Is ThreadLocal pattern *really* thread safe?

Cue · **Joined:** Tue May 04, 2004 6:15 am **Posts:** 51

I'm trying to get my head around the ThreadLocal pattern (although I can't find it anymore in the Wiki) and I would really appreciate your help. I did have a read through the forum but I couldn't really find an answer.

Lets see the standard getSession() method of the pattern

Code:

Session s = (Session) threadSession.get();
if (s == null) {
   s = getSessionFactory().openSession();
   threadSession.set(s);
}

return s;   

Isn't it possible that two threads would get two different Session objects since the block isn't synchronized?

Seeing as one thread comes around, finds Session to be null tries to acquire a new one, at that time a second one comes around and since the first one hasn't really assigned the Session to the Threadlocal, gets null as well.

So the two threads end up with 2 sessions out of which only 1 of them gets closed when calling the closeSession().

Code:

Session s = (Session) threadSession.get();
threadSession.set(null);
if (s != null && s.isOpen()) {
   log.debug("Closing Session of this thread.");
        s.close();
}

Thanks in advance.

LaLiLuna · **Posted:** Thu Mar 02, 2006 9:18 am

I suppose you are using Hibernate 2 as in Hibernate 3 you can let the sessionFactory bind the session to the current thread automatically.

In my opionion you are correct. I will check this in the SessionFactory implementation to see if this could not be a problem there.

Regards Sebastian

Cue · **Joined:** Tue May 04, 2004 6:15 am **Posts:** 51

As I was reading through this I realized that the following statement is false

Quote:

So the two threads end up with 2 sessions out of which only 1 of them gets closed when calling the closeSession().

since each thread will get its associated Session and close it.

My confusion actually started when I saw the "reference counting" implementation of the ThreadLocal.

Code:

Session s = (Session) threadSession.get();
if (s == null) {
   s = getSessionFactory().openSession();
   threadSession.set(s);
   level++;
}

return s;  

Why is level incremented for all threads and not per thread using a ThreadLocal as well? :/