假设我们已经有了以下两张表： 一级菜单表 first_menu： 字段名 类型 描述 id bigint 主键 name varchar 菜单名称 url varchar 菜单链接 二级菜单表 second_menu： 字段名 类型 描述 id bigint 主键 name varchar 菜单名称 url varchar 菜单链接 first_menu_id bigint 所属的一级菜单id 现在我们需要创建一张标记是否被选中的用户-菜单表 user_menu： 字段名 类型 描述 id bigint 主键 user_id bigint 用户id menu_id bigint 菜单id selected boolean 是否被选中 java 详细设计，使用mybatis 操作数据库

首先，我们需要建立三个实体类，分别对应三张表：

FirstMenu.java

public class FirstMenu {
    private Long id;
    private String name;
    private String url;
    
    // 省略getter和setter
}

SecondMenu.java

public class SecondMenu {
    private Long id;
    private String name;
    private String url;
    private Long firstMenuId;
    
    // 省略getter和setter
}

UserMenu.java

public class UserMenu {
    private Long id;
    private Long userId;
    private Long menuId;
    private boolean selected;
    
    // 省略getter和setter
}

然后，我们需要创建对应的Mapper接口和XML文件：

FirstMenuMapper.java

public interface FirstMenuMapper {
    List<FirstMenu> findAll();
}

FirstMenuMapper.xml

<mapper namespace="com.example.mapper.FirstMenuMapper">
    <resultMap id="firstMenuMap" type="com.example.entity.FirstMenu">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="url" property="url"/>
    </resultMap>

    <select id="findAll" resultMap="firstMenuMap">
        select id, name, url from first_menu;
    </select>
</mapper>

SecondMenuMapper.java

public interface SecondMenuMapper {
    List<SecondMenu> findByFirstMenuId(Long firstMenuId);
}

SecondMenuMapper.xml

<mapper namespace="com.example.mapper.SecondMenuMapper">
    <resultMap id="secondMenuMap" type="com.example.entity.SecondMenu">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="url" property="url"/>
        <result column="first_menu_id" property="firstMenuId"/>
    </resultMap>

    <select id="findByFirstMenuId" parameterType="Long" resultMap="secondMenuMap">
        select id, name, url, first_menu_id from second_menu where first_menu_id = #{firstMenuId};
    </select>
</mapper>

UserMenuMapper.java

public interface UserMenuMapper {
    void save(UserMenu userMenu);
    UserMenu findByUserIdAndMenuId(Long userId, Long menuId);
    void updateSelected(Long id, boolean selected);
}

UserMenuMapper.xml

<mapper namespace="com.example.mapper.UserMenuMapper">
    <insert id="save" parameterType="com.example.entity.UserMenu">
        insert into user_menu (user_id, menu_id, selected) values (#{userId}, #{menuId}, #{selected});
    </insert>

    <resultMap id="userMenuMap" type="com.example.entity.UserMenu">
        <id column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="menu_id" property="menuId"/>
        <result column="selected" property="selected"/>
    </resultMap>

    <select id="findByUserIdAndMenuId" parameterType="map" resultMap="userMenuMap">
        select * from user_menu where user_id = #{userId} and menu_id = #{menuId};
    </select>

    <update id="updateSelected" parameterType="map">
        update user_menu set selected = #{selected} where id = #{id};
    </update>
</mapper>

最后，我们在Service层进行对应的逻辑操作：

MenuService.java

@Service
public class MenuService {
    @Autowired
    private FirstMenuMapper firstMenuMapper;

    @Autowired
    private SecondMenuMapper secondMenuMapper;

    @Autowired
    private UserMenuMapper userMenuMapper;

    public List<FirstMenu> findAllFirstMenu() {
        return firstMenuMapper.findAll();
    }

    public List<SecondMenu> findByFirstMenuId(Long firstMenuId) {
        return secondMenuMapper.findByFirstMenuId(firstMenuId);
    }

    public void saveUserMenu(Long userId, Long menuId) {
        UserMenu userMenu = userMenuMapper.findByUserIdAndMenuId(userId, menuId);
        if (userMenu == null) {
            userMenu = new UserMenu();
            userMenu.setUserId(userId);
            userMenu.setMenuId(menuId);
            userMenu.setSelected(true);
            userMenuMapper.save(userMenu);
        } else {
            userMenuMapper.updateSelected(userMenu.getId(), !userMenu.isSelected());
        }
    }
}

这样，我们就完成了对应的需求。其中，第一个方法用于获取所有一级菜单；第二个方法用于根据一级菜单id获取对应的二级菜单；第三个方法用于保存或更新用户-菜单表中的选中状态。