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