Issue

When following Drools community document page 7.5.2. Active Mode to run rules engine in the below way

KieSessionConfiguration config = KieServices.Factory.get().newKieSessionConfiguration();
config.setOption( ClockTypeOption.get("realtime") );
KieSession session = kbase.newKieSession( conf, null );

new Thread( new Runnable() {
  @Override
  public void run() {
      session.fireUntilHalt();
  }
} ).start();

Hanging or long time waiting threads can be observed (application becomes unresponsive) when calling StatefulKnowledgeSessionImpl.insert(fact) method in another thread.

Checking the thread dumps, there is a HashMap (which is thread unsafe) being used by 2 (or more) different threads like below:

"Thread-83" prio=10 tid=0x00007f1dcc900000 nid=0x659c runnable [0x00007f1d798d7000]
   java.lang.Thread.State: RUNNABLE
    at java.util.HashMap.put(HashMap.java:498)
    at org.drools.core.time.impl.TrackableTimeJobFactoryManager.addTimerJobInstance(TrackableTimeJobFactoryManager.java:65)
    at org.drools.core.time.impl.JDKTimerService.internalSchedule(JDKTimerService.java:126)
    at org.drools.core.time.impl.JDKTimerService.scheduleJob(JDKTimerService.java:99)
    ... ...
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:1541)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:1487)

"Thread--fireUntilHalt" prio=10 tid=0x00007f1dcc840800 nid=0x1d7b runnable [0x00007f1d7adec000]
   java.lang.Thread.State: RUNNABLE
    at java.util.HashMap.removeEntryForKey(HashMap.java:694)
    at java.util.HashMap.remove(HashMap.java:660)
    at org.drools.core.time.EnqueuedSelfRemovalJobContext$1.execute(EnqueuedSelfRemovalJobContext.java:35)
    ... ...
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireUntilHalt(StatefulKnowledgeSessionImpl.java:1404)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireUntilHalt(StatefulKnowledgeSessionImpl.java:1383)