How long are Entities persited?

Question

I am trying to understand how JPA works. From what I know, if you persist an Entity, that object will remain in the memory until the application is closed. This means, that when I look for a previously persisted entity, there will be no query made on the database. Assuming that no insert, update or delete is made, if the application runs long enough, all the information in it might become persistent. Does this mean that at some point, I will no longer need the database?

Edit

My problem is not with the database. I am sure that the database can not be modified from outside the application. I am managing transactions by myself, so the data gets stored in the database as soon as I commit. My question is: What happens with the entities after I commit? Are they kept in the memory and act like a cache? If so, how long are they kept there? After I commit a persist, I make a select query. This select should return the object I persisted before. Will that object be brought from memory, or will the application query the database?

Answer 1

Not really. Think about it.

Your application probably isn't the only thing that will use the database. If an entity was persisted once and stored in memory, how can you be sure that, let's say, one hour later, it won't be changed by some other means? If that happens, you will have stale data that can harm logic of your application.

Storing data in memory and hoping that everything will be alright won't bring any benefits. That's why data stored in database is your primary source of information, and you should query it every time, unless you are absolutely sure that a subset of data won't change.