When not to use Drools If your business logic is well defined/static and doesn't change often, you don't need to change rules at run-time. If your rules are simple, self-contained, and usually spanning only a single object (for example, a check that user's age is less than 21). If you have a memory constrained environment. If your project is a one-shot effort and it will never be used again or maintained over time.
Alternatives to Drools ILOG (now IBM)—JRules Fair Isaac—Blaze Advisor Corticon's BRMS, Haley (now Oracle) Business Rules Engine Pegasystems—PegaRules, Production Systems Technologies—OPSJ CLIPS OpenRules or Jess