With the attached unit test (inside the maven project, execute with mvn test) and against drools-api/drools-core 5.0.1.SNAPSHOT I get very reproducible hangs in AbstractRuleBase#lock().

Usually those happen after a few update threads are done with their work, and from the stack traces it looks like all blocked threads wait for some thread that already is done (looked at also in the debugger to check the lock's exclusiveOwnerThread).

On rarer occasions the test also triggers:

• NPE inside AbstractRuleBase when unlocking the working memories
• ConcurrentModificationException in ClassObjectTypeConf#getMatchingObjectTypes() when walking over the objectTypeNodes values
• Too many open files when building the rules (only seen when running the test in eclipse)