I have what looks to me like a Drools bug on a simple drl with declared classes. I'm using version drools-distribution-5.3.0.Final. I'm invoking it with this: StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession(); List<Object> facts = new ArrayList<Object>(); ksession.execute( facts ); ============================= I get this crash: Exception in thread "main" java.lang.NoClassDefFoundError: com/sample/Cap at ASMAccessorImpl_129541121334339192620.getValue(Unknown Source) at org.mvel2.optimizers.dynamic.DynamicGetAccessor.getValue(DynamicGetAccessor.java:73) at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:103) at org.mvel2.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:42) at org.mvel2.MVEL.executeExpression(MVEL.java:952) ... ========================== Here's the drl file: package com.sample import com.sample.*; declare Item id : int; end declare Priority name : String; priority : int; end declare Cap item : Item; name : String end rule "split cart into items" when then insert(new Item(1)); insert(new Item(2)); insert(new Item(3)); end rule "Priorities" when then insert(new Priority("A", 3)); insert(new Priority("B", 2)); insert(new Priority("C", 5)); end rule "Caps" when $i : Item() $p : Priority($name : name) then insert(new Cap($i, $name)); end rule "test" when $i : Item() Cap(item.id == $i.id) then System.out.println("Cap"); end ==================== Some observations: - If I comment out any of the insert lines, the crash goes away - If I change the first field of class Cap to be id:int instead of item:Item and make the corresponding changes, the crash goes away