JPA 和 Hibernate 中的 persist() 和 merge() 有什么区别？

Question

What is the difference between persist() and merge() in Hibernate?

persist() can create a UPDATE & INSERT query, eg:

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

in this case query will be generated like this:

Hibernate: insert into A (NAME, ID) values (?, ?)
Hibernate: update A set NAME=? where ID=?

so persist() method can generate an Insert and an Update.

Now with merge() :

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

This is what I see in the database:

SINGER_ID   SINGER_NAME
1           Ricky Martin
2           Madonna
3           Elvis Presley
4           Luciano Pavarotti

Now update a record using merge()

SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
Singer singer = new Singer();
singer.setId(2);
singer.setName("Luciano Pavarotti");
session.merge(singer);
session.flush();

This is what I see in the database:

SINGER_ID   SINGER_NAME
1           Ricky Martin
2           Luciano Pavarotti
3           Elvis Presley

Answer 1

JPA specification contains a very precise description of semantics of these operations, better than in javadoc:

The semantics of the persist operation, applied to an entity X are as follows:

• If X is a new entity, it becomes managed. The entity X will be entered into the database at or before transaction commit or as a result of the flush operation.

• If X is a preexisting managed entity, it is ignored by the persist operation. However, the persist operation is cascaded to entities referenced by X, if the relationships from X to these other entities are annotated with the cascade=PERSIST or cascade=ALL annotation element value or specified with the equivalent XML descriptor element.

• If X is a removed entity, it becomes managed.

• If X is a detached object, the EntityExistsException may be thrown when the persist operation is invoked, or the EntityExistsException or another PersistenceException may be thrown at flush or commit time.

• For all entities Y referenced by a relationship from X, if the relationship to Y has been annotated with the cascade element value cascade=PERSIST or cascade=ALL , the persist operation is applied to Y.

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The semantics of the merge operation applied to an entity X are as follows:

• If X is a detached entity, the state of X is copied onto a pre-existing managed entity instance X' of the same identity or a new managed copy X' of X is created.

• If X is a new entity instance, a new managed entity instance X' is created and the state of X is copied into the new managed entity instance X'.

• If X is a removed entity instance, an IllegalArgumentException will be thrown by the merge operation (or the transaction commit will fail).

• If X is a managed entity, it is ignored by the merge operation, however, the merge operation is cascaded to entities referenced by relationships from X if these relationships have been annotated with the cascade element value cascade=MERGE or cascade=ALL annotation.

• For all entities Y referenced by relationships from X having the cascade element value cascade=MERGE or cascade=ALL , Y is merged recursively as Y'. For all such Y referenced by X, X' is set to reference Y'. (Note that if X is managed then X is the same object as X'.)

• If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.

Answer 2

This is coming from JPA . In a very simple way:

• persist(entity) should be used with totally new entities, to add them to DB (if entity already exists in DB there will be EntityExistsException throw).

• merge(entity) should be used, to put entity back to persistence context if the entity was detached and was changed.

Answer 3

Persist should be called only on new entities, while merge is meant to reattach detached entities.

If you're using the assigned generator, using merge instead of persist can cause a redundant SQL statement.

Also, calling merge for managed entities is also a mistake since managed entities are automatically managed by Hibernate, and their state is synchronized with the database record by the dirty checking mechanism upon flushing the Persistence Context.

Answer 4

The most important difference is this:

• In case of persist method, if the entity that is to be managed in the persistence context, already exists in persistence context, the new one is ignored. (NOTHING happened)

• But in case of merge method, the entity that is already managed in persistence context will be replaced by the new entity (updated) and a copy of this updated entity will return back. (from now on any changes should be made on this returned entity if you want to reflect your changes in persistence context)