【正文】 ession tracking and caching of previous putations. 4． Portable Servlets are written in the Java programming language and follow a standard API. Servlets are supported directly or by a plugin on virtually every major Web server. Consequently, servlets written for, say, Macromedia JRun can run virtually unchanged on Apache Tomcat, Microsoft Inter Information Server (with a separate plugin), IBM WebSphere, iPla Enterprise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition (J2EE。 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。 當(dāng)然，僅僅是流行并不能證明技術(shù)的優(yōu)越性。例如，在 C和 C＋＋中，可以分配一 個 100 個元素的數(shù)組，然后向第 999 個 “ 元素“ 寫入數(shù)據(jù) —— 實際上是程序內(nèi)存的隨機部分，這完全合法。因此，他們可以切換到 BEA WebLogic 或 Oracle9i AS。因此，從美國公司那里購買專用 Web 服務(wù)器會消耗掉項目的大部分前期資金。這與其他 CGI方案形成鮮明的對比，這些 CGI 方案在初期都需要為購買專利軟件包投入大量的資金。實際上都直接或通過插件支持 servlet。而在 CGI 中，大部分工作都需要我們資金完成。而使用 servelt， Java 虛擬機會一直運行，并用輕量級的 Java 線程處理每個請求，而非重量級的操作系統(tǒng)進(jìn)程。從客戶端到 Web 層再到數(shù)據(jù)庫（三層結(jié)構(gòu)），要比從 applet 直接到數(shù)據(jù)庫（二層結(jié)構(gòu)）更靈活，也更安全，而性能上的損失很少甚至沒有。 如果頁面需要根據(jù)每個具體的請求做出相應(yīng)的改變，當(dāng)然需要在請求發(fā)生時構(gòu)建響應(yīng)。然而，許多情況下靜態(tài)的結(jié)果不能滿足要求，我們需要針對每個請求生成一個頁面 。 這個文檔可以用各種格式發(fā)送，包括文本（ HTML 或 XML），二進(jìn)制（ GIF圖），甚至可以式建立在其他底層格式之上的壓縮格式，如 gzip。 （ 3） 生成結(jié)果。Servlet 的工作是執(zhí)行西門的任務(wù)，如圖 所示 。然而，數(shù)據(jù)還有可能來自 applet或定 制的 HTTP 客戶程序。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)行會話。 圖 Web 中間層到客戶端的單箭頭。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。例如，天氣預(yù)報或新聞網(wǎng)站可能會動態(tài)地構(gòu)建頁面，也有可能會返回之前構(gòu)建的頁面（如果它還是最新的話）。例如， servlet 能夠嵌入到 FTP 或郵件 服務(wù)器中，擴展他們的功能。這種方式減少了服務(wù)器的內(nèi)存需求，通過實例化更少的對象從而節(jié)省了時間。例如，與 Web 服務(wù)器的通信使得講相對 URL 轉(zhuǎn)換成具體的路徑名變得更為容易。 iPla Enterprise Server。印度接近列表的頂端，可能 僅次于美國。他們的 servelt 和 jsp 不需要重寫編寫。他們還是不需要做出任何更改。即使 servelt 執(zhí)行系統(tǒng)調(diào)用激活本地操作系統(tǒng)上的程序，它也不會用到外殼來完成這項任務(wù)。 An Overview of Servlet and JSP Technology Abstract: Servlet program running in the serverside, dynamically generated Web page with the traditional CGI and many other similar pared to CGI technology, Java Servlet with a more efficient, easier to use, more powerful and has better portability, more savings to invest . Key words: JSP Technology, Servlet, HTTP server A Servlet39。 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