hibernate session三种状态
Session对象是有生命周期的，它以Transaction对象的事务开始和结束边界
session就像数据库连接池的一个连接，用于操作在hibernate里面操作对象，不用于保存东西
save 并没保存到数据库中，可以自动设置id在session中保存一份对象的缓存，提高访问速度（不用每次都访问数据库） persistent状态
/**
* Persist the given transient instance, first assigning a generated identifier. (Or
* using the current value of the identifier property if the <tt>assigned</tt>
* generator is used.) This operation cascades to associated instances if the
* association is mapped with <tt>cascade="save-update"</tt>.
*
* @param object a transient instance of a persistent class
* @return the generated identifier
* @throws HibernateException
*/
public Serializable save(String entityName, Object object) throws HibernateException;
commit 发出sql语句 ，将关闭session，对象引用不在session中 detached状态
相当于一个缓存和数据库的一个同步
commit时调用了flush方法
（ps：commit后，session对象消失，如果该对象引用到另一个事物中，如果修改对象属性，事物提交时不会将该游离态对象同步，需手动调用saveOrUpdate方法，手动调用如果session中存在另一个id一样的session对象，将报hibernate session id冲突问题，merge可以解决，但这种不规范，易忽视的代码不建议这么干）
delete 对象必须有id号，也就是只能删除persistent和detached状态的对象
load 将数据库记录转化为对象 Teacher t = (Teacher)session.load(Teacher. class, 1); 并没发sql语句，只是一个代理对象，在需要用的时候发sql语句
比如：session.getTransaction().commit();
System. out.println(t.getClass());
System. out.println(t.getBirthday());
commit后，如果commit前没有使用过对象t，那么之后使用将报错，commit后session中的对象缓存将不存在，所以不能用对象t
get 将数据库记录转化为对象 立即发sql语句将记录封装成对象
get和load都是先查缓存 clear可以强制清除缓存
saveOrUpdate
/**
* Either {@link #save(String, Object)} or {@link #update(String, Object)}
* the given instance, depending upon resolution of the unsaved-value checks
* (see the manual for discussion of unsaved-value checking).
* <p/>
* This operation cascades to associated instances if the association is mapped
* with <tt>cascade="save-update"</tt>.
*
* @see Session#save(String,Object)
* @see Session#update(String,Object)
* @param object a transient or detached instance containing new or updated state
* @throws HibernateException
*/
public void saveOrUpdate(String entityName, Object object) throws HibernateException;
update 更新detached或者persistent对象或者transient 需有主键
update默认更新所有字段，效率低，可设置字段不参与更新，或xml里面设置有变更时更新
persistent对象设置不同字段在commit时就会更新
当和数据库相同是不会更新
/**
* Update the persistent instance with the identifier of the given detached
* instance. If there is a persistent instance with the same identifier,
* an exception is thrown. This operation cascades to associated instances
* if the association is mapped with <tt>cascade="save-update"</tt>.
*
* @param object a detached instance containing updated state
* @throws HibernateException
*/
public void update(String entityName, Object object) throws HibernateException;
merge 先从数据库load记录比较和对象的不同
/**
* Copy the state of the given object onto the persistent object with the same
* identifier. If there is no persistent instance currently associated with
* the session, it will be loaded. Return the persistent instance. If the
* given instance is unsaved, save a copy of and return it as a newly persistent
* instance. The given instance does not become associated with the session.
* This operation cascades to associated instances if the association is mapped
* with <tt>cascade="merge"</tt>.<br>
* <br>
* The semantics of this method are defined by JSR-220.
*
* @param object a detached instance with state to be copied
* @return an updated persistent instance
*/
public Object merge(Object object) throws HibernateException;
createQuery hql语句返回Query对象
clear 清除缓存
flush 强制同步缓存和数据库内容
session用于管理事务增删改查
Session创建：
//当上下文里面没session则创建session，有则不创建
Session session = sf.getCurrentSession();
最后不需加session.close();
永远打开新的session
Session session = sf.openSession();
最后需加session.close();
session.commit()之后session将被关闭（getcurr
getCurrentSession创建的线程会在事务回滚或事物提交后自动关闭,而openSession必须手动关闭
and 必须设定current_session_context_class
转载于:https://my.oschina.net/yugj/blog/521926