Error in OptaPlanner when multithreading (External object cannot be looked up) - optaplanner

Question

I have developped an optimization module using OptaPlanner but have encountered an error when multithreading. Note that the module works when multithreading is disabled. I tried several OptaPlanner versions (8.4, 8.1, 7.51) and different JDKs (16 and 14).
I used a simple solver configuration file
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My planning entity is also straghtforward :
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#PlanningSolution public class SuccessPromotion {
#ProblemFactCollectionProperty
public ArrayList<RCT> rcts;
#PlanningEntityCollectionProperty
public ArrayList<RCT_Planning> rcts_promos;
#ValueRangeProvider(id = "availablePromotion")
#ProblemFactCollectionProperty
public List<int[]> promotions;
#PlanningScore
public HardSoftScore score;
#PlanningId
public String name; //Constructors //Getter & Setter }
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The error I get is as follows :
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Exception in thread "main" java.lang.IllegalStateException: The move
thread with moveThreadIndex (3) has thrown an exception. Relayed here
in the parent thread. at
org.optaplanner.core.impl.heuristic.thread.OrderByMoveIndexBlockingQueue.take(OrderByMoveIndexBlockingQueue.java:147)
at
org.optaplanner.core.impl.localsearch.decider.MultiThreadedLocalSearchDecider.forageResult(MultiThreadedLocalSearchDecider.java:188)
at
org.optaplanner.core.impl.localsearch.decider.MultiThreadedLocalSearchDecider.decideNextStep(MultiThreadedLocalSearchDecider.java:159)
at
org.optaplanner.core.impl.localsearch.DefaultLocalSearchPhase.solve(DefaultLocalSearchPhase.java:71)
at
org.optaplanner.core.impl.solver.AbstractSolver.runPhases(AbstractSolver.java:99)
at
org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:189)
at test.RealSolver.main(RealSolver.java:140) Caused by:
java.lang.IllegalArgumentException: The externalObject ([I#5ce1e52a)
cannot be looked up. Some functionality, such as multithreaded
solving, requires this ability. Maybe add an #PlanningId annotation on
an identifier property of the class (class [I). Or otherwise, maybe
change the #PlanningSolution annotation's LookUpStrategyType (not
recommended). at
org.optaplanner.core.impl.domain.lookup.NoneLookUpStrategy.lookUpWorkingObject(NoneLookUpStrategy.java:44)
at
org.optaplanner.core.impl.domain.lookup.LookUpManager.lookUpWorkingObject(LookUpManager.java:76)
at
org.optaplanner.core.impl.score.director.AbstractScoreDirector.lookUpWorkingObject(AbstractScoreDirector.java:512)
at
org.optaplanner.core.impl.heuristic.selector.move.generic.ChangeMove.rebase(ChangeMove.java:83)
at
org.optaplanner.core.impl.heuristic.selector.move.generic.ChangeMove.rebase(ChangeMove.java:31)
at
org.optaplanner.core.impl.heuristic.thread.MoveThreadRunner.run(MoveThreadRunner.java:140)
at
java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at
java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at
java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
**

Answer 1

Your error message is
The externalObject ([I#5ce1e52a) cannot be looked up. Some functionality, such as multithreaded solving, requires this ability. Maybe add an #PlanningId annotation on an identifier property of the class (class [I).
That class[I talks about your planning value class (int[]), not your planning entity class (ArrayList<RCT_Planning>).
The planning value class int[] is neither immutable, nor has a #PlanningId, so it can't rebase it easily to another thread (although in theory it could copy it, it doesn't do so at the moment). The error message could be better.
Workaround: wrap int[] in a planning value class.