java – Hibernate中的transaction.commit()是什么？

Account account = new Account();
account.setId(100);
account = (Account) session.get(Account.class,account.getId());
System.out.println("Before Transaction: Balance = " + account.getBalance());
double preBal = account.getBalance();
account.setBalance(50000000);
Transaction transaction = session.beginTransaction();
session.update(account);
account = (Account) session.get(Account.class,account.getId());
System.out.println("After Transaction: Balance = " + account.getBalance());
// transaction.commit();    
account = (Account) session.get(Account.class,account.getId());
System.out.println("Pev-Bal=" + preBal + " Curr-Bal=" + account.getBalance());

这给我结果：

Hibernate: select account0_.id as id0_1_,account0_.balance as ..........
Before Transaction: Balance = 300.0
After Transaction: Balance = 5.0E7
Pev-Bal=300.0 Curr-Bal=5.0E7

但是由于我没有调用transaction.commit(),所以数据库没有变化.

这是否意味着一切都只在某些实例/对象上完成,而不会真正改变数据库？

我是Hibernate的新手,所以请帮我理解.
我正在使用hibernate 4.

更新：

如果我调用transaction.commit(),那么结果有这一行

Hibernate: update account set balance=? where id=?

数据库也更新了.

这是否意味着没有调用transaction.commit()一切都只在实例级别完成,而不会真正更改数据库？