Hibernate学习之四-------The Persistence Life Cycle

最新推荐文章于 2021-03-03 11:14:56 发布

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最新推荐文章于 2021-03-03 11:14:56 发布

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分类专栏： Hibernate 文章标签： Hibernate UP

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本文介绍了Hibernate中对象的三种状态：瞬时、持久和分离，并详细讲解了如何通过Hibernate的各种方法实现这些状态间的转换，包括保存、更新、删除实体对象等。

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1. Life Cycle

Given an instance of an object that is mapped to Hibernate, it can be in any one of three different states: transient, persistent, or detached.

映射到Hibernate中的类可以是以下三种状态中的一种：瞬时的，持久的，分离的。

Transient objects exist in memory, Hibernate does not manage transient objects or persist changes to transient objects.

Persistent objects exist in the database, and Hibernate manages the persistence for persistent objects. If fields or propertieschange on a persistent object, Hibernate will keep the database representation up-to-date.

Detached objects have a representation in the database, but changes to the object will not be reflected in the database, and vice versa(反之亦然). A detached object can be created by closing the session that it was associated with, or by evicting it from the session with a call to the session’ s evict() method.

当关闭当前的连接此时的对象和数据库立之间的联系的断开了。

In order to persist changes made to a detached object, the application must reattach it to a valid Hibernate session. A detachedinstance can be associated with a new Hibernate session when your application calls one of the load(), refresh(), merge(), update(), or save()methods on the new session with a reference to the detached object. After the call, the detached object would be a persistent object managed by the new Hibernate session.重新获得连接后，分离的对象就会转变成持久的对象。

2. Associations 关系

3.Saving Entities 保存实体对象

也就是将瞬时对象持久化到数据库中去，提供的方法：

public Serializable save(Object object) throws HibernateException
public void save(Object object, Serializable id) throws HibernateException
public Serializable save(String entityName,Object object) throws HibernateException

参数中的对象均是瞬时对象（必须要有对应的映射文件）

public void saveOrUpdate(Object object) throws HibernateException Hibernate自动判断是更新还是插入新记录。

4.Object Equality and Identity 对象的相等性和同一性

从一个Hibernate会话中重复请求一个持久化对象，就会返回一个Java类的相同实例，可以用==来比较。但是，如果是从多个Hibernate会话中请求一个持久化对象，Hibernate就会从每个会话中提供不同的实例，==会返回fase,这时要比较对象，就应该用equals()方法了。

数据库中的一个表中的一行记录相当于对应的实体类的一个对象实例。

5.Updating Entities ,Deleting Entities ,Refreshing Entities ,Loading Entities

6. Cascading Operations

7.Lazy Loading,Proxies,and Collection Wrappers(需要深入学习)