first of all Drool is a good AI(Artificial Inteligence) rule engine.
But Drools full of shortage:

1. You can't use generics in the drl file; that if you tried to define java function inside the drl file with the following Signature
   function List getLoans(LoanBO loan) {...}
   it will complains and will give the following error message
   org.drools.rule.InvalidRulePackage: unknown:24:15 Unexpected token '<'
   unknown:24:15 Unexpected token '<'




2. if your code encountered sentex error, or throw exception or you even forget to import a class that you are using all what drools will dump is the following error message org.drools.rule.InvalidRulePackage: Function Compilation error
   at org.drools.rule.Package.checkValidity(Unknown Source)
   at org.drools.common.AbstractRuleBase.addPackage(Unknown Source)