hibernate-demo/src/test/java/com/ankurm/hibernatedemo/bootstrap/EntityManagerBootstrapTest.java

package com.ankurm.hibernatedemo.bootstrap;

import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatThrownBy;

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import jakarta.persistence.Persistence;
import jakarta.persistence.PersistenceConfiguration;
import jakarta.persistence.PersistenceException;
import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Backs ankurm.com post 4855 (bootstrapping EntityManager). Docs: docs/17-entitymanager-bootstrap.md.
 *
 * <p>Deliberately plain JUnit, NOT {@code @SpringBootTest} -- the whole point of this chapter is
 * bootstrapping a raw JPA {@link EntityManagerFactory} with no Spring involved, exactly as a
 * Java SE application or a unit test outside a Spring context would. The XML path reads
 * {@code src/test/resources/META-INF/persistence.xml}; the programmatic path uses Jakarta
 * Persistence 3.2's {@link PersistenceConfiguration}, new in this version.
 *
 * <p>Run with {@code ./mvnw -Dtest=EntityManagerBootstrapTest test}.
 */
class EntityManagerBootstrapTest {

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

    @Test
    void xmlBootstrap_createsFactoryFromPersistenceXmlAndPersistsAUser() {
        // No properties map passed here at all -- everything comes from
        // META-INF/persistence.xml on the test classpath, resolved purely by unit name.
        try (EntityManagerFactory emf = Persistence.createEntityManagerFactory("XmlBootstrapPU")) {
            EntityManager em = emf.createEntityManager();
            em.getTransaction().begin();
            BootstrapUser user = new BootstrapUser("Ankur", "[email protected]");
            em.persist(user);
            em.getTransaction().commit();
            Long id = user.getId();
            em.close();

            EntityManager reader = emf.createEntityManager();
            BootstrapUser reloaded = reader.find(BootstrapUser.class, id);
            assertThat(reloaded).isNotNull();
            assertThat(reloaded.getEmail()).isEqualTo("[email protected]");
            reader.close();
            DEMO.info("xmlBootstrap: persisted and reloaded user id={}", id);
        }
    }

    @Test
    void programmaticBootstrap_needsNoPersistenceXmlUnitOnClasspath() {
        // "ProgrammaticPU" has NO matching <persistence-unit> anywhere in persistence.xml --
        // this only works at all if PersistenceConfiguration genuinely bypasses XML lookup.
        PersistenceConfiguration config = new PersistenceConfiguration("ProgrammaticPU")
                .provider("org.hibernate.jpa.HibernatePersistenceProvider")
                .managedClass(BootstrapUser.class)
                .property(PersistenceConfiguration.JDBC_DRIVER, "org.h2.Driver")
                .property(PersistenceConfiguration.JDBC_URL, "jdbc:h2:mem:bootstrap-programmatic;DB_CLOSE_DELAY=-1")
                .property(PersistenceConfiguration.JDBC_USER, "sa")
                .property(PersistenceConfiguration.JDBC_PASSWORD, "")
                .property("hibernate.hbm2ddl.auto", "create-drop");

        try (EntityManagerFactory emf = config.createEntityManagerFactory()) {
            EntityManager em = emf.createEntityManager();
            em.getTransaction().begin();
            BootstrapUser user = new BootstrapUser("Programmatic", "[email protected]");
            em.persist(user);
            em.getTransaction().commit();
            Long id = user.getId();
            em.close();

            EntityManager reader = emf.createEntityManager();
            assertThat(reader.find(BootstrapUser.class, id)).isNotNull();
            reader.close();
            DEMO.info("programmaticBootstrap: persisted user id={} with zero persistence.xml units named 'ProgrammaticPU'", id);
        }
    }

    @Test
    void persistenceUnitName_inProgrammaticConfig_doesNotTriggerXmlSearch() {
        // Reusing "XmlBootstrapPU" -- the EXACT SAME name as the real XML unit -- but with
        // completely different properties built purely in code, pointed at a third database.
        // If the name alone triggered an XML lookup/merge, this factory would end up
        // connected to bootstrap-xml (the XML unit's database) instead. It doesn't.
        PersistenceConfiguration config = new PersistenceConfiguration("XmlBootstrapPU")
                .provider("org.hibernate.jpa.HibernatePersistenceProvider")
                .managedClass(BootstrapUser.class)
                .property(PersistenceConfiguration.JDBC_DRIVER, "org.h2.Driver")
                .property(PersistenceConfiguration.JDBC_URL, "jdbc:h2:mem:bootstrap-namecollision;DB_CLOSE_DELAY=-1")
                .property(PersistenceConfiguration.JDBC_USER, "sa")
                .property(PersistenceConfiguration.JDBC_PASSWORD, "")
                .property("hibernate.hbm2ddl.auto", "create-drop");

        try (EntityManagerFactory emf = config.createEntityManagerFactory()) {
            EntityManager em = emf.createEntityManager();
            @SuppressWarnings("unchecked")
            java.util.List<Object> row = em.createNativeQuery("SELECT DATABASE()").getResultList();
            String actualDb = (String) row.get(0);
            assertThat(actualDb).as("the programmatic config's own JDBC URL won, not the XML unit's")
                    .isEqualToIgnoringCase("BOOTSTRAP-NAMECOLLISION");
            em.close();
            DEMO.info("persistenceUnitNameCollision: connected database = {} (unit name 'XmlBootstrapPU' reused on purpose)", actualDb);
        }
    }

    @Test
    void unconfiguredUnitName_failsWithPersistenceException_notSilently() {
        // "TotallyUnknownPU" exists neither in persistence.xml nor as a PersistenceConfiguration
        // -- Persistence.createEntityManagerFactory(name) with no config map has nowhere left
        // to look.
        assertThatThrownBy(() -> Persistence.createEntityManagerFactory("TotallyUnknownPU"))
                .isInstanceOf(PersistenceException.class)
                .satisfies(ex -> {
                    DEMO.info("unconfiguredUnitName: {}: {}", ex.getClass().getName(), ex.getMessage());
                    assertThat(ex.getMessage()).contains("TotallyUnknownPU");
                });
    }

    @Test
    void repeatedFactoryCreation_isMeasurablyExpensive() {
        // Not a Metaspace OOM reproduction -- that needs sustained, uncollectable class-loader
        // growth over many thousands of factories and isn't something to actually trigger in a
        // shared CI sandbox. What IS safely measurable: EntityManagerFactory creation is not
        // cheap, which is the whole reason "never create one per request" is the rule in the
        // first place.
        PersistenceConfiguration config = new PersistenceConfiguration("TimingPU")
                .provider("org.hibernate.jpa.HibernatePersistenceProvider")
                .managedClass(BootstrapUser.class)
                .property(PersistenceConfiguration.JDBC_DRIVER, "org.h2.Driver")
                .property(PersistenceConfiguration.JDBC_URL, "jdbc:h2:mem:bootstrap-timing;DB_CLOSE_DELAY=-1")
                .property(PersistenceConfiguration.JDBC_USER, "sa")
                .property(PersistenceConfiguration.JDBC_PASSWORD, "")
                .property("hibernate.hbm2ddl.auto", "create-drop");

        long factoryStart = System.nanoTime();
        EntityManagerFactory emf = config.createEntityManagerFactory();
        long factoryMillis = (System.nanoTime() - factoryStart) / 1_000_000;

        long emStart = System.nanoTime();
        EntityManager em = emf.createEntityManager();
        long emMillis = (System.nanoTime() - emStart) / 1_000_000;
        em.close();
        emf.close();

        assertThat(factoryMillis).as("factory creation happened and was timed").isGreaterThanOrEqualTo(0);
        DEMO.info("repeatedFactoryCreation: createEntityManagerFactory() took {} ms, createEntityManager() took {} ms -- "
                + "the factory call is the one doing schema validation, service registry bootstrap, and metadata scanning; "
                + "the EntityManager call is comparatively trivial", factoryMillis, emMillis);
    }
}