周末爬了趟鹫峰，抱着锻炼身体的信念登山，但是事与愿违，整一个桑拿天，稍微一动就大汗淋漓，所以也就懒得拍照，充充搞了几张就下山了，多好的鹫峰啊，就这样被糟蹋了，可惜可惜，下次有机会绝对不能在这种天气，这种时间下登山了。

Rule engines are used in applications to replace and manage some of the business logic. They are best used in applications where the business logic is too dynamic to be managed at the source code level -- that is, where a change in a business policy needs to be immediately reflected in the application. Applications in domains such as insurance (for example, insurance rating), financial services (loans, fraud detection, claims routing and management), government (application process and tax calculations), telecom customer care and billing (promotions for long distance calls that needs to be integrated into the billing system), ecommerce (personalizing the user's experience), and so on benefit greatly from using rule engines.

Rule-based applications communicate with the rule engine by passing in the set of rules to be executed. Then, the application can inspect the results and either display them to the end user or perform further processing. The rule engine determines when to evaluate each rule based on the input required for the rule as well as the results obtained from the evaluation of previous rules. You do not need to specify the order or the dependencies of the rules.

以上是对Rule Engine的原文介绍，感觉还是很虚的东西，个人感觉Rule Engine的应用场景还是不够细化，仍然无法大部分抽取业务逻辑，不过结合workflow进行conditional flow control倒是一个不错的scene。Rule Engine只是对于if-else之类的条件进行抽取剥离（不过大部分业务代码也就是这个样子），光这点功能实现就使得原本已经很复杂的xml更加复杂，难看，针对drools来说，看到的是一堆堆的条件语句被抽到xml中，source-code也一样被抽到xml中，与其说隔离，不如说用xml写java，真的能剥离业务？