Can you explain what the first-level cache in EntityManager is and how it works?

The First Level Cache in EntityManager is an built-in caching mechanism for objects loaded from the database within a single session (transaction).

How it works:

• When an entity is loaded through EntityManager, the object is stored in the first level cache.
• If the same entity with the same identifier is requested during the same session, EntityManager returns the object from the cache instead of making a new database query.
• The first level cache only operates within the scope of one EntityManager (session).
• When the session is closed, the cache is cleared.

This improves performance by reducing database access and ensures data consistency within a single transaction.

Example:

EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();

User user1 = em.find(User.class, 1); // database query
User user2 = em.find(User.class, 1); // from first level cache, no database query

em.getTransaction().commit();
em.close();

Thus, the first level cache helps optimize database operations by reducing redundant queries.