Mybatis中的延迟加载案例解析

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

package cn.zhang.entity;
import java.util.HashSet;
import java.util.Set;
public class Dept {
 private Integer deptno;
 private String deptname;
 private Set<Emp> emp = new HashSet<Emp>();
 @Override
 public String toString() {
  return "Dept [deptno=" + deptno + ", deptname=" + deptname + ", emp="
    + emp + "]";
 }
 public Integer getDeptno() {
  return deptno;
 }
 public void setDeptno(Integer deptno) {
  this.deptno = deptno;
 }
 public String getDeptname() {
  return deptname;
 }
 public void setDeptname(String deptname) {
  this.deptname = deptname;
 }
 public Set<Emp> getEmp() {
  return emp;
 }
 public void setEmp(Set<Emp> emp) {
  this.emp = emp;
 }
}

package cn.zhang.entity;
public class Emp {
 private Integer empno;
 private String empname;
 @Override
 public String toString() {
  return "Emp [empno=" + empno + ", empname=" + empname + "]";
 }
 public Integer getEmpno() {
  return empno;
 }
 public void setEmpno(Integer empno) {
  this.empno = empno;
 }
 public String getEmpname() {
  return empname;
 }
 public void setEmpname(String empname) {
  this.empname = empname;
 }
}

package cn.zhang.util;
import java.io.IOException;
import java.io.Reader;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
/**
 * 工具类
 * 
 */
public class MybatisUtil {
 private static String config = "mybatis-config.xml";
 static Reader reader;
 static {
  try {
   reader = Resources.getResourceAsReader(config);
  } catch (IOException e) {
   e.printStackTrace();
  }
 }
 private static SqlSessionFactory factory = new SqlSessionFactoryBuilder()
   .build(reader);
 // 提供一个可以获取到session的方法
 public static SqlSession getSession() throws IOException {
  SqlSession session = factory.openSession();
  return session;
 }
}

package cn.zhang.dao;
import java.io.IOException;
import cn.zhang.entity.Dept;
public interface DeptDao {
 /**
  * 查询指定记录
  * @return
  * @throws IOException
  */
 public Dept findById(Integer id) throws IOException;
}

<?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.zhang.dao.DeptDao">
 <!-- 3.根据员工id查询员工信息 -->
 <select id="selectEmpByDeptNo" resultType="Emp">
  select empno,empname
  from emp where deptno=#{deptno}
 </select>
 <!-- 2.对部门实体的映射 -->
 <resultMap type="Dept" id="deptMapper">
  <id property="deptno" column="deptno" />
  <result property="deptname" column="deptname" />
  <!-- 一对多部门关联的员工 -->
  <!--select:关联员工查询 -->
  <!--column:关联员工查询所需要的条件（来源于1） -->
  <collection property="emp" ofType="Emp" select="selectEmpByDeptNo"
   column="deptno" />
 </resultMap>
 <!--1.根据部门id查询部门信息 -->
 <select id="findById" resultMap="deptMapper">
  select deptno,deptname from dept
  where deptno=#{deptno}
 </select>
</mapper>

<?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>
 <!--lazyLoadingEnabled：设置懒加载，默认为false。如果为false：则所有相关联的都会被初始化加载。
  aggressiveLazyLoading：默认为true。当设置为true时，懒加载的对象可能被任何懒属性全部加载；否则，每个属性按需加载。 -->
 <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>
 <!-- 配置别名 -->
 <typeAliases>
  <!--方式一: 按类型名定制别名 -->
  <!--方式二: 拿当前指定包下的简单类名作为别名 -->
  <package name="cn.zhang.entity" />
 </typeAliases>
 <environments default="oracle">
  <environment id="oracle">
   <!-- 使用jdbc的事务 -->
   <transactionManager type="JDBC" />
   <!-- 使用自带的连接池 -->
   <dataSource type="POOLED">
    <!-- 我用的Oracle数据库 -->
    <property name="driver" value="oracle.jdbc.driver.OracleDriver" />
    <property name="url" value="jdbc:oracle:thin:@localhost:1521:orcl" />
    <property name="username" value="study" />
    <property name="password" value="123" />
   </dataSource>
  </environment>
 </environments>
 <mappers>
  <mapper resource="cn/zhang/dao/DeptDAO.xml" />
 </mappers>
</configuration>

package cn.zhang.test;
//一对多
import java.io.IOException;
import org.apache.ibatis.session.SqlSession;
import org.junit.Before;
import org.junit.Test;
import cn.zhang.dao.DeptDao;
import cn.zhang.entity.Dept;
import cn.zhang.util.MybatisUtil;
public class MyTest {
 DeptDao dao;
 @Before
 public void initData() throws IOException{
  SqlSession session = MybatisUtil.getSession();
  dao = session.getMapper(DeptDao.class);
 }
 /**
  * 查询指定记录
  * @throws IOException
  */
 @Test
 public void findAll() throws IOException{
  Dept dept = dao.findById(1);
  System.out.println(dept);
 }
}

<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>