第三章
创建一个Stateless Session Bean
本章我们讲述如何创建一个stateless session EJB component。这个bean通过封装了Java Database Connectivity (JDBC) 代码的Data Access Object (DAO)来和数据库进行通信来鉴别用户身份。DAO具备我们所使用的bean的所有属性（(fields) ）和行为(methods)
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MyStore中所有的customer, supplier 和 manager都被赋予了唯一的username 和userid 获取服务。MyStore, but in order to access these services all these entities have to first login into the system (MyStore). The method for authentication is named loginUser, which takes two String parameters, username and password and returns the userID if authentication is successful.