Revision

1. Entity States:
   • Transient: A new entity not yet associated with a persistence context (e.g., new Restaurant()).
   • Persistent: An entity managed by the persistence context (e.g., after session.persist(restaurant) or entityManager.persist(restaurant)).
   • Detached: An entity no longer managed by the persistence context (e.g., after session.evict(restaurant) or session closure).
   • Removed: An entity marked for deletion (e.g., after session.remove(restaurant)), deleted upon transaction commit.
2. Auto Dirty Checking:
   • Hibernate automatically detects changes to persistent entities within a session and synchronizes them with the database during the flush or commit phase.
   • Example: Modifying a Restaurant’s name in a transaction updates the database without explicit session.update().
3. Session API:
   • persist(entity): Makes a transient entity persistent.
   • merge(entity): Copies the state of a detached entity to a persistent one.
   • remove(entity): Marks a persistent entity for deletion.
   • find(Class, id): Retrieves an entity by ID (replaces get).
   • getReference(Class, id): Returns a proxy (lazy loading, replaces load).
   • flush(): Synchronizes the session with the database.
   • evict(entity): Detaches an entity from the session.
   • clear(): Detaches all entities from the session.

Any Specific Questions?: Since none were provided, I’ll proceed with the new topics and objectives. If you have questions about entity states, dirty checking, or Session API, please clarify.

New Topics

1. Image Handling

Objective: Store and retrieve user images in/from the database using the FileUtils class from Apache Commons IO, avoiding traditional File I/O methods.

Dependencies: Assume commons-io is in pom.xml:

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.16.1</version>
</dependency>

User Entity (with image storage):

import jakarta.persistence.Entity;
import jakarta.persistence.Lob;
import lombok.Getter;
import lombok.Setter;
import lombok.NoArgsConstructor;
import lombok.ToString;

@Entity
@Getter
@Setter
@NoArgsConstructor
@ToString(callSuper = true)
public class User extends BaseEntity {
    private String email;
    private String name;

    @Lob // Stores large binary data (BLOB)
    private byte[] profileImage;
}

1.1 Save User Image in DB

• Input: User ID, image file name (path).

• Output: Message (e.g., "Image saved successfully").

• Implementation (in UserDao):

  import jakarta.persistence.EntityManager;
  import jakarta.persistence.PersistenceContext;
  import org.apache.commons.io.FileUtils;
  import org.springframework.stereotype.Repository;
  import org.springframework.transaction.annotation.Transactional;
  import java.io.File;
  import java.io.IOException;
  
  @Repository
  public class UserDao {
      @PersistenceContext
      private EntityManager entityManager;
  
      @Transactional
      public String saveUserImage(Long userId, String imageFilePath) {
          User user = entityManager.find(User.class, userId);
          if (user != null) {
              try {
                  byte[] imageData = FileUtils.readFileToByteArray(new File(imageFilePath));
                  user.setProfileImage(imageData);
                  return "Image saved successfully";
              } catch (IOException e) {
                  return "Error saving image: " + e.getMessage();
              }
          }
          return "User not found";
      }
  }
  
  

• Explanation:

  • Use FileUtils.readFileToByteArray to convert the image file to a byte[].
  • Store the byte[] in the profileImage field (annotated with @Lob for BLOB storage).
  • Update the User entity; Hibernate’s dirty checking persists the change.

1.2 Restore User Image from DB

• Input: User email, new image file name (path for storing the retrieved image).