mybatis实现延迟加载多对一

1、数据库表

CREATE TABLE `country` (
  `cid` int(4) NOT NULL AUTO_INCREMENT COMMENT '国家id',
  `cname` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`cid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci；

CREATE TABLE `provincial` (
  `pid` int(4) NOT NULL AUTO_INCREMENT,
  `pname` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `countryid` int(4) NOT NULL,
  PRIMARY KEY (`pid`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci；

2、实体类

package cn.bdqn.bean;

/**
 *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

    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();
    }

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

}

package cn.bdqn.bean;

/**
 *
 *省会对应的实体类
 */
public class Provincial {
    private Integer pId; // 省会的编号
    private String pName; // 省会名称
    // 关联的国家属性
    private Country country;

    public Country getCountry() {
        return country;
    }

    public void setCountry(Country country) {
        this.country = country;
    }

    public Integer getpId() {
        return pId;
    }

    public void setpId(Integer pId) {
        this.pId = pId;
    }

    public String getpName() {
        return pName;
    }

    public void setpName(String pName) {
        this.pName = pName;
    }

    public Provincial(Integer pId, String pName) {
        super();
        this.pId = pId;
        this.pName = pName;
    }

    public Provincial() {
        super();
    }

    /*@Override
    public String toString() {
        return "Provincial [pId=" + pId + ", pName=" + pName + ", country="
                + country + "]";
    }*/

}

3、mybatis.xml配置文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <!-- 通过properties文件配置连接数据库的四要素 -->
    <properties resource="jdbc.properties" />

    <settings>
        <!-- 打开延迟加载的开关 -->
        <setting name="lazyLoadingEnabled" value="true" />
        <!-- 将积极加载改为消息加载即按需加载 -->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

    <!-- 设置别名 两种方式 -->
    <typeAliases>
           <!--  01.这种方式别名可以随意取  但是如果有多个 类 就需要配置多个typeAlias
           <typeAlias type="cn.bdqn.bean.Student" alias="student"/> -->
           <package name="cn.bdqn.bean"/><!-- 02.这种方式要求别名必须使用简写的类名  -->
    </typeAliases>

    <!-- 配置运行环境 可以有多个 environment -->
    <environments default="mysql"><!-- 默认采用的环境 -->
        <environment id="mysql"> <!-- 环境的名称 -->
            <!--配置事务管理 采用jdbc默认的事务管理 之后整合的时候 肯定是交给了 spring来处理了 -->
            <transactionManager type="JDBC" />
            <!-- 数据源采用的是连接池技术 POOLED:mybatis自带的数据源，JNDI:基于tomcat的数据源 -->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
    </environments>
    <!-- 加载映射文件信息 -->
    <mappers>
        <mapper resource="cn/bdqn/dao/StduentMapper.xml" />
        <!-- <mapper resource="cn/bdqn/dao/CountryMapper.xml" /> -->
        <mapper resource="cn/bdqn/dao/ProvincialMapper.xml" />
    </mappers>
</configuration>

4、Dao层接口

package cn.bdqn.dao;

import cn.bdqn.bean.Provincial;

public interface ProvincialDao {
    /**
     * 根据省会的id查询出省会和对应国家的信息
     */
    Provincial selectProvincialById(Integer pId);
}

5、Mapper.xml

<?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.ProvincialDao">
    <select id="selectCountryByProvincialId" resultType="country">
        select cid,cname from country
        where cid = #{countryid}
    </select>

   <resultMap type="provincial" id="provincialMap">
       <id column="pid" property="pId"/>
       <result column="pname" property="pName"/>
       <association property="country" javaType="Country"
           select="selectCountryByProvincialId" column="countryid" >
       </association>
   </resultMap>

   <select id="selectProvincialById" resultMap="provincialMap">
       select pid,pname,countryid from provincial
       where pid = #{pid}
   </select>
 </mapper>

6、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.Provincial;
import cn.bdqn.dao.ProvincialDao;
import cn.bdqn.util.MybatisUtil;

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

    @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);
    }

    @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());*/
    }
}