Add hibernate-demo: get() vs load(), merge() vs refresh(), inserting objects (Hibernate 7.4.1.Final + Spring Boot 4.1.0)
Adds a JUnit test suite (GetVsGetReferenceTest, MergeRefreshTest, OptimisticLockTest, IdentityBatchTest, SequenceBatchTest, AllocationSizeSweepTest, BatchSizeSweepTest) so every surprising behavior described in the three companion posts has a reproducible test, alongside the original CommandLineRunner scenarios. Rewrites all three doc chapters and the README around the new experiments: the get()/getReference() same-session matrix, the merge()/refresh() experiments (including exactly when OptimisticLockException surfaces and a corrected LAZY-plus- cascade merge() result), and two new sweeps (allocationSize, batch_size) for batch inserts.
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package com.ankurm.hibernatedemo;
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import static org.assertj.core.api.Assertions.assertThat;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import com.ankurm.hibernatedemo.model.Book;
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import jakarta.persistence.EntityManager;
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import jakarta.persistence.EntityManagerFactory;
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import jakarta.persistence.EntityNotFoundException;
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import java.util.HashSet;
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import java.util.Set;
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import org.hibernate.LazyInitializationException;
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import org.hibernate.Session;
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import org.hibernate.SessionFactory;
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import org.hibernate.stat.Statistics;
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import org.junit.jupiter.api.Test;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import org.springframework.beans.factory.annotation.Autowired;
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import org.springframework.boot.test.context.SpringBootTest;
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/**
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* Backs ankurm.com post 4859. Docs: docs/01-get-vs-load.md.
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*
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* <p>Run with {@code ./mvnw -Dtest=GetVsGetReferenceTest test}. Every assertion here was first
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* observed by running the same code and reading the log, then pinned down as an assertion --
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* none of the outcomes below were assumed going in.
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*/
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@SpringBootTest
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class GetVsGetReferenceTest {
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private static final Logger DEMO = LoggerFactory.getLogger("DEMO");
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@Autowired
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private EntityManagerFactory emf;
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private Long seedBook(String title) {
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Book book = new Book(title, "Test Author");
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em.persist(book);
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em.getTransaction().commit();
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Long id = book.getId();
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em.close();
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return id;
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}
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private Statistics stats() {
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return emf.unwrap(SessionFactory.class).getStatistics();
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}
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// ---- Part 1: four calls, four outcomes ----
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@Test
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void getOnExistingId_firesSelect_returnsRealEntity() {
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Long id = seedBook("Effective Java");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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stats().clear();
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Book book = session.get(Book.class, id);
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assertThat(stats().getPrepareStatementCount()).as("get() on an existing id must fire a SELECT").isEqualTo(1);
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assertThat(book).isNotNull();
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assertThat(book.getClass()).isEqualTo(Book.class);
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void getOnMissingId_firesSelect_returnsNull() {
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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stats().clear();
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Book book = session.get(Book.class, 999_111_222L);
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assertThat(stats().getPrepareStatementCount()).as("get() on a missing id still fires a SELECT").isEqualTo(1);
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assertThat(book).isNull();
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void getReferenceOnExistingId_noSelectUntilPropertyAccessed() {
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Long id = seedBook("Domain-Driven Design");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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stats().clear();
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Book proxy = session.getReference(Book.class, id);
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assertThat(stats().getPrepareStatementCount())
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.as("getReference() must not fire a SELECT at the call site")
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.isEqualTo(0);
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String title = proxy.getTitle();
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assertThat(stats().getPrepareStatementCount())
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.as("the SELECT is deferred until a non-id accessor is called")
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.isEqualTo(1);
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assertThat(title).isEqualTo("Domain-Driven Design");
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void getReferenceOnMissingId_noExceptionUntilAccessed() {
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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Book proxy = session.getReference(Book.class, 999_333_444L);
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// No exception yet -- constructing the proxy never touched the database.
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assertThat(proxy).isNotNull();
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EntityNotFoundException ex = assertThrows(EntityNotFoundException.class, proxy::getTitle);
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DEMO.info("getReference() on a missing id, once accessed, threw: {}: {}", ex.getClass().getName(), ex.getMessage());
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em.getTransaction().rollback();
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em.close();
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}
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// ---- Part 2: same-session matrix ----
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// For each combination, both calls target the SAME id in the SAME session.
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@Test
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void sessionMatrix_getThenGet_secondCallHitsL1Cache_sameInstance() {
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Long id = seedBook("Matrix: get/get");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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Book first = session.get(Book.class, id);
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stats().clear();
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Book second = session.get(Book.class, id);
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assertThat(stats().getPrepareStatementCount())
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.as("second get() in the same session must NOT re-fire a SELECT (L1 cache hit)")
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.isEqualTo(0);
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assertThat(second).isSameAs(first);
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void sessionMatrix_getReferenceThenGetReference_secondCallHitsL1Cache_sameInstance() {
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Long id = seedBook("Matrix: getReference/getReference");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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Book first = session.getReference(Book.class, id);
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stats().clear();
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Book second = session.getReference(Book.class, id);
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assertThat(stats().getPrepareStatementCount())
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.as("second getReference() in the same session must not fire anything either -- still just a reference")
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.isEqualTo(0);
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assertThat(second).isSameAs(first);
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void sessionMatrix_getThenGetReference_returnsTheSameAlreadyInitializedInstance() {
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Long id = seedBook("Matrix: get/getReference");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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Book real = session.get(Book.class, id);
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stats().clear();
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Book second = session.getReference(Book.class, id);
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assertThat(stats().getPrepareStatementCount())
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.as("getReference() after get() must not fire a SELECT -- the real entity is already in the L1 cache")
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.isEqualTo(0);
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assertThat(second)
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.as("getReference() returns the SAME already-managed real instance, not a new proxy, once one exists in this session")
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.isSameAs(real);
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assertThat(second.getClass()).isEqualTo(Book.class);
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em.getTransaction().commit();
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em.close();
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}
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@Test
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void sessionMatrix_getReferenceThenGet_getReturnsTheExistingProxyAndDoesNotForceInitialization() {
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Long id = seedBook("Matrix: getReference/get");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Session session = em.unwrap(Session.class);
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Book proxy = session.getReference(Book.class, id);
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stats().clear();
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Book second = session.get(Book.class, id);
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assertThat(second)
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.as("get() after getReference() returns the SAME proxy already sitting in the L1 cache")
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.isSameAs(proxy);
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DEMO.info("get() after getReference(): prepareStatementCount for this call = {}, returned class = {}",
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stats().getPrepareStatementCount(), second.getClass().getName());
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em.getTransaction().commit();
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em.close();
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}
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// ---- Part 3: proxy identity experiment ----
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@Test
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void proxyIdentity_instanceofSurvives_equalsDoesNot() {
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Long id = seedBook("Proxy Identity");
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EntityManager em1 = emf.createEntityManager();
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em1.getTransaction().begin();
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Book real = em1.unwrap(Session.class).get(Book.class, id);
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EntityManager em2 = emf.createEntityManager();
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em2.getTransaction().begin();
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Book proxy = em2.unwrap(Session.class).getReference(Book.class, id);
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assertThat(proxy).isInstanceOf(Book.class);
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assertThat(org.hibernate.Hibernate.getClass(proxy)).isEqualTo(Book.class);
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assertThat(proxy.getClass()).isNotEqualTo(Book.class);
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// Book does not override equals()/hashCode() -- this is the point of the test.
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assertThat(real.equals(proxy)).isFalse();
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assertThat(proxy.equals(real)).isFalse();
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Set<Book> set = new HashSet<>();
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set.add(real);
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assertThat(set.contains(proxy))
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.as("a HashSet built on default equals()/hashCode() cannot recognise the proxy and the real instance as the same row")
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.isFalse();
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em1.getTransaction().commit();
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em1.close();
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em2.getTransaction().commit();
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em2.close();
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}
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@Test
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void proxyOutlivesItsSession_throwsLazyInitializationExceptionOnAccess() {
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Long id = seedBook("Outlives Session");
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EntityManager em = emf.createEntityManager();
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em.getTransaction().begin();
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Book proxy = em.unwrap(Session.class).getReference(Book.class, id);
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em.getTransaction().commit();
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em.close();
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assertThrows(LazyInitializationException.class, proxy::getTitle);
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}
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}
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