How does Hibernate detect dirty state of an entity object?

Is it using some kind of byte codes modification to the original classes?

Or, maybe Hibernate get the dirty state by compare the given object with previously persisted version?

I'm having a problem with hashCode() and equals() methods for complicated objects. I feel it would be very slow to compute hash code if the object has collection members, and cyclic references are also a problem.

If Hibernate won't use hashCode()/equals() to check the dirty state, I guess I should not use equals()/hashCode() for the entity object (not value object), but I'm also afraid if the same operator (==) is not enough.

So, the questions are:

1. How does Hibernate know if a property of an object is changed?

2. Do you suggest to override the hashCode()/equals() methods for complicated objects? What if they contains cyclic references?

   And, also,

3. Would hashCode()/equals() with only the id field be enough?

Hibernate uses a strategy called inspection, which is basically this: when an object is loaded from the database a snapshot of it is kept in memory. When the session is flushed Hibernate compares the stored snapshot with the current state. If they differ the object is marked as dirty and a suitable SQL command is enqueued. If the object is still transient then it is always dirty.

Source: book Hibernate in Action (appendix B: ORM implementation strategies)

It's important to notice however that Hibernate's dirty-checking is independent of the methods equals/hascode. Hibernate does not look at these methods at all (except when using java.util.Set's, but this is unrelated to dirty-checking, only to the Collections API) The state snapshot I mentioned earlier is something similar to an array of values. It would be a very bad decision to leave such a core aspect of the framework in the hands of developers (to be honest, developers should not care about dirty-checking). Needless to say that equals/hascode can be implemented in many ways according to your needs. I recommend you to read the cited book, there the author discuss equals/hascode implementation strategies. Very insightful reading.

Hibernate default dirty checking mechanism will match all mapped properties of all currently attached entities against their initial loading-time values.

You can better visualize this process in the following diagram: