今天在写项目的mapper.xml文件的时候，出现了个别字段查询结果为null的情况，但sql语句没有错误，仔细查看了一遍，才发现错误，现在记录下来
先讲一讲sql语句和ResultMap的顺序：
首先会执行sql语句，sql语句返回字段信息，然后才是ResultMap映射字段信息。
实体类UserInfo
package com.school.oauth.endpoint.domain;
import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonIgnore;
import com.school.parent.domain.BaseDomain;
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
import org.apache.ibatis.annotations.Insert;
import org.apache.ibatis.annotations.Update;
import org.hibernate.validator.constraints.Length;
import org.springframework.format.annotation.DateTimeFormat;
import javax.persistence.*;
import javax.validation.constraints.Email;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Pattern;
import java.time.ZonedDateTime;
import java.util.Date;
import java.util.List;
@Table(name = "oauth_user")
@ApiModel(value = "用户表")
@Data
public class UserInfo extends BaseDomain {
@Id
@GeneratedValue(generator = "JDBC")
@ApiModelProperty(value = "用户主键，提供给其他表做外键")
private Long userId;
@Column(unique = true)
@NotBlank
@Length(max = 32)
@ApiModelProperty(value = "用户名，必须唯一")
private String username;
@NotBlank
@Length(max = 32)
@ApiModelProperty(value = "昵称，可以重复")
private String nickname;
@Length(max = 128)
@ApiModelProperty(value = "加密密码")
private String encryptedPassword;
@Length(max = 32)
@Pattern(regexp = "^[1][3,4,5,7,8][0-9]{9}$"
,message = "手机号码格式错误")
@ApiModelProperty(value = "用户手机号码")
private String phone;
@Email
@Length(max = 128)
@ApiModelProperty(value = "用户邮箱")
private String email;
@Length(max = 8)
@ApiModelProperty(value = "国际冠码，默认 +86")
private String idd;
@Length(max = 16)
@ApiModelProperty(value = "性别：男/女")
private String gender;
@Length(max = 1024)
@ApiModelProperty(value = "头像地址")
private String avatar;
@NotBlank
@Length(max = 128)
@ApiModelProperty(value = "所属学校")
private String school;
@DateTimeFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@JsonFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@ApiModelProperty(value = "账户过期时间")
private Date accountExpiredTime;
@DateTimeFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@JsonFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@ApiModelProperty(value = "账户锁定时间")
private Date accountLockedTime;
@DateTimeFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@JsonFormat(pattern = "yyyy-MM-dd hh:mm:ss")
@ApiModelProperty(value = "凭证过期时间")
private Date credentialsExpiredTime;
@Column(name = "is_admin")
@ApiModelProperty(value = "是否是管理员")
private Boolean admin;
@Column(name = "is_able")
@ApiModelProperty(value = "账户是否启用")
private Boolean able;
@Length(max = 32)
@NotBlank(groups = Insert.class)
@ApiModelProperty(value = "用户注册平台：WEB(默认)/AliPay/WeChat")
private String userType;
@Length(max = 64)
@ApiModelProperty(value = "微信用户识别ID")
private String weChatUserId;
@Length(max = 64)
@ApiModelProperty(value = "支付宝用户识别ID")
private String aliPayUserId;
@Transient
@NotBlank(groups = Insert.class)
@ApiModelProperty(value = "密码")
private String password;
/**
* 一个用户只能对应一个角色（最高权限角色）
*/
@Transient
@ApiModelProperty(value = "用户角色集合：guest/user/admin")
private List<Role> roleList;
public UserInfo() {
}
}
UserMapper.xml错误代码：
<resultMap id="BaseMap" type="com.school.oauth.endpoint.domain.UserInfo">
<id column="user_id" property="userId"/>
<result column="username" property="username"/>
<result column="nickname" property="nickname"/>
<result column="encrypted_password" property="encryptedPassword"/>
<result column="phone" property="phone"/>
<result column="email" property="email"/>
<result column="idd" property="idd"/>
<result column="gender" property="gender"/>
<result column="avatar" property="avatar"/>
<result column="school" property="school"/>
<result column="account_expired_time" property="accountExpiredTime"/>
<result column="account_locked_time" property="accountLockedTime"/>
<result column="credentials_expired_time" property="credentialsExpiredTime"/>
<result column="is_admin" property="admin" jdbcType="TINYINT"/>
<result column="is_able" property="able" jdbcType="TINYINT"/>
<result column="user_type" property="userType"/>
<result column="we_char_user_id" property="weChatUserId"/>
<result column="ali_pay_user_id" property="aliPayUserId"/>
<collection property="roleList" ofType="com.school.oauth.endpoint.domain.Role"
column="user_id" select="com.school.oauth.endpoint.mapper.RoleMapper.selectRoleByUserId">
</collection>
</resultMap>
<select id="selectByUsername" resultMap="BaseMap">
SELECT
u.user_id,
u.username,
u.nickname,
u.encrypted_password,
u.phone,
u.email,
u.idd,
u.gender,
u.avatar,
u.school,
u.account_expired_time,
u.account_locked_time,
u.credentials_expired_time,
u.is_admin AS admin,
u.is_able AS able,
u.user_type,
u.we_chat_user_id,
u.ali_pay_user_id
FROM oauth_user u
WHERE u.username = #{username}
</select>
这样查询出来的User对象的admin和able字段为空。这是因为我们的selectByUsername 引用了我们定义的ResultMap。
注意看sql语句，其中有两个字段我们取了别名
u.is_admin AS admin,
u.is_able AS able,
,也就是说，当sql语句查询出来的is_admin字段和is_able 字段已经变为了admin字段和able字段，而ResultMap里面这两个字段的映射：
<result column="is_admin" property="admin" jdbcType="TINYINT"/>
<result column="is_able" property="able" jdbcType="TINYINT"/>
当ResultMap去映射字段信息的时候，发现找不到is_admin字段和is_able字段，因为在sql语句我们已经为这两个字段去了别名，现在这两个字段叫admin和able。所有ResultMap映射到实体类的时候，就会出现这两个字段为空的情况。
正确写法
去掉sql语句里面的别名，就可以咯。这样resultMap就可以找到相应的字段了。
<resultMap id="BaseMap" type="com.school.oauth.endpoint.domain.UserInfo">
<id column="user_id" property="userId"/>
<result column="username" property="username"/>
<result column="nickname" property="nickname"/>
<result column="encrypted_password" property="encryptedPassword"/>
<result column="phone" property="phone"/>
<result column="email" property="email"/>
<result column="idd" property="idd"/>
<result column="gender" property="gender"/>
<result column="avatar" property="avatar"/>
<result column="school" property="school"/>
<result column="account_expired_time" property="accountExpiredTime"/>
<result column="account_locked_time" property="accountLockedTime"/>
<result column="credentials_expired_time" property="credentialsExpiredTime"/>
<result column="is_admin" property="admin" jdbcType="TINYINT"/>
<result column="is_able" property="able" jdbcType="TINYINT"/>
<result column="user_type" property="userType"/>
<result column="we_char_user_id" property="weChatUserId"/>
<result column="ali_pay_user_id" property="aliPayUserId"/>
<collection property="roleList" ofType="com.school.oauth.endpoint.domain.Role"
column="user_id" select="com.school.oauth.endpoint.mapper.RoleMapper.selectRoleByUserId">
</collection>
</resultMap>
<select id="selectByUsername" resultMap="BaseMap">
SELECT
u.user_id,
u.username,
u.nickname,
u.encrypted_password,
u.phone,
u.email,
u.idd,
u.gender,
u.avatar,
u.school,
u.account_expired_time,
u.account_locked_time,
u.credentials_expired_time,
u.is_admin,
u.is_able,
u.user_type,
u.we_chat_user_id,
u.ali_pay_user_id
FROM oauth_user u
WHERE u.username = #{username}
</select>