Add hibernate-demo: get() vs load(), merge() vs refresh(), inserting objects (Hibernate 7.4.1.Final + Spring Boot 4.1.0)

src/main/java/com/ankurm/hibernatedemo/scenario/InsertIdentityRunner.java

@@ -0,0 +1,58 @@
+package com.ankurm.hibernatedemo.scenario;
+
+import com.ankurm.hibernatedemo.model.WidgetIdentity;
+import jakarta.persistence.EntityManager;
+import jakarta.persistence.EntityManagerFactory;
+import org.hibernate.SessionFactory;
+import org.hibernate.stat.Statistics;
+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 4861 ("inserting objects efficiently") and
+ * docs/03-inserting-objects.md. Captured verbatim into docs/output/insert-identity.txt by
+ * {@code scripts/run.sh insert-identity}.
+ *
+ * <p>Same {@code hibernate.jdbc.batch_size} and {@code hibernate.order_inserts} settings as
+ * {@link InsertSequenceRunner} -- the only difference is {@link WidgetIdentity}'s
+ * {@code GenerationType.IDENTITY} strategy. Compare the two captured output files directly;
+ * the diff between them is the entire point of this pair.
+ */
+@Component
+@Profile("insert-identity")
+public class InsertIdentityRunner implements CommandLineRunner {
+
+    private static final Logger DEMO = LoggerFactory.getLogger("DEMO");
+    private static final int ROW_COUNT = 30;
+
+    private final EntityManagerFactory emf;
+
+    public InsertIdentityRunner(EntityManagerFactory emf) {
+        this.emf = emf;
+    }
+
+    @Override
+    public void run(String... args) {
+        SessionFactory sessionFactory = emf.unwrap(SessionFactory.class);
+        Statistics stats = sessionFactory.getStatistics();
+        stats.clear();
+
+        DEMO.info("--- inserting {} WidgetIdentity rows (GenerationType.IDENTITY) ---", ROW_COUNT);
+        EntityManager em = emf.createEntityManager();
+        em.getTransaction().begin();
+        for (int i = 1; i <= ROW_COUNT; i++) {
+            em.persist(new WidgetIdentity("identity-" + i));
+        }
+        em.getTransaction().commit();
+        em.close();
+
+        DEMO.info("entityInsertCount     = {}", stats.getEntityInsertCount());
+        DEMO.info("prepareStatementCount = {}", stats.getPrepareStatementCount());
+        DEMO.info("(with IDENTITY, expect prepareStatementCount to land close to entityInsertCount --" +
+                " each insert has to go to the database immediately to hand back the generated key,"
+                + " so there is nothing left for hibernate.jdbc.batch_size to batch)");
+    }
+}