mybatis resultmap 如何为对象赋值的调用顺序

目录

• resultmap 为对象赋值的调用顺序
• java bean定义如下
• 部分mybatis映射文件如下
• createResultObject方法的代码如下
• 使用resultMap时需注意的问题
• 直接用原始方式就行
• 如果是实体中是list集合
• 使用association 标签来写
• 应该使用collection标签

resultmap 为对象赋值的调用顺序

写了一个mybatis的mapper映射文件，

java bean定义如下

public class GroupCourseResult extends GroupResult {
    private String cid;
    private String cname;
    public GroupCourseResult(int stuschool, int nian, String stuclass, String cid, String cname) {
        super(stuschool, nian, stuclass);
        this.cid = cid;
        this.cname = cname;
    }
    public String getCid() {
        return cid;
    }
    public void setCid(String cid) {
        this.cid = cid;
    }
    public String getCname() {
        return cname;
    }
    public void setCname(String cname) {
        this.cname = cname;
    }
}

部分mybatis映射文件如下

<select id="selectFailedCourseRationByGroupIdAndCourseName" resultType="GroupCourseResult">
      ...
  </select>

实体类中的属性名和查询的列名完全匹配，但是没有查询stuclass，则封装后的实体类中的stuclass属性应该为空。

然而程序运行后，stuclass属性不仅不为空，还与cname完全相同，百思不得其解，故翻了翻mybatis的源码。

在mybatis中的DefaultResultSetHandler类中，

createResultObject方法的代码如下

  private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
      throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
    final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
    if (hasTypeHandlerForResultObject(rsw, resultType)) {
      return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
    } else if (!constructorMappings.isEmpty()) {
      return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
    } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
      return objectFactory.create(resultType);
    } else if (shouldApplyAutomaticMappings(resultMap, false)) {
      return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs);
    }
    throw new ExecutorException("Do not know how to create an instance of " + resultType);
  }

经过调试发现，mybatis会先查找该javabean有无默认构造方法，如果有则采用设值注入，若没有，则根据javabean的有参构造方法进行设值，而在8以前的jdk版本中,我们利用反射只能获取到参数类型，不能获取到参数名称，这其中设值可能出现了匹配失误，将cname的值同时赋给了cname和stuclass。

想要解决这个问题，只须在javabean中添加默认构造方法即可。

使用resultMap时需注意的问题

如果是实体中是直接引用别的对象的具体参数字段，

直接用原始方式就行

    <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>

如果是实体中是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>

如果实体中引用的是别的对象，

使用association 标签来写

    <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>

如果实体中是引用的别的对象的list集合，

应该使用collection标签

<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的时候，应该直接用as后面的字段名，即自己命的名字

如果没有使用as的话，直接使用数据库中原本的名字

resultMap中各个标签的含义

tips：

在一个 resultMap 元素中，这些子元素出现的先后顺序是有严格规定的，它们从前到后依次是：constructor–>id --> result–> association–>collection -->discriminator, 不然就会报错。

以上为个人经验，希望能给大家一个参考，也希望大家多多支持我们。