这里定义了三个表，用户表、角色表、中间表，每个人可以有多个角色，每个角色可以由多个人来扮演。
在用户表和角色表中，它们的ID为各自的主键，中间表为这俩个表的连接，各自表的ID在中间表中为外键。

1、创建表

用户表

中间表

角色表

2、创建接受数据类

User.java

public class User implements Serializable{private Integer id;private String username;private Date birthday;private String sex;private String address;private List<Role> roles;public List<Role> getRoles() {return roles;}public void setRoles(List<Role> roles) {this.roles = roles;}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getUsername() {return username;}public void setUsername(String username) {this.username = username;}public Date getBirthday() {return birthday;}public void setBirthday(Date birthday) {this.birthday = birthday;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}@Overridepublic String toString() {return "User{" +"id=" + id +", username='" + username + '\'' +", birthday=" + birthday +", sex='" + sex + '\'' +", address='" + address + '\'' +'}';}
}

Role.java

public void setRoleId(Integer roleId) {this.roleId = roleId;}public String getRoleName() {return roleName;}public void setRoleName(String roleName) {this.roleName = roleName;}public String getRoleDesc() {return roleDesc;}public void setRoleDesc(String roleDesc) {this.roleDesc = roleDesc;}@Overridepublic String toString() {return "Role{" +"roleId=" + roleId +", roleName='" + roleName + '\'' +", roleDesc='" + roleDesc + '\'' +'}';}
}

3、创建用户层接口

IUserDao.java

public interface IUserDao {/*** 查询所有操作* @return*/
//    @Select("select * from user")List<User> findAll();
//    根据ID查询用户User findById(Integer id);}

IRoleDao.java

public interface IRole {// 查询所有角色List<Role> findAll();
}

4、编写映射配置文件

IRoleDao.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.dao.IUserDao"><resultMap id="resultfindAll" type="com.itheima.domain.User"><id property="id" column="id"></id><result property="username" column="username"></result><result property="sex" column="sex"></result><result property="birthday" column="birthday"></result><result property="address" column="address"></result><collection property="roles" ofType="Role"><id property="roleId" column="rid"></id><id property="roleName" column="role_name"></id><id property="roleDesc" column="role_desc"></id></collection></resultMap><!--配置查询所有--><select id="findAll" resultMap="resultfindAll">select u.*,r.id as rid,r.role_name,r.role_desc from user u left outer join user_role ur on u.id = ur.uidleft outer join role  r on ur.rid = r.id;</select><!-- 根据用户ID查询 --><select id="findById" parameterType="Integer" resultType="com.itheima.domain.User">select *from user where id = #{uid};</select></mapper>

IUserDao.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.dao.IRole"><resultMap id="roleMap" type="com.itheima.domain.Role"><id property="roleId" column="ID"></id><result property="roleName" column="ROLE_NAME"></result><result property="roleDesc" column="ROLE_DESC"></result><collection property="users" ofType="User"><id property="id" column="uid"></id><result property="username" column="username"></result><result property="sex" column="sex"></result><result property="address" column="address"></result><result property="birthday" column="birthday"></result></collection></resultMap><!--配置查询所有--><select id="findAll" resultMap="roleMap">select r.*, u.id as uid,u.username,u.birthday,u.sex,u.address from role r left outer join user_role ur on r.id = ur.ridleft outer join user u on ur.uid = u.id;</select></mapper>

5、测试

RoleTest.java

@Testpublic void testSearchById() throws IOException {//5.使用代理对象执行方法List<Role> roles = roleDao.findAll();for(Role role : roles){System.out.println(role);System.out.println(role.getUsers());}}

如图：有的角色有两个人扮演，有的一个人，有的没人。

UserTest.java

 @Testpublic void testSearchById() throws IOException {//5.使用代理对象执行方法List<User> users = userDao.findAll();for(User user : users){System.out.println(user);System.out.println(user.getRoles());}}

如图：有的人扮演连个角色，有的一个，有的没有。