1.問題闡述:
在Mybatis中,當根據id查詢用戶信息時,映射文件userMapper.xml進行配置查詢時,如果數據庫字段名與實體類屬性名稱不一致,查詢時,會出現爲Null。
t_user表字段:
create table t_user(
u_id int pramary key not null,
u_name varchar(20) not null,
u_pwd varchar(20) not null
)
User.java實體類屬性字段
public class User{
private int uid;
private String name;
private String pwd;
}
2.解決方式:
1)在userMapper.xml中配置sql查詢語句時,給表中的字段起別名,最好字段別名與實體類中屬性名保持一致,如下:
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">
<!-- namespace:命名空間,對應dao接口 -->
<mapper namespace="com.casv.entity.userMapper">
<select id="getuser" parameterType="int" resultType="users" >
select u_id uid,u_name name,u_pwd pwd from t_user where u_id=#{uid};
</select>
</mapper>
2)在userMapper.xml中配置resultMap屬性,進行表字段與實體類屬性映射,如下:
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">
<!-- namespace:命名空間,對應dao接口 -->
<mapper namespace="com.casv.entity.userMapper">
<!-- 配置resultMap屬性,進行表字段與實體類屬性映射 -->
<resultMap id="BaseResultMap" type="users" >
<result column="u_id" property="uid"></result>
<result column="u_name" property="name"></result>
<result column="u_pwd" property="pwd"></result>
</resultMap>
<!-- select中引入resultMap屬性,通過id將表字段與實體屬性映射加載進來 -->
<select id="getuser" resultMap="BaseResultMap" parameterType="int" resultType="users" >
select * from t_user where u_id=#{sid}
</select>
</mapper>