（五）测试学习JavaWeb之MyBatis上篇

前言

Web开发离不开数据库的操作，该篇文章咱们一起来学习MyBatis 这款优秀的持久层框架，官网对MyBatis描述如下。

MyBatis支持定制化 SQL、存储过程以及高级映射。它避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。

原理

每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或一个预先定制的 Configuration 的实例构建出 SqlSessionFactory 的实例。

下面以mysql为例，通过实现数据库增删改查、多表关联、动态sql、整合Spring等功能来逐步深入学习MyBatis。

pom依赖

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.2.8</version>
</dependency>

<!-- mysql-connector -->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.29</version>
</dependency>

MyBatis增删改查入门

项目结构

1、建表

CREATE TABLE mybatis(
idno VARCHAR(30) PRIMARY KEY,
idtype INT(2),
username VARCHAR(30),
phone VARCHAR(20),
address VARCHAR(100)
);

2、新建实体类

建立与数据库映射的是实体类DataPoolEntity。

package com.entity;

public class DataPoolEntity{

    private String idno;
    private int idtype;
    private String username;
    private String phone;
    private String address;

    public DataPoolEntity(){
    }

    public DataPoolEntity(String idno,int idtype,String username,String phone,String address){
        this.idno = idno;
        this.idtype = idtype;
        this.username = username;
        this.phone = phone;
        this.address = address;
    }

    public String getIdno() {
        return idno;
    }

    public int getIdtype() {
        return idtype;
    }

    public String getUsername() {
        return username;
    }

    public String getPhone() {
        return phone;
    }

    public String getAddress() {
        return address;
    }

    public void setIdno(String idno) {
        this.idno = idno;
    }

    public void setIdtype(int idtype) {
        this.idtype = idtype;
    }

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

    public void setPhone(String phone) {
        this.phone = phone;
    }

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

3、新建映射接口

package com.mapper;

import com.entity.DataPoolEntity;

import java.util.List;

public interface DataPoolMapper {

    //根据idno查询
    public DataPoolEntity selectByIdno(String idno);

    //根据姓名和手机查询
    public List<DataPoolEntity> selectByUsernameAndPhone(String username, String phone);

    //新增记录
    public void insertData(DataPoolEntity dataPoolEntity);

    //删除记录
    public void deleteByIdno(String idno);

    //更新记录
    public void updateData(DataPoolEntity dataPoolEntity);
}

4、新建工具类

package com.utils;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;

public class MybatisUtil {
    private static SqlSessionFactory sqlSessionFactory;

    public static SqlSessionFactory getSqlSessionFactory(){
        if(sqlSessionFactory == null){
            InputStream inputStream = MybatisUtil.class.getClassLoader().getResourceAsStream("mybatisConfig.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        }

        return sqlSessionFactory;
    }

    public static SqlSession openSession(){
        return getSqlSessionFactory().openSession();
    }
}

5、新建配置文件

mysql.properties定义数据库的链接信息，下文的mybatisConfig.xml配置文件可通过properties属性引用。

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://127.0.0.1:3306/iiaccount_datapool
jdbc.username=root
jdbc.password=root

mybatisMapper.xml为映射文件，定义了sql语句的参数及返回类型等，与第3点的映射接口相关联。如果表的列名与实体类的成员变量名不一致的话，此时不应该使用resultType，而应该使用resultMap，否则mybatis将无法进行映射，下文会举例验证。

<?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">
<!--namespace为映射接口全路径，否则会报错-->
<mapper namespace="com.mapper.DataPoolMapper">
    <!--id名称与DataPoolMapper接口的方法名一致-->
    <insert id="insertData" useGeneratedKeys="true" keyProperty="idno">
        insert into mybatis(idno,idtype,username,phone,address) values (#{idno},#{idtype},
        #{username},#{phone},#{address})
    </insert>
    
    <update id="updateData">
        update mybatis set username=#{username},phone=#{phone},address=#{address} where idno
        = #{idno}
    </update>

    <delete id="deleteByIdno">
        delete from mybatis where idno = #{idno}
    </delete>

    <!--id与mapper类方法名一致-->
    <!--entity为mybatisConfig.xml创建的别名-->
    <!--可以使用resultType或resultMap-->
    <!--如果是多参数则不能使用parameterType， 改用#｛index｝来表示selectByUsernameAndPhone方法的第几个参数，索引从0开始-->
    <select id="selectByUsernameAndPhone" resultType="entity">
        select * from mybatis where username = #{0} and phone = #{1}
    </select>

    <!--单个参数可以使用parameterType-->
    <select id="selectByIdno" parameterType="String" resultType="entity">
        select * from mybatis where idno = #{idno}
    </select>
</mapper>

mybatisConfig.xml是 mybatis 用来建立 SqlSessionFactory用的，具体运用见第4点的工具类MybatisUtil 。该配置文件包含了数据库的链接信息以及数据库映射文件mybatisMapper.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>
    <properties resource="mysql.properties">
    </properties>

    <typeAliases>
        <!--定义别名-->
        <typeAlias type="com.entity.DataPoolEntity" alias="entity"/>
    </typeAliases>


    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"></transactionManager>
            <!-- mybatis提供了3种数据源类型，分别是：POOLED,UNPOOLED,JNDI -->
            <!-- POOLED 表示支持JDBC数据源连接池 -->
            <!-- UNPOOLED 表示不支持数据源连接池 -->
            <!-- JNDI 表示支持外部数据源连接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="mybatisMapper.xml"/>
    </mappers>
</configuration>

6、增删改查验证

新建测试类MybatisTest验证增删改查操作

package com.mybatis;

import com.entity.DataPoolEntity;
import com.mapper.DataPoolMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class MybatisTest {

    public static void main(String[] args) {
        insert();
        //delete();
        selectByUsernameAndPhone();
        update();
        selectById();

    }

    public static void insert() {
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        DataPoolEntity dataPoolEntity = new DataPoolEntity("4408810", 1, "Tomandy", "13692464000", "PK");
        try {
            dataPoolMapper.insertData(dataPoolEntity);
            sqlSession.commit();
            System.out.println("新增成功！");
            System.out.println("----------------");
        } catch (Exception e) {
            sqlSession.rollback();
        } finally {
            sqlSession.close();
        }

    }

    public static void update() {
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        //先查询再修改
        DataPoolEntity dataPoolEntity = dataPoolMapper.selectByIdno("4408810");
        dataPoolEntity.setAddress("XIAN");
        dataPoolEntity.setPhone("13692464999");
        dataPoolEntity.setUsername("Tomandy123");

        try {
            dataPoolMapper.updateData(dataPoolEntity);
            sqlSession.commit();
            System.out.println("修改成功");
            System.out.println("----------------");
        } catch (Exception e) {
            sqlSession.rollback();
        } finally {
            sqlSession.close();
        }
    }

    public static void delete() {
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        try {
            dataPoolMapper.deleteByIdno("4408810");
            sqlSession.commit();
            System.out.println("删除成功！");
            System.out.println("----------------");
        } catch (Exception e) {
            sqlSession.rollback();
        } finally {
            sqlSession.close();
        }

    }

    public static void selectById(){
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        DataPoolEntity dataPoolEntity = dataPoolMapper.selectByIdno("4408810");
        System.out.println("selectById查询信息如下：");
        System.out.println("idno:"+dataPoolEntity.getIdno());
        System.out.println("idtype:"+dataPoolEntity.getIdtype());
        System.out.println("username:"+dataPoolEntity.getUsername());
        System.out.println("phone:"+dataPoolEntity.getPhone());
        System.out.println("address:"+dataPoolEntity.getAddress());
        System.out.println("----------------");
    }

    public static void selectByUsernameAndPhone(){
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        List<DataPoolEntity> dataPoolEntityList  = dataPoolMapper.selectByUsernameAndPhone("Tomandy","13692464000");
        for(DataPoolEntity dataPoolEntity:dataPoolEntityList){
            System.out.println("selectByUsernameAndPhone查询信息如下：");
            System.out.println("idno:"+dataPoolEntity.getIdno());
            System.out.println("idtype:"+dataPoolEntity.getIdtype());
            System.out.println("username:"+dataPoolEntity.getUsername());
            System.out.println("phone:"+dataPoolEntity.getPhone());
            System.out.println("address:"+dataPoolEntity.getAddress());
            System.out.println("----------------");
        }
    }

}

执行结果如下

新增成功！
----------------
selectByUsernameAndPhone查询信息如下：
idno:4408810
idtype:1
username:Tomandy
phone:13692464000
address:PK
----------------
修改成功
----------------
selectById查询信息如下：
idno:4408810
idtype:1
username:Tomandy123
phone:13692464999
address:XIAN
----------------

7、xml映射文件resultType和resultMap的区别

上文提到，如果表的列名与实体类的成员变量名不一致的话，此时不应该使用resultType，而应该使用resultMap，下面通过例子来进行验证。
修改实体类原成员变量username为username_，其他配置文件还是跟上文的一致。

package com.entity;

public class DataPoolEntity{

    private String idno;
    private int idtype;
    private String username_;
    private String phone;
    private String address;

    public DataPoolEntity(){
    }

    public DataPoolEntity(String idno,int idtype,String username,String phone,String address){
        this.idno = idno;
        this.idtype = idtype;
        this.username_ = username;
        this.phone = phone;
        this.address = address;
    }

    public String getIdno() {
        return idno;
    }

    public int getIdtype() {
        return idtype;
    }

    public String getUsername_() {
        return username_;
    }

    public String getPhone() {
        return phone;
    }

    public String getAddress() {
        return address;
    }

    public void setIdno(String id_no) {
        this.idno = id_no;
    }

    public void setIdtype(int idtype) {
        this.idtype = idtype;
    }

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

    public void setPhone(String phone) {
        this.phone = phone;
    }

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

编写测试类进行验证。

package com.mybatis;

import com.entity.DataPoolEntity;
import com.mapper.DataPoolMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class MybatisTest {

    public static void main(String[] args) {
        selectById();
    }

    public static void selectById(){
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        DataPoolEntity dataPoolEntity = dataPoolMapper.selectByIdno("4408810");
        System.out.println("selectById查询信息如下：");
        System.out.println("idno:"+dataPoolEntity.getIdno());
        System.out.println("idtype:"+dataPoolEntity.getIdtype());
        System.out.println("username:"+dataPoolEntity.getUsername_());
        System.out.println("phone:"+dataPoolEntity.getPhone());
        System.out.println("address:"+dataPoolEntity.getAddress());
        System.out.println("----------------");
    }
}

运行测试，输出如下，可发现username输出了null，这说明映射失败。

selectById查询信息如下：
idno:4408810
idtype:1
username:null
phone:13692464000
address:XIAN
----------------

我们把mybatisMapper.xml改为resultMap的形式试试。

<?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.mapper.DataPoolMapper">
    
    <resultMap id="map" type="entity">
        <!-- id 属性专门用来映射主键信息，其他信息用result节点
        column 数据库字段
        property 实体类成员属性 -->
        <id column="idno" property="idno"/>
        <result column="idtype" property="idtype"/>
        <result column="username" property="username_"/>
        <result column="phone" property="phone"/>
        <result column="address" property="address"/>
    </resultMap>

    <!--单个参数可以使用parameterType-->
    <select id="selectByIdno" parameterType="String" resultMap="map">
        select * from mybatis where idno = #{idno}
    </select>
</mapper>

接着运行上面的测试类，可发现输出结果中username不再为null。

selectById查询信息如下：
idno:4408810
idtype:1
username:Tomandy
phone:13692464000
address:XIAN
----------------

由此可见，resultMap 仅仅是作用数据库表字段跟实体类的属性的映射关系而存在，当需要映射的实体类的属性跟数据库字段不一样的时候使用，
但是如果数据库字段名跟需要映射的实体类的属性名完全一致，resultMap可以不用，resultType足够了。

8、映射文件属性“useGeneratedKeys”和“keyProperty”详解

上文的mybatisMapper.xml配置文件insert中用到了“useGeneratedKeys”和“keyProperty”属性（由于上文例子没有用到自增列主键，这两个属性没有实际意义），其中，useGeneratedKeys设置为 true 时，表示如果插入的表id以自增列为主键，则允许 JDBC 支持自动生成主键，并可将自动生成的主键id返回，举例如下：

<insert id="insert" parameterType="user"   
        useGeneratedKeys="true" keyProperty="id">  
        insert into user(user_id,user_name,  
            user_phone)  
        values(#{id},#{name},#{phone})  
    </insert> 

Mybatis执行完插入语句后，自动将自增长值赋值给对象user的属性id，可通过user实体类对应的getterId()方法获取！
另外，对于useGeneratedKeys和keyProperty的用法，官网也给了相应的说明。

<insert id="insertAuthor">
  insert into Author (id,username,password,email,bio)
  values (#{id},#{username},#{password},#{email},#{bio})
</insert>

如果你的数据库支持自动生成主键的字段（比如 MySQL 和 SQL Server），那么你可以设置 useGeneratedKeys=”true”，然后再把 keyProperty 设置到目标属性上就OK了。例如，如果上面的 Author 表已经对 id 使用了自动生成的列类型，那么语句可以修改为:

<insert id="insertAuthor" useGeneratedKeys="true"
    keyProperty="id">
  insert into Author (username,password,email,bio)
  values (#{username},#{password},#{email},#{bio})
</insert>

MyBatis多表关联操作

1、ResultMap详解

在开始多表关联操作学习之前，首先来深入了解一下ResultMap，前文提到ResultMap是作用数据库表字段跟实体类属性的映射关系而存在的。结合官网的相关例子，我们来学习下ResultMap的高级用法。
resultMap元素里包含了以下子元素。

名称	用途
id	一个 ID 结果;标记出作为 ID 的结果可以帮助提高整体性能
result	注入到字段或 JavaBean 属性的普通结果
constructor	用于在实例化类时，注入结果到构造方法中
association	一个复杂类型的关联;许多结果将包装成这种类型
collection	一个复杂类型的集合
discriminator	使用结果值来决定使用哪个 resultMap

id和result子元素上文已举例说明，下面通过举例对其他几个子元素做详细的说明。

• constructor子元素举例说明
  mabatisMapper.xml配置，column属性顺序需与实体类构造函数入参顺序一致，官网也提供了顺序不一致的解决方法，详情参考官网。

<?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.mapper.DataPoolMapper">
    
    <resultMap id="map" type="entity">
        <!-- id 属性专门用来映射主键信息，其他信息用result节点
        column 数据库字段
        property 实体类成员属性 -->
        <!--
        <id column="idno" property="idno"/>
        <result column="idtype" property="idtype"/>
        <result column="username" property="username_"/>
        <result column="phone" property="phone"/>
        <result column="address" property="address"/>
        -->
        <constructor>
            <idArg column="idno" javaType="String"/>
            <arg column="idtype" javaType="_int"/>
            <arg column="username" javaType="String"/>
            <arg column="phone" javaType="String"/>
            <arg column="address" javaType="String"/>
        </constructor>
    </resultMap>
    
    <!--单个参数可以使用parameterType-->
    <select id="selectByIdno" parameterType="String" resultMap="map">
        select * from mybatis where idno = #{idno}
    </select>
</mapper>

编写实体类DataPoolEntity

package com.entity;

public class DataPoolEntity{

    private String idno;
    private int idtype;
    private String username_;
    private String phone;
    private String address;

    public DataPoolEntity(){
    }

    public DataPoolEntity(String idno,int idtype,String username,String phone,String address){
        this.idno = idno;
        this.idtype = idtype;
        this.username_ = username;
        this.phone = phone;
        this.address = address;
    }

    public String getIdno() {
        return idno;
    }

    public int getIdtype() {
        return idtype;
    }

    public String getUsername_() {
        return username_;
    }

    public String getPhone() {
        return phone;
    }

    public String getAddress() {
        return address;
    }
}

编写测试类MybatisTest

package com.mybatis;

import com.entity.DataPoolEntity;
import com.mapper.DataPoolMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class MybatisTest {

    public static void main(String[] args) {
        selectById();
    }
     public static void selectById(){
        SqlSession sqlSession = MybatisUtil.openSession();
        DataPoolMapper dataPoolMapper = sqlSession.getMapper(DataPoolMapper.class);

        DataPoolEntity dataPoolEntity = dataPoolMapper.selectByIdno("4408810");
        System.out.println("selectById查询信息如下：");
        System.out.println("idno:"+dataPoolEntity.getIdno());
        System.out.println("idtype:"+dataPoolEntity.getIdtype());
        System.out.println("username:"+dataPoolEntity.getUsername_());
        System.out.println("phone:"+dataPoolEntity.getPhone());
        System.out.println("address:"+dataPoolEntity.getAddress());
        System.out.println("----------------");
    }

执行后输出结果如下：

selectById查询信息如下：
idno:4408810
idtype:1
username:Tomandy
phone:13692464000
address:PK
----------------

对比前文配置的mybatisMapper.xml文件和实体类，可发现使用constructor子元素的话，mybatis也可映射成功。

• association子元素举例说明
  association用于一对一的关联，常见的有三种用法，下面举例说明。

场景：
一个用户有一张携程的会员卡。分别对应客户信息表和会员卡信息表，通过id关联。

先建表并insert记录。

#客户信息表
drop table if exists customer;
create table customer(
id varchar(20) primary key,
idtype int(2),  #证件类型
idno varchar(30), #证件号码
username varchar(50) not null, #名称
sex varchar(10) not null #性别
);

#会员卡表
drop table if exists membershipCard;
create table membershipCard(
id varchar(20) primary key,
level int(2) not null, #会员等级
phone varchar(20) not null, #电话
address varchar(100) #地址
);

insert into customer(id,idtype,idno,username,sex) values("123",1,"44088118","Tomandy","male")；
insert into membershipcard(id,level,phone,address) values("123",1,"18601111","Pk");

新建实体类CustomerEntity及MembershipCardEntity。

package com.entity;

public class CustomerEntity {

    private String id;
    private int idtype;
    private String idno;
    private String username;
    private String sex;
    private MembershipCardEntity membershipCardEntity;

    public String getId() {
        return id;
    }

    public int getIdtype() {
        return idtype;
    }

    public String getIdno() {
        return idno;
    }

    public String getUsername() {
        return username;
    }

    public String getSex() {
        return sex;
    }

    public MembershipCardEntity getMembershipCardEntity() {
        return membershipCardEntity;
    }

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

    public void setIdtype(int idtype) {
        this.idtype = idtype;
    }

    public void setIdno(String idno) {
        this.idno = idno;
    }

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

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

    public void setMembershipCardEntity(MembershipCardEntity membershipCardEntity) {
        this.membershipCardEntity = membershipCardEntity;
    }
}

package com.entity;

public class MembershipCardEntity {

    private String id;
    private int level;
    private String phone;
    private String address;

    public String getId() {
        return id;
    }

    public int getLevel() {
        return level;
    }

    public String getPhone() {
        return phone;
    }

    public String getAddress() {
        return address;
    }

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

    public void setLevel(int level) {
        this.level = level;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

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

新建映射接口XieChengMapper

package com.mapper;

import com.entity.CustomerEntity;

public interface XieChengMapper {

    CustomerEntity queryById(String id);
}

association第一种用法：
新建映射配置文件xiechengMapper.xml，注意resultMap里面用到了association子元素。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="cutomerEntity">
        <id property="id" column="id"/>
        <result property="idtype" column="idtype"/>
        <result property="idno" column="idno"/>
        <result property="username" column="username"/>
        <result property="sex" column="sex"/>
        <!--property属性值需要在CustomerEntity有相应的getter方法-->
        <!--javaType为mybatisConfig.xml定义com.entity.MembershipCardEntity的别名-->
        <association property="membershipCardEntity" javaType="membershipCardEntity">
            <id property="id" column="id"/>
            <result property="level" column="level"/>
            <result property="phone" column="phone"/>
            <result property="address" column="address"/>
        </association>
    </resultMap>
    
    <select id="queryById" resultMap="cmmap" parameterType="String">
        select
        a.id,a.sex,a.idno,a.username,b.phone,b.address
        from customer a,membershipCard b
        where a.id = b.id
        and a.id = #{id}
    </select>

</mapper>

添加映射文件到mybatisConfig.xml配置文件，相比上文的配置文件，此处增加了cutomerEntity，membershipCardEntity两个别名，另外也引入了xiechengMapper.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>
    <properties resource="mysql.properties">
    </properties>

    <typeAliases>
        <!--定义别名-->
        <typeAlias type="com.entity.DataPoolEntity" alias="entity"/>
        <typeAlias type="com.entity.CustomerEntity" alias="cutomerEntity"/>
        <typeAlias type="com.entity.MembershipCardEntity" alias="membershipCardEntity"/>
    </typeAliases>


    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"></transactionManager>
            <!-- mybatis提供了3种数据源类型，分别是：POOLED,UNPOOLED,JNDI -->
            <!-- POOLED 表示支持JDBC数据源连接池 -->
            <!-- UNPOOLED 表示不支持数据源连接池 -->
            <!-- JNDI 表示支持外部数据源连接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="mybatisMapper.xml"/>
        <!--此处新加了映射配置文件-->
        <mapper resource="xiechengMapper.xml"/>
    </mappers>
</configuration>

编写测试类

package com.mybatis;

import com.entity.CustomerEntity;
import com.mapper.XieChengMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

public class XieChengTest {

    public static void main(String args[]){
        queryByid();
    }

    public static void queryByid(){
        SqlSession sqlSession = MybatisUtil.openSession();
        XieChengMapper xieChengMapper = sqlSession.getMapper(XieChengMapper.class);
        CustomerEntity customerEntity = xieChengMapper.queryById("123");

        System.out.println("id: "+customerEntity.getId());
        System.out.println("idno: "+customerEntity.getIdno());
        System.out.println("username: "+customerEntity.getUsername());
        System.out.println("sex: "+customerEntity.getSex());
        System.out.println("address: "+customerEntity.getMembershipCardEntity().getAddress());
        System.out.println("phone: "+customerEntity.getMembershipCardEntity().getPhone());
    }
}

执行后运行结果输出如下。

id: 123
idno: 44088118
username: Tomandy
sex: male
address: Pk
phone: 18601111

association第二种用法：
修改xiechengMapper.xml如下，再执行测试类，也可得到同样的输出结果。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="cutomerEntity">
        <id property="id" column="id"/>
        <result property="idtype" column="idtype"/>
        <result property="idno" column="idno"/>
        <result property="username" column="username"/>
        <result property="sex" column="sex"/>
        <!--property属性值需要在CustomerEntity有相应的getter方法-->
        <!--javaType为mybatisConfig.xml定义com.entity.MembershipCardEntity的别名-->
        <!--
        <association property="membershipCardEntity" javaType="membershipCardEntity">
            <id property="id" column="id"/>
            <result property="level" column="level"/>
            <result property="phone" column="phone"/>
            <result property="address" column="address"/>
        </association>
        -->
        <association property="membershipCardEntity" resultMap="mscmap"/>
    </resultMap>

    <resultMap id="mscmap" type="membershipCardEntity">
        <id property="id" column="id"/>
        <result property="level" column="level"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
    </resultMap>
    
    <select id="queryById" resultMap="cmmap" parameterType="String">
        select
        a.id,a.sex,a.idno,a.username,b.phone,b.address
        from customer a,membershipCard b
        where a.id = b.id
        and a.id = #{id}
    </select>

</mapper>

对比一下association第一种用法，区别在于把membershipCardEntity的映射独立放在了一个resultMap里。

association第三种用法：该方法通过association的select属性来实现。假如给membershipCard表加上一列cardname（MembershipCardEntity实体类也加上了cardname对应的getter和setter方法），其值与customer表的username字段一样，修改xiechengMapper.xml如下。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="cutomerEntity">
        <id property="id" column="id"/>
        <result property="idtype" column="idtype"/>
        <result property="idno" column="idno"/>
        <result property="username" column="username"/>
        <result property="sex" column="sex"/>
        <!--property属性值需要在CustomerEntity有相应的getter方法-->
        <!--javaType为mybatisConfig.xml定义com.entity.MembershipCardEntity的别名-->
        <!--
        <association property="membershipCardEntity" javaType="membershipCardEntity">
            <id property="id" column="id"/>
            <result property="level" column="level"/>
            <result property="phone" column="phone"/>
            <result property="address" column="address"/>
        </association>
        -->
        <!--
        <association property="membershipCardEntity" resultMap="mscmap"/>
        -->
        <!--如果涉及多参数查询的话，可通过column="{id=id,phone=phone}"格式来传递变量 -->
        <association property="membershipCardEntity" column="{id=id,cardname=username}" select="queryByCardId"/>
    </resultMap>

    <!--
    <resultMap id="mscmap" type="membershipCardEntity">
        <id property="id" column="id"/>
        <result property="level" column="level"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
    </resultMap>
    -->

    <!--对于多参数传递的场景，parameterType属性值需使用Map-->
    <select id="queryByCardId" parameterType="java.util.Map" resultType="membershipCardEntity">
        select id,level,phone,address from membershipCard where id = #{id} and cardname =#{cardname}
    </select>
    <!--
    <select id="queryById" resultMap="cmmap" parameterType="String">
        select
        a.id,a.sex,a.idno,a.username,b.phone,b.address
        from customer a,membershipCard b
        where a.id = b.id
        and a.id = #{id}
    </select>
    -->
    <select id="queryById" resultMap="cmmap" parameterType="String">
        select
        *
        from customer 
        where id = #{id}
    </select>

</mapper>

执行上文的测试类，可发现输出结果也是一样的。由xiechengMapper.xml配置文件的内容，我们也可以推断出先执行了queryById的select语句，然后再把对应列的值传递给queryByCardId的select语句变量进行查询，从而获取查询结果。

• collection子元素举例说明
  collection用于一对多的关联，与association类似，常见也有三种用法，下面举例说明。

场景：
携程会员可享受多种产品优惠（一对多）。

在上文association例子的基础上，再新建一张产品优惠信息表并insert数据。

drop table if exists product;
create table product(
id varchar(20) not null,
productno varchar(30) not null,
productname varchar(100) not null,
PRIMARY KEY (`id`,`productno`)
);

insert into product(id,productno,productname) values("123","01","hotel");
insert into product(id,productno,productname) values("123","02","train tickets");
insert into product(id,productno,productname) values("123","03","plane tickets");

新增实体类ProductEntity，并修改MembershipCardEntity。

package com.entity;

public class ProductEntity {

    private String id;
    private String productno;
    private String productname;

    public String getId() {
        return id;
    }

    public String getProductno() {
        return productno;
    }

    public String getProductname() {
        return productname;
    }

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

    public void setProductno(String productno) {
        this.productno = productno;
    }

    public void setProductname(String productname) {
        this.productname = productname;
    }
}

package com.entity;

import java.util.List;

public class MembershipCardEntity {

    private String id;
    private int level;
    private String phone;
    private String address;
    private String cardname;
    private List<ProductEntity> productEntities;

    public String getId() {
        return id;
    }

    public int getLevel() {
        return level;
    }

    public String getPhone() {
        return phone;
    }

    public String getAddress() {
        return address;
    }

    public String getCardname() {
        return cardname;
    }

    public List<ProductEntity> getProductEntities() {
        return productEntities;
    }

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

    public void setLevel(int level) {
        this.level = level;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

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

    public void setCardname(String cardname) {
        this.cardname = cardname;
    }

    public void setProductEntities(List<ProductEntity> productEntities) {
        this.productEntities = productEntities;
    }
}

修改映射接口XieChengMapper如下。

package com.mapper;

import com.entity.CustomerEntity;
import com.entity.MembershipCardEntity;

public interface XieChengMapper {

    CustomerEntity queryById(String id);

    //增加根据id查询优惠产品信息
    MembershipCardEntity queryProductById(String id);
}

collection第一种用法如下：
修改映射文件xiechengMapper.xml如下。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="membershipCardEntity">
        <id property="id" column="id"/>
        <result property="level" column="level"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
        <result property="cardname" column="cardname"/>
        <collection property="productEntities" javaType="java.util.List" ofType="com.entity.ProductEntity">
            <id property="id" column="id"/>
            <id property="productno" column="productno"/>
            <result property="productname" column="productname"/>
        </collection>
    </resultMap>

    <select id="queryProductById" parameterType="String" resultMap="cmmap">
        select * from membershipCard a,product b
        where a.id = b.id
        and
        a.id = #{id}
    </select>

</mapper>

修改测试类如下。

package com.mybatis;

import com.entity.CustomerEntity;
import com.entity.MembershipCardEntity;
import com.entity.ProductEntity;
import com.mapper.XieChengMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

public class XieChengTest {

    public static void main(String args[]){
        queryProductByid();
    }

    public static void queryProductByid(){
        SqlSession sqlSession = MybatisUtil.openSession();
        XieChengMapper xieChengMapper = sqlSession.getMapper(XieChengMapper.class);
        MembershipCardEntity membershipCardEntity = xieChengMapper.queryProductById("123");

        System.out.println("id: "+membershipCardEntity.getId());
        System.out.println("phone: "+membershipCardEntity.getPhone());
        System.out.println("address: "+membershipCardEntity.getAddress());
        System.out.println("cardname: "+membershipCardEntity.getCardname());
        System.out.println("Listsize: "+membershipCardEntity.getProductEntities().size());

        for(ProductEntity productEntity:membershipCardEntity.getProductEntities()){
            System.out.println("------------------------");
            System.out.println("id: "+productEntity.getId());
            System.out.println("productno: "+productEntity.getProductno());
            System.out.println("productname: "+productEntity.getProductname());

        }
    }

}

运行后，输出结果为如下。

id: 123
phone: 18601111
address: Pk
cardname: Tomandy
Listsize: 3
------------------------
id: 123
productno: 01
productname: hotel
------------------------
id: 123
productno: 02
productname: train tickets
------------------------
id: 123
productno: 03
productname: plane tickets

collection第二种用法如下：
修改xiechengMapper.xml如下。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="membershipCardEntity">
        <id property="id" column="id"/>
        <result property="level" column="level"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
        <result property="cardname" column="cardname"/>
        
        <collection property="productEntities" resultMap="pmap"/>

        <!--
        <collection property="productEntities" javaType="java.util.List" ofType="com.entity.ProductEntity">
            <id property="id" column="id"/>
            <id property="productno" column="productno"/>
            <result property="productname" column="productname"/>
        </collection>
        -->
    </resultMap>

    <resultMap id="pmap" type="com.entity.ProductEntity">
        <id property="id" column="id"/>
        <id property="productno" column="productno"/>
        <result property="productname" column="productname"/>
    </resultMap>

    <select id="queryProductById" parameterType="String" resultMap="cmmap">
        select * from membershipCard a,product b
        where a.id = b.id
        and
        a.id = #{id}
    </select>

</mapper>

执行上文的测试类，可发现输出同样的结果。
collection第三种用法如下：
与association第三种用法类似，可通过select属性来实现collection一对多的映射。修改xiechengMapper.xml如下。

<?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.mapper.XieChengMapper">
    <resultMap id="cmmap" type="membershipCardEntity">
        <id property="id" column="id"/>
        <result property="level" column="level"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
        <result property="cardname" column="cardname"/>

        <!--
        <collection property="productEntities" resultMap="pmap"/>
        -->

        <!--
        <collection property="productEntities" javaType="java.util.List" ofType="com.entity.ProductEntity">
            <id property="id" column="id"/>
            <id property="productno" column="productno"/>
            <result property="productname" column="productname"/>
        </collection>
        -->

        <collection property="productEntities" column="id" select="queryProductById"/>
    </resultMap>

    <!--
    <resultMap id="pmap" type="com.entity.ProductEntity">
        <id property="id" column="id"/>
        <id property="productno" column="productno"/>
        <result property="productname" column="productname"/>
    </resultMap>
    -->
    
    <select id="queryProductById" parameterType="java.util.Map" resultType="com.entity.ProductEntity">
        select * from product where id = #{id}
    </select>

    <select id="queryCardById" parameterType="String" resultMap="cmmap">
        select * from membershipCard where id = #{id}
    </select>

</mapper>

修改XieChengMapper映射接口如下。

package com.mapper;

import com.entity.CustomerEntity;
import com.entity.MembershipCardEntity;
import com.entity.ProductEntity;

import java.util.List;

public interface XieChengMapper {

    CustomerEntity queryById(String id);

    MembershipCardEntity queryCardById(String id);

    List<ProductEntity> queryProductById(String id);
}

编写测试类

package com.mybatis;

import com.entity.CustomerEntity;
import com.entity.MembershipCardEntity;
import com.entity.ProductEntity;
import com.mapper.XieChengMapper;
import com.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

public class XieChengTest {

    public static void main(String args[]){
        queryProductByid();
    }

    public static void queryProductByid(){
        SqlSession sqlSession = MybatisUtil.openSession();
        XieChengMapper xieChengMapper = sqlSession.getMapper(XieChengMapper.class);
        MembershipCardEntity membershipCardEntity = xieChengMapper.queryCardById("123");

        System.out.println("id: "+membershipCardEntity.getId());
        System.out.println("phone: "+membershipCardEntity.getPhone());
        System.out.println("address: "+membershipCardEntity.getAddress());
        System.out.println("cardname: "+membershipCardEntity.getCardname());
        System.out.println("Listsize: "+membershipCardEntity.getProductEntities().size());

        for(ProductEntity productEntity:membershipCardEntity.getProductEntities()){
            System.out.println("------------------------");
            System.out.println("id: "+productEntity.getId());
            System.out.println("productno: "+productEntity.getProductno());
            System.out.println("productname: "+productEntity.getProductname());

        }
    }

}

运行后发现输出结果与上文的一致。

• discriminator子元素举例说明
  discriminator称为鉴别器，依照官网的描述，主要用于以下场景。

有时一个单独的数据库查询也许返回很多不同 (但是希望有些关联) 数据类型的结果集。 鉴别器元素就是被设计来处理这个情况的, 还有包括类的继承层次结构。 鉴别器非常容易理 解,因为它的表现很像 Java 语言中的 switch 语句。

关于鉴别器的说明参考文章《MyBatis级联探讨第二篇——鉴别器（discriminator）》。

参考资料

《开发测试的Spring应用》
mybatis实战教程(mybatis in action),mybatis入门到精通
官方文档