2022.5.14 mybatis解决属性名和字段名不一致的问题

5、解决属性名和字段名不一致的问题

数据库的字段

测试实体类字段不一致的情况

 package com.xing.pojo;
 ​
 import org.apache.ibatis.type.Alias;
 ​
 //实体类
 public class User {
     private int id;
     private String name;
     private String password;
 ​
     public User() {
     }
 ​
     public User(int id, String name, String password) {
         this.id = id;
         this.name = name;
         this.password = password;
     }
 ​
     public int getId() {
         return id;
     }
 ​
     public void setId(int id) {
         this.id = id;
     }
 ​
     public String getName() {
         return name;
     }
 ​
     public void setName(String name) {
         this.name = name;
     }
 ​
     public String getPassword() {
         return password;
     }
 ​
     public void setPassword(String password) {
         this.password = password;
     }
 ​
     @Override
     public String toString() {
         return "User{" +
                 "id=" + id +
                 ", name='" + name + '\'' +
                 ", password='" + password + '\'' +
                 '}';
     }
 }

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="com.xing.dao.UserMapper">
 ​
     <select id="getUserById" parameterType="int" resultType="com.xing.pojo.User">
         select id,name,pwd
         from mybatis.user
         where id = #{id};    # 接收方法中的参数id
     </select>
 ​
 </mapper>

测试出现问题

UserDaoTest

 package com.xing.dao;
 ​
 import com.xing.pojo.User;
 import com.xing.utils.MybatisUtils;
 import org.apache.ibatis.session.SqlSession;
 import org.junit.Test;
 ​
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 ​
 public class UserDaoTest {
 ​
     @Test
     public void test() {
         SqlSession sqlSession = MybatisUtils.getSqlSession();
         //方式一：执行  得到接口对象调用其中方法  接口的class对象
         UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
         User userById = userMapper.getUserById(2);
 ​
         System.out.println(userById);
 ​
         //关闭SqlSession
         sqlSession.close();
     }
 }

解决方法

• 起别名

 <?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.xing.dao.UserMapper">
 ​
     <select id="getUserById" parameterType="int" resultType="com.xing.pojo.User">
         select id,name,pwd as password
         from mybatis.user
         where id = #{id};    # 接收方法中的参数id
     </select>
 ​
 </mapper>

• resultMap

mybatis-config.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>
 ​
     <!--引入外部配置文件-->
     <properties resource="db.properties">
         <property name="username" value="root"/>
         <property name="pwd" value="123456"/>
     </properties>
 <!--    可以给实体类起别名-->
     <typeAliases>
         <typeAlias type="com.xing.pojo.User" alias="User"/>
     </typeAliases>
 ​
     <environments default="development">
         <environment id="development">
             <transactionManager type="JDBC"/>
             <dataSource type="POOLED">
                 <property name="driver" value="${driver}"/>
                 <property name="url" value="${url}"/>
                 <property name="username" value="${username}"/>
                 <property name="password" value="${pwd}"/>
             </dataSource>
         </environment>
 ​
     </environments>
 ​
     <mappers>
         <mapper resource="com/xing/dao/UserMapper.xml"/>
     </mappers>
 </configuration>

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="com.xing.dao.UserMapper">
 <!--结果集映射 id ：参数名  type：结果集的类型 对应实体类的名字或别名 可以写com.xing.pojo.User-->
     <resultMap id="UserMap" type="User">
 <!--coLumn数据库中的字毁,property实体类中的属姓 一致可以不用写-->
 <!--        <result column="id" property="id"/>-->
 <!--        <result column="name" property="name"/>-->
         <result column="pwd" property="password"/>
     </resultMap>
 <!--                                    根据参数名引用上面的resultMap-->
     <select id="getUserById" resultMap="UserMap">
         select id,name,pwd
         from mybatis.user
         where id = #{id};    # 接收方法中的参数id
     </select>
 ​
 </mapper>

• ResultMap元素是MyBatis中最重要最强大的元素

• ResultMap的设计思想是，对于简单的语句根本不需要配置显式的结果映射，而对于复杂一点的语句只需要描述它们的关系就行了。