【正文】 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 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 progra m 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。t know how to use them, what good are they? Servlet and JSP technology is supported by servers from Apache, Oracle, IBM, Sybase, BEA, Macromedia, Caucho, Sun/iPla, New Atlanta, ATG, Fujitsu, Lutris, Silverstream, the World Wide Web Consortium (W3C), and many others. Several lowcost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM mainframe operating systems. They are the single most popular application of the Java programming language. They are arguably the most popular choice for developing medium to large Web applications. They are used by the airline industry (most United Airlines and Delta Airlines Web sites), emerce (), online banking (First USA Bank, Banco Popular de Puerto Rico), Web search engines/portals (), large financial sites (American Century Investments), and hundreds of other sites that you visit every day. Of course, popularity alone is no proof of good technology. Numerous counterexamples abound. But our point is that you are not experimenting with a new and unproven technology when you work with serverside Java. Servlet 和 JSP 技術(shù)簡(jiǎn)述 Gildas Avoine and Philippe Oechslin EPFL, Lausanne, Switzerland Servlet 的功能 Servlets 是運(yùn)行在 Web 或應(yīng)用服務(wù)器上的 Java 程序，它是一個(gè)中間層，負(fù)責(zé)連接來自 Web 瀏覽器或其他 HTTP 客戶程序的請(qǐng)求和 HTTP 服務(wù)器上的數(shù)據(jù)庫或應(yīng)用程序。然而，數(shù)據(jù)還有可能來自 applet 或定制的 HTTP 客戶程 序。 HTTP 信息包括 cookie、瀏覽器所能識(shí)別的媒體類型和壓縮模式等。該數(shù)據(jù)庫可能不理解 HTTP，或者不能返回 HTML 形式的結(jié)果，所有 Web 瀏覽器不 能直接與數(shù)據(jù)庫進(jìn)行會(huì)話。 （ 4） 向客戶發(fā)送顯式數(shù)據(jù)（即文檔）。 圖 中顯示了一條從 Web 中間層到客戶端的單箭頭。 動(dòng)態(tài)構(gòu)建網(wǎng)頁的原因 預(yù)先建立的文檔可以滿足客戶的許多請(qǐng)求，服務(wù)器無需調(diào)用 servlet 就可以處理這些請(qǐng)求。在沒有讀取到用戶提交的數(shù)據(jù)之前，我們不知道應(yīng)該顯示什么。 頁面由頻繁改變的數(shù)據(jù)導(dǎo)出。例如，天氣預(yù)報(bào)或新聞網(wǎng)站可能會(huì)動(dòng)態(tài)地構(gòu)建頁面，也有可能會(huì)返回之前構(gòu)建的頁面（如果它還是最新的話）。顯然，這樣很愚蠢；這種情況下，我們需要與數(shù)據(jù)庫進(jìn)行會(huì)話。例如， servlet 能夠嵌入到 FTP 或郵件服務(wù)器中，擴(kuò)展他 們的功能。如果 CGI程序自身相對(duì)比 較簡(jiǎn)短，那么啟動(dòng)進(jìn)程的開銷會(huì)占用大部分執(zhí)行時(shí)間。這種方式減少了服務(wù)器的內(nèi)存需求，通過實(shí)例化更少的對(duì)象從而節(jié)省了時(shí)間。 便利 Servelt 提供大量的基礎(chǔ)構(gòu)造，可以自動(dòng)分析和解碼 HTML 的表單數(shù)據(jù)，讀取和設(shè)置 HTTP 報(bào)頭，處理 cookie，跟蹤會(huì)話，以及其他次類高級(jí)功能。例如，與 Web 服務(wù)器的通信使得講相對(duì) URL 轉(zhuǎn)換成具體的路徑名變得更為容易。所有主要的 Web 服務(wù)器。 iPla Enterprise Server。因此，通過使用 servelt 和 jsp，我們可以從免費(fèi)或廉價(jià)的服務(wù)器開始，在項(xiàng)目獲得初步成功后，在移植到更高性能或高級(jí)管理工具的昂貴的服務(wù)器上。印度接近列表的頂端，可能僅次于美國。其次，這兩個(gè)國家的貨幣對(duì)美元的匯率都極為不利。他們的 servelt 和 jsp 不需要重寫編寫。如果項(xiàng)目變得極為龐大，錯(cuò)綜復(fù)雜，他們或許希望使用 Enterprise JavaBeans 來封裝他們的商業(yè)邏輯。他們還是不需要做出任何更改。 問題的第