MyBatis多表查询操作_11:06:25create table `account` ( `userid` int(11) -优快云博客

本文链接：https://blog.youkuaiyun.com/qq_40867650/article/details/106204777

往期内容，如下
一、MyBatis简介
二、MyBatis环境搭建
三、MyBatis入门案例
四、MyBatis自定义
五、MyBatis CRUD操作
六、Mybatis中参数和返回值的深入了解
七、MyBatis 配置文件标签
八、MyBatis POOLED连接池深入了解
九、MyBatis中的动态sql语句

最终代码目录结构如下：
在这里插入图片描述

一对一、一对多环境搭建

数据库数据

User实体类数据

CREATE TABLE `user` (
  `id` int(11) NOT NULL auto_increment,
  `username` varchar(32) NOT NULL COMMENT '用户名称',
  `birthday` datetime default NULL COMMENT '生日',
  `sex` char(1) default NULL COMMENT '性别',
  `address` varchar(256) default NULL COMMENT '地址',
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;



insert  into `user`(`id`,`username`,`birthday`,`sex`,`address`) values (41,'老王','2018-02-27 17:47:08','男','北京'),(42,'小二王','2018-03-02 15:09:37','女','北京金燕龙'),(43,'小二王','2018-03-04 11:34:34','女','北京金燕龙'),(45,'传智播客','2018-03-04 12:04:06','男','北京金燕龙'),(46,'老王','2018-03-07 17:37:26','男','北京'),(48,'小马宝莉','2018-03-08 11:44:00','女','北京修正');

account实体类数据

DROP TABLE IF EXISTS `account`;

CREATE TABLE `account` (
  `ID` int(11) NOT NULL COMMENT '编号',
  `UID` int(11) default NULL COMMENT '用户编号',
  `MONEY` double default NULL COMMENT '金额',
  PRIMARY KEY  (`ID`),
  KEY `FK_Reference_8` (`UID`),
  CONSTRAINT `FK_Reference_8` FOREIGN KEY (`UID`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;



insert  into `account`(`ID`,`UID`,`MONEY`) values (1,41,1000),(2,45,1000),(3,41,2000);

实体类代码

com.itheima.domain.User

package com.itheima.domain;

import java.io.Serializable;
import java.util.Date;

public class User implements Serializable {

    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    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 String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", address='" + address + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday=" + birthday +
                '}';
    }
}

com.itheima.domain.Account

package com.itheima.domain;

import java.io.Serializable;

public class Account implements Serializable {

    private Integer id;
    private Integer uid;
    private Double money;


    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUid() {
        return uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    public Double getMoney() {
        return money;
    }

    public void setMoney(Double money) {
        this.money = money;
    }

    @Override
    public String toString() {
        return "Account{" +
                "id=" + id +
                ", uid=" + uid +
                ", money=" + money +
                '}';
    }
}

接口

com.itheima.dao.IUserDao

package com.itheima.dao;

import com.itheima.domain.User;

import java.util.List;

/**
 * 用户的持久层接口
 */
public interface IUserDao {

    /**
     * 查询所有用户
     * @return
     */
    List<User> findAll();


    /**
     * 根据id查询用户信息
     * @param userId
     * @return
     */
    User findById(Integer userId);


}

com.itheima.dao.IAccountDao

package com.itheima.dao;

import com.itheima.domain.Account;

import java.util.List;

public interface IAccountDao {

    /**
     * 查询所有账户
     * @return
     */
    List<Account> findAll();
}

配置文件

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.itheima</groupId>
    <artifactId>day03_eesy_03one2many</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.6</version>
        </dependency>

        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.12</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.10</version>
        </dependency>
    </dependencies>


</project>

log4j.properties

# Set root category priority to INFO and its only appender to CONSOLE.
#log4j.rootCategory=INFO, CONSOLE            debug   info   warn error fatal
log4j.rootCategory=debug, CONSOLE, LOGFILE

# Set the enterprise logger category to FATAL and its only appender to CONSOLE.
log4j.logger.org.apache.axis.enterprise=FATAL, CONSOLE

# CONSOLE is set to be a ConsoleAppender using a PatternLayout.
log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender
log4j.appender.CONSOLE.layout=org.apache.log4j.PatternLayout
log4j.appender.CONSOLE.layout.ConversionPattern=%d{ISO8601} %-6r [%15.15t] %-5p %30.30c %x - %m\n

# LOGFILE is set to be a File appender using a PatternLayout.
log4j.appender.LOGFILE=org.apache.log4j.FileAppender
log4j.appender.LOGFILE.File=d:\axis.log
log4j.appender.LOGFILE.Append=true
log4j.appender.LOGFILE.layout=org.apache.log4j.PatternLayout
log4j.appender.LOGFILE.layout.ConversionPattern=%d{ISO8601} %-6r [%15.15t] %-5p %30.30c %x - %m\n

jdbcConfig.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/eesy_mybatis
jdbc.username=root
jdbc.password=1234

SqlMapConfig.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- 配置properties-->
    <properties resource="jdbcConfig.properties"></properties>

    <!--使用typeAliases配置别名，它只能配置domain中类的别名 -->
    <typeAliases>
        <package name="com.itheima.domain"></package>
    </typeAliases>

    <!--配置环境-->
    <environments default="mysql">
        <!-- 配置mysql的环境-->
        <environment id="mysql">
            <!-- 配置事务 -->
            <transactionManager type="JDBC"></transactionManager>

            <!--配置连接池-->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"></property>
                <property name="url" value="${jdbc.url}"></property>
                <property name="username" value="${jdbc.username}"></property>
                <property name="password" value="${jdbc.password}"></property>
            </dataSource>
        </environment>
    </environments>
    <!-- 配置映射文件的位置 -->
    <mappers>
        <package name="com.itheima.dao"></package>
    </mappers>
</configuration>

com/itheima/dao/IUserDao.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.itheima.dao.IUserDao">

    <!-- 查询所有 -->
    <select id="findAll" resultType="user">
        select * from user 
    </select>

    <!-- 根据id查询用户 -->
    <select id="findById" parameterType="INT" resultType="user">
        select * from user where id = #{uid}
    </select>

</mapper>

com/itheima/dao/IAccountDao.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.itheima.dao.IAccountDao">

    <!-- 查询所有 -->
    <select id="findAll" resultType="account">
        select * from account 
    </select>

</mapper>

测试类

com.itheima.test.AccountTest

package com.itheima.test;

import com.itheima.dao.IAccountDao;
import com.itheima.domain.Account;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class AccountTest {

    private InputStream in;
    private SqlSession sqlSession;
    private IAccountDao accountDao;

    @Before//用于在测试方法执行之前执行
    public void init()throws Exception{
        //1.读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //3.获取SqlSession对象
        sqlSession = factory.openSession(true);
        //4.获取dao的代理对象
        accountDao = sqlSession.getMapper(IAccountDao.class);
    }

    @After//用于在测试方法执行之后执行
    public void destroy()throws Exception{
        //提交事务
        // sqlSession.commit();
        //6.释放资源
        sqlSession.close();
        in.close();
    }

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<Account> accounts = accountDao.findAll();
        for(Account account : accounts){
            System.out.println(account);
        }
    }
}

运行测试类结果如下：
在这里插入图片描述

一对一查询操作

效果：查询所有用户，同时还要获取当前用户的所属用户信息，很显然这有以下代码不可能实现的
com.itheima.dao.IAccountDao

    /**
     * 查询所有账户
     *
     * @return
     */
    List<Account> findAll();

需要先编写sql语句

select * from account a, user u where u.id = a.uid;

查询结果：
在这里插入图片描述
由于User和Account表的id都叫id，所以第二个出现的id自动起别名为id1，我们可以手动起别名

select u.*,a.id as aid,a.uid,a.money from account a, user u where u.id = a.uid;

查询结果：
在这里插入图片描述
去除重复和不需要的部分：保留主键、名称和地址

解决方法一：通过Account的子类方式查询

在com.itheima.dao.IAccountDao添加方法

package com.itheima.dao;

import com.itheima.domain.Account;
import com.itheima.domain.AccountUser;

import java.util.List;

public interface IAccountDao {

    /**
     * 查询所有账户，同时还要获取到当前账户的所属用户信息
     * @return
     */
    List<Account> findAll();

    /**添加方法
     * 查询所有账户，并且带有用户名称和地址信息
     * @return
     */
    List<AccountUser> findAllAccount();
}

创建类com.itheima.domain.AccountUser

package com.itheima.domain;

public class AccountUser extends Account {

    private String username;
    private String address;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return super.toString()+"        AccountUser{" +
                "username='" + username + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

配置IAccountDao

<?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.itheima.dao.IAccountDao">

    <!-- 查询所有 -->
    <select id="findAll" resultType="account">
        select * from account
    </select>

    <!--查询所有账户同时包含用户名和地址信息-->
    <select id="findAllAccount" resultType="accountuser">
        select a.*,u.username,u.address from account a , user u where u.id = a.uid;
    </select>
</mapper>

编写测试方法com.itheima.test.AccountTest

    /**
     * 测试查询所有账户，同时包含用户名称和地址
     */
    @Test
    public void testFindAllAccountUser(){
        List<AccountUser> aus = accountDao.findAllAccount();
        for(AccountUser au : aus){
            System.out.println(au);
        }
    }

运行测试方法，结果如下：
在这里插入图片描述

解决方法二（常用）：建立实体类关系的方式查询

无论是一对一、一对多还是多对多，从表的实体类和主表的实体类的关系应该为，从Mybatis体现一对一应该是，从表实体类应该包含一个主表实体的对象引用
所以应该是Account类中引用User
com.itheima.domain.Account

package com.itheima.domain;

import java.io.Serializable;

public class Account implements Serializable {

	//省略部分set/get方法，保留新加入的成员变量、set/get方法和部分代码
    private Integer id;
    private Integer uid;
    private Double money;

    //从表实体应该包含一个主表实体的对象引用
    private User user;

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

}

因为我们在封装Account的时候，希望把User的信息也给封装进来，而我们的IAccountDao.xml中只能封装到Account，于是我们需要定义封装Account和User的resultMap
com/itheima/dao/IAccountDao.xml

    <!-- 定义封装account和uesr的resultMap -->
    <resultMap id="accountUserMap" type="account">
        <!-- 这一段定义只能封装account数据 -->
        <id property="id" column="aid"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
        <!-- 一对一的关系配置：封装User的内容-->
        <!-- javaType的正确写法应该是com.itheima.domain.User，但是因为我们在mybatis配置文件中定义了别名，所以直接写user-->
        <association property="user" column="uid" javaType="user">
            <id property="id" column="id"></id>
            <result column="username" property="username"></result>
            <result column="address" property="address"></result>
            <result column="sex" property="sex"></result>
            <result column="birthday" property="birthday"></result>
        </association>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="accountUserMap">
        select u.*,a.id as aid,a.uid,a.money from account a, user u where u.id = a.uid;
    </select>

运行com.itheima.test.AccountTest中的testFindAll方法
在这里插入图片描述
如果需要User的数据，修改com.itheima.test.AccountTest中的testFindAll方法

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<Account> accounts = accountDao.findAll();
        for(Account account : accounts){
            System.out.println("--------每个account的信息------------");
            System.out.println(account);
            System.out.println(account.getUser());
        }
    }

再次运行com.itheima.test.AccountTest中的testFindAll方法
在这里插入图片描述

一对多查询

在查询用户的同时，我们需要获取到用户下所有用户的信息
com.itheima.dao.IUserDao

    /**
     * 查询所有用户，同时获取到用户下所有账户的信息
     * @return
     */
    List<User> findAll();

所以需要在User实体类中建立关系映射
com.itheima.domain.User

package com.itheima.domain;

import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {

	//省略部分set/get方法，保留新加入的成员变量、set/get方法和部分代码
    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    //一对多关系映射：主表实体应该包含从表实体的集合引用
    private List<Account> accounts;

    public List<Account> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<Account> accounts) {
        this.accounts = accounts;
    }

}

定义User的resultMap
com/itheima/dao/IUserDao.xml

    <!-- 定义User的resultMap-->
    <resultMap id="userAccountMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!-- 配置user对象中accounts集合的映射 -->
        <collection property="accounts" ofType="account">
            <id column="aid" property="id"></id>
            <result column="uid" property="uid"></result>
            <result column="money" property="money"></result>
        </collection>
    </resultMap>
    
    <!-- 查询所有 -->
    <select id="findAll" resultMap="userAccountMap">
        select u.*,a.id as aid,a.uid,a.money from user u left outer join account a on u.id = a.uid
    </select>

建立一个测试类和测试方法
com.itheima.test.UserTest

package com.itheima.test;

import com.itheima.dao.IUserDao;
import com.itheima.domain.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class UserTest {

    private InputStream in;
    private SqlSession sqlSession;
    private IUserDao userDao;

    @Before//用于在测试方法执行之前执行
    public void init()throws Exception{
        //1.读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //3.获取SqlSession对象
        sqlSession = factory.openSession(true);
        //4.获取dao的代理对象
        userDao = sqlSession.getMapper(IUserDao.class);
    }

    @After//用于在测试方法执行之后执行
    public void destroy()throws Exception{
        //提交事务
        // sqlSession.commit();
        //6.释放资源
        sqlSession.close();
        in.close();
    }

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<User> users = userDao.findAll();
        for(User user : users){
            System.out.println("-----每个用户的信息------");
            System.out.println(user);
            System.out.println(user.getAccounts());
        }
    }

}

运行com.itheima.test.UserTest中的testFindAll方法，结果如下
在这里插入图片描述

多对多查询操作

分析

示例：用户和角色的关系

一个用户可以有多个角色
一个角色可以赋予多个用户

步骤：

建立两张表：用户表，角色表
让用户表和角色表具有多对多的关系。需要使用中间表，中间表中包含各自的主键，在中间表中是外键。
建立两个实体类：用户实体类和角色实体类
让用户和角色的实体类能体现出来多对多的关系，各自包含对方一个集合引用
建立两个配置文件
用户的配置文件、角色的配置文件
实现配置：
当我们查询用户时，可以同时得到用户所包含的角色信息
当我们查询角色时，可以同时得到角色的所赋予的用户信息

环境搭建

最终的目录结构入下：
在这里插入图片描述
初始环境如下：

pom.xml log4j.properties jdbcConfig.properties SqlMapConfig.xml和上面的环境搭建一样，这里不在赘述

数据库数据

DROP TABLE IF EXISTS `role`;

CREATE TABLE `role` (
  `ID` int(11) NOT NULL COMMENT '编号',
  `ROLE_NAME` varchar(30) default NULL COMMENT '角色名称',
  `ROLE_DESC` varchar(60) default NULL COMMENT '角色描述',
  PRIMARY KEY  (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;



insert  into `role`(`ID`,`ROLE_NAME`,`ROLE_DESC`) values (1,'院长','管理整个学院'),(2,'总裁','管理整个公司'),(3,'校长','管理整个学校');





DROP TABLE IF EXISTS `user_role`;

CREATE TABLE `user_role` (
  `UID` int(11) NOT NULL COMMENT '用户编号',
  `RID` int(11) NOT NULL COMMENT '角色编号',
  PRIMARY KEY  (`UID`,`RID`),
  KEY `FK_Reference_10` (`RID`),
  CONSTRAINT `FK_Reference_10` FOREIGN KEY (`RID`) REFERENCES `role` (`ID`),
  CONSTRAINT `FK_Reference_9` FOREIGN KEY (`UID`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert  into `user_role`(`UID`,`RID`) values (41,1),(45,1),(41,2);

实体类代码

com.itheima.domain.User

package com.itheima.domain;

import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {

    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    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 String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", address='" + address + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday=" + birthday +
                '}';
    }
}

接口

com.itheima.dao.IUserDao

package com.itheima.dao;

import com.itheima.domain.User;

import java.util.List;

/**
 * 用户的持久层接口
 */
public interface IUserDao {

    /**
     * 查询所有用户，同时获取到用户下所有账户的信息
     * @return
     */
    List<User> findAll();


    /**
     * 根据id查询用户信息
     * @param userId
     * @return
     */
    User findById(Integer userId);


}

配置文件

IUserDao.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.itheima.dao.IUserDao">

    <!-- 定义User的resultMap-->
    <resultMap id="userAccountMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="userAccountMap">
        select * from user 
    </select>

    <!-- 根据id查询用户 -->
    <select id="findById" parameterType="INT" resultType="user">
        select * from user where id = #{uid}
    </select>

</mapper>

测试类

package com.itheima.test;

import com.itheima.dao.IUserDao;
import com.itheima.domain.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class UserTest {

    private InputStream in;
    private SqlSession sqlSession;
    private IUserDao userDao;

    @Before//用于在测试方法执行之前执行
    public void init()throws Exception{
        //1.读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //3.获取SqlSession对象
        sqlSession = factory.openSession(true);
        //4.获取dao的代理对象
        userDao = sqlSession.getMapper(IUserDao.class);
    }

    @After//用于在测试方法执行之后执行
    public void destroy()throws Exception{
        //提交事务
        // sqlSession.commit();
        //6.释放资源
        sqlSession.close();
        in.close();
    }

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<User> users = userDao.findAll();
        for(User user : users){
            System.out.println("-----每个用户的信息------");
            System.out.println(user);
        }
    }
}

创建Role类
com.itheima.domain.Role

package com.itheima.domain;

import java.io.Serializable;
import java.util.List;

public class Role implements Serializable {
	
	//这里我们是按照java的规范，驼峰命名法，所以后面要在配置文件中配置resultMap	
    private Integer roleId;
    private String roleName;
    private String roleDesc;

    public Integer getRoleId() {
        return roleId;
    }

    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;
    }

    @Override
    public String toString() {
        return "Role{" +
                "roleId=" + roleId +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }
}

创建接口IRoleDao
com.itheima.dao.IRoleDao

package com.itheima.dao;

import com.itheima.domain.Role;

import java.util.List;

public interface IRoleDao {

    /**
     * 查询所有角色
     * @return
     */
    List<Role> findAll();
}

创建配置文件IRoleDao.xml
com/itheima/dao/IRoleDao.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.itheima.dao.IRoleDao">

    <!--定义role表的ResultMap-->
    <resultMap id="roleMap" type="role">
        <id property="roleId" column="id"></id>
        <!-- windows系统下MySQL不区分大小写，linux是区分的，所以要注意一下 -->
        <result property="roleName" column="role_name"></result>
        <result property="roleDesc" column="role_desc"></result>
    </resultMap>

    <!--查询所有-->
    <select id="findAll" resultMap="roleMap">
       select * from role
    </select>
</mapper>

创建测试类和测试方法
com.itheima.test.RoleTest

package com.itheima.test;

import com.itheima.dao.IRoleDao;
import com.itheima.domain.Role;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class RoleTest {

    private InputStream in;
    private SqlSession sqlSession;
    private IRoleDao roleDao;

    @Before//用于在测试方法执行之前执行
    public void init()throws Exception{
        //1.读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //3.获取SqlSession对象
        sqlSession = factory.openSession(true);
        //4.获取dao的代理对象
        roleDao = sqlSession.getMapper(IRoleDao.class);
    }

    @After//用于在测试方法执行之后执行
    public void destroy()throws Exception{
        //提交事务
        // sqlSession.commit();
        //6.释放资源
        sqlSession.close();
        in.close();
    }

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<Role> roles = roleDao.findAll();
        for(Role role : roles){
            System.out.println("---每个角色的信息----");
            System.out.println(role);
        }
    }
}

运行测试方法testFindAll，结果如下：
在这里插入图片描述

查询角色的时候同时获得用户的信息

在实体类Role中引用User

package com.itheima.domain;

import java.io.Serializable;
import java.util.List;

public class Role implements Serializable {

    private Integer roleId;
    private String roleName;
    private String roleDesc;

    //多对多的关系映射：一个角色可以赋予多个用户
    private List<User> users;

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

}

编写sql语句

 select u.*,r.id as rid,r.role_name,r.role_desc from role r left outer join user_role ur  on r.id = ur.rid left outer join user u on u.id = ur.uid;

配置Role实体类对User对象的数据映射
com/itheima/dao/IRoleDao.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.itheima.dao.IRoleDao">

    <!--定义role表的ResultMap-->
    <resultMap id="roleMap" type="role">
        <!-- 角色表中的id起别名为rid，所以这里需要写上rid -->
        <id property="roleId" column="rid"></id>
        <!-- windows系统下MySQL不区分大小写，linux是区分的，所以要注意一下 -->
        <result property="roleName" column="role_name"></result>
        <result property="roleDesc" column="role_desc"></result>
        <collection property="users" ofType="user">
            <id column="id" property="id"></id>
            <result column="username" property="username"></result>
            <result column="address" property="address"></result>
            <result column="sex" property="sex"></result>
            <result column="birthday" property="birthday"></result>
        </collection>
    </resultMap>

    <!--查询所有-->
    <select id="findAll" resultMap="roleMap">
    	<!-- sql语句在换行的时候一定要在行末加上一个或几个空格，防止mybatis解析的时候把换行符清除掉，语句中间没有空格导致错误-->
        select u.*,r.id as rid,r.role_name,r.role_desc from role r
        left outer join user_role ur  on r.id = ur.rid
        left outer join user u on u.id = ur.uid
    </select>
</mapper>

测试类方法com.itheima.test.RoleTest

    /**
     * 测试查询所有
     */
    @Test
    public void testFindAll(){
        List<Role> roles = roleDao.findAll();
        for(Role role : roles){
            System.out.println("---每个角色的信息----");
            System.out.println(role);
            System.out.println(role.getUsers());
        }
    }