金九银十: Hibernate快速入门到一级缓存

矫情吗；* 2023-02-21 11:46 14阅读 0赞

***1***

Hibernate介绍

Hibernate,让程序员更方便访问数据库,一款全ORM的框架,通过操作对象来操作数据库......

***2***

Hibernate入坑

**2.1  准备数据库**

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**2.2  实体类**

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**2.3 元数据文件**

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**2.4 主配置文件**

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**2.5 测试CRUD**

**2.5.1 get**

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**2.5.2 save**

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**2.5.3 update**

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**2.5.4 delete**

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***3***

一级缓存

从获取单个用户可以看出,调用session的get方法会打印一次sql语句,也就是执行一次查询,那如果调用两次呢?

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从控制台可以看出其实是只打印出了一次sql,也就是只操作了一次数据库,那么第二次调用get方法就是从缓存中获取的,那么hibernate的一级缓存是如何定义的呢?

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从图中可以看出,当我们的程序调用session的get方法的时候,首先会先从session缓存中查找出是否符合我们所需要查询对象,如果存在我们所需要的对象,会直接返回该对象到程序中,从而并不会操作数据库,减少了数据库查询的时间,如果在缓存中查询不到我们所需要的数据对象,会从数据库中查询,返回给程序并且将该对象存入session缓存中。

其实在session中定义了一系列的集合来存储数据，而他们则构成了session缓存。只要session没有关闭，它就一直会存在。

***4***

缓存进阶之快照

当我们从数据库中取出查询结果之后，有一份数据会保存到session缓存中，其中话会有一份保存到快照中，当我们同时使用get获取对象之后，进行update操作，如果update操作的对象属性与快照中所存储的对象一致，不会进行update操作。

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控制台中没有输入update语句，因为user对象的address属性值就是beijing，执行保存的时候，会到快照中对比属性值，如果属性值与数据库一致，也就是没有修改新的值，不会执行udpate语句。

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