What is pessimistic locking in Hibernate 7 and what are the improvements?

Pessimistic locking acquires a DB-level lock on a row when loaded. Hibernate 7 improves this with the Timeouts API from Jakarta Persistence 3.2.

// PESSIMISTIC_WRITE: SELECT ... FOR UPDATE
Product p = session.find(Product.class, id, LockModeType.PESSIMISTIC_WRITE);

// Hibernate 7: Timeouts API
import jakarta.persistence.Timeouts;

// Fail immediately if lock unavailable:
Product p2 = session.find(Product.class, id,
    LockModeType.PESSIMISTIC_WRITE, Timeouts.NO_WAIT);

// Wait up to 5 seconds:
Product p3 = session.find(Product.class, id,
    LockModeType.PESSIMISTIC_WRITE, Timeouts.of(5, TimeUnit.SECONDS));

// Lock an already-managed entity:
session.lock(managed, LockModeType.PESSIMISTIC_WRITE, Timeouts.NO_WAIT);

// Lock modes:
// PESSIMISTIC_READ  -> SELECT FOR SHARE
// PESSIMISTIC_WRITE -> SELECT FOR UPDATE

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