hibernate學習6_session之clear與flush

標籤：

1、session的clear()方法

　 hibernate從資料庫中get或load記錄時，或先去session一級緩衝中尋找對應資料，如果緩衝中沒有記錄，則去對應資料庫中查詢，

如果有記錄則直接從緩衝中擷取。

    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();    }

執行時只會發出一次查詢

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()方法後，會清空當前session緩衝，資料庫查詢兩次，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方法會強制同步當前session快取資料至資料庫中，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();    }

此時資料庫中load出對應記錄後，雖然前後連續更改了兩次屬性，資料庫只執行一次update

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

加上flush後：

    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();    }

資料庫連續執行兩次update

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

 

hibernate學習6_session之clear與flush