java.lang.RuntimeException: no debugger registered to handle breakpoint
    at org.mvel2.debug.DebuggerContext.checkBreak(DebuggerContext.java:98)
    at org.mvel2.MVELRuntime.execute(MVELRuntime.java:76)
    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.core.util.MVELSafeHelper$RawMVELEvaluator.executeExpression(MVELSafeHelper.java:496)
    at org.drools.core.rule.constraint.MvelConditionEvaluator.evaluate(MvelConditionEvaluator.java:92)
    at org.drools.core.rule.constraint.MvelConditionEvaluator.evaluate(MvelConditionEvaluator.java:77)
    at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:248)
    at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:204)
    at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:141)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:494)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:374)
    at org.drools.core.reteoo.ObjectTypeNode.propagateAssert(ObjectTypeNode.java:298)
    at org.drools.core.phreak.PropagationEntry$Insert.execute(PropagationEntry.java:93)
    at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:96)
    at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:69)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.flushPropagations(StatefulKnowledgeSessionImpl.java:1993)
    at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1289)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.internalFireAllRules(StatefulKnowledgeSessionImpl.java:1294)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1281)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1260)
    at org.drools.impl.adapters.StatefulKnowledgeSessionAdapter.fireAllRules(StatefulKnowledgeSessionAdapter.java:72)
    at step1.TestFibonacci.main(TestFibonacci.java:47)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

final List<String> knownVariables = new ArrayList<String>();
    MVELRuntime.resetDebugger();
    MVELDebugHandler.setDebugMode(true);
    MVELRuntime.setThreadDebugger(new Debugger() {
           public int onBreak(Frame frame) {
               System.out.println("onBreak");
               for (String var: frame.getFactory().getKnownVariables()) {
                   System.out.println("  " + var);
                   knownVariables.add(var);
               }
               return 0;
           }
       });