如何在Spring Data JPA中映射特定的列？

Question

Whenever I on click submit button with some value, I want to select some columns from database and show it on my jsp page. The form is successfully submitted but I'm getting this error:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.psc.Entity.CandidateappearagainstadvtcodeEntity

So I think whatever data the repository is returning is not mapped with Entity . Please suggest, how to resolve this issue?

Controller:

   @RequestMapping(value = "/getCandidateDetails", method = RequestMethod.POST)
    public String getCandidateDetails(Model model,@RequestParam("studentmasterid") String studentmasterid){
        System.out.println(studentmasterid);
        List<CandidateappearagainstadvtcodeEntity> candidates=candidateappearagainstadvtcodeEntityRepository.findBystudentmasterid(studentmasterid);

        return "payments";
    }

Model looks like this:

@Entity
@Table(name = "CANDIDATEAPPEARAGAINSTADVTCODE", schema = "PSCNEPALCOMMERCIALDATABASEU1", catalog = "")
public class CandidateappearagainstadvtcodeEntity {
    private long id;
    private String advertisementcode;
    private Integer ageonlastdateday;
    private Integer ageonlastdatemonth;
    private Integer ageonlastdateyear;
    private String applicationnumber;
    private String attendancestatus;
    private String candidatefirstname;
    private String dateofbirthinnepali;
    private String documentverificationstatus;
    private String examinationcenterid;
    private String examresultsstatus;
    private String fathername;
    private Long firstcoding;
    private String studentmasterid;
    //getters setters//

Repository:

public interface CandidateappearagainstadvtcodeEntityRepository extends JpaRepository<CandidateappearagainstadvtcodeEntity,Long> {

    @Query("select c.studentmasterid,c.advertisementcode,c.candidatefirstname from CandidateappearagainstadvtcodeEntity c where c.studentmasterid=?1")
    List<CandidateappearagainstadvtcodeEntity> findBystudentmasterid(String masterId);
}

ApplicationContext.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:util="http://www.springframework.org/schema/util"
       xmlns:jpa="http://www.springframework.org/schema/data/jpa"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
           http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd
           http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.2.xsd
           http://www.springframework.org/schema/data/jpa http://www.springframework.org/schema/data/jpa/spring-jpa-1.0.xsd">

    <context:annotation-config/>
    <context:component-scan base-package="com.psc"/>

    <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="location">
    <value>/WEB-INF/db.properties</value>
    </property>
    </bean>

    <!-- BoneCP configuration -->
    <bean id="mainDataSource" class="com.jolbox.bonecp.BoneCPDataSource" destroy-method="close">
        <property name="driverClass" value="${jdbc.driver}" />
        <property name="jdbcUrl" value="${jdbc.url}" />
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
        <property name="idleConnectionTestPeriodInMinutes" value="60"/>
        <property name="idleMaxAgeInMinutes" value="240"/>
        <property name="maxConnectionsPerPartition" value="30"/>
        <property name="minConnectionsPerPartition" value="10"/>
        <property name="partitionCount" value="3"/>
        <property name="acquireIncrement" value="5"/>
        <property name="statementsCacheSize" value="100"/>
        <property name="releaseHelperThreads" value="3"/>
    </bean>

    <!-- SPRING - JPA -->
    <jpa:repositories
            base-package="com.psc" />

    <bean class="org.springframework.orm.jpa.JpaTransactionManager"
          id="transactionManager">
        <property name="entityManagerFactory"
                  ref="entityManagerFactory" />
        <property name="jpaDialect">
            <bean class="org.springframework.orm.jpa.vendor.HibernateJpaDialect" />
        </property>
    </bean>

    <bean id="entityManagerFactory"
          class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
        <property name="dataSource" ref="mainDataSource" />
        <property name="packagesToScan" value="com.psc"/>
        <property name="jpaVendorAdapter">
            <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
                <property name="generateDdl" value="true" />
                <property name="showSql" value="false"/>
                <property name="databasePlatform" value="org.hibernate.dialect.Oracle12cDialect"/>
                <property name="database" value="ORACLE"/>
            </bean>
        </property>
        <property name="jpaProperties">
            <props>
                <prop key="hibernate.hbm2ddl.auto">update</prop>
                <prop key="hibernate.dialect">org.hibernate.dialect.Oracle12cDialect</prop>
            </props>
        </property>
    </bean>

</beans>

Answer 1

A JPA repository usually returns the domain model. From the docs

Spring Data Repositories usually return the domain model when using query methods. However, sometimes, you may need to alter the view of that model for various reasons. In this section, you will learn how to define projections to serve up simplified and reduced views of resources.

Your repository method findBystudentmasterid will be returning a list Object[] hence the ClassCastException .If you want to get specific columns try Projections .

Chekout the docs and this answer on how to use projections.