What's the difference between EntityManagerFactory and EntityManager, and why does that difference matter?

EntityManagerFactory is heavyweight, thread-safe, and created exactly once per application/datasource — it holds expensive parsed mappings and connection pool configuration. EntityManager is lightweight, NOT thread-safe, and created fresh per unit of work. The difference matters because recreating the factory per request is a severe, avoidable performance mistake, and sharing an EntityManager across threads causes real concurrency bugs, since it's explicitly not designed for that.