J2EE教程汇.pptVIP

声明 本课件仅用于北京航空航天大学计算机学院的教学； 本课件修改采用了一些网络资源（论文、研究报告、技术报告等），在采用的时候并没有准确标注引用信息。 * * * * * * * * * * * * * * * * * * * * * J2EE应用程序通常由三层构成，因为他们分布于三个不同的位置：客户机、服务器、后台数据库服务器。 * * * What is Servlet? A servlet is Java objects that extend the functionality of HTTP server. That is, the Servlet technology was designed to provide the capability of dynamic contents generation on a HTTP server. Now you might want to ask people were able to do dynamic contents generation using CGI? What is wrong with CGI? Lots of things. They are efficient because in CGI every client HTTP request results in a new process, which is quite inefficient and furthermore it is not scalable. It is also platform-specific. What about NSAPI or ISAPI. Well they address some issues of CGI but they are vendor-specific or product specific and thus not portable. The servlet technology is also designed to solve these problems associated with using CGI, or proprietary APIs such as NSAPI or ISAPI. For example, Servlet handles the HTTP client requests much more efficiently than CGI because it does not have to create new process every time there is a new client request. It also handles session management for you so that you don’t have to deal with it yourself. By being Java code, it is cross-platform technology as opposed to vendor-specific or product-specific technologies such as NSAPI or ISAPI. * * * * * * * * * * * * * * * In this slide, JCA represents J2EE Connector Architecture. While this acronym is commonly used for brevity, the technology is most appropriately referred to by its full name, or just “Connectors”, as there are several other Java technologies that share its initials. 3. 业务逻辑组件 业务逻辑代码是解决特定业务领域的需求的程序代码，在J2EE中，通常指EJB。 What is EJB Technology? J2EE的基石 服务器端的组件 共有三种EJB： 会话 (session) bean：代表短暂的与客户的会话，当客户结束执行时，会话bean及它的数据就消失了 实体 (entity) bean：实体bean代表存储在数据库的表，如果客户结束程序或服务器关闭，潜在的服务方法会将数据存储。 消息驱动(message-driven) bean。 1.3 J2EE容器 瘦客户端的多层应用程序总是很难开发，因为它包括各个层的事务处理、状态管理、多线程、资源池和其他复杂底层细节等等的错综复杂的编码。