mybatis延迟加载一对多

Posted 2020-10-02 羽哲

1、实体类

package cn.bdqn.bean;

import java.util.Set;

/**
 *国家的实体类
 */
public class Country {

    private Integer cId; // 国家的编号
    private String cName; // 国家的名称

    private Set<Provincial> provincials;

    public Integer getcId() {
        return cId;
    }

    public void setcId(Integer cId) {
        this.cId = cId;
    }

    public String getcName() {
        return cName;
    }

    public void setcName(String cName) {
        this.cName = cName;
    }

    public Country(Integer cId, String cName) {
        super();
        this.cId = cId;
        this.cName = cName;
    }

    public Country() {
        super();
    }

    public Set<Provincial> getProvincials() {
        return provincials;
    }

    public void setProvincials(Set<Provincial> provincials) {
        this.provincials = provincials;
    }

    @Override
    public String toString() {
        return "Country [cId=" + cId + ", cName=" + cName;
    }

}

2、dao层

package cn.bdqn.dao;

import cn.bdqn.bean.Country;

public interface CountryDao {
    /**
     * 根据国家的id查询出国家的信息  以及国家下面的省会信息
     */
    Country selectCountryById(Integer cId);
}

3、Mapper文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-Mapper.dtd">
<mapper namespace="cn.bdqn.dao.CountryDao">
    
    
    <select id="selectProvincialByCountryId" resultType="Provincial">
        select pid,pname from provincial
         where countryid=#{cid}
    </select>
    
    <resultMap type="Country" id="countryMap">
        <id property="cId" column="cid"/>
        <result property="cName" column="cname"/>
        <!--设置关联的集合属性  
          select:需要关联的查询语句
         column: select关联语句中需要的参数 -->
         <collection property="provincials" ofType="Provincial"
          select="selectProvincialByCountryId"
          column="cid"/>
     </resultMap>
    
    <!-- 多表的查询  经常使用  可以使用延迟加载策略  -->
    <select id="selectCountryById" resultMap="countryMap">
          select  cid,cname from  country where cid=#{cid} 
    </select> 
    
</mapper>

4、测试类

package cn.bdqn.test;

import org.apache.ibatis.session.SqlSession;
import org.apache.log4j.Logger;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import cn.bdqn.bean.Country;
import cn.bdqn.bean.Provincial;
import cn.bdqn.dao.CountryDao;
import cn.bdqn.dao.ProvincialDao;
import cn.bdqn.util.MybatisUtil;

public class Test1 {
    private Logger logger = Logger.getLogger(Test1.class);
    SqlSession session;
    ProvincialDao dao;
    CountryDao dao2;

    @Before
    public void before() {
        // 因为需要关闭session 需要把session提取出去
        session = MybatisUtil.getSqlSession();
        dao = session.getMapper(ProvincialDao.class);
        dao2 = session.getMapper(CountryDao.class);
    }

    @After
    public void after() {
        if (session != null) {
            session.close();
        }
    }

    @Test
    public void test1() {
        Provincial provincial = dao.selectProvincialById(1);
        /*logger.debug("provincialId=1======>provincial:" + provincial);*/
        /*logger.debug("provincialId=1======>country:" + provincial.getCountry());*/
    }

    @Test
    public void test2() {
        Country country = dao2.selectCountryById(1);
        /*Set<Provincial> provincials = country.getProvincials();
        for (Provincial provincial : provincials) {
            logger.debug(provincial);
        }*/
        /*logger.debug("provincialId=1======>provincial:" + provincial);*/
        /*logger.debug("provincialId=1======>country:" + provincial.getCountry());*/
    }
}