【正文】 Figure 111．Read the explicit data sent by the client.The end user normally enters this data in an HTML form on a Web page. However, the data could also e from an applet or a custom HTTP client program.2．Read the implicit HTTP request data sent by the browser.Figure 11 shows a single arrow going from the client to the Web server (the layer where servlets and JSP execute), but there are really two varieties of data: the explicit data that the end user enters in a form and the behindthescenes HTTP information. Both varieties are critical. The HTTP information includes cookies, information about media types and pression schemes the browser understands, and so on.3．Generate the results.This process may require talking to a database, executing an RMI or EJB call, invoking a Web service, or puting the response directly. Your real data may be in a relational database. Fine. But your database probably doesn39。t talk directly to the database. Even if it could, for security reasons, you probably would not want it to. The same argument applies to most other applications. You need the Web middle layer to extract the results inside a document.4．Send the explicit data (., the document) to the client.This document can be sent in a variety of formats, including text (HTML or XML), binary (GIF images), or even a pressed format like gzip that is layered on top of some other underlying format. But, HTML is by far the most mon format, so an important servlet/JSP task is to wrap the results inside of HTML.5．Send the implicit HTTP response data.Figure 11 shows a single arrow going from the Web middle layer (the servlet or JSP page) to the client. But, there are really two varieties of data sent: the document itself and the behindthescenes HTTP information. Again, both varieties are critical to effective development. Sending HTTP response data involves telling the browser or other client what type of document is being returned (., HTML), setting cookies and caching parameters, and other such tasks. Why Build Web Pages Dynamically?Many client requests can be satisfied by prebuilt documents, and the server would handle these requests without invoking servlets. In many cases, however, a static result is not sufficient, and a page needs to be generated for each request. There are a number of reasons why Web pages need to be built onthefly:1． The Web page is based on data sent by the client.For instance, the results page from search engines and orderconfirmation pages at online stores are specific to particular user requests. You don39。 you need to talk to the database. Going from the client to the Web tier to the database (a threetier approach) instead of from an applet directly to a database (a twotier approach) provides increased flexibility and security with little or no performance penalty. After all, the database call is usually the ratelimiting step, so going through the Web server does not slow things down. In fact, a threetier approach is often faster because the middle tier can perform caching and connection pooling.In principle, servlets are not restricted to Web or application servers that handle HTTP requests but can be used for other types of servers as well. For example, servlets could be embedded in FTP or mail servers to extend their functionality. And, a servlet API for SIP (Session Initiation Protocol) servers was recently standardized (see ). In practice, however, this use of servlets has not caught on, and we39。re already convinced that Java technology makes for more reliable and reusable code than does Visual Basic, VBScript, or C++. Why