从 0 开始实现一个网页聊天室 (小型项目)（上）

@Data
public class User {

    private int userId;
    private String username = "";
    private String password = "";

    public User() {
    }

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
}

// 使用一个 Friend 对象表示一个好友, 对应数据库的 friend 表
@Data
public class Friend {
    private int friendId;
    private String friendName;

    public Friend() {
    }

    public Friend(int friendId, String friendName) {
        this.friendId = friendId;
        this.friendName = friendName;
    }
}

// 本类表示一条消息的相关信息
// (没有 postTime 是因为: 在查询的时候就是一次性查出所有的时间, 按照时间结果排序后返回, 我们这里就不需要再获取时间了)
@Data
public class Message {
    private Integer messageId;
    private int fromId;
    private String fromName;
    private int sessionId;
    private String content;

    public Message() {
    }

    public Message( int fromId, String fromName, int sessionId, String content) {
        this.fromId = fromId;
        this.fromName = fromName;
        this.sessionId = sessionId;
        this.content = content;
    }
}

// 使用该类表示一个会话
@Data
public class MessageSession {
    private int sessionId;
    private List<Friend> friends;
    private String lastMessage;
}

// 该类对象表示 message_session_user 表里的一个记录
@Data
public class MessageSessionUserItem {
    private int sessionId;
    private int userId;

    public MessageSessionUserItem() {
    }

    public MessageSessionUserItem(int sessionId, int userId) {
        this.sessionId = sessionId;
        this.userId = userId;
    }
}

// WebSocket请求
@Data
public class MessageRequest {
    private String type = "message";
    private int sessionId;
    private String content;
}

// WebSocket响应
@Data
public class MessageResponse {
    private String type = "message";
    private int fromId;
    private String fromName;
    private int sessionId;
    private String content;

    public MessageResponse() {
    }

    public MessageResponse(int fromId, String fromName, int sessionId, String content) {
        this.fromId = fromId;
        this.fromName = fromName;
        this.sessionId = sessionId;
        this.content = content;
    }
}

@Mapper
public interface FriendMapper {
    // 查询用户好友列表
    List<Friend> selectFriendList(@Param("userId") int userId);
}

<?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.java_chatroom.model.FriendMapper">

    <select id="selectFriendList" resultType="com.example.java_chatroom.model.Friend">
        select userId as friendId, username as friendName
        from user
        where userId in
        (select friendId from friend where userId = #{userId})
    </select>
</mapper>

@Mapper
public interface MessageMapper {

    // 获取指定会话的最后一条消息
    String getLastMessageBySessionId(@Param("sessionId") int sessionId);

    // 获取指定会话的历史消息 (限制100条)
    List<Message> getMessagesBySessionId(@Param("sessionId") int sessionId);

    // 插入一条消息到数据库表中
    void add(@Param("message") Message message);
}

<?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.java_chatroom.model.MessageMapper">

    <select id="getLastMessageBySessionId" resultType="java.lang.String">
        select content from message
        where sessionId = #{sessionId}
        order by postTime desc
        limit 1
    </select>

    <select id="getMessagesBySessionId" resultType="com.example.java_chatroom.model.Message">
        select
            messageId, sessionId, fromId, content, username as fromName
        from
            message, user
        where
            sessionId = #{sessionId}
            and fromId = userId
        order by
            postTime desc
        limit 100 offset 0
    </select>

    <insert id="add">
        insert into message values(null, #{message.fromId}, #{message.sessionId}, #{message.content}, now());
    </insert>
</mapper>

@Mapper
public interface MessageSessionMapper {
    // 1.根据 userId 获取到该用户在哪些会话中存在, 返回结果是一组 sessionId.
    List<Integer> getSessionIdsByUserId(@Param("userId") int userId);

    // 2. 根据 sessionId 查询这个会话包含哪些用户(刨除掉最初的 user)
    List<Friend> getFriendsBySessionId(@Param("sessionId") int sessionId,@Param("selfUserId") int selfUserId);

    // 3. 新增会话记录, 返回会话 id
    int addMessageSession(@Param("messageSession") MessageSession messageSession);

    // 4.给 message_session_user 表新增对应记录
    int addMessageSessionUser(@Param("messageSessionUserItem") MessageSessionUserItem messageSessionUserItem);
}

<?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.java_chatroom.model.MessageSessionMapper">

    <select id="getSessionIdsByUserId" resultType="java.lang.Integer">
        select sessionId from message_session
        where sessionId in
            ( select sessionId from message_session_user
             where userId = #{userId} )
        order by lastTime desc
    </select>

    <select id="getFriendsBySessionId" resultType="com.example.java_chatroom.model.Friend">
        select userId as friendId, username as friendName
        from user
        where userId in
            ( select userId from message_session_user
                where sessionId = #{sessionId}
                and userId != #{selfUserId} )
    </select>

    <insert id="addMessageSession" useGeneratedKeys="true" keyProperty="messageSession.sessionId">
        insert into message_session values(null, now())
    </insert>

    <insert id="addMessageSessionUser">
        insert into message_session_user values(
            #{messageSessionUserItem.sessionId},
            #{messageSessionUserItem.userId}
        )
    </insert>
</mapper>