Java Persistence with Hibernate

9.1. The persistence lifecycle

Because Hibernate is a transparent persistence mechanism—classes are unaware of their own persistence capability—it's possible to write application logic that is unaware whether the objects it operates on represent persistent state or temporary state that exists only in memory. The application shouldn't necessarily need to care that an object is persistent when invoking its methods. You can, for example, invoke the calculateTotalPrice() business method on an instance of the Item class without having to consider persistence at all; e.g., in a unit test.

Any application with persistent state must interact with the persistence service whenever it needs to propagate state held in memory to the database (or vice versa). ...