hibernate学习6_session之clear与flush

1、session的clear()方法

　 hibernate从数据库中get或load记录时，或先去session一级缓存中查找对应数据，如果缓存中没有记录，则去对应数据库中查询，

如果有记录则直接从缓存中获取。

    public void testClear() {
        Session session1 = sf.getCurrentSession();
        session1.beginTransaction();
        Student t1 = (Student) session1.get(Student.class, 45);
        //session1.clear();
        Student t2 = (Student) session1.get(Student.class, 45);
        session1.getTransaction().commit();
    }

执行时只会发出一次查询

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=?

加上session的clear()方法后，会清空当前session缓存，数据库查询两次，如下：

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=?
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=?

2、session的flush方法

flush方法会强制同步当前session缓存数据至数据库中，如下：

    @Test
    public void testFlush() {
        Session session1 = sf.getCurrentSession();
        session1.beginTransaction();
        Student t1 = (Student) session1.get(Student.class, 45);
        t1.setAge(30);
        t1.setAge(40);
        session1.getTransaction().commit();
    }

此时数据库中load出对应记录后，虽然前后连续更改了两次属性，数据库只执行一次update

Hibernate: update Student set age=?, name=? where id=?

加上flush后：

    public void testFlush() {
        Session session1 = sf.getCurrentSession();
        session1.beginTransaction();
        Student t1 = (Student) session1.get(Student.class, 45);
        t1.setAge(30);
        session1.flush();
        t1.setAge(40);
        session1.getTransaction().commit();
    }

数据库连续执行两次update

Hibernate: update Student set age=?, name=? where id=?
Hibernate: update Student set age=?, name=? where id=?