一级缓存

会话对象保存第一级缓存数据。默认启用。第一级缓存数据将对整个应用程序不可用。一个应用程序可以使用多个会话对象。

二级缓存

SessionFactory对象保存二级缓存数据。存储在第二级缓存中的数据将对整个应用程序可用。但是我们需要显式地启用它。

1.1)一级缓存

一级缓存始终与Session对象关联。Hibernate默认使用这个缓存。这里，它处理一个 一个接一个的交易，意味着不会处理一个交易很多 次了。主要是它减少了SQL查询的数量 在给定事务中生成。这代替了之后的更新 在事务中所做的每一次修改，都会更新事务 只有在交易结束时。

1.2)二级缓存

Second-level cache always associates with the Session Factory object. While running the transactions, in between it loads the objects at the Session Factory level, so that those objects will be available to the entire application, not bound to single user. Since the objects are already loaded in the cache, whenever an object is returned by the query, at that time no need to go for a database transaction. In this way the second level cache works. Here we can use query level cache also.

引用自:http://javabeat.net/introduction-to-hibernate-caching/

在streamlined Logic博客上有一个关于一级缓存的很好的解释。

基本上，第一级缓存发生在每个会话的基础上，而第二级缓存可以跨多个会话共享。

在二级缓存中，域hbm文件的键可以是可变的，值可以是false。 例如, 在这个领域类中，一天中的某些持续时间作为普遍真理保持不变。因此，它可以在应用程序中被标记为不可变。

这里有一些hibernate缓存的基本解释…

First level cache is associated with “session” object. The scope of cache objects is of session. Once session is closed, cached objects are gone forever. First level cache is enabled by default and you can not disable it. When we query an entity first time, it is retrieved from database and stored in first level cache associated with hibernate session. If we query same object again with same session object, it will be loaded from cache and no sql query will be executed. The loaded entity can be removed from session using evict() method. The next loading of this entity will again make a database call if it has been removed using evict() method. The whole session cache can be removed using clear() method. It will remove all the entities stored in cache.

Second level cache is apart from first level cache which is available to be used globally in session factory scope. second level cache is created in session factory scope and is available to be used in all sessions which are created using that particular session factory. It also means that once session factory is closed, all cache associated with it die and cache manager also closed down. Whenever hibernate session try to load an entity, the very first place it look for cached copy of entity in first level cache (associated with particular hibernate session). If cached copy of entity is present in first level cache, it is returned as result of load method. If there is no cached entity in first level cache, then second level cache is looked up for cached entity. If second level cache has cached entity, it is returned as result of load method. But, before returning the entity, it is stored in first level cache also so that next invocation to load method for entity will return the entity from first level cache itself, and there will not be need to go to second level cache again. If entity is not found in first level cache and second level cache also, then database query is executed and entity is stored in both cache levels, before returning as response of load() method.

一级缓存

会话对象保存第一级缓存数据。默认启用。第一级缓存数据将对整个应用程序不可用。一个应用程序可以使用多个会话对象。

二级缓存

SessionFactory对象保存二级缓存数据。存储在第二级缓存中的数据将对整个应用程序可用。但是我们需要显式地启用它。