7、one2many单向

one2many单向

1 1的一方的hbm.xml文件的配置

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.baowei.entity">
<class name="Classroom" table="classroom">
<id name="id">
<generator class="native" />
</id>
<property name="name" />
<property name="grade" />

<!-- 使用了lazy=extra之后会稍微智能一些，会根据去的值的不同来判断是调用count和获取投影 -->
<set name="stus" lazy="extra">
<key column="mid" />  <!-- key用来指定在对方的外键的名称 -->
<one-to-many class="Student" />  <!-- class用来设置列表中的对象类型 -->
</set>

</class>
</hibernate-mapping>

2 关于one2many的测试代码

2.1 测试Add，会发出五条sql，三条插入，两条更新

public void testAdd01() {
Session session = null;
try {
session = HibernateUtil.openSession();
session.beginTransaction();

Student stu1 = new Student();
stu1.setName("猪八戒");
stu1.setNo("001");

Student stu2 = new Student();
stu2.setName("孙悟空");
stu2.setNo("002");
session.save(stu2);

session.save(stu1);
session.save(stu2);

Classroom c = new Classroom();
c.setGrade(2012);
c.setName("计算机网络技术");
// //设置关联关系
c.getStus().add(stu1);
c.getStus().add(stu2);
session.save(c);
// 此时会发出5条sql，三条查询，两条更新

session.getTransaction().commit();
} catch (Exception e) {
e.printStackTrace();
if (session != null)
session.getTransaction().rollback();
} finally {
// HibernateUtil.close(session);
}
}

2.2 测试懒加载-----再用到的时候才回去发出sql语句

2.3 关于lazy=extra，会智能的用一些sql语句去查询,如果输出的是int类型，会去select count(*)

public void testAdd02() {
Session session = null;
try {
session = HibernateUtil.openSession();
session.beginTransaction();

Classroom c = (Classroom) session.load(Classroom.class, 1);
System.out.println(c.getStus().size());
//如果使用了 lazy=extra,sql语句会进行一些优化
session.getTransaction().commit();
} catch (Exception e) {
e.printStackTrace();
if (session != null)
session.getTransaction().rollback();
} finally {
HibernateUtil.close(session);
}
}

2.4