What is persistence? How does it work?

Hi,
I'm a total noob. I've been reading and following examples from books but I haven't read anything about how persistence really works? How does it help my session beans to access or make transactions to the database? How does it actually work? What if I just make direct queries from my session beans without the help of the EntityManager or the Peristence API? What's the difference?
Like this for example:
Customer requestCustomer = em.find(Customer.class.customerId);Does it mean that the Persistence API keeps the object alive in the Customer Entity? For how long? Until the application is closed? If so, what happens if the user opens the application again and looks for that object?
Why can't I just make a direct query to see if that customerId exists in the database?
I hope you guys don't mind my questions... I just really want to learn this stuff and I gotta let things out of my head. Hope you can help. Thanks!

morphian wrote:
Hi,
I'm a total noob. I've been reading and following examples from books but I haven't read anything about how persistence really works? How does it help my session beans to access or make transactions to the database?
How does it actually work? What if I just make direct queries from my session beans without the help of the EntityManager or the Peristence API? What's the difference?
You have to code the stuff that EntityManager and Persistence are doing for you.
Like this for example:
Customer requestCustomer = em.find(Customer.class.customerId);Does it mean that the Persistence API keeps the object alive in the Customer Entity? For how long? Until the application is closed? Complicated. Not enough info.
If so, what happens if the user opens the application again and looks for that object?
Open the app and the object will be refreshed from that data store. Hence the word "persistent".
Why can't I just make a direct query to see if that customerId exists in the database?
You can.
I hope you guys don't mind my questions... I just really want to learn this stuff and I gotta let things out of my head. Hope you can help. Thanks!This isn't the way to do it, but it's a start.
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