jpa：如何在java se开发中使用jpa

Question

I am working on a java se project which use jpa to persist data with database.

Right now i get a new entity manager for each database operation and close it when function call end. like

public void save(T entity){
  em = getNewEntity();
  em.persisty(entity);
  em.close;
}

but its very hard to manager the many-to-many relationship. for example
i have A entity who have a many-to-many relationship with B entity. and they have CascadeType.MERGE.

once i create A, i create B , and run multiple thread to generate information for both A and B. but sometime B keep the outdated of A, if i merge B then A's data will be purged.

for example . A is music , B is music album , so one B will have a A list. Another thread will read music metadata and get music tag info and update A , B will be updated when a new A found in the FS.
Since entity manager closed , the entities in B's A list is out of date which means when A update itself , B doesn't know A updated . Once Merge B , jpa will help us merge A using the entity in B 's A list.

in order to solve this problem, i decide to use one entity manager and put all database operation into one single thread. Then new problem comes out.

when the transaction is active and the transaction is supposed to open for the entity between

transaction.begin();
....
transaction.end();

however there are many threads running , and those thread may change entity too.

my question is: how to use jpa in java se development ?. thanks

Answer 1

If the relationship gets out of sync, you cannot blame JPA. It is the application's responsibility to keep the relationship sane.

In JPA, as in Java in general it is the responsibility of the application, or the object model to maintain relationships.

Source

As @home said, you need to synchronize properly.

That being said, the EntityManager is not thread safe so you need to solve this in some way. Synchronize your application, use one EntityManager per thread, use @PersistenceContext, etc.

Edit: Here is a question asked about synchrinizing access to the EntityManager. I think it may be of use to you.