Running multiple JPA transactions in parallel

Question

I have two (or more) Java Threads creating, updating and deleting entities from a mysql database using JPA. To achieve this I have a PersistenceLayer class creating the EntityManager and providing save, update and delete methods for all my entities looking like:

public void saveEntity(Entity entity) { manager.getTransaction().begin(); manager.persist(entity); manager.getTransaction().commit(); } public void saveEntity2(Entity2 entity) { manager.getTransaction().begin(); manager.persist(entity); manager.getTransaction().commit(); }

If one thread enters operation saveEntity and the other saveEntity2 at the same time, both will try to get the transaction from the EntityManager wich will fail. However I always thought that the underlying database is able to manage multiple transactions, especially if both are working on different rows or even different tables. Of course I could synchronize the blocks, but that would mean only one DB connection is possible at a time which will not scale to multiple users creating several threads.

What wrong assumption am I doing here? Is it possible to submit multiple transactions to a database via JPA and let the DB handle concurrency issues?

Jahan Zinedine · Accepted Answer · 2015-06-10 12:05:44Z

6

EntityManager is not intended to be used by multiple threads. You need to obtain separate instances of EntityManager for each thread.

Actually, if you use EJB or Spring you can use a transaction-scoped EntityManager, which can be used from multiple threads (it's a proxy which delegates actual work to separate thread-bound instances of EntityManager), but I think it's not your case.

edited Jun 10, 2015 at 12:05

Jahan Zinedine

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answered May 11, 2011 at 15:34

axtavt

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3 Comments

ali Over a year ago

Thanks but unfortunately I am not using EJB. That is why I have to call manager.getTransaction().begin(). But your comment might be helpful anyways. Maybe it is possible to create an EntityManager per Thread. I will try that.

FrankTan Over a year ago

Please @axtavt i m interested in the Ejb transaction-scoped EntityManager can you link me to an example on how to use multiple threads for run multiple queries ? thx.

user725455 Over a year ago

@frank, have you bean able to use multiple threads for run multiple queries?

Nayan Wadekar · Accepted Answer · 2011-05-11 18:44:15Z

To handle concurrency issues when the object is being processed accross multiple transactions, lock can be acquired on the object. Either Optimistic or Pessimistic locking can be used with appropriate lock-modes.

Locking the versioned object by entityManager.lock(entity, LockModeType.READ) will ensure that it will prevent any Dirty Read & Non-Repeatable Read.

LockModeType.WRITE will force incrementing/updating the version column of the entity.

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