hibernate-demo/src/main/java/com/ankurm/hibernatedemo/scenario/GetVsLoadRunner.java

package com.ankurm.hibernatedemo.scenario;

import com.ankurm.hibernatedemo.model.Book;
import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import org.hibernate.Session;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Profile;
import org.springframework.stereotype.Component;

/**
 * Backs ankurm.com post 4859 ("Hibernate 7: get() vs load()") and
 * docs/01-get-vs-load.md. Captured verbatim into docs/output/get-vs-load.txt by
 * {@code scripts/run.sh getvsload}.
 *
 * <p>Each step opens its own {@link EntityManager} deliberately, so the SQL log lines that
 * bracket a step are unambiguously that step's own traffic &mdash; there is no shared session
 * whose first-level cache could quietly answer a later {@code get()} for free.
 */
@Component
@Profile("getvsload")
public class GetVsLoadRunner implements CommandLineRunner {

    private static final Logger DEMO = LoggerFactory.getLogger("DEMO");

    private final EntityManagerFactory emf;

    public GetVsLoadRunner(EntityManagerFactory emf) {
        this.emf = emf;
    }

    @Override
    public void run(String... args) {
        Long existingId = seedOneBook();
        long missingId = existingId + 999_000L;

        step1_getExisting(existingId);
        step2_getMissing(missingId);
        step3_getReferenceExisting_noSelectUntilAccessed(existingId);
        step4_getReferenceMissing_exceptionOnlyOnAccess(missingId);
        step5_getReferenceThenSessionClosed_lazyInitException(existingId);
        step6_proxyVsRealIdentity(existingId);
    }

    private Long seedOneBook() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Book book = new Book("Effective Java", "Joshua Bloch");
        em.persist(book);
        em.getTransaction().commit();
        Long id = book.getId();
        em.close();
        DEMO.info("SEED: inserted Book id={}", id);
        return id;
    }

    private void step1_getExisting(Long id) {
        DEMO.info("--- Step 1: session.get() on an existing id ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        DEMO.info("about to call session.get(Book.class, {})", id);
        Book book = session.get(Book.class, id);
        DEMO.info("get() returned: {}", book);
        em.getTransaction().commit();
        em.close();
    }

    private void step2_getMissing(long missingId) {
        DEMO.info("--- Step 2: session.get() on a missing id ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        DEMO.info("about to call session.get(Book.class, {})", missingId);
        Book book = session.get(Book.class, missingId);
        DEMO.info("get() returned: {} (no exception thrown)", book);
        em.getTransaction().commit();
        em.close();
    }

    private void step3_getReferenceExisting_noSelectUntilAccessed(Long id) {
        DEMO.info("--- Step 3: session.getReference() on an existing id ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        Book proxy = session.getReference(Book.class, id);
        DEMO.info("getReference() returned proxy of class {} -- no SELECT above this line", proxy.getClass().getName());
        DEMO.info("now calling proxy.getTitle() ...");
        String title = proxy.getTitle();
        DEMO.info("getTitle() returned '{}' -- the SELECT for this ran just above this line", title);
        em.getTransaction().commit();
        em.close();
    }

    private void step4_getReferenceMissing_exceptionOnlyOnAccess(long missingId) {
        DEMO.info("--- Step 4: session.getReference() on a missing id ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        Book proxy = session.getReference(Book.class, missingId);
        DEMO.info("getReference() returned a proxy for a row that does not exist -- no exception yet: {}", proxy.getClass().getName());
        try {
            proxy.getTitle();
            DEMO.info("no exception -- this line should be unreachable");
        } catch (RuntimeException e) {
            DEMO.info("accessing the proxy threw {}: {}", e.getClass().getName(), e.getMessage());
        }
        em.getTransaction().rollback();
        em.close();
    }

    private void step5_getReferenceThenSessionClosed_lazyInitException(Long id) {
        DEMO.info("--- Step 5: proxy accessed after its session is closed ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        Book proxy = session.getReference(Book.class, id);
        em.getTransaction().commit();
        em.close();
        DEMO.info("session closed. proxy in hand: {}", proxy.getClass().getName());
        try {
            proxy.getTitle();
            DEMO.info("no exception -- this line should be unreachable");
        } catch (RuntimeException e) {
            DEMO.info("accessing the proxy after close threw {}: {}", e.getClass().getName(), e.getMessage());
        }
    }

    private void step6_proxyVsRealIdentity(Long id) {
        DEMO.info("--- Step 6: proxy identity vs a real loaded instance ---");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        Book real = session.get(Book.class, id);
        // second em/session so this is a genuinely separate proxy, not the same cached instance
        EntityManager em2 = emf.createEntityManager();
        em2.getTransaction().begin();
        Session session2 = em2.unwrap(Session.class);
        Book proxy = session2.getReference(Book.class, id);

        DEMO.info("real.getClass()  = {}", real.getClass().getName());
        DEMO.info("proxy.getClass() = {}", proxy.getClass().getName());
        DEMO.info("proxy instanceof Book.class:            {}", Book.class.isInstance(proxy));
        DEMO.info("real.getClass() == proxy.getClass():    {}", real.getClass() == proxy.getClass());
        DEMO.info("real.equals(proxy) before proxy access: {}", real.equals(proxy));

        em.getTransaction().commit();
        em.close();
        em2.getTransaction().commit();
        em2.close();
    }
}