hibernate的懒加载与抓取策略

<hibernate-mapping>
    <class name="cn.jbit.houserent.bean.District" table="district">
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="district_name" length="50" not-null="true" />
        </property>
        <!--设置lazy属性为false，立即加载关联的对象-->
        <set name="streets" table="street" cascade="all" inverse="true" lazy="false">
            <key>
                <column name="district_id" not-null="true" />
            </key>
            <one-to-many class="cn.jbit.houserent.bean.Street" />
        </set>
    </class>
</hibernate-mapping>

/**
	 * 类的懒加载
	 */
	@Test
	public void testClass_lazy(){
		session = HibernateUtils.openSession();
		transaction = session.beginTransaction();
		Classes classes = (Classes) session.load(Classes.class, 1L);
		//classes.setName("asdj");
		System.out.println(classes.getName());//发出sql
		transaction.commit();
		session.close();
	}

try {
	config=new Configuration().configure();
	sf=config.buildSessionFactory();
	session= sf.openSession();
	ts=session.beginTransaction();
	user=(UserBean)session.load(UserBean.class, 1);
	System.out.println("**"+user.getUserId());
	System.out.println("**"+user.getUsername());
	ts.commit();
} catch (Exception e) {
	e.printStackTrace();
	ts.rollback();
}
finally{
	session.close();
}

  //不会发出sql语句
  District district=(District)session.load(District.class, 83);
  System.out.println("**");
  //调用对象发出sql语句
  System.out.println(district.getDisName());
  //不会发sql语句
  Set<Street> set=district.getSetStreet();
  System.out.println("**");
  //调用对象发出sql语句
  Iterator<Street> it=set.iterator();
  while(it.hasNext()){
     Street street=it.next();
     System.out.println(street.getStrName());
}

<class name="cn.jbit.houserent.bean.User" table="users" schema="jbit" lazy="true">
…
</class>

// 省略部分代码
User user = (User)session.load(User.class, new Integer("281"));
System.out.println("获取用户ID为1000的用户");
user.getName();
// 省略部分代码

/**
 * 集合的懒加载
 */
@Test
public void testCollect_lazy(){
	session = HibernateUtils.openSession();
	transaction = session.beginTransaction();
	
	Classes classes = (Classes) session.get(Classes.class, 1L);
	Set<Student> students = classes.getStudents();//不发出
	for (Student student : students) {//迭代的时候发出sql
		System.out.println(student.getName());
	}
	transaction.commit();
	session.close();
}
/**
 * 更进一步的懒加载,设置extra,对于查询行数,最大值,最小值等
 */
@Test
public void testlazy_extra(){
	session = HibernateUtils.openSession();
	transaction = session.beginTransaction();
	
	Classes classes = (Classes) session.get(Classes.class, 1L);
	Set<Student> students = classes.getStudents();
	System.out.println(students.size());
	transaction.commit();
	session.close();
}

<class name="cn.jbit.houserent.bean.House" table="house">
    <!--省略其他配置-->
    <property name="description" type="text" lazy="true">
        <column name="description"/>
    </property>
</class>

Query query = session.createQuery("from House as h where h.price = 2000");
System.out.println("获取价格为2000 元的房屋信息");
List<House> result = query.list();
for(House house:result){
    System.out.println("标题:"+house.getTitle());
    System.out.println("描述:"+house.getDescription());
}

public class FetchTest {
 
	private Session session;
	private Transaction transaction;
	private SessionFactory sessionFactory;
 
	/**
	 * fetch : select情况:
	 * 先查找Classes的id,在根据id查Student
	 * 	n+1条数据
	 * 		n:classes表中的条数
	 * 		1:classes表本身
	 */
	@Test
	public void testQueryAllStudent(){
		session = HibernateUtils.openSession();
		List<Classes> classes = session.createQuery("from Classes").list();
		for (Classes classes2 : classes) {
			System.out.println(classes2.getName());
			Set<Student> students = classes2.getStudents();
			for (Student student : students) {
				System.out.println(student.getName());
			}
		}
		session.close();
		
	}
	/**
	 * fetch : subselect情况:
	 * 因为该需求含有子查询,所以fetch 使用 subselect
	 * 2条sql
	 */
	@Test
	public void testQueryAllStudent2(){
		session = HibernateUtils.openSession();
		List<Classes> classes = session.createQuery("from Classes").list();
		for (Classes classes2 : classes) {
			System.out.println(classes2.getName());
			Set<Student> students = classes2.getStudents();
			for (Student student : students) {
				System.out.println(student.getName());
			}
		}
		session.close();
		
	}
	/**
	 * select
	 *    先查询一的一方的所有的对象(Classes)，再根据每一个对象的id值查询其关联对象(Student)
	 */
	@Test
	public void testQueryClassesByCidAndStudents_Select(){
		Session session = HibernateUtils.openSession();
		Classes classes = (Classes)session.get(Classes.class, 1L);
		System.out.println(classes.getName());
		Set<Student> students = classes.getStudents();
		for (Student student : students) {
			System.out.println(student.getName());
		}
		session.close();
	}
	
	/**
	 * join  利用左外连接一条SQL语句把classes和student表全部查询出来了
	 * 
	 * 在含有子查询的需求分析中，利用join的抓取策略是不取的
	 * 1条sql
	 */
	@Test
	public void testQueryClassesByCidAndStudents_Join(){
		Session session = HibernateUtils.openSession();
		Classes classes = (Classes)session.get(Classes.class, 1L);//发出sql,故fetch为join的抓取,会导致懒加载的失效
		System.out.println(classes.getName());
		Set<Student> students = classes.getStudents();
		
		for (Student student : students) {
			System.out.println(student.getName());
		}
		session.close();
	}
}