英文文獻(xiàn)科技類原文及翻譯66java(編輯修改稿)

　

【文章內(nèi)容簡(jiǎn)介】 t Window Toolkit (AWT) APIs, but a mandline interface is certainly possible. Application clients directly access enterprise beans running in the business tier. However, if the J2EE application client requirements warrant it, an application client can open an HTTP connection to establish munication with a servlet running in the web tier. Web Browsers The user39。s web browser downloads static or dynamic Hypertext Markup Language (HTML), Wireless Markup Language (WML), or Extensible Markup Language (XML) web pages from the web tier. Dynamic web pages are generated by servlets or JSP pages running in the web tier. 聯(lián)系 Q709604208 獲取 Applets A web page downloaded from the web tier can include an embedded applet. An applet is a small client application written in the Java programming language that executes in the Java VM installed in the web browser. However, client systems will likely need Java Plugin and possibly a security policy file so the applet can successfully execute in the web browser. JSP pages are the preferred API for creating a webbased client program because no plugins or security policy files are needed on the client systems. Also, JSP pages enable cleaner and more modular application design because they provide a way to separate applications programming from web page design. This means personnel involved in web page design do not need to understand Java programming language syntax to do their jobs. Applets that run in other workbased systems such as handheld devices or car phones can render Wireless Markup Language (WML) pages generated by a JSP page or servlet running on the J2EE server. The WML page is delivered over Wireless Application Protocol (WAP) and the work configuration requires a gateway to translate WAP to HTTP and back again. The gateway translates the WAP request ing from the handheld device to an HTTP request for the J2EE server, and then translates the HTTP server response and WML page to a WAP server response and WML page for display on the handheld device. JavaBeans Component Architecture The client tier might also include a ponent based on the JavaBeans ponent architecture (JavaBeans ponent) to manage the data flow between an application client or applet and ponents running on the J2EE server. JavaBeans ponents are not considered ponents by the J2EE specification. JavaBeans ponents written for the J2EE platform have instance variables and get and set methods for accessing the data in the instance variables. JavaBeans ponents used in this way are typically simple in design and 聯(lián)系 Q709604208 獲取 implementation, but should conform to the naming and design conventions outlined in the JavaBeans ponent architecture. J2EE Server Communications Figure 2 shows the various elements that can make up the client tier. The client municates with the business tier running on the J2EE server either directly, or as in the case of a client running in a browser, by going through JSP pages or servlets running in the web tier. Figure 2: Server Communications Thin Clients J2EE applications use a thin client. A thin client is a lightweight interface to the application that does not do things like query databases, execute plex business rules, or connect to legacy applications. Heavyweight operations like these are offloaded to web or enterprise beans executing on the J2EE server where they can leverage the security, speed, services, and reliability of J2EE serverside technologies. Web Components J2EE web ponents can be either JSP pages or servlets. Servlets are Java programming language classes that dynamically process requests and construct responses. JSP pages are textbased documents that contain static 聯(lián)系 Q709604208 獲取 content and snippets of Java programming language code to generate dynamic content. When a JSP page loads, a background servlet executes the code snippets and returns a response. Static HTML pages and applets are bundled with web ponents during application assembly, but are not considered web ponents by the J2EE specification. Serverside utility classes can also be bundled with web ponents, and like HTML pages, are not considered web ponents. Like the client tier and as shown in Figure 3, the web tier might include a JavaBeans object to manage the user input and send that input to enterprise beans running in the business tier for processing. Figure 3: Web Tier and J2EE Application Business Components Business code, which is logic that solves or meets the needs of a particular business domain such as banking, retail, or finance, is handled by enterprise beans running in the business tier. Figure 4 shows how an enterprise bean receives data from client programs, processes it (if necessary), and sends it to the enterprise information system tier for storage. An enterprise bean also retrieves data from storage, processes it (if necessary), and sends it back to the client program. There are three kinds of enterprise beans: session beans, entity beans, and messagedriven beans. A session bean represents a transient conversation 聯(lián)系 Q709604208 獲取 with a client. When the client finishes executing, the session bean and its data are gone. In contrast, an entity bean represents persistent data stored in one row of a database table. If the client terminates or if the server shuts down, the underlying services ensure the entity bean data is saved. A messagedriven bean bines features of a session bean and a Java Message Service (JMS) message listener, allowing a business ponent to receive JMS messages asynchronously. This introduction describes entity beans and session beans. For information on messagedriven beans, see the Java Message Ser