Differences Between Update and Merge Methods in Hibernate

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Question:

What is the difference between update and merge method in Hibernate?

Answer:

In Hibernate, both the update and merge methods are used to synchronize the state of an object with the database, but they serve different purposes and have different behaviors. Here’s a breakdown of the differences between the two:

update Method

  1. Purpose: The update method is used to make a transient (new) instance persistent or to reattach a detached instance to the current Hibernate session.

  2. Session Context: When you call update, Hibernate expects that the object you are updating is already associated with the session or is a detached instance. If the object is transient (not yet persisted), it will be made persistent.

  3. Entity State: If the entity is already persistent in the session, calling update will throw an exception (specifically, a NonUniqueObjectException), as Hibernate does not allow two instances of the same entity to be associated with the same session.

  4. Use Case: Use update when you are sure that the object you are working with is either detached or transient and you want to reattach it to the current session.

merge Method

  1. Purpose: The merge method is used to merge the state of a detached instance with the current persistent instance in the session. It can also be used with transient instances.

  2. Session Context: When you call merge, Hibernate will check if the object is already persistent in the session. If it is, it will return the persistent instance. If it is detached, it will copy the state of the detached instance into a new persistent instance.

  3. Entity State: merge does not throw an exception if the entity is already persistent in the session. Instead, it will return the persistent instance that is associated with the session.

  4. Use Case: Use merge when you want to ensure that the state of a detached instance is synchronized with the database, regardless of whether the instance is already persistent or not.

Summary

  • update: Used for reattaching a detached instance or making a transient instance persistent. Throws an exception if the entity is already persistent in the session.
  • merge: Used for merging the state of a detached instance with the current persistent instance. It can handle both detached and transient instances without throwing exceptions.

Example

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

// Using update
MyEntity entity = session.get(MyEntity.class, id);
entity.setSomeProperty(newValue);
session.update(entity); // Reattaches the entity

// Using merge
MyEntity detachedEntity = new MyEntity();
detachedEntity.setId(id);
detachedEntity.setSomeProperty(newValue);
MyEntity mergedEntity = (MyEntity) session.merge(detachedEntity); // Merges the state

In summary, choose update when you are certain about the state of the entity, and use merge when you want to ensure that the state is synchronized without worrying about the entity's current state.

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