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写了一个mybatis的mapper映射文件，
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;
}
}<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，
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。
如果是实体中是直接引用别的对象的具体参数字段，
<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>
<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，
<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，
<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。
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