典型的J2EE三层结构，分为表现层、中间层（业务逻辑层）和数据服务层。三层体系将业务规则、数据访问及合法性校验等工作放在中间层处理。客户端不直接与数据库交互，而是通过组件与中间层建立连接，再由中间层与数据库交互。-A typical J2EE three-tier structure, divided into the presentation layer, middle layer (business logic) and data services layer. Three-tier system, the business rules, data access and verify the legitimacy of such work on the intermediate layer processing. Clients do not directly interact with the database, but by the middle tier components and establish a connection, then the middle layer and database interaction.