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Java?Mybatis使用resultMap時(shí),屬性賦值順序錯(cuò)誤的巨坑

更新時(shí)間：2022年01月20日 09:49:25 作者：百事可樂_

這篇文章主要介紹了Java?Mybatis使用resultMap時(shí),屬性賦值順序錯(cuò)誤的巨坑，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教

Mybatis使用resultMap屬性賦值順序錯(cuò)誤

今天發(fā)現(xiàn)個(gè)坑，新建的表使用生成工具生成的mapper文件和實(shí)體類后，發(fā)現(xiàn)少了個(gè)字段就又手動(dòng)加了下，結(jié)果發(fā)現(xiàn)一個(gè)問題

ids是后加入的字段

@Data
@Builder
public class QueryRecordPo {
?
? ? ?//若干其他屬性....
? ? private String outputField;
? ? ?//后加的
? ? private String ids;
?
? ? //若干其他屬性
? ? //...?
}

resultMap中是這樣寫的

? ? <resultMap id="BaseResultMap" type="....">
? ? ? ? <id column="id" jdbcType="BIGINT" property="id"/>
? ? ? ? ..若干其他屬性
? ? ? ? <result column="ids" jdbcType="VARCHAR" property="ids"/>
? ? ? ? <result column="output_field" jdbcType="VARCHAR" property="outputField"/>
? ? ? ? ..若干其他屬性
? ? </resultMap>

可以發(fā)現(xiàn)ids加的位置是不一樣的，實(shí)體類中在outputField屬性下面，但resultMap中在其上面。然后測(cè)試數(shù)據(jù)中ids字段為null，查詢出來時(shí)卻發(fā)現(xiàn)ids的值和outputField的值是一樣的。但如果ids的字段有值，就可以正確賦值。

mybatis在生成目標(biāo)類進(jìn)行映射時(shí)，會(huì)先檢查構(gòu)造函數(shù)聲明情況，但如果Data注解和Builder注解一塊使用的話就只會(huì)生成全屬性參數(shù)構(gòu)造函數(shù)，不會(huì)有默認(rèn)無參構(gòu)造函數(shù)。全屬性構(gòu)造函數(shù)的參數(shù)順序是和類中屬性聲明順序一致的

在把數(shù)據(jù)庫(kù)字段映射到實(shí)體類的時(shí)候發(fā)現(xiàn)實(shí)體類沒有默認(rèn)無參構(gòu)造函數(shù)，就會(huì)把數(shù)據(jù)庫(kù)中的字段按照全屬性構(gòu)造函數(shù)參數(shù)的順序依次賦值給實(shí)體類的屬性。但如果實(shí)體類的屬性定義順序與數(shù)據(jù)庫(kù)中字段順序不一致，就會(huì)出現(xiàn)賦值錯(cuò)誤的情況。

然后再為outputField字段賦值時(shí)調(diào)用了set方法這樣就出現(xiàn)了兩個(gè)不同名但同值的屬性。

解決辦法

1 修改屬性順序保持一致

2 為實(shí)體類加上@NoArgsConstructor和 @AllArgsConstructor注解使其可以生成無參數(shù)構(gòu)造函數(shù)即可

之前生成時(shí) 順序都保持了一致，還真沒發(fā)現(xiàn)這個(gè)問題

Mybatis使用resultMap時(shí)需注意

    <resultMap id="baseMap" type="com.ei.medical.modules.model.EduEducationKnowledge">
        <id column="id" property="id"/>
        <result column="visitNumber" property="visitNumber"/>
        <result column="patientName" property="patientName"/>
        <result column="sendTime" property="sendTime"/>
        <result column="wardCode" property="wardCode"/>
        <result column="wardName" property="wardName"/>
        <result column="categoryCode" property="categoryCode"/>
        <result column="title" property="title"/>
        <result column="content" property="content"/>
        <result column="cover" property="cover"/>
    </resultMap>
    
    <select id="getAllBy" resultMap="baseMap">
        SELECT
            eer.visit_number as visitNumber,
            eer.patient_name as patientName,
            eer.send_time as sendTime,
            eek.id as id,
            eek.ward_code as wardCode,
            eek.ward_name as wardName,
            eek.category_code as categoryCode,
            eek.title as title,
            eek.content as content,
            eek.cover as cover
        FROM
            edu_education_record AS eer,
            edu_education_knowledge AS eek
        WHERE
            eer.education_knowledge_id=eek.id
    </select>

如果是實(shí)體中是直接引用別的對(duì)象的具體參數(shù)字段，直接用原始方式就行

如果是實(shí)體中是list集合

    <resultMap id="baseMap" type="com.ei.medical.modules.model.EduEducationKnowledge">
    	<id column="id" property="id"/>
        <result column="visitNumber" property="visitNumber"/>
        <result column="patientName" property="patientName"/>
        <result column="sendTime" property="sendTime"/>
        <result column="wardCode" property="wardCode"/>
        <result column="wardName" property="wardName"/>
        <result column="categoryCode" property="categoryCode"/>
        <result column="title" property="title"/>
        <result column="content" property="content"/>
        <result column="cover" property="cover"/>
        
        <collection property="pic" ofType="string">
            <result column="pic"/>
        </collection>
    </resultMap>
    
    <select id="getAllBy" resultMap="baseMap">
        SELECT
            eer.visit_number as visitNumber,
            eer.patient_name as patientName,
            eer.send_time as sendTime,
            eek.id as id,
            eek.ward_code as wardCode,
            eek.ward_name as wardName,
            eek.category_code as categoryCode,
            eek.title as title,
            eek.content as content,
            eek.cover as cover
        FROM
            edu_education_record AS eer,
            edu_education_knowledge AS eek
        WHERE
            eer.education_knowledge_id=eek.id
    </select>

如果實(shí)體中引用的是別的對(duì)象，可以使用association 標(biāo)簽來寫

    <resultMap id="baseMap" type="com.ei.medical.modules.model.EduEducationKnowledge">
        <id column="id" property="id"/>
        <result column="wardCode" property="wardCode"/>
        <result column="wardName" property="wardName"/>
        <result column="categoryCode" property="categoryCode"/>
        <result column="title" property="title"/>
        <result column="content" property="content"/>
        <result column="cover" property="cover"/>
        
        <association property="eduEducationRecord" javaType="com.ei.medical.modules.model.EduEducationRecord">
	        <result column="visitNumber" property="visitNumber"/>
	        <result column="patientName" property="patientName"/>
	        <result column="sendTime" property="sendTime"/>
        </association>
    </resultMap>
    
    <select id="getAllBy" resultMap="baseMap">
        SELECT
            eer.visit_number as visitNumber,
            eer.patient_name as patientName,
            eer.send_time as sendTime,
            eek.id as id,
            eek.ward_code as wardCode,
            eek.ward_name as wardName,
            eek.category_code as categoryCode,
            eek.title as title,
            eek.content as content,
            eek.cover as cover
        FROM
            edu_education_record AS eer,
            edu_education_knowledge AS eek
        WHERE
            eer.education_knowledge_id=eek.id
    </select>

如果實(shí)體中是引用的別的對(duì)象的list集合，應(yīng)該使用collection 標(biāo)簽

<resultMap id="baseMap" type="com.ei.medical.modules.model.EduEducationKnowledge">
        <id column="id" property="id"/>
        <result column="wardCode" property="wardCode"/>
        <result column="wardName" property="wardName"/>
        <result column="categoryCode" property="categoryCode"/>
        <result column="title" property="title"/>
        <result column="content" property="content"/>
        <result column="cover" property="cover"/>
        
        <collection property="eduEducationRecordList" ofType="com.ei.medical.modules.model.EduEducationRecord">
            <result column="visitNumber" property="visitNumber"/>
            <result column="patientName" property="patientName"/>
            <result column="sendTime" property="sendTime"/>
        </collection>
    </resultMap>
    <select id="getAllBy" resultMap="baseMap">
        SELECT
            eer.visit_number as visitNumber,
            eer.patient_name as patientName,
            eer.send_time as sendTime,
            eek.id as id,
            eek.ward_code as wardCode,
            eek.ward_name as wardName,
            eek.category_code as categoryCode,
            eek.title as title,
            eek.content as content,
            eek.cover as cover
        FROM
            edu_education_record AS eer,
            edu_education_knowledge AS eek
        WHERE
            eer.education_knowledge_id=eek.id
    </select>

tips：

使用resultMap的時(shí)候，應(yīng)該直接用as后面的字段名，即自己命的名字

如果沒有使用as的話，直接使用數(shù)據(jù)庫(kù)中原本的名字

resultMap中各個(gè)標(biāo)簽的含義

在這里插入圖片描述

tips：

在一個(gè) resultMap 元素中，這些子元素出現(xiàn)的先后順序是有嚴(yán)格規(guī)定的，它們從前到后依次是：constructor–>id --> result–> association–>collection -->discriminator, 不然就會(huì)報(bào)錯(cuò)。

以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持腳本之家。

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