【正文】 ,VMS，和 IBM 大型機(jī)操作系統(tǒng)之上。例如，在 C 和 C＋＋中，可以分配一個(gè) 100 個(gè)元素的數(shù)組，然后向第 999個(gè) “元素 “寫入數(shù)據(jù) ——實(shí)際上是程序內(nèi)存的隨機(jī)部分，這完全合法。因此，從美國公司那里購買專用 Web 服務(wù)器會(huì)消耗掉項(xiàng)目的大部分前期資金。實(shí)際上都直接或通過插件支持 servlet。而使用 servelt， Java虛擬機(jī)會(huì)一直運(yùn)行，并用輕量級(jí)的 Java 線程處理每個(gè)請(qǐng)求，而非重量級(jí)的操作系統(tǒng)進(jìn)程。 如果頁面需要根據(jù)每個(gè)具體的請(qǐng)求做出相應(yīng)的改變，當(dāng)然需要在請(qǐng)求發(fā)生時(shí)構(gòu)建響應(yīng)。 這個(gè)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。 Servlet 的工作是執(zhí)行西門的任務(wù)，如圖 所示 。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 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。 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。 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。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。 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。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。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。它們用在航空業(yè)、電子商務(wù)、在線銀行、 web 搜索引擎、門戶、大型金融網(wǎng)站、以及成百上千您日常光顧的其他網(wǎng)站。 問題的第二個(gè)來源是，一些 CGI 程序用不自動(dòng) 檢查數(shù)組和字符串邊界的語言編寫而成。其次，這兩個(gè)國家的貨幣對(duì)美元的匯率都極為不利。所有主要的 Web 服務(wù)器。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。 頁面由頻繁改變的數(shù)據(jù)導(dǎo)出。 （ 4） 向客戶發(fā)送顯式數(shù)據(jù)（即文檔）。 關(guān)鍵字 ： JSP 技術(shù)， Servlet， HTTP 服務(wù) 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。t know what to display until you read the data that the user submits. Just remember that the user submits two kinds of data: explicit (., HTML form data) and implicit (., HTTP request headers). Either kind of input can be used to build the output page. In particular, it is quite mon to build a userspecific page based on a cookie value. 2． The Web page is derived from data that changes frequently. 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 info