EJB 3.0 Stateless SessionBean concurrency

Question

Hi everyone, i have this problem with an applcation. I have a @Stateless session bean that is accessed by (60-100) clients at the same time. In some of the services of the stateless bean i need to make some operations that involve the use of transactions, my problem is that i dont know if i can use an entity manager injected like:
@PersistenceContext(unitName="pu1")
private EntityManager em;
Because if 5 clients access the bean at the same time they are going to share the same persistent context in the EntityManager em, and they wont have a transactional context.
To avoid this, i`m thinking in using an EntityManagerfactory and use it everytime a client mekes a request to guarantee the transactional context, the problem is that i have to do something like this every time for a transaction to work:
EntityManager em = emf.createEntityManager();
try {
em.getTransaction().begin();
try {
// work
em.getTransaction().commit();
} catch (Throwable t) {
em.getTransaction().rollback();
throw t;
} finally {
em.close();
I dont know if im missing the advantages of the session bean, dont know if i have to use a Statefull session bean, to aviod the concurrency inconvinient.

Answer 1

Concurrency is automatically handled by the ejb container so there's no need to write special code to handle it. The container will guarantee that at most one thread is executing within each stateless session bean instance at a time, so there won't be any concurrency issues regarding the access of the container-managed EntityManager.
The issue you raise can be a problem when injecting a non-sharable resource into a servlet instance, since the web container can use the same servlet instance to process multiple concurrent threads.
--ken