本章节将介绍 <discriminator> 标签的用法，<discriminator> 标签使用比其他标签稍微复杂一些。该标签用来根据列的值，选择不同的结果映射。例如：
<discriminator column="type" javaType="INTEGER"> <case value="1" resultMap="BOOK_RESULT_MAP" /> <case value="2" resultMap="CONTACT_RESULT_MAP" /> </discriminator>
上面映射中，将根据 type 数据类的值，动态决定使用 BOOK_RESULT_MAP 或 CONTACT_RESULT_MAP 结果集映射。其中：
column：指定一个数据列名，该列名将用来实现动态选择结果集映射，如：type
javaType：指定 column 指定的数据列在 Java 中的数据类型，如：INTEGER
<case> 标签用来映射一个条件分支，上面就使用两个 <case> 标签映射两个映射分支。其中：
value：用来指定匹配当前条件分支的数据列值，如：type 列等于 1
resultMap：用来指定一个结果集映射的ID，如：BOOK_RESULT_MAP
其实，<discriminator> 标签就是一个标签样式的 switch 语句。如上示例，当 type 等于 1，则选择 BOOK_RESULT_MAP 结果集映射；当 type 等于 2，则选择 CONTACT_RESULT_MAP 结果集映射。转成 switch 语句如下：
String resultMap = "";
switch(type) {
case 1: resultMap = "BOOK_RESULT_MAP"; break;
case 2: resultMap = "CONTACT_RESULT_MAP"; break;
}（1）MyBatis 配置文件 mybatis-cfg.xml 内容如下：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="database.properties"/>
<environments default="MySqlDatabase" >
<environment id="MySqlDatabase" >
<transactionManager type="JDBC" />
<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="com\hxstrive\mybatis\resultmap\demo8\UserMapper.xml" />
</mappers>
</configuration>（2）UserBean、SubBean、BookBean 和 ContactBean 代码如下：
a、UserBean.java
public class UserBean {
private Integer userId;
private String name;
private String sex;
private Integer age;
private List<SubBean> subList;
public UserBean(Integer userId, String name) {
this.userId = userId;
this.name = name;
}
// 忽略 getter 和 setter
@Override
public String toString() {
StringBuilder subStrs = new StringBuilder();
for(SubBean sub : subList) {
subStrs.append("\n\t").append(sub.toString());
}
return "UserBean{" +
"userId=" + userId +
", name='" + name + '\'' +
", sex='" + sex + '\'' +
", age=" + age +
"} " + subStrs;
}
}b、SubBean.java
package com.hxstrive.mybatis.resultmap.demo8;
public class SubBean {
private Integer subId;
}c、BookBean.java
package com.hxstrive.mybatis.resultmap.demo8;
public class BookBean extends SubBean {
private String bookName;
private Float price;
private String content;
// 忽略 getter 和 setter
@Override
public String toString() {
return "BookBean{" +
"bookId=" + getSubId() +
", bookName='" + bookName + '\'' +
", price=" + price +
", content='" + content + '\'' +
'}';
}
}d、ContactBean.java
package com.hxstrive.mybatis.resultmap.demo8;
public class ContactBean extends SubBean {
private String usage;
private String number;
// 忽略 getter 和 setter
@Override
public String toString() {
return "ContactBean{" +
"id=" + getSubId() +
", usage='" + usage + '\'' +
", number='" + number + '\'' +
'}';
}
}（3）Mapper 接口文件 UserMapper.java 代码如下：
package com.hxstrive.mybatis.resultmap.demo8;
import java.util.List;
public interface UserMapper {
/** 查询所有用户信息 */
List<UserBean> findAll();
}（4）Mapper XML 文件 UserMapper.xml 代码如下：
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.hxstrive.mybatis.resultmap.demo8.UserMapper"> <!-- 定义 book 表的映射 --> <resultMap id="BOOK_RESULT_MAP" type="com.hxstrive.mybatis.resultmap.demo8.BookBean"> <result column="book_name" jdbcType="VARCHAR" property="bookName" /> <result column="price" jdbcType="FLOAT" property="price" /> <result column="content" jdbcType="VARCHAR" property="content" /> </resultMap> <!-- 定义 contact 表的映射 --> <resultMap id="CONTACT_RESULT_MAP" type="com.hxstrive.mybatis.resultmap.demo8.ContactBean"> <result column="contact_usage" jdbcType="VARCHAR" property="usage" /> <result column="contact_number" jdbcType="VARCHAR" property="number" /> </resultMap> <!-- 定义SQL语句映射 --> <resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.resultmap.demo8.UserBean"> <constructor> <idArg column="user_id" javaType="int" /> <arg column="name" javaType="String" /> </constructor> <result column="sex" jdbcType="VARCHAR" property="sex" /> <result column="age" jdbcType="INTEGER" property="age" /> <collection property="subList" ofType="com.hxstrive.mybatis.resultmap.demo8.SubBean"> <id column="sub_id" jdbcType="INTEGER" property="subId" /> <!-- 根据 type 列来选择不同的 resultMap --> <discriminator column="type" javaType="INTEGER"> <case value="1" resultMap="BOOK_RESULT_MAP" /> <case value="2" resultMap="CONTACT_RESULT_MAP" /> </discriminator> </collection> </resultMap> <select id="findAll" resultMap="RESULT_MAP"> select u.`user_id`, u.`name`, u.`sex`, u.`age`, ifnull(bk.book_id, ct.id) as sub_id, ifnull(bk.type, ct.type) as type, bk.book_name, bk.price, bk.content, ct.`usage` as contact_usage, ct.number as contact_number from `user` u left join ( select 1 as type, ub.user_id, b.* from `user_book` ub join `book` b on b.book_id=ub.book_id ) bk on bk.user_id=u.user_id left join ( select 2 as type, t.* from `user_contact` t ) ct on ct.user_id=u.user_id </select> </mapper>
（5）客户端代码如下：
package com.hxstrive.mybatis.resultmap.demo8;
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 java.io.InputStream;
import java.util.List;
public class ResultMapDemo8 {
public static void main(String[] args) throws Exception {
// 获取 Mapper 对象
String cfgName = "com/hxstrive/mybatis/resultmap/demo8/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<UserBean> userBeanList = userMapper.findAll();
for(UserBean userBean : userBeanList) {
System.out.println("结果：" + userBean);
}
}
}
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