【正文】 s derived from data that changes frequently. 8 If the page changes for every request, then you certainly need to build the response at request time. If it changes only periodically, however, you could do it two ways: you could periodically build a new Web page on the server (independently of client requests), or you could wait and only build the page when the user requests it. The right approach depends on the situation, but sometimes it is more convenient to do the latter: wait for the user request. For example, a weather report or news headlines site might build the pages dynamically, perhaps returning a previously built page if that page is still up to date. 3． The Web page uses information from corporate databases or other serverside sources. If the information is in a database, you need serverside processing even if the client is using dynamic Web content such as an applet. Imagine using an applet by itself for a search engine site: Downloading 50 terabyte applet, please wait! Obviously, that is silly。ll only be discussing HTTP servlets. The Advantages of Servlets Over Traditional CGI Java servlets are more efficient, easier to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGIlike technologies. 1． Efficient With traditional CGI, a new process is started for each HTTP request. If the CGI program itself is relatively short, the overhead of starting the process can dominate the execution time. With servlets, the Java virtual machine stays running and handles each request with a lightweight Java thread, not a heavyweight operating system process. Similarly, in traditional CGI, if there are N requests to the same CGI program, the code for the CGI program is loaded into memory N times. With servlets, however, there would be N threads, but only 9 a single copy of the servlet class would be loaded. This approach reduces server memory requirements and saves time by instantiating fewer objects. Finally, when a CGI program finishes handling a request, the program terminates. This approach makes it difficult to cache putations, keep database connections open, and perform other optimizations that rely on persistent data. Servlets, however, remain in memory even after they plete a response, so it is straightforward to store arbitrarily plex data between client requests. 2． Convenient Servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, and many other such highlevel utilities. In CGI, you have to do much of this yourself. Besides, if you already know the Java programming language, why learn Perl too? You39。 see so industry support for servlets is being even more pervasive. 5． Inexpensive A number of free or very inexpensive Web servers are good for development use or deployment of low or mediumvolume Web sites. Thus, with servlets and JSP you can start with a free or inexpensive server and migrate to more expensive servers with highperformance capabilities or advanced administration utilities only after your project meets initial success. This is in contrast to many of the other CGI alternatives, which require a significant initial investment for 10 the purchase of a proprietary package. Price and portability are somewhat connected. For example, Marty tries to keep track of the countries of readers that send him questions by . India was near the top of the list, probably 2 behind the . Marty also taught one of his JSP and servlet training courses (see in Manila, and there was great interest in servlet and JSP technology there. Now, why are India and the Philippines both so interested? We surmise that the answer is twofold. First, both countries have large pools of wel