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Mybatis中从数据库中获取值为null ResultMap

2020-08-01 12:08 36 查看

ResultMap和返回值为空的的问题

要解决的问题:属性名和字段名不一致

代码块如下:

接口:

package com.lx.dao;

import com.lx.pojo.User;

public interface UserMapper {

User getUserById(int id);

}

穿插:
要想使用@Alias注解的话,必须要在mybatis-config.xml配置typeAlias,例如:

<typeAliases>
<package name="com.lx.pojo"/>
</typeAliases>

实体类:

package com.lx.pojo;

import org.apache.ibatis.type.Alias;

@Alias("user")
public class User {

private int id;

private String name;

private String pwd;

public User() {
}

public User(int id, String name, String pwd) {
this.id = id;
this.name = name;
this.pwd = pwd;
}

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 getPwd() {
return pwd;
}

public void setPwd(String pwd) {
this.pwd = pwd;
}

@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", pwd='" + pwd + '\'' +
'}';
}
}

resouce目录下的数据库配置文件:

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=false&useUnicode=true&characterEncoding=UTF-8
username=root
pwd=123456
<?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="db.properties" />

<!--    可以给实体类起别名-->

<typeAliases>
<package name="com.lx.pojo"/>
</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\lx\dao\UserMapper.xml"/>
</mappers>

</configuration>

工具类:获取sqlSession

package com.lx.utils;

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.IOException;
import java.io.InputStream;

public class MybatisUtils {

private static SqlSessionFactory sqlSessionFactory;

static{
//使用Mybatis第一步:获取sqlSessionFactory对象
String resource = "mybatis-config.xml";
InputStream inputStream = null;
try {
inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
e.printStackTrace();
}
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
}

//有了SqlSessionFactory ,可以从中获取SqlSession 的实例
//SqlSession 在其中包含了面向数据库执行 SQL 命令的所有方法

public static SqlSession getSqlSession(){
SqlSession sqlSession = sqlSessionFactory.openSession();
return sqlSession;
}

}

执行sql语句,帮定UserMapper接口的xml配置文件:

<?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=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.lx.dao.UserMapper">

<select id="getUserById" parameterType="int" resultType="user">
select * from user where id= #{id}
</select>

</mapper>

测试类:

import com.lx.dao.UserMapper;
import com.lx.pojo.User;
import com.lx.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class test3 {

@Test
public void userbyid(){
SqlSession sqlSession = MybatisUtils.getSqlSession();

UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = mapper.getUserById(1);

System.out.println(user);

sqlSession.close();
}

}

测试结果:

pwd的值为null

mybatis会根据这些查询的列名(会将列名转化为小写,数据库不区分大小写) , 去对应的实体类中查找
相应列名的set方法设值 , 由于找不到setPassword() , 所以pwd返回null ; 【自动映射】

解决方法

使用结果集映射:ResultMap

column是数据库表的列名 , property是对应实体类的属性名

<?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">

<!--namespace=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.lx.dao.UserMapper">

<resultMap id="usermap" type="user">
<!-- id为主键 -->
<id column="id" property="id"/>

<!-- column是数据库表的列名 , property是对应实体类的属性名 -->
<result column="name" property="name"/>
<result column="password" property="pwd"/>
</resultMap>

<select id="getUserById" resultMap="usermap">
select * from user where id=#{id}
</select>

<!--  <select id="getUserById" parameterType="int" resultType="user">-->
<!--      select * from user where id= #{id}-->
<!--  </select>-->

</mapper>

测试结果:

good good study ! day day up !

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