Behind the Scenes

Before moving on, it is useful to look at what's going on behind the scenes. One thing you should know is that the Hibernate cache does not store object instances. Instead, it stores objects in their "dehydrated" form (to use Hibernate terminology), that is, as a set of property values. The following is a sample of the contents of the Country cache:

{
30  => [bw,Botswana,30],
214 => [uy,Uruguay,214],
158 => [pa,Panama,158],
31  => [by,Belarus,31]
95  => [in,India,95]
...
}

Notice how each ID is mapped to an array of property values. You may also have noticed that only the primitive properties are stored; there is no sign of the airports property. This is because the airports property is actually an association: a set of references to other persistent objects.

By default, Hibernate does not cache associations. It's up to you to decide which associations should be cached, and which associations should be reloaded whenever the cached object is retrieved from the second-level cache.

Association caching is a very powerful functionality. The next section takes a more detailed look at it.