[AMQ-3506] Access to ConnectionPool.createSession needs to be synchronized

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 5.5.0
• Fix Version/s: 5.6.0
• Component/s: Broker
• Labels: activemq-pool maximumActive sessionPool

Environment

activemq-pool, PooledConnectionFactory with maximumActive=1 and blockIfSessionPoolIsFull=true (default behavior)

• Patch Info: Patch Available
• Assignee: Unassigned
• Reporter: Torsten Mielke
• Created: 20/Sep/11 14:03
• Updated: 21/Sep/11 13:57
• Resolved: 21/Sep/11 13:57

Description

When configuring a PooledConnectionFactory with maximumActive=1 and blockIfSessionPoolIsFull=true (default behavior for latter config) it is possible that multiple threads that concurrently try to use the same JMS connection to create a new session might create more sessions than the configured maximumActive limit.

That's because the call to ConnectionPool.createSession() is not synchronized and if multiple threads try to call this method concurrently (on the same underlying JMS connection) then the if-condition in

SessionKey key = new SessionKey(transacted, ackMode);
SessionPool pool = cache.get(key);
if (pool == null) {
  pool = createSessionPool(key);
  cache.put(key, pool);
}

will evaluate to true for all threads and they all end up creating their own sessionPool using the same SessionKey properties.
Access to the if-condition needs to be synchronized so that only one session pool gets created. That will ensure that not more than the configured maximumActive number of sessions can get created.