This Issue seems to be very similar to DROOL-1156. Unfortunatly the implemented solution seems not to work on a tomcat application server as there is no managed executor service.

It seems that the deployed knowledgebase need at least some level complexity as trivial examples do not run in this behaviour. If my debugging was correct it seems that this is related drools internal optimization, which would explain that simple knowledgebases will not be optimized.

The log that the tomcat emits for each left over thread looks like that:

WARNING [Thread-3] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [xxx] appears to have started a thread named [drools-worker-3] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread: 

As we usually only see this message when we shutdown the server this memory leak issue is not that severe for our usecase, it is still anoying and causes confusion for our customer. Therefore we would be happy if they would not appear.