JPA Fundamentals

JPA is a specification that defines standard interfaces and annotations for ORM, while Hibernate is the most popular implementation that generates SQL, manages the persistence context, and talks to JDBC.

Every JPA entity is in one of four states: Transient (new), Managed (tracked by persistence context), Detached (disconnected), or Removed (scheduled for DELETE) – persist() inserts, merge() re-attaches.

persist() attaches the same new object to the persistence context and schedules an INSERT, while merge() copies a detached object's state into a managed copy and returns that copy.

EntityManager is the gateway to the persistence context, providing lifecycle operations (persist, merge, remove), lookup by primary key (find, getReference), and JPQL query creation (createQuery with parameter binding).

The persistence context is the EntityManager's first-level cache and identity map of managed entities; it does automatic dirty checking and flushes changes at commit, and in Spring it is transaction-scoped.