status != 2
        and from_id != 1
        and conversation_id = #{conversationId}
        order by id desc
        limit #{offset}, #{limit}
    </select>

    <select id="selectLetterCount" resultType="int">
        select count(id)
        from message
        where status != 2
        and from_id != 1
        and conversation_id = #{conversationId}
    </select>

    <select id="selectLetterUnreadCount" resultType="int">
        select count(id)
        from message
        where status = 0
        and from_id != 1
        and to_id = #{userId}
        <if test="conversationId!=null">
            and conversation_id = #{conversationId}
        </if>
    </select>

</mapper>

二、业务层

MessageService.java

@Service
public class MessageService {

    @Autowired
    private MessageMapper messageMapper;

    @Autowired
    private SensitiveFilter sensitiveFilter;

    public List<Message> findConversations(int userId, int offset, int limit) {
        return messageMapper.selectConversations(userId, offset, limit);
    }

    public int findConversationCount(int userId) {
        return messageMapper.selectConversationCount(userId);
    }

    public List<Message> findLetters(String conversationId, int offset, int limit) {
        return messageMapper.selectLetters(conversationId, offset, limit);
    }

    public int findLetterCount(String conversationId) {
        return messageMapper.selectLetterCount(conversationId);
    }

    public int findLetterUnreadCount(int userId, String conversationId) {
        return messageMapper.selectLetterUnreadCount(userId, conversationId);
    }

    public int addMessage(Message message) {
        message.setContent(HtmlUtils.htmlEscape(message.getContent()));
        message.setContent(sensitiveFilter.filter(message.getContent()));
        return messageMapper.insertMessage(message);
    }

    public int readMessage(List<Integer> ids) {
        return messageMapper.updateStatus(ids, 1);
    }

}

三、表现层

MessageController.java

@Controller
public class MessageController {

    @Autowired
    private MessageService messageService;

    @Autowired
    private HostHolder hostHolder;

    @Autowired
    private UserService userService;

    // 私信列表
    @RequestMapping(path = "/letter/list", method = RequestMethod.GET)
    public String getLetterList(Model model, Page page) {
        User user = hostHolder.getUser();
        // 分页信息
        page.setLimit(5);
        page.setPath("/letter/list");
        page.setRows(messageService.findConversationCount(user.getId()));

        // 会话列表
        List<Message> conversationList = messageService.findConversations(
                user.getId(), page.getOffset(), page.getLimit());
        List<Map<String, Object>> conversations = new ArrayList<>();
        if (conversationList != null) {
            for (Message message : conversationList) {
                Map<String, Object> map = new HashMap<>();
                map.put("conversation", message);
                map.put("letterCount", messageService.findLetterCount(message.getConversationId()));
                map.put("unreadCount", messageService.findLetterUnreadCount(user.getId(), message.getConversationId()));
                int targetId = user.getId() == message.getFromId() ? message.getToId() : message.getFromId();
                map.put("target", userService.findUserById(targetId));

                conversations.add(map);
            }
        }
        model.addAttribute("conversations", conversations);

        // 查询未读消息数量
        int letterUnreadCount = messageService.findLetterUnreadCount(user.getId(), null);
        model.addAttribute("letterUnreadCount", letterUnreadCount);

        return "/site/letter";
    }
    @RequestMapping(path = "/letter/detail/{conversationId}", method = RequestMethod.GET)
    public String getLetterDetail(@PathVariable("conversationId") String conversationId, Page page, Model model) {
        // 分页信息
        page.setLimit(5);
        page.setPath("/letter/detail/" + conversationId);
        page.setRows(messageService.findLetterCount(conversationId));

        // 私信列表
        List<Message> letterList = messageService.findLetters(conversationId, page.getOffset(), page.getLimit());
        List<Map<String, Object>> letters = new ArrayList<>();
        if (letterList != null) {
            for (Message message : letterList) {
                Map<String, Object> map = new HashMap<>();
                map.put("letter", message);
                map.put("fromUser", userService.findUserById(message.getFromId()));
                letters.add(map);
            }
        }
        model.addAttribute("letters", letters);

        // 私信目标
        model.addAttribute("target", getLetterTarget(conversationId));

        // 设置已读
        List<Integer> ids = getLetterIds(letterList);
        if (!ids.isEmpty()) {
            messageService.readMessage(ids);
        }

        return "/site/letter-detail";
    }

    private User getLetterTarget(String conversationId) {
        String[] ids = conversationId.split("_");
        int id0 = Integer.parseInt(ids[0]);
        int id1 = Integer.parseInt(ids[1]);

        if (hostHolder.getUser().getId() == id0) {
            return userService.findUserById(id1);
        } else {
            return userService.findUserById(id0);
        }
    }
}

页面处理（略）letter.html letter-detil.html