When Drools is being run to process rules on a number of threads on occasion the rules engine freezes with the stack trace below and does not complete. There are a number of other threads that are also frozen out waiting to lock 0x00002aab9d4caf58:

Frozen Thread

"pool-3-thread-8" prio=10 tid=0x00002ab53d16e000 nid=0x1c29 runnable [0x0000000049fc7000]
java.lang.Thread.State: RUNNABLE
at java.util.WeakHashMap.expungeStaleEntries(WeakHashMap.java:408)

• locked \<0x00002aab9d4caf58\> (a java.lang.ref.ReferenceQueue)
  at java.util.WeakHashMap.getTable(WeakHashMap.java:417)
  at java.util.WeakHashMap.get(WeakHashMap.java:464)
  at org.mvel2.util.ParseTools.getBestCandidate(ParseTools.java:241)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:1039)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:987)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:377)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.optimizeAccessor(ReflectiveAccessorOptimizer.java:143)
  at org.mvel2.ast.ASTNode.optimize(ASTNode.java:159)
  at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:115)
  at org.mvel2.MVELRuntime.execute(MVELRuntime.java:85)
  at org.mvel2.compiler.CompiledExpression.getDirectValue(CompiledExpression.java:123)
  at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:119)
  at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:113)
  at org.mvel2.MVEL.executeExpression(MVEL.java:930)
  at org.drools.base.mvel.MVELConsequence.evaluate(MVELConsequence.java:104)
  at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1287)
• locked \<0x00002aaec5b1b160\> (a org.drools.common.DefaultAgenda)
  at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1221)
  at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1456)
  at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:710)
  at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:674)
  at com.iontrading.arc.liquidityrules.engine.BatchRulesEngine.processAll(BatchRulesEngine.java:64)
  at com.iontrading.arc.liquidityrules.api.rulestoresource.AbstractRuleStoreSourceService.performBaselineProcessing(AbstractRuleStoreSourceService.java:860)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService.access$600(RuleStoreSourceService.java:32)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService$RuleStoreProcess.call(RuleStoreSourceService.java:66)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService$RuleStoreProcess.call(RuleStoreSourceService.java:38)
  at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
  at java.util.concurrent.FutureTask.run(FutureTask.java:166)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
  at java.lang.Thread.run(Thread.java:722)

Example of other frozen threads:

"pool-3-thread-9" prio=10 tid=0x00002ab53d16f800 nid=0x1c2a waiting for monitor entry [0x000000004a0c9000]
java.lang.Thread.State: BLOCKED (on object monitor)
at java.util.WeakHashMap.expungeStaleEntries(WeakHashMap.java:386)

• waiting to lock \<0x00002aab9d4caf58\> (a java.lang.ref.ReferenceQueue)
  at java.util.WeakHashMap.getTable(WeakHashMap.java:417)
  at java.util.WeakHashMap.get(WeakHashMap.java:464)
  at org.mvel2.util.ParseTools.getBestCandidate(ParseTools.java:241)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:1039)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:987)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:377)
  at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.optimizeAccessor(ReflectiveAccessorOptimizer.java:143)
  at org.mvel2.ast.ASTNode.optimize(ASTNode.java:159)
  at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:115)
  at org.mvel2.MVELRuntime.execute(MVELRuntime.java:85)
  at org.mvel2.compiler.CompiledExpression.getDirectValue(CompiledExpression.java:123)
  at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:119)
  at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:113)
  at org.mvel2.MVEL.executeExpression(MVEL.java:930)
  at org.drools.base.mvel.MVELConsequence.evaluate(MVELConsequence.java:104)
  at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1287)
• locked \<0x00002aaeedca2870\> (a org.drools.common.DefaultAgenda)
  at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1221)
  at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1456)
  at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:710)
  at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:674)
  at com.iontrading.arc.liquidityrules.engine.BatchRulesEngine.processAll(BatchRulesEngine.java:64)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService.performViewProcessing(RuleStoreSourceService.java:133)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService$RuleStoreProcess.call(RuleStoreSourceService.java:78)
  at com.rbc.erm.rulestoresource.RuleStoreSourceService$RuleStoreProcess.call(RuleStoreSourceService.java:38)
  at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
  at java.util.concurrent.FutureTask.run(FutureTask.java:166)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
  at java.lang.Thread.run(Thread.java:722)