深入理解什么是Mybatis懒加载(延迟加载)

#创建数据库
CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
#创建数据表
CREATE TABLE `test`.`student` ( `sid` INT(10) NOT NULL AUTO_INCREMENT ,`sname` VARCHAR(20) NOT NULL ,PRIMARY KEY(`sid`)) ENGINE = MyISAM;
CREATE TABLE `test`.`course` ( `cid` INT(10) NOT NULL AUTO_INCREMENT ,`cname` VARCHAR(20) NOT NULL ,PRIMARY KEY(`cid`)) ENGINE = MyISAM;
CREATE TABLE `test`.`middle` (
`id` INT(10) NOT NULL AUTO_INCREMENT ,`studentId` INT(10) NOT NULL ,`courseId` INT(10) NOT NULL ,PRIMARY KEY(`id`)) ENGINE = MyISAM;
#初始化数据表
INSERT INTO `course` (`cid`, `cname`) VALUES ('1', 'JAVA') ;
INSERT INTO `course` (`cid`, `cname`) VALUES ('2', 'C++') ;
INSERT INTO `course` (`cid`, `cname`) VALUES ('3', 'JS') ;

INSERT INTO `student` (`sid`, `sname`) VALUES ('1', 'Jam') ;
INSERT INTO `student` (`sid`, `sname`) VALUES ('2', 'Lina') ;

INSERT INTO `middle` (`id`, `studentId`, `courseId`) VALUES ('1', '1', '1');
INSERT INTO `middle` (`id`, `studentId`, `courseId`) VALUES ('2', '1', '2');
INSERT INTO `middle` (`id`, `studentId`, `courseId`) VALUES ('3', '2', '1');
INSERT INTO `middle` (`id`, `studentId`, `courseId`) VALUES ('4', '2', '3');

public class Country {
	private Integer cid;
	private String cname;

	private Set<Minister> ministers;
	public Integer getCid() {
		return cid;
	}
	public void setCid(Integer cid) {
		this.cid = cid;
	}
	public String getName() {
		return cname;
	}
	public void setName(String cname) {
		this.cname = cname;
	}
	public Set<Minister> getMinisters() {
		return ministers;
	}
	public void setMinisters(Set<Minister> ministers) {
		this.ministers = ministers;
	}
	@Override
	public String toString() {
		return "Country [cid=" + cid + ", cname=" + cname + ", ministers="
				+ ministers + "]";
	}
}

public class Minister {
	private Integer mid;
	private String mname;
	@Override
	public String toString() {
		return "Minister [mid=" + mid + ", mname=" + mname + "]";
	}
	public Integer getMid() {
		return mid;
	}
	public void setMid(Integer mid) {
		this.mid = mid;
	}
	public String getMname() {
		return mname;
	}
	public void setMname(String mname) {
		this.mname = mname;
	}
	
}

<?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>
    <!-- 配置数据库文件 -->
    <properties resource="jdbc_mysql.properties">

    </properties>
    <settings>
        <setting name="lazyLoadingEnabled" value="false"/>
        <!--<setting name="aggressiveLazyLoading" value="false"/>-->
    </settings>
    <!-- 别名，对数据对象操作全名太长，需要使用别名 -->
    <typeAliases>
        <!--<typeAlias type="bean.Student" alias="Student"/>-->
        <!--直接使用类名即可，对于整个包的路径配置（别名），简单快捷 -->
        <package name="beans"/>
    </typeAliases>
    <!-- 配置运行环境 -->
    <!-- default 表示默认使用哪一个环境，可以配置多个，比如开发时的测试环境，上线后的正式环境等 -->
    <environments default="mysqlEM">
        <environment id="mysqlEM">
            <transactionManager type="JDBC">
            </transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.user}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 注册映射文件 -->
    <mappers>
        <mapper resource="mapper/mapper.xml"/>
    </mappers>
</configuration>

<?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="dao.ICountryDao">
    <!-- 	resultMap 能解决字段和属性不一样的问题 -->
    <!-- 以后用得比较多 ，是因为可以使用延迟加载-->
    <!-- 嵌套查询 -->
    <select id="selectMinisterByCountry" resultType="Minister">
	select mid,mname from minister where countryId=#{ooo}
	</select>
    <resultMap type="Country" id="countryMapper">
        <id column="cid" property="cid"/>
        <result column="cname" property="cname"/>
        <!-- country中有一个成员变量是ministers，它的泛型是Minister -->
        <collection property="ministers"
                    ofType="Minister"
                    select="selectMinisterByCountry"
                    column="cid">
        </collection>
    </resultMap>
    <select id="selectCountryById" resultMap="countryMapper">
		select cid,cname
		from country
		where
		cid=#{cid}
	</select>
</mapper>

public interface ICountryDao {
	Country selectCountryById(int cid);
}

public class MyBatisUtils {
  private static SqlSessionFactory sqlSessionFactory;

  public static SqlSession getSqlSession() {
    InputStream is;
    try {
      is = Resources.getResourceAsStream("mybatis.xml");
      if (sqlSessionFactory == null) {
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
      }
      return sqlSessionFactory.openSession();
    } catch (IOException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
    return null;
  }
}

    <properties resource="jdbc_mysql.properties">
    </properties>
    <settings>
        <setting name="lazyLoadingEnabled" value="flase"/>
    </settings>
    <!-- 别名，对数据对象操作全名太长，需要使用别名 -->
    <typeAliases>
        <!--<typeAlias type="bean.Student" alias="Student"/>-->
        <!--直接使用类名即可，对于整个包的路径配置（别名），简单快捷 -->
        <package name="bean"/>
    </typeAliases>

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
	}

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressivelazyLoading" value="true"/>
    </settings>

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
	}

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
		System.out.println(country.getCid());
	}

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressivelazyLoading" value="false"/>
    </settings>

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
	}

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
		System.out.println(country.getCid());
	}

	@Test
	public void TestselectCountryById(){
		Country country=dao.selectCountryById(1);
		System.out.println(country.getMinisters());
	}