In Hibernate, detached objects are objects that were previously associated with a Hibernate Session, but that Session has since been closed, leaving the object in a detached state. Detached objects are not currently managed by any Session, so they are not automatically synchronized with the database. If you want to reattach these objects to a new session, Hibernate provides a couple of operations to handle them:

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1. Using Session.update()

The update() method reattaches a detached object to the current session and marks it as persistent. The object must represent a row that already exists in the database. If there are issues (e.g., the object doesn’t exist in the database), an exception will be thrown.

Example:

Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();

MyEntity detachedEntity = getDetachedEntity(); // Detached entity

// Reattach the detached entity to the session
session.update(detachedEntity);

// After reattachment, changes to the entity will be synchronized with the database
detachedEntity.setSomeField("newValue");

transaction.commit();
session.close();

Note: Use update() only when you are sure that the detached entity exists in the database.

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2. Using Session.merge()

The merge() method is often a better approach for reattaching detached objects, as it handles the entity more flexibly. If the object exists in the database, it merges the changes from the detached entity into the persistent object in the session. If it doesn’t exist, it creates a new database row.

Example:

Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();

MyEntity detachedEntity = getDetachedEntity(); // Detached entity

// Merge the detached entity with the session
MyEntity mergedEntity = (MyEntity) session.merge(detachedEntity);

// Use the merged entity for further operations
mergedEntity.setSomeField("anotherValue");

transaction.commit();
session.close();

Key Differences Between update() and merge():
– update() can throw NonUniqueObjectException if an object with the same identifier is already associated with the session.
– merge() does not throw an exception—it creates a new instance in the session if an object with the same identifier is already associated.

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3. Using Session.saveOrUpdate() (Not Recommended for Detached Objects)

The saveOrUpdate() method can accept both transient and detached objects. For detached objects, it either updates the corresponding row in the database or saves it if it’s not already present. However, this method is not as commonly used for handling detached objects as update() or merge().

Example:

Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();

MyEntity detachedEntity = getDetachedEntity(); // Detached entity

// Save or update the detached entity
session.saveOrUpdate(detachedEntity);

transaction.commit();
session.close();

Note: This method can be less predictable when dealing with detached objects compared to merge().

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Common Scenarios and Approaches

• When you need to reattach a detached object and persist its changes: Use merge().
• When you know the object already exists in the database: Use update().
• Avoid session conflicts: If a different object with the same identifier is already in the new session, prefer merge() because update() will result in an exception.

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Best Practices

• Ensure entities have properly defined identifiers (@Id) to avoid Hibernate-related issues when reattaching.
• Use merge() when you are uncertain about whether the object is persistent or detached, as it adapts to the situation.
• Keep sessions short and transactions small to reduce occurrences of detached objects.

By understanding and implementing these methods appropriately, you can effectively handle detached objects and ensure smooth database operations in a Hibernate-based application.