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hibernate学习5_session之load与get区别
hibernate从数据库中查询对应记录到java实体中时，一般可通过session.load或者session.get来完成
(1)load方法
@Test public void testLoad(){ Session session1 = sf.getCurrentSession(); session1.beginTransaction(); Student t = (Student) session1.load(Student.class, 45); System.out.println(t.getAge()); session1.getTransaction().commit(); }
在session关闭前使用返回对象时，hibernate加载正常
Hibernate: select student0_.id as id1_0_0_, student0_.age as age2_0_0_, student0_.name as name3_0_0_ from Student student0_ where student0_.id=?
如果将调用去掉。如下
@Test public void testLoad(){ Session session1 = sf.getCurrentSession(); session1.beginTransaction(); Student t = (Student) session1.load(Student.class, 45); //System.out.println(t.getAge()); session1.getTransaction().commit(); }
则hibernate执行上述程序时没有调用查询语句。
如果将调用放在session关闭之后，如下
@Test public void testLoad(){ Session session1 = sf.getCurrentSession(); session1.beginTransaction(); Student t = (Student) session1.load(Student.class, 45); session1.getTransaction().commit(); System.out.println(t.getAge()); }
则出现如下异常
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:165)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:286)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185)
.....
(2)get方法
@Test public void testGet(){ Session session1 = sf.getCurrentSession(); session1.beginTransaction(); Student t = (Student) session1.get(Student.class, 45); session1.getTransaction().commit(); System.out.println(t.getAge()); }
get方法返回对应对象后，无论是在session关闭前调用或者关闭后调用，都能正常进行查询
Hibernate: select student0_.id as id1_0_0_, student0_.age as age2_0_0_, student0_.name as name3_0_0_ from Student student0_ where student0_.id=?
get方法返回对应对象后，即使不调用对象，依然能够正常查询。
综上：
(1)session的load方法返回对象时不是立即从数据库中执行查询，而是采用动态代理，在对象中属性被调用时hibernate再执行查询，即所谓的延迟加载。
如果此时session关闭，则出现加载异常。
(2)session的get方法则是在返回对象时立即从数据库中执行对应查询，将返回记录封装进对应实体中
hibernate学习5_session之load与get区别