What is Multitiered Application? 出自sun对宠物商店例子的综述文档?
Partitioning a design into tiers allows designers to choosethe appropriate technology for a given situation. The JavaPet Store demo, illustrates how its design is divided intomultiple tiers.使用分层的方式来进行设计，可以使设计者针对具体的情况使用合适的技术。这个Java 宠物商店的例子展示了如何把一个设计分成多层结构。
The Client tier is often the only part of the application theuser sees. The Java Pet Store demo Client tier consists of abrowser displaying web pages generated from JSP pages in theWeb tier. An administrator interface uses a spreadsheet client,such as StarOffice software, to communicate with the Web TierXML messages. The Client tier communicates with other tiersthrough defined interfaces.客户层通常是用户唯一可以看到的应用程序的部分。这个宠物商店例子的客户层是一个浏览器，它负责显示Web层中JSP页产生的web页面。一个管理员接口可以使用一个spreadsheet客户端（例如StarOffice软件）来和Web层交换XML消息。客户层通过定义好的接口和其他层交互。
The Web tier performs web-related processing, such as servingHTML, instantiating web page templates, and formatting JSP pagesfor display. Some objects within the Web tier act as proxies tocomponents in the Enterprise JavaBeans components tier.Web层负责Web相关的处理，比如提供HTML服务，产生web页面的模板的实作，格式化JSP页面来显示。Web层中有一些物件可以作为EJB组件层中组件的代理。
The Enterprise JavaBeans components tier is responsible for anyprocessing involving Enterprise JavaBeans components. EnterpriseJavaBeans components contain the business logic of the Java PetStore demo, such as processing orders. EJB components alsorepresent the model objects of the Java Pet Store demo, includingAccount and placed Orders.EJB组件层负责任何与EJB组件相关的处理。EJB组件中包含这个Java宠物商店的商业逻辑，比如处理订单。EJB组件也含有这个Java宠物商店的物件模型，包括账目和处理过的订单。
Stand-alone clients and web applications in the Web tier can useEnterprise JavaBeans components hosted by the Enterprise JavaBeanscomponents tier. The Java Pet Store demo demonstrates the properuse of Enterprise JavaBeans components.单机版客户端和网络层中的web应用都可以使用EJB组件层中的EJB组件。这个Java宠物商店的例子展示了如何恰当地使用EJB组件。
The Enterprise Information System (EIS) tier provides a layer ofsoftware that maps existing data and application resources intothe design of a J2EE application in an implementation-neutral way.The EIS tier can also access applications of other organizationsor businesses, providing seamless business-to-business integration.企业信息系统（EIS）层提供了一层软件，以一种与实现无关的方式把已存在的数据和应用资源一起映射到一个J2EE应用程序的设计之中。EIS层通过B2B的无缝集成使得其它的组织或商业机构也可以对其进行使用。
