Java.Hibernate.Beginner.Explain “Can leave the entity in a transient state if there’s no transaction (e.g., changes may not actually persist)”

🔹 What happens technically:
When you call session.save(entity), Hibernate assigns an identifier (e.g., the primary key) and puts the entity into the session’s first-level cache — so it becomes persistent in the session context.

But if you haven’t started a transaction, then:

• There’s no active database transaction to guarantee that the pending insert will be executed and committed.
• Hibernate will still queue the SQL INSERT statement internally, but it won’t necessarily send it to the database immediately.
• If you close the session or an error happens before a transaction is committed, the entity’s changes may never actually get flushed to the database — leaving it effectively transient outside the session.

🔹 Example showing the problem:

Session session = sessionFactory.openSession(); // but NO transaction started!

User user = new User();
user.setUsername("john");

session.save(user); // Hibernate assigns ID, but no transaction active
// No commit happens, so changes might never be sent to DB!

session.close();    // Session closes → pending SQL is discarded → user is not saved in DB

🔹 Why doesn’t persist() do this?

• The JPA persist() method requires an active transaction — if you try to call persist() outside a transaction, it throws a TransactionRequiredException.
  ✅ This prevents you from accidentally thinking you saved something when no transaction is active.

🔹 Key takeaway:

• save() can mislead you into thinking you’ve persisted data when you haven’t committed a transaction → resulting in lost changes.
• Always use save() (or persist()) inside an active transaction, and prefer persist() for standard, safer behavior.