Threading issues with Hibernate - multithreading

Question

I'm experiencing a strange behaviour with Hibernate. I've been banging my head against the wall for a while now, and will award any answer which leads to a solution with a +100 bounty.
I have a JAX-RS (Jersey) REST server, with a filter that associates one Hibernate-session per request.
In one request a client POSTs some data which is stored in the database using one session (and one transaction). In a subsequent call, the client tries to GET this entity, but Hibernate can't find it.
Some observations:
I can only reproduce this if I run multiple simultaneous clients. I've never managed to reproduce it by running one client at a time.)
I can see the entity ID in the database, and if I restart the server, the entity is found by Hibernate as it should.
The error does not occur if I use a thread pool of size 1 (regardless of how many clients I run simultaneously).
Here's the code, with some logging:
chargeables.setId(new SecureRandom().nextLong());
System.out.printf("%s, session: %s [%s]%n",
Thread.currentThread(),
System.identityHashCode(session),
"session.beginTransaction()");
session.beginTransaction();
System.out.printf("%s, session: %s [%s]%n",
Thread.currentThread(),
System.identityHashCode(session),
"session.save(id = "+chargeables.getId()+")");
session.save(chargeables);
System.out.printf("%s, session: %s [%s]%n",
Thread.currentThread(),
System.identityHashCode(session),
"session.getTransaction().commit()");
session.getTransaction().commit();
The code for getting the entity:
System.out.printf("%s, session: %s [%s]%n",
Thread.currentThread(),
System.identityHashCode(session),
"session.get("+id+")");
Chargeables entity = (Chargeables) session.get(Chargeables.class, id);
if (entity == null)
System.out.printf("%s, session: %s [%s]%n",
Thread.currentThread(),
System.identityHashCode(session),
"ENTITY NOT FOUND!");
Now here is an excerpt of the resulting log (with some additional open/close session output):
Thread[Grizzly(5),5,main], session: 2041842357 [factory.openSession()]
Thread[Grizzly(5),5,main], session: 2041842357 [session.beginTransaction()]
Thread[Grizzly(5),5,main], session: 2041842357 [session.save(id = 7939229356942262438)]
Thread[Grizzly(5),5,main], session: 2041842357 [session.getTransaction().commit()]
Thread[Grizzly(5),5,main], session: 2041842357 [session.close()]
[...]
Thread[Grizzly(7),5,main], session: 1717445911 [factory.openSession()]
Thread[Grizzly(7),5,main], session: 1717445911 [session.get(7939229356942262438)]
Thread[Grizzly(7),5,main], session: 1717445911 [ENTITY NOT FOUND!]
Thread[Grizzly(7),5,main], session: 1717445911 [session.close()]
Why on earth do I reach ENTITY NOT FOUND!?
Hibernate version: 4.1.9.Final
MySQL verison: 14.14 Distrib 5.5.29
Mapping file for Chargeables:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping
default-cascade="all"
package="se.package.common.chargeables"
default-lazy="false">
<class name="Chargeables">
<id name="id" type="long">
<generator class="assigned"/>
</id>
<property name="startTimeStamp" />
<property name="endTimeStamp" />
<list name="chargeables">
<key column="chargeableId" />
<list-index column="pos" />
<many-to-many class="Chargeable"/>
</list>
</class>
<class name="Chargeable">
<id column="id" type="long">
<generator class="native"/>
</id>
<discriminator />
<property name="timestamp" />
</class>
<subclass name="DataTransfer" extends="Chargeable">
<property name="bytesSent" />
<property name="bytesReceived" />
</subclass>
<subclass name="TelephonyChargeable" extends="Chargeable">
<many-to-one name="num" />
</subclass>
<subclass name="Call" extends="TelephonyChargeable">
<property name="duration" />
</subclass>
<subclass name="OutgoingCall" extends="Call" />
<subclass name="IncomingCall" extends="Call" />
<subclass name="Message" extends="TelephonyChargeable" />
<subclass name="Sms" extends="Message" />
<subclass name="IncomingSms" extends="Sms" />
<subclass name="OutgoingSms" extends="Sms" />
<subclass name="Mms" extends="Message" />
<subclass name="IncomingMms" extends="Mms" />
<subclass name="OutgoingMms" extends="Mms" />
</hibernate-mapping>

Answer 1

IMHO it's an isolation problem. Your second session start before the transaction of the 1st one is commited. As default hibernate isolation level is read_commited, the 2nd session can't so retrieve the entity. How are you passing id between the 2 threads ?

Answer 2

If you can see the commit in the database, it cannot be an isolation issue at the MySQL level: console operations are also client operations, you don't get special powers when running them, so they conform to the isolation policy you selected.
Looking out for solution, I discovered that Hibernate provide some facilities to cache DB results: there are actually 2 levels of cache implemented there. Perhaps your hibernate installation comes bundled with a cache mechanism?
The first level works on a per session basis, which is consistent with the behaviour you observe when running the code on one thread, and seemingly happens to be activated by default. I am not Hibernate proficient, I can't say that for sure, but my opinion is that your setup has this first level of cache set in read-only mode, while you would like it to be in read-write mode.