What are the First and Second Level caches in (N)Hibernate?

1.1) First-level cache

First-level cache always Associates with the Session object. Hibernate uses this cache by default. Here, it processes one transaction after another one, means wont process one transaction many times. Mainly it reduces the number of SQL queries it needs to generate within a given transaction. That is instead of updating after every modification done in the transaction, it updates the transaction only at the end of the transaction.

1.2) Second-level cache

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.

Quoted from: http://javabeat.net/introduction-to-hibernate-caching/

There's a pretty good explanation of first level caching on the Streamline Logic blog.

Basically, first level caching happens on a per session basis where as second level caching can be shared across multiple sessions.

Here some basic explanation of hibernate cache...

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.