Mybatis查询数据部分字段显示为null，怎么转成空串("")

1、先定义一个handler，来把字段为null的转成空串（""）
2、在Mapper.xml中，把可能为空的字段，加上typeHandler属性，指定处理的handler类的全路径。

CustomStringTypeHandler.java

package com.wang.common.mybatis.handler;

import org.apache.ibatis.executor.result.ResultMapException;
import org.apache.ibatis.type.BaseTypeHandler;
import org.apache.ibatis.type.JdbcType;
import org.apache.ibatis.type.MappedJdbcTypes;
import org.apache.ibatis.type.MappedTypes;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

/**
 * @Name: CustomStringTypeHandler
 * @Desc: 自定义mybatis处理类，将null返回为空串（‘’）
 * @Author: Administrator
 * @Date: 2019-09-03 18:20
 */
@MappedTypes({String.class})
@MappedJdbcTypes(JdbcType.VARCHAR)
public class CustomStringTypeHandler extends BaseTypeHandler<String> {


    @Override
    public String getResult(ResultSet rs, String columnName) {
        String result;
        try {
            result = getNullableResult(rs, columnName);
        } catch (Exception e) {
            throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set.  Cause: " + e, e);
        }
        return result;
    }

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)
            throws SQLException {
        ps.setString(i, parameter);
    }

    @Override
    public String getNullableResult(ResultSet rs, String columnName)
            throws SQLException {
        return rs.getString(columnName) == null? "" : rs.getString(columnName);
    }

    @Override
    public String getNullableResult(ResultSet rs, int columnIndex)
            throws SQLException {
        return rs.getString(columnIndex) == null? "" : rs.getString(columnIndex);
    }

    @Override
    public String getNullableResult(CallableStatement cs, int columnIndex)
            throws SQLException {
        return cs.getString(columnIndex) == null? "" : cs.getString(columnIndex);
    }
}

Mapper.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.wang.sss.fw.mapper.BusinessTripMapper">
  <resultMap id="BaseResultMap" type="com.wang.sss.fw.pojo.BusinessTrip">
    <result column="REQUEST_ID" jdbcType="VARCHAR" property="requestId" />
    <result column="JOB_NUMBER" jdbcType="VARCHAR" property="jobNumber" />
    <result column="REQUEST_DATE" jdbcType="VARCHAR" property="requestDate" />
    <result column="DEPARTMENT" jdbcType="VARCHAR" property="department" />
    <result column="BUSINESS_DAYS" jdbcType="VARCHAR" property="businessDays"/>
    <result column="CFD" jdbcType="VARCHAR" property="cfd" />
    <result column="MDD" jdbcType="VARCHAR" property="mdd" />
    <result column="START_TIME" jdbcType="VARCHAR" property="startTime" />
    <result column="END_TIME" jdbcType="VARCHAR" property="endTime" />
    <result column="REASON" jdbcType="VARCHAR" property="reason" typeHandler="com.wang.common.mybatis.handler.CustomStringTypeHandler"/>
    <result column="REMARK" jdbcType="VARCHAR" property="remark" typeHandler="com.wang.common.mybatis.handler.CustomStringTypeHandler"/>
  </resultMap>
</mapper>  

没有加typeHandler属性，处理之前的查询结果：

BusinessTrip(requestId=11925, jobNumber=5721, requestDate=2019-05-06, department=57, businessDays=21, cfd=上海, mdd=南京, startTime=2019-05-06 13:36, endTime=2019-05-07 13:36, reason=null, remark=null)

增加typeHandler属性，处理后的结果：（reason和remark字段都变成了空串）

BusinessTrip(requestId=11925, jobNumber=5721, requestDate=2019-05-06, department=57, businessDays=21, cfd=上海, mdd=南京, startTime=2019-05-06 13:36, endTime=2019-05-07 13:36, reason=, remark=)