mybatis的配置 mybatis-config.xml

<settings>
<!-- mybatis resultType equal map void null value -->
<setting name="callSettersOnNulls" value="true"/>
</settings>
2，springBoot配置 application.properties 新增

#mybatis resultType equal map void null value
mybatis.configuration.call-setters-on-nulls=true

//導包
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.serializer.SerializerFeature;

//程式碼
JSON.toJSONString(new HashMap<>(),SerializerFeature.WriteMapNullValue)；

一、查詢sql新增每個欄位的判斷空

IFNULL(rate,'') as rate
二、ResultType利用實體返回，不用map

三、springMVC+mybatis查詢資料，返回resultType=”map”時，如果資料為空的欄位，則該欄位省略不顯示，可以通過新增配置檔案，規定查詢資料為空是則返回null。

 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD SQL MAP Config 3.1//EN"
    "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
  <settings>
    <setting name="callSettersOnNulls" value="true"/>
  </settings>
</configuration>

spring
 
-mybatis.xml

<!-- spring和MyBatis完美整合，新增mybatis的配置對映檔案 -->
  <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="configLocation" value="classpath:mybatis-configuration.xml"/>
    <!-- 自動掃描mapping.xml檔案 -->
    <property name="mapperLocations" value="classpath:mapping/*.xml"></property>
  </bean>

如果想要配置rate的預設值，例如“”字串，則可以建立一個類，實現Mybatis的TypeHandler介面

public class EmptyStringIfNull implements TypeHandler<String> {

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

    @Override
    public String getResult(ResultSet rs, int columnIndex) throws SQLException {
     return (rs.getString(columnIndex) == null) ? "" : rs.getString(columnIndex);
    }
    @Override
    public String getResult(CallableStatement cs, int columnIndex)   throws SQLException {
     return (cs.getString(columnIndex) == null) ? "" : cs.getString(columnIndex);
    }
    @Override
    public void setParameter(PreparedStatement ps, int arg1, String str, JdbcType jdbcType) throws SQLException { }}

在sql.xml檔案定義與使用如下如下

<resultMap id="find" type="java.util.LinkedHashMap">
    <result property="name" column="name" />
    <result property="phone" column="phone" />
    <result property="rate" column="rate" typeHandler="com.mybatis.EmptyStringIfNull"/>
</resultMap>