[ https://issues.jboss.org/browse/DROOLS-2111?page=com.atlassian.jira.plugi... ] Tibor Zimányi commented on DROOLS-2111: --------------------------------------- New findings about this: In JDK 9 it is now not possible to use a mapping function that also modifies the underlying map in HashMap.computeIfAbsent() method. See this JavaDoc: "@throws ConcurrentModificationException if it is detected that the mapping function modified this map" The part which cause this problem is here [1]. The problem there is that getCompiler() mapping function also modifies the map. The question now is if we consider this a bad habit (to use a mapping function that changes the map) or not and therefore we can try to replace the HashMap with some other map implementation. [1] https://github.com/kiegroup/drools/blob/7a6db1342925872a898dee3f1d03f835b... -- This message was sent by Atlassian JIRA (v7.5.0#75005)