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What is the proper way to re-attach detached objects in Hibernate?

java
hibernate-entity-management
entity-states
detached-objects
Nikita BarsukovbyNikita Barsukov·Dec 11, 2024
TLDR

When dealing with detached entities in Hibernate, the go-to method to re-attach them is session.merge(detachedObject). This method does not require a check if session already contains the entity and also neatly avoids the NonUniqueObjectException.

YourEntity reattachedEntity = (YourEntity) session.merge(detachedObject);

Choosing to merge() serves the benefit of merging the state of your detached object with any existing persisted entity having the same identifier. This effectively simplifies your re-attachment process and minimizes potential errors.

Exploring Entity States and Transitions

To avoid wandering into pitfalls and exceptions, it's critical to comprehend the lifecycle states of a Hibernate entity:

  • Transient: The entity is just born, but not associated with a session nor has any representation in the DB.
  • Persistent: The entity is now in a session, and it maps to a row in the DB.
  • Detached: The entity was in a session but not now. Though it still has a DB counterpart.
  • Removed: The entity is scheduled for termination. It's still in a session, but not in the DB anymore.

Understanding these states are like the ABCs of Hibernate. When it comes to dealing with detached entities, remember they've lost their session and require careful handling.

Handling State Synchronization - Update() or Merge()?

You have a detachedObject and you want to get it back into a session. Here are your choices, choose wisely!

if (!session.contains(detachedObject)) { session.update(detachedObject); // Pay heed, NonUniqueObjectException lurks around this corner. }

update() reattaches the object to your session, but only apply this when you're certain it doesn't lead to a NonUniqueObjectException.

On the other hand, merge() is your safe bet. It combines changes without checking if a session contains the entity, and thus, circumvents potential errors.

YourEntity reattachedEntity = (YourEntity) session.merge(detachedObject);

And then, there's your versatile saveOrUpdate(). Consider it as a Hybrid, that knows what to do based on the situation.

session.saveOrUpdate(detachedObject);

While lock(entity, LockMode.NONE) won't re-attach your entity, it prevents LazyInitException.

Preventing Stale State - The SelectBeforeUpdate Way

If enabled, @SelectBeforeUpdate causes Hibernate to issue a SELECT just before an update to make sure the object's state is not stale.

@Entity @SelectBeforeUpdate(true) public class YourEntity { ... }

In optimistic locking situations, this can be a lifesaver. It helps prevent unnecessary exceptions thrown because of overlapping updates.

Clearing customs - Re-attaching Visualised

To clear customs:

📦->🛬: Declare your luggage and clear customs.

These methods are your customs forms:

📝 update(): "My luggage was here before, just bringing it back." 📝 merge(): "This is a new piece, but similar to my old one." 📝 saveOrUpdate(): "Check my luggage, if it is new, register it; if not, just accept it back." 📝 lock(): "Ensure this luggage is mine, no updates needed."

Employing Hibernate Spring Data is like having a super-efficient VIP customs clearance for your luggage. Fast and swift, no detentions.

🛅->🛃: With Spring Data, your luggage gets a VIP clearance.

Bon voyage traversing sessions! ✈️ 📦 🛬

Understanding Extended Persistence Context

Working with Extended persistence context in Seam Framework (or equivalent) helps maintain the entity manager's lifespan throughout transactions. In other words, detached entities are much less of a concern, and also fetching fresh instances from the database becomes less frequent.

YourEntity persistentInstance = session.get(detachedObject.getClass(), detachedObject.getId()); // Look, fresh entity!

It's like having an on-demand fresh entity at your disposal, thereby negating the need for reattachment in some scenarios.

Traverse Wisely - Using Diagrams and References

JPA Entity State Transition Diagrams help visualize the lifecycle and transitions of entities. Coupling these diagrams with wisdom from the Hibernate community or from references such as the "Java Persistence with Hibernate" book forms a potent blend to avoid pitfalls.