JPA’s EntityManager class provides persist() and merge() methods to save the state of an entity in the database. Though both methods look the same, they have subtle differences which we must know to effectively use in the application program.
1. Difference between Merge and Persist
Let’s start with a quick summary before diving deep into the methods.
| Feature | Merge | Persist |
|---|---|---|
| Behavior | Adds or updates an entity to the persistence context, merging its state if entity already exists | Adds a new entity to the persistence context |
| When to Use | When dealing with detached entities that need to be synchronized with the database | When adding a new entity to the database |
| Managed vs. Detached Entities | Works with both managed and detached entities | Specifically for new entities not managed by the persistence context |
| Exception Handling | Throws EntityExistsException if the entity already exists in the persistence context or database | Throws EntityExistsException if the entity already exists in the persistence context or database |
| Return Value | Returns the managed entity instance | N/A (Void method) |
| Cascade Behavior | Cascades to related entities if specified in mappings | Cascades to related entities if specified in mappings |
| Performance Impact | Can potentially result in additional database calls and performance overhead due to state synchronization | Generally performs better as it only inserts entities |
2. EntityManager.persist() Method
In JPA, the persist() method adds a new entity to the persistence context. The entity is saved into the database when the transaction is committed. Note that the persist() throws an EntityExistsException if the entity already exists in the persistence context or if an entity with the same identifier already exists in the database.
The persist() method is used to when we want to add a new record to the database.
@Transactional
public void addPerson(Person newPerson) {
entityManager.persist(newPerson);
}3. EntityManager.merge() Method
The merge() method can be used for adding new entities as well as updating existing entities, also. More specifically, the merge() method adds entities to the persistence context or updates detached entities.
The main difference between merge() and persist() is seen in the update operation. add entities to the persistence context or update detached entities. In merge() operation, if an entity with the same identifier already exists in the persistence context, its state will be copied over to the managed entity instance. If no such entity exists, a new one will be created.
After the merge operation is finished, it returns the managed entity instance.
@Transactional
public Person updatePerson(Person updatedPerson) {
return entityManager.merge(updatedPerson);
}4. Summary
As discussed in this short JPA tutorial, we should use merge when dealing with detached entities; and use persist to explicitly add a new entity to the database without checking if it already exists.
Both methods show different behavior when an entity with the same identifier already exists, so we should use the appropriate method based on the requirements.
Happy Learning!!
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