【正文】 方式要根據(jù)具體情況而定，但后一種方式常常更為方便，因為它只需簡單地等待用戶的請求。 理論上講， servlet并非只用于處理 HTTP請求的 Web服務(wù)器或應(yīng)用服務(wù)器，它同樣可以用于其他類型的服務(wù)器。同樣的情況，如果使用servlet,則啟動 N 個線程， 但是 僅僅載入 servlet 類的單一副本。 Servlet 能夠直接于 Web服務(wù)器對話，而常規(guī)的 CGI程序做不到這一點，至少在不使用服務(wù)器專有 API的情況下是這樣。為 Macromedia JRun編寫的 servlet，可以不經(jīng)過任何修改地在 Apache Tomcat， Microsoft Inter Information Server，IBM WebSphere， iPla Enterprise Serve， Oracle9i AS 或者 StrNine WebStar 上運行。Marty曾在馬尼拉講授過 jsp 和 servlet 培訓課程， 那兒對 servlet 和 jsp 技術(shù)抱很大的興趣。如果他們 的項目變得更龐大，他們或許希望轉(zhuǎn)移到分布式環(huán)境。 安全 傳統(tǒng) CGI程序中主要的漏洞來源之一就是， CGI程序常常由通過的操作系統(tǒng)外殼來執(zhí)行。當然，數(shù)組邊界的檢查以及其他內(nèi)存包含特性是 java編程語言的核心部分。但我們的立場是：服務(wù)器端 Java本非一項新的、為經(jīng)證實的技術(shù) 。 Servlet不存在這些問題。最后，如果他們的項目變得更龐大，他們或許將他從 Linux轉(zhuǎn)移到運行 IBM WebSphere 的 IBM大型機上。項目取得成功之后，他們可以轉(zhuǎn)移到性能更高、管理更容易，但需要付費的服務(wù)器。例如， Marty 記錄了所有通過電子郵件向他發(fā)送問題的讀者的所在國。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servlet， 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），二進制（ GIF圖 像 ），甚至可以 是 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 下面 的任務(wù)，如圖 11所示 。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 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。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 d