MyBatis多对多映射查询

5、MyBatis 入门实例  多对多  基于xml配置

　　这里我们以 users 表和 groups 表为例，一个 users 可能加入多个 groups，而一个 groups 可能包含多个 users，故构成 多对多 的关联

　　①、在数据库中建立相应的表

　　users 表

　　

　　groups 表

　　

　　两者之间的关联表users_groups表

　　

　　②、建立对应的实体类

　　Users.java

package many.to.many;
import java.util.Set;
public class Users {
    private int uid;
    private String uname;
    private Set<Groups> groups;
    public int getUid() {
        return uid;
    }
    public void setUid(int uid) {
        this.uid = uid;
    }
    public String getUname() {
        return uname;
    }
    public void setUname(String uname) {
        this.uname = uname;
    }
    public Set<Groups> getGroups() {
        return groups;
    }
    public void setGroups(Set<Groups> groups) {
        this.groups = groups;
    }
    @Override
    public String toString() {
        return "User [uid=" + uid + ", uname=" + uname + ", groups=" + groups + "]";
    }

}

　　　　Groups.java

package many.to.many;
import java.util.Set;
public class Groups {
    private int gid;
    private String gname;
    private Set<Users> users;
    public int getGid() {
        return gid;
    }
    public void setGid(int gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }
    public Set<Users> getUsers() {
        return users;
    }
    public void setUsers(Set<Users> users) {
        this.users = users;
    }
    @Override
    public String toString() {
        return "Group [gid=" + gid + ", gname=" + gname + ", users=" + users + "]";
    }

}

　　　　Users_Groups.java

package many.to.many;
public class Users_Groups {
    private Users user;
    private Groups group;
    public Users getUser() {
        return user;
    }
    public void setUser(Users user) {
        this.user = user;
    }
    public Groups getGroup() {
        return group;
    }
    public void setGroup(Groups group) {
        this.group = group;
    }
}

　　③、多对多：定义操作 sql映射文件userMapper.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="many.to.many.userMapper">
    <!-- 给一个用户 id，查看该用户下的所有用户组信息 -->
    <select id="getUsers" resultMap="getGroupMap">
        select g.gid,g.gname from users_groups ug,groups g
            where ug.group_id=g.gid and ug.user_id=#{uid}
    </select>
    <resultMap type="many.to.many.Groups" id="getGroupMap">
        <id column="gid" property="gid"/>
        <result column="gname" property="gname"/>
        <collection property="users" ofType="many.to.many.Users">
            <id column="uid" property="uid"/>
        <result column="uname" property="uname"/>
        </collection>
    </resultMap>

</mapper>

　　

　　⑤、向 mybatis-configuration.xml 配置文件中注册 userMapper.xml文件

　　⑥、编写测试类

//多对多：根据根据用户 id 查询所有的用户组信息
    @Test
    public void testGetGroups(){
        String statement = "many.to.many.userMapper.getUsers";
        List<Groups> listGroup = session.selectList(statement,1);
        for(Groups g : listGroup){
            System.out.println(g.toString());
        }
    }