BAEL-5439: PersistentObjectException: detached entity passed to persist thrown by JPA and Hibernate (#12303)

* BAEL-5439: added code samples for article

* BAEL-5439: added static import for assertj

persistence-modules/hibernate-exceptions/src/main/java/com/baeldung/hibernate/exception/detachedentity/HibernateUtil.java

@@ -0,0 +1,46 @@
+package com.baeldung.hibernate.exception.detachedentity;
+
+import java.util.Properties;
+
+import org.hibernate.SessionFactory;
+import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
+import org.hibernate.cfg.Configuration;
+import org.hibernate.cfg.Environment;
+import org.hibernate.service.ServiceRegistry;
+
+import com.baeldung.hibernate.exception.detachedentity.entity.Comment;
+import com.baeldung.hibernate.exception.detachedentity.entity.Post;
+
+public class HibernateUtil {
+    private static SessionFactory sessionFactory;
+
+    public static SessionFactory getSessionFactory() {
+        if (sessionFactory == null) {
+            try {
+                Configuration configuration = new Configuration();
+                Properties settings = new Properties();
+                settings.put(Environment.DRIVER, "org.hsqldb.jdbcDriver");
+                settings.put(Environment.URL, "jdbc:hsqldb:mem:transient");
+                settings.put(Environment.USER, "sa");
+                settings.put(Environment.PASS, "");
+                settings.put(Environment.DIALECT, "org.hibernate.dialect.HSQLDialect");
+                settings.put(Environment.SHOW_SQL, "true");
+                settings.put(Environment.FORMAT_SQL, "true");
+                settings.put(Environment.USE_SQL_COMMENTS, "true");
+                settings.put(Environment.HBM2DDL_AUTO, "update");
+                configuration.setProperties(settings);
+
+                configuration.addAnnotatedClass(Comment.class);
+                configuration.addAnnotatedClass(Post.class);
+
+                ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties())
+                    .build();
+                sessionFactory = configuration.buildSessionFactory(serviceRegistry);
+
+            } catch (Exception e) {
+                e.printStackTrace();
+            }
+        }
+        return sessionFactory;
+    }
+}

persistence-modules/hibernate-exceptions/src/main/java/com/baeldung/hibernate/exception/detachedentity/entity/Comment.java

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+package com.baeldung.hibernate.exception.detachedentity.entity;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+
+@Entity
+public class Comment {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    private String text;
+
+    public Comment(String text) {
+        this.text = text;
+    }
+
+    public Comment() {
+    }
+
+    @ManyToOne
+    private Post post;
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getText() {
+        return text;
+    }
+
+    public void setText(String text) {
+        this.text = text;
+    }
+
+    public Post getPost() {
+        return post;
+    }
+
+    public void setPost(Post post) {
+        this.post = post;
+    }
+
+    @Override
+    public String toString() {
+        return "Comment{" + "id=" + id + ", name='" + text + '\'' + ", post=" + post + '}';
+    }
+}

persistence-modules/hibernate-exceptions/src/main/java/com/baeldung/hibernate/exception/detachedentity/entity/Post.java

@@ -0,0 +1,44 @@
+package com.baeldung.hibernate.exception.detachedentity.entity;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+
+@Entity
+public class Post {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    private String title;
+
+    public Post() {
+    }
+
+    public Post(String title) {
+        this.title = title;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    @Override
+    public String toString() {
+        return "Post{" + "id=" + id + ", text='" + title + '\'' + '}';
+    }
+}

persistence-modules/hibernate-exceptions/src/test/java/com/baeldung/hibernate/exception/detachedentity/DetachedEntityUnitTest.java

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+package com.baeldung.hibernate.exception.detachedentity;
+
+import com.baeldung.hibernate.exception.detachedentity.entity.Comment;
+import com.baeldung.hibernate.exception.detachedentity.entity.Post;
+
+import org.assertj.core.api.Assertions;
+import org.hibernate.Session;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import javax.persistence.PersistenceException;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class DetachedEntityUnitTest {
+
+    private static Session session;
+    private Post detachedPost;
+
+    @Before
+    public void beforeEach() {
+        session = HibernateUtil.getSessionFactory()
+            .openSession();
+        session.beginTransaction();
+        this.detachedPost = new Post("Hibernate Tutorial");
+        session.persist(detachedPost);
+        session.evict(detachedPost);
+    }
+
+    @After
+    public void afterEach() {
+        clearDatabase();
+        session.close();
+    }
+
+    @Test
+    public void givenDetachedPost_whenTryingToPersist_thenThrowException() {
+        detachedPost.setTitle("Hibernate Tutorial for Absolute Beginners");
+
+        assertThatThrownBy(() -> session.persist(detachedPost))
+            .isInstanceOf(PersistenceException.class)
+            .hasMessageContaining("org.hibernate.PersistentObjectException: detached entity passed to persist");
+    }
+
+    @Test
+    public void givenDetachedPost_whenTryingToMerge_thenNoExceptionIsThrown() {
+        detachedPost.setTitle("Hibernate Tutorial for Beginners");
+
+        session.merge(detachedPost);
+        session.getTransaction()
+            .commit();
+
+        List<Post> posts = session.createQuery("Select p from Post p", Post.class)
+            .list();
+        assertThat(posts).hasSize(1);
+        assertThat(posts.get(0)
+            .getTitle()).isEqualTo("Hibernate Tutorial for Beginners");
+    }
+
+    @Test
+    public void givenDetachedPost_whenPersistingNewCommentWithIt_thenThrowException() {
+        Comment comment = new Comment("nice article!");
+        comment.setPost(detachedPost);
+
+        session.persist(comment);
+        session.getTransaction()
+            .commit();
+
+        assertThatThrownBy(() -> session.persist(detachedPost))
+            .isInstanceOf(PersistenceException.class)
+            .hasMessageContaining("org.hibernate.PersistentObjectException: detached entity passed to persist");
+    }
+
+    @Test
+    public void givenDetachedPost_whenMergeAndPersistComment_thenNoExceptionIsThrown() {
+        Comment comment = new Comment("nice article!");
+        Post mergedPost = (Post) session.merge(detachedPost);
+        comment.setPost(mergedPost);
+
+        session.persist(comment);
+        session.getTransaction()
+            .commit();
+
+        List<Comment> comments = session.createQuery("Select c from Comment c", Comment.class)
+            .list();
+        Comment savedComment = comments.get(0);
+        assertThat(savedComment.getText()).isEqualTo("nice article!");
+        assertThat(savedComment.getPost()
+            .getTitle()).isEqualTo("Hibernate Tutorial");
+    }
+
+    private void clearDatabase() {
+        if (!session.getTransaction()
+            .isActive()) {
+            session.beginTransaction();
+        }
+        session.createQuery("DELETE FROM Comment")
+            .executeUpdate();
+        session.createQuery("DELETE FROM Post")
+            .executeUpdate();
+    }
+}