hibernate-demo/src/test/java/com/ankurm/hibernatedemo/GetVsGetReferenceTest.java

package com.ankurm.hibernatedemo;

import static org.assertj.core.api.Assertions.assertThat;
import static org.junit.jupiter.api.Assertions.assertThrows;

import com.ankurm.hibernatedemo.model.Book;
import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import jakarta.persistence.EntityNotFoundException;
import java.util.HashSet;
import java.util.Set;
import org.hibernate.LazyInitializationException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.stat.Statistics;
import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

/**
 * Backs ankurm.com post 4859. Docs: docs/01-get-vs-load.md.
 *
 * <p>Run with {@code ./mvnw -Dtest=GetVsGetReferenceTest test}. Every assertion here was first
 * observed by running the same code and reading the log, then pinned down as an assertion --
 * none of the outcomes below were assumed going in.
 */
@SpringBootTest
class GetVsGetReferenceTest {

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

    @Autowired
    private EntityManagerFactory emf;

    private Long seedBook(String title) {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Book book = new Book(title, "Test Author");
        em.persist(book);
        em.getTransaction().commit();
        Long id = book.getId();
        em.close();
        return id;
    }

    private Statistics stats() {
        return emf.unwrap(SessionFactory.class).getStatistics();
    }

    // ---- Part 1: four calls, four outcomes ----

    @Test
    void getOnExistingId_firesSelect_returnsRealEntity() {
        Long id = seedBook("Effective Java");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        stats().clear();
        Book book = session.get(Book.class, id);
        assertThat(stats().getPrepareStatementCount()).as("get() on an existing id must fire a SELECT").isEqualTo(1);
        assertThat(book).isNotNull();
        assertThat(book.getClass()).isEqualTo(Book.class);
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void getOnMissingId_firesSelect_returnsNull() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        stats().clear();
        Book book = session.get(Book.class, 999_111_222L);
        assertThat(stats().getPrepareStatementCount()).as("get() on a missing id still fires a SELECT").isEqualTo(1);
        assertThat(book).isNull();
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void getReferenceOnExistingId_noSelectUntilPropertyAccessed() {
        Long id = seedBook("Domain-Driven Design");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        stats().clear();
        Book proxy = session.getReference(Book.class, id);
        assertThat(stats().getPrepareStatementCount())
                .as("getReference() must not fire a SELECT at the call site")
                .isEqualTo(0);

        String title = proxy.getTitle();

        assertThat(stats().getPrepareStatementCount())
                .as("the SELECT is deferred until a non-id accessor is called")
                .isEqualTo(1);
        assertThat(title).isEqualTo("Domain-Driven Design");
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void getReferenceOnMissingId_noExceptionUntilAccessed() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);
        Book proxy = session.getReference(Book.class, 999_333_444L);
        // No exception yet -- constructing the proxy never touched the database.
        assertThat(proxy).isNotNull();

        EntityNotFoundException ex = assertThrows(EntityNotFoundException.class, proxy::getTitle);
        DEMO.info("getReference() on a missing id, once accessed, threw: {}: {}", ex.getClass().getName(), ex.getMessage());
        em.getTransaction().rollback();
        em.close();
    }

    // ---- Part 2: same-session matrix ----
    // For each combination, both calls target the SAME id in the SAME session.

    @Test
    void sessionMatrix_getThenGet_secondCallHitsL1Cache_sameInstance() {
        Long id = seedBook("Matrix: get/get");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);

        Book first = session.get(Book.class, id);
        stats().clear();
        Book second = session.get(Book.class, id);

        assertThat(stats().getPrepareStatementCount())
                .as("second get() in the same session must NOT re-fire a SELECT (L1 cache hit)")
                .isEqualTo(0);
        assertThat(second).isSameAs(first);
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void sessionMatrix_getReferenceThenGetReference_secondCallHitsL1Cache_sameInstance() {
        Long id = seedBook("Matrix: getReference/getReference");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);

        Book first = session.getReference(Book.class, id);
        stats().clear();
        Book second = session.getReference(Book.class, id);

        assertThat(stats().getPrepareStatementCount())
                .as("second getReference() in the same session must not fire anything either -- still just a reference")
                .isEqualTo(0);
        assertThat(second).isSameAs(first);
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void sessionMatrix_getThenGetReference_returnsTheSameAlreadyInitializedInstance() {
        Long id = seedBook("Matrix: get/getReference");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);

        Book real = session.get(Book.class, id);
        stats().clear();
        Book second = session.getReference(Book.class, id);

        assertThat(stats().getPrepareStatementCount())
                .as("getReference() after get() must not fire a SELECT -- the real entity is already in the L1 cache")
                .isEqualTo(0);
        assertThat(second)
                .as("getReference() returns the SAME already-managed real instance, not a new proxy, once one exists in this session")
                .isSameAs(real);
        assertThat(second.getClass()).isEqualTo(Book.class);
        em.getTransaction().commit();
        em.close();
    }

    @Test
    void sessionMatrix_getReferenceThenGet_getReturnsTheExistingProxyAndDoesNotForceInitialization() {
        Long id = seedBook("Matrix: getReference/get");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Session session = em.unwrap(Session.class);

        Book proxy = session.getReference(Book.class, id);
        stats().clear();
        Book second = session.get(Book.class, id);

        assertThat(second)
                .as("get() after getReference() returns the SAME proxy already sitting in the L1 cache")
                .isSameAs(proxy);
        DEMO.info("get() after getReference(): prepareStatementCount for this call = {}, returned class = {}",
                stats().getPrepareStatementCount(), second.getClass().getName());
        em.getTransaction().commit();
        em.close();
    }

    // ---- Part 3: proxy identity experiment ----

    @Test
    void proxyIdentity_instanceofSurvives_equalsDoesNot() {
        Long id = seedBook("Proxy Identity");
        EntityManager em1 = emf.createEntityManager();
        em1.getTransaction().begin();
        Book real = em1.unwrap(Session.class).get(Book.class, id);

        EntityManager em2 = emf.createEntityManager();
        em2.getTransaction().begin();
        Book proxy = em2.unwrap(Session.class).getReference(Book.class, id);

        assertThat(proxy).isInstanceOf(Book.class);
        assertThat(org.hibernate.Hibernate.getClass(proxy)).isEqualTo(Book.class);
        assertThat(proxy.getClass()).isNotEqualTo(Book.class);

        // Book does not override equals()/hashCode() -- this is the point of the test.
        assertThat(real.equals(proxy)).isFalse();
        assertThat(proxy.equals(real)).isFalse();

        Set<Book> set = new HashSet<>();
        set.add(real);
        assertThat(set.contains(proxy))
                .as("a HashSet built on default equals()/hashCode() cannot recognise the proxy and the real instance as the same row")
                .isFalse();

        em1.getTransaction().commit();
        em1.close();
        em2.getTransaction().commit();
        em2.close();
    }

    @Test
    void proxyOutlivesItsSession_throwsLazyInitializationExceptionOnAccess() {
        Long id = seedBook("Outlives Session");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        Book proxy = em.unwrap(Session.class).getReference(Book.class, id);
        em.getTransaction().commit();
        em.close();

        assertThrows(LazyInitializationException.class, proxy::getTitle);
    }
}