## Persist() vs. Merge(): When Should You Use Which JPA Method?

Exploring the Differences Between persist() and merge() in JPA and Hibernate

In the realm of data persistence, JPA (Java Persistence API) and Hibernate play a crucial role. At their core lie two fundamental methods for managing objects in the context of a database: persist() and merge().

persist()

The persist() method is primarily responsible for creating a new entity in the database or updating an existing one. Its behavior varies based on the state of the entity:

• If the entity is new (not yet persisted), persist() will insert it into the database and assign it an identifier.
• If the entity is managed (already persisted), persist() will either ignore it or cascade the operation to related entities if configured using the cascade attribute.
• If the entity is detached (removed from context), an exception may be thrown.

An example using persist() that illustrates its dual functionality:

SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
A a = new A();
session.persist(a);
a.setName("Mario");
session.flush();

In this scenario, persist() will initially insert the "A" entity into the database and subsequently update it with the specified name.

merge()

Unlike persist(), merge() focuses solely on merging the state of a detached entity into a managed one. Its behavior is somewhat more straightforward:

• If the detached entity represents a new entity, a managed copy will be created and its state copied over.
• If the detached entity represents an existing entity, its state will be copied onto the matching managed entity.
• If the detached entity represents a removed entity, an exception will be thrown.
• If the detached entity has relationships annotated with cascade=MERGE or cascade=ALL, these related entities will be merged recursively.

An example using merge() demonstrates its operation:

SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
Singer singer = new Singer();
singer.setName("Luciano Pavarotti");
session.merge(singer);
session.flush();

After execution, the provided Singer object will be merged into an existing or newly created managed entity, resulting in the respective database changes.

Conclusion

Persist() and merge() serve distinct purposes in JPA and Hibernate. persist() allows for both insertion and update operations, while merge() is tailored specifically for merging detached entities into managed ones. Understanding their different behaviors is essential for effectively managing entities in a database-driven application.

The above is the detailed content of ## Persist() vs. Merge(): When Should You Use Which JPA Method?. For more information, please follow other related articles on the PHP Chinese website!