What is the first-level cache (persistence context) in Hibernate 7?

The first-level cache is the per-Session in-memory cache for all entities loaded or persisted within an open Session. It is mandatory - every Session has its own first-level cache.

Session session = sf.openSession();
session.beginTransaction();

Product p1 = session.find(Product.class, 1L);  // DB hit
Product p2 = session.find(Product.class, 1L);  // CACHE HIT - no SQL!
Product p3 = session.find(Product.class, 1L);  // CACHE HIT

System.out.println(p1 == p2);  // true! Same object reference

// Evict specific entity:
session.detach(p1);  // p1 is now DETACHED
Product p4 = session.find(Product.class, 1L);  // DB hit again

// Clear entire cache:
session.clear();  // all entities become DETACHED

// Bulk processing pattern (prevent OutOfMemoryError):
for (int i = 0; i < 100_000; i++) {
    session.persist(products.get(i));
    if (i % 50 == 0) {
        session.flush();   // write to DB
        session.clear();   // free cache memory
    }
}

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