外文翻譯--- servlet和jsp技術(shù)概要

1、<p><b>　　鄭州輕工業(yè)學(xué)院</b></p><p>　　本科畢業(yè)設(shè)計(jì)（論文）</p><p><b>　　—————英文翻譯</b></p><p>　　題 目 Servlet和Jsp技術(shù)概要 </p><p>　　學(xué)生姓名 婁文

2、 </p><p>　　專業(yè)班級(jí) 軟件工程08-1 </p><p>　　學(xué) 號(hào) 520813130116 </p><p>　　院 （系） 軟件學(xué)院 </p><p>　　指導(dǎo)教師

3、 聶南（副教授） </p><p>　　完成時(shí)間 2012 年 6 月 2 日 </p><p>　　An Overview of Servlet and JSP Technology</p><p>　　Abstract: Servlet program running in the server-sid

4、e, dynamically generated Web page with the traditional CGI and many other similar compared to CGI technology, Java Servlet with a more efficient, easier to use, more powerful and has better portability, more savings to i

5、nvest .</p><p>　　Key words: JSP Technology, Servlet, HTTP server</p><p>　　1.1 A Servlet's Job</p><p>　　Servlets are Java programs that run on Web or application servers, acting

6、 as a middle layer between requests coming from Web browsers or other HTTP clients and databases or applications on the HTTP server. Their job is to perform the following tasks, as illustrated in Figure 1-1.</p>&

7、lt;p>　　Figure 1-1</p><p>　　1．Read the explicit data sent by the client.</p><p>　　The end user normally enters this data in an HTML form on a Web page. However, the data could also come from a

8、n applet or a custom HTTP client program.</p><p>　　2．Read the implicit HTTP request data sent by the browser.</p><p>　　Figure 1-1 shows a single arrow going from the client to the Web server (th

9、e layer where servlets and JSP execute), but there are really two varieties of data: the explicit data that the end user enters in a form and the behind-the-scenes HTTP information. Both varieties are critical. The HTTP

10、information includes cookies, information about media types and compression schemes the browser understands, and so on.</p><p>　　3．Generate the results.</p><p>　　This process may require talking

11、 to a database, executing an RMI or EJB call, invoking a Web service, or computing the response directly. Your real data may be in a relational database. Fine. But your database probably doesn't speak HTTP or return

12、results in HTML, so the Web browser can'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 We

13、b middle layer to extract the result</p><p>　　inside a document.</p><p>　　4．Send the explicit data (i.e., the document) to the client.</p><p>　　This document can be sent in a variet

14、y of formats, including text (HTML or XML), binary (GIF images), or even a compressed format like gzip that is layered on top of some other underlying format. But, HTML is by far the most common format, so an important s

15、ervlet/JSP task is to wrap the results inside of HTML.</p><p>　　5．Send the implicit HTTP response data.</p><p>　　Figure 1-1 shows a single arrow going from the Web middle layer (the servlet or J

16、SP page) to the client. But, there are really two varieties of data sent: the document itself and the behind-the-scenes HTTP information. Again, both varieties are critical to effective development. Sending HTTP response

17、 data involves telling the browser or other client what type of document is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks. </p><p>　　1.2 Why Build Web Pages Dynam

18、ically?</p><p>　　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,

19、and a page needs to be generated for each request. There are a number of reasons why Web pages need to be built on-the-fly:</p><p>　　1． The Web page is based on data sent by the client.</p><p>　

20、　For instance, the results page from search engines and order-confirmation pages at online stores are specific to particular user requests. You don't know what to display until you read the data that the user submits

21、. Just remember that the user submits two kinds of data: explicit (i.e., HTML form data) and implicit (i.e., HTTP request headers). Either kind of input can be used to build the output page. In particular, it is quite co

22、mmon to build a user-specific page based on a cookie value.</p><p>　　2．The Web page is derived from data that changes frequently.</p><p>　　If the page changes for every request, then you certain

23、ly 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 coul

24、d 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 new

25、s hea</p><p>　　3．The Web page uses information from corporate databases or other server-side sources.</p><p>　　If the information is in a database, you need server-side processing even if the cl

26、ient is using dynamic Web content such as an applet. Imagine using an applet by itself for a search engine site:</p><p>　　"Downloading 50 terabyte applet, please wait!" Obviously, that is silly; yo

27、u need to talk to the database. Going from the client to the Web tier to the database (a three-tier approach) instead of from an applet directly to a database (a two-tier approach) provides increased flexibility and secu

28、rity with little or no performance penalty. After all, the database call is usually the rate-limiting step, so going through the Web server does not slow things down. In fact, a three-tier approach is ofte</p><

29、;p>　　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

30、 to extend their functionality. And, a servlet API for SIP (Session Initiation Protocol) servers was recently standardized (see http://jcp.org/en/jsr/detail?id=116). In practice, however, this use of servlets has not cau

31、ght on, and we'll only be discussing HTTP servlets.</p><p>　　1.3 The Advantages of Servlets Over "Traditional" CGI</p><p>　　Java servlets are more efficient, easier to use, more p

32、owerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies.</p><p>　　1．Efficient</p><p>　　With traditional CGI, a new process is started for each

33、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 lig

34、htweight 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,

35、 how</p><p>　　2．Convenient</p><p>　　Servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking

36、sessions, and many other such high-level utilities. In CGI, you have to do much of this yourself. Besides, if you already know the Java programming language, why learn Perl too? You're already convinced that Java tec

37、hnology makes for more reliable and reusable code than does Visual Basic, VBScript, or C++. Why go back to those languages for</p><p>　　3．Powerful</p><p>　　Servlets support several capabilities

38、that are difficult or impossible to accomplish with regular CGI. Servlets can talk directly to the Web server, whereas regular CGI programs cannot, at least not without using a server-specific API. Communicating with the

39、 Web server makes it easier to translate relative URLs into concrete path names, for instance. Multiple servlets can also share data, making it easy to implement database connection pooling and similar resource-sharing o

40、ptimizations. Servlets</p><p>　　4．Portable</p><p>　　Servlets are written in the Java programming language and follow a standard API. Servlets are supported directly or by a plugin on virtually e

41、very major Web server. Consequently, servlets written for, say, Macromedia JRun can run virtually unchanged on Apache Tomcat, Microsoft Internet Information Server (with a separate plugin), IBM WebSphere, iPlanet Enterpr

42、ise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition (J2EE; see http://java.sun.com/j2ee</p><p>　　B/S ( Browser/Server ) structure browser and server structu

43、re. It is along with the technology of Internet spring up , it is for the structure of improvement or a kind of change of the structure of C/S. Under this kind of structure, user working interface is to realize through W

44、WW browser, lose the logic of general affairs very much in front( Browser) realization, but the major logic of general affairs in server end( Server) realization, form the three-layer claimed 3-tier structure. So, have s

45、i</p><p>　　( 1 ) The Maintenance of inferior position and upgrading way are simple. </p><p>　　Now upgrading and the improvement of software system more and more frequently, the product of the co

46、nfiguration of B/S embodies more convenient property obviously. For one a little a little bit big unit , if systematic administrator needs , between hundreds of 1000 even last computers round trip run , efficiency and wo

47、rkload is to can imagine, but the configuration of B/S software needs management server have been all right , all customer ends are browser only, need not do any maintenance at all</p><p>　　( 2 ) Cost reduct

48、ion, it is more to select. </p><p>　　All know windows in the computer of top of a table on nearly one Tong world, browser has become standard disposition, but on server operating system, windows is in absolu

49、te dominance position not. Current tendency is the application management software that uses the configuration of B/S all , need to install only in Linux server on , and safety is high. The so server option of operating

50、 system is many, no matter choosing those operating system, can let the most of ones use windows in order to t</p><p>　　Say, many persons on daily, "Sina website" nets , so long as having installed

51、 browser for can , and what need not know the server of " Sina website " to use is that what operating system, and in fact the most of websites do not use windows operating system really, but the computer of us

52、er is most of as installing to be windows operating system. </p><p>　　( 3 ) Application server operation data load value comparatively. </p><p>　　Since B/S configures management, software instal

53、lation in server end ( Server ) on, it is been all right that network administrator need to manage server only, the user interface major logic of general affairs in server ( Server ) end pass through WWW browser complete

54、ly realization, lose the logic of general affairs very much in front( Browser) realization, all customer ends has only browser, network administrator need to do hardware maintenance only. But application server operation

55、 data load i</p><p>　　5．Inexpensive</p><p>　　A number of free or very inexpensive Web servers are good for development use or deployment of low- or medium-volume Web sites. Thus, with servlets a

56、nd JSP you can start with a free or inexpensive server and migrate to more expensive servers with high-performance capabilities or advanced administration utilities only after your project meets initial success. This is

57、in contrast to many of the other CGI alternatives, which require a significant initial investment for the purchase of a proprietary</p><p>　　Price and portability are somewhat connected. For example, Marty t

58、ries to keep track of the countries of readers that send him questions by email. India was near the top of the list, probably #2 behind the U.S. Marty also taught one of his JSP and servlet training courses (see http://c

59、ourses.coreservlets.com/) in Manila, and there was great interest in servlet and JSP technology there.</p><p>　　Now, why are India and the Philippines both so interested? We surmise that the answer is twofol

60、d. First, both countries have large pools of well-educated software developers. Second, both countries have (or had, at that time) highly unfavorable currency exchange rates against the U.S. dollar. So, buying a special-

61、purpose Web server from a U.S. company consumed a large part of early project funds.</p><p>　　But, with servlets and JSP, they could start with a free server: Apache Tomcat (either standalone, embedded in th

62、e regular Apache Web server, or embedded in Microsoft IIS). Once the project starts to become successful, they could move to a server like Caucho Resin that had higher performance and easier administration but that is no

63、t free. But none of their servlets or JSP pages have to be rewritten. If their project becomes even larger, they might want to move to a distributed (clustered) enviro</p><p><b>　　6．Secure</b><

64、;/p><p>　　One of the main sources of vulnerabilities in traditional CGI stems from the fact that the programs are often executed by general-purpose operating system shells. So, the CGI programmer must be carefu

65、l to filter out characters such as backquotes and semicolons that are treated specially by the shell. Implementing this precaution is harder than one might think, and weaknesses stemming from this problem are constantly

66、being uncovered in widely used CGI libraries.</p><p>　　A second source of problems is the fact that some CGI programs are processed by languages that do not automatically check array or string bounds. For ex

67、ample, in C and C++ it is perfectly legal to allocate a 100-element array and then write into the 999th "element," which is really some random part of program memory. So, programmers who forget to perform this

68、check open up their system to deliberate or accidental buffer overflow attacks.</p><p>　　Servlets suffer from neither of these problems. Even if a servlet executes a system call (e.g., with Runtime.exec or J

69、NI) to invoke a program on the local operating system, it does not use a shell to do so. And, of course, array bounds checking and other memory protection features are a central part of the Java programming language. <

70、;/p><p>　　7．Mainstream</p><p>　　There are a lot of good technologies out there. But if vendors don't support them and developers don't know how to use them, what good are they? Servlet and

71、JSP technology is supported by servers from Apache, Oracle, IBM, Sybase, BEA, Macromedia, Caucho, Sun/iPlanet, New Atlanta, ATG, Fujitsu, Lutris, Silverstream, the World Wide Web Consortium (W3C), and many others. Severa

72、l low-cost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM main</p><p>　　Of course, popularity alone is no proof of good technology. Numerous count

73、er-examples abound. But our point is that you are not experimenting with a new and unproven technology when you work with server-side Java.</p><p><b>　　英文翻譯</b></p><p>　　Servlet和Jsp技

74、術(shù)概要</p><p>　　摘要：Servlet程序在服務(wù)器端運(yùn)行，動(dòng)態(tài)地生成Web頁(yè)面與傳統(tǒng)的CGI和許多其他類似CGI的技術(shù)相比，Java Servlet具有更高的效率，更容易使用，功能更強(qiáng)大，具有更好的可移植性，更節(jié)省投資。</p><p>　　關(guān)鍵字：JSP技術(shù)，Servlet，HTTP服務(wù)</p><p>　　1.1Servlet的功能</p>

75、<p>　　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(lái)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(kù)或應(yīng)用程序。Servlet的工作是執(zhí)行西門的任務(wù)，如圖1.1所示 。</p><p>　　圖1.1Web中間件的作用</p><p>　　讀取客戶發(fā)送的顯式數(shù)據(jù)。</p><p>　　最終

76、用戶一般在頁(yè)面的HTML表單中輸入這些數(shù)據(jù)。然而，數(shù)據(jù)還有可能來(lái)自applet或定制的HTTP客戶程序。</p><p>　　讀取由瀏覽器發(fā)送的隱式請(qǐng)求數(shù)據(jù)。</p><p>　　圖1.1中顯示了一條從客戶端到Web服務(wù)器的單箭頭，但實(shí)際上從客戶端傳送到Web服務(wù)器的數(shù)據(jù)有兩種，它們分別為用戶在表單中輸入的顯式數(shù)據(jù)，以及后臺(tái)的HTTP信息。兩種數(shù)據(jù)都很重要。HTTP信息包括cookie、瀏

77、覽器所能識(shí)別的媒體類型和壓縮模式等。</p><p><b>　　生成結(jié)果。</b></p><p>　　這個(gè)過(guò)程可能需要訪問(wèn)數(shù)據(jù)庫(kù)、執(zhí)行RMI或EJB調(diào)用、調(diào)用Web服務(wù)，或者直接計(jì)算得出對(duì)應(yīng)的響應(yīng)。實(shí)際的數(shù)據(jù)可能存儲(chǔ)在關(guān)系型數(shù)據(jù)庫(kù)中。該數(shù)據(jù)庫(kù)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(kù)進(jìn)行會(huì)話。即使它能夠做到這一點(diǎn)，為了安全

78、上的考慮，我們也不希望讓它這么做。對(duì)應(yīng)大多數(shù)其他應(yīng)用程序，也存在類似的問(wèn)題。因此，我們需要Web中間層從HTTP流中提取輸入數(shù)據(jù)，與應(yīng)用程序會(huì)話，并將結(jié)果嵌入到文檔中。</p><p>　　向客戶發(fā)送顯式數(shù)據(jù)（即文檔）。</p><p>　　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。但是，到目前為止

79、，HTML式最常用的格式，故而servelt和JSP的重要任務(wù)之一就式將結(jié)果包裝到HTML中。</p><p>　　發(fā)送隱式的HTTP響應(yīng)數(shù)據(jù)。</p><p>　　圖1.1中顯示了一條從Web中間層到客戶端的單箭頭。但是，實(shí)際發(fā)送的數(shù)據(jù)有兩種：文檔本身，以及后臺(tái)的HTTP信息。同樣，兩種數(shù)據(jù)對(duì)開(kāi)發(fā)來(lái)說(shuō)都式至關(guān)重要的。HTTP響應(yīng)數(shù)據(jù)的發(fā)送過(guò)程涉及告知瀏覽器或其他客戶程序所返回文檔的類型（

80、如HTML），設(shè)置cookie和緩存參數(shù)，以及其他類似的任務(wù)。</p><p>　　1.2動(dòng)態(tài)構(gòu)建網(wǎng)頁(yè)的原因</p><p>　　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(wú)需調(diào)用servlet就可以處理這些請(qǐng)求。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(duì)每個(gè)請(qǐng)求生成一個(gè)頁(yè)面。實(shí)時(shí)構(gòu)建頁(yè)面的理由有很多種：</p><p>　　1、網(wǎng)頁(yè)基于客戶發(fā)送的數(shù)據(jù)。

81、</p><p>　　例如，搜索引擎生成的頁(yè)面，以及在線商店的訂單確認(rèn)頁(yè)面，都要針對(duì)特定的用戶請(qǐng)求而產(chǎn)生。在沒(méi)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。要記住，用戶提交兩種類型的數(shù)據(jù)：顯示（即HTML表單的數(shù)據(jù)）和隱式（即HTTP請(qǐng)求的報(bào)頭）。兩種輸入都可用來(lái)構(gòu)建輸出頁(yè)面?；赾ookie值針對(duì)具體用戶構(gòu)建頁(yè)面的情況尤其普遍。</p><p>　　2、頁(yè)面由頻繁改變的數(shù)據(jù)導(dǎo)出。&

82、lt;/p><p>　　如果頁(yè)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)。但是，如果頁(yè)面周期性地改變，我們可以用兩種方式來(lái)處理它：周期性地在服務(wù)器上構(gòu)建新的頁(yè)面（和客戶請(qǐng)求無(wú)關(guān)），或者僅僅在用戶請(qǐng)求該頁(yè)面時(shí)再構(gòu)建。具體應(yīng)該采用哪種方式要根據(jù)具體情況而定，但后一種方式常常更為方便，因?yàn)樗恍韬?jiǎn)單地等待用戶的請(qǐng)求。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(yè)面，也有可能會(huì)返回之前構(gòu)建的頁(yè)面（如果它

83、還是最新的話）。</p><p>　　3、頁(yè)面中使用了來(lái)自公司數(shù)據(jù)庫(kù)或其他數(shù)據(jù)庫(kù)斷數(shù)據(jù)源的信息。</p><p>　　如果數(shù)據(jù)存儲(chǔ)在數(shù)據(jù)庫(kù)中，那么，即使客戶端使用動(dòng)態(tài)Web內(nèi)容，比如applet，我們依舊需要執(zhí)行服務(wù)器端處理。想象以下，如果一個(gè)搜索引擎網(wǎng)站完全使用applet，那么用戶將會(huì)看到：“正在下載50TB的applet，請(qǐng)等待！”。顯然，這樣很愚蠢；這種情況下，我們需要與數(shù)據(jù)庫(kù)進(jìn)行

84、會(huì)話。從客戶端到Web層再到數(shù)據(jù)庫(kù)（三層結(jié)構(gòu)），要比從applet直接到數(shù)據(jù)庫(kù)（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(méi)有。畢竟數(shù)據(jù)庫(kù)調(diào)用通常是對(duì)速度影響最大的步驟，因而，經(jīng)過(guò)中間層可以執(zhí)行高速緩存和連接共享。</p><p>　　理論上講，servelt并非只用于處理HTTP請(qǐng)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。例如，servlet能夠嵌入到FTP或郵件服務(wù)器中，擴(kuò)展他們的

85、功能。而且，用于會(huì)話啟動(dòng)協(xié)議服務(wù)器的servlet API最近已經(jīng)被標(biāo)準(zhǔn)化（參見(jiàn)http://jcp.org/en/jsr/detail?id=116）。但在實(shí)踐中，servelt的這種用法尚不流行，在此，我們只論述HTTP Servlet。</p><p>　　1.3 Servlet相對(duì)于“傳統(tǒng)”CGI的優(yōu)點(diǎn)</p><p>　　和傳統(tǒng)CGI及許多類CGI技術(shù)相比，Java servel

86、t效率更高、更易用、更強(qiáng)大、更容易移植、更安全、也更廉價(jià)。</p><p><b>　　1、效率</b></p><p>　　應(yīng)用傳統(tǒng)的CGI，針對(duì)每個(gè)HTTP請(qǐng)求都用啟動(dòng)一個(gè)新的進(jìn)程。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(kāi)銷會(huì)占用大部分執(zhí)行時(shí)間。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。

87、類似地，應(yīng)用傳統(tǒng)的CGI技術(shù)，如果存在對(duì)同一CGI程序的N個(gè)請(qǐng)求，那么CGI程序的代碼會(huì)載入內(nèi)存N次。同樣的情況，如果使用servlet則啟動(dòng)N個(gè)線程，單僅僅載入servlet類的單一副本。這種方式減少了服務(wù)器的內(nèi)存需求，通過(guò)實(shí)例化更少的對(duì)象從而節(jié)省了時(shí)間。最后，當(dāng)CGI程序結(jié)束對(duì)請(qǐng)求的處理之后，程序結(jié)束。這種方式難以緩存計(jì)算結(jié)果，保持?jǐn)?shù)據(jù)庫(kù)連接打開(kāi)，或是執(zhí)行依靠持續(xù)性數(shù)據(jù)的其他優(yōu)化。然而，servelt會(huì)一直停留在內(nèi)存中（即使請(qǐng)求處理

88、完畢），因而可以直接存儲(chǔ)客戶請(qǐng)求之間的任意復(fù)雜數(shù)據(jù)。</p><p><b>　　2、便利</b></p><p>　　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。而在CGI中，大部分工作都需要我們資金完成。另外，如果您已經(jīng)了解了Java編程語(yǔ)言，為什么還有學(xué)校Per

89、l呢？您已經(jīng)承認(rèn)應(yīng)用Java技術(shù)編寫的代碼要比Visual Basic，VBScript或C＋＋編寫的代碼更可靠，且更易重用，為什么還有倒退回去選擇那些語(yǔ)言來(lái)開(kāi)發(fā)服務(wù)器端的程序呢？</p><p><b>　　3、強(qiáng)大</b></p><p>　　Servlet支持常規(guī)CGI難以實(shí)現(xiàn)或根本不能實(shí)現(xiàn)的幾項(xiàng)功能。Servlet能夠直接于Web服務(wù)器對(duì)話，而常規(guī)的CGI程序

90、做不到這一點(diǎn)，至少在不使用服務(wù)器專有API的情況下是這樣。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。多個(gè)servelt還可以共享數(shù)據(jù)，從而易于實(shí)現(xiàn)數(shù)據(jù)庫(kù)連接共享和類似的資源共享優(yōu)化。Servelt還能維護(hù)請(qǐng)求之間的信息，使得諸如會(huì)話跟蹤和計(jì)算結(jié)果緩存等技術(shù)變得更為簡(jiǎn)單。</p><p><b>　　4、可移植性</b></p><p>

91、　　Servelt使用Java編程語(yǔ)言，并且遵循標(biāo)準(zhǔn)的API。所有主要的Web服務(wù)器。實(shí)際上都直接或通過(guò)插件支持servlet。因此。為Macromedia JRun編寫的servlet，可以不經(jīng)過(guò)任何修改地在Apache Tomcat，Microsoft Internet Information Server，IBM WebSphere 。iPlanet Enterprise Server。Oracle9i AS 或者StrNine

92、WebStar上運(yùn)行。他們是java2平臺(tái)企業(yè)版的一部分，所以對(duì)servlet的支持越來(lái)越普遍。</p><p>　　通過(guò)B/S（Browser/Server）結(jié)構(gòu)即瀏覽器和服務(wù)器結(jié)構(gòu)。它是隨著Internet技術(shù)的興起，對(duì)C/S結(jié)構(gòu)的一種變化或者改進(jìn)的結(jié)構(gòu)。在這種結(jié)構(gòu)下，用戶工作界面是通過(guò)WWW瀏覽器來(lái)實(shí)現(xiàn)，極少部分事務(wù)邏輯在前端（Browser）實(shí)現(xiàn)，但是主要事務(wù)邏輯在服務(wù)器端（Server）實(shí)現(xiàn)，形成所謂三

93、層3-tier結(jié)構(gòu)。這樣就大大簡(jiǎn)化了客戶端電腦載荷，減輕了系統(tǒng)維護(hù)與升級(jí)的成本和工作量，降低了用戶的總體成本（TCO）。以目前的技術(shù)看，局域網(wǎng)建立B/S結(jié)構(gòu)的網(wǎng)絡(luò)應(yīng)用，并通過(guò)Internet/Intranet模式下數(shù)據(jù)庫(kù)應(yīng)用，相對(duì)易于把握、成本也是較低的。它是一次性到位的開(kāi)發(fā)，能實(shí)現(xiàn)不同的人員，從不同的地點(diǎn)，以不同的接入方式（比如LAN, WAN, Internet/Intranet等）訪問(wèn)和操作共同的數(shù)據(jù)庫(kù)；它能有效地保護(hù)數(shù)據(jù)平臺(tái)和管

94、理訪問(wèn)權(quán)限，服務(wù)器數(shù)據(jù)庫(kù)也很安全 。目前我院內(nèi)網(wǎng)（Intranet）、外網(wǎng)（Internet）和北京東方清大公司“案件、辦公管理軟件”就是B/S 結(jié)構(gòu)管理軟件，干警在局域網(wǎng)各工作站通過(guò)WWW瀏覽器就能實(shí)現(xiàn)工作業(yè)務(wù)。特別是在JAVA這樣的跨平臺(tái)語(yǔ)言出現(xiàn)之后，B/S</p><p><b>　　解決方案：</b></p><p>　?。?）維護(hù)和升級(jí)方式簡(jiǎn)單。</p

95、><p>　　目前，軟件系統(tǒng)的改進(jìn)和升級(jí)越來(lái)越頻繁，B/S架構(gòu)的產(chǎn)品明顯體現(xiàn)著更為方便的特性。對(duì)一個(gè)稍微大一點(diǎn)單位來(lái)說(shuō)，系統(tǒng)管理人員如果需要在幾百甚至上千部電腦之間來(lái)回奔跑，效率和工作量是可想而知的，但B/S架構(gòu)的軟件只需要管理服務(wù)器就行了，所有的客戶端只是瀏覽器，根本不需要做任何的維護(hù)。無(wú)論用戶的規(guī)模有多大，有多少分支機(jī)構(gòu)都不會(huì)增加任何維護(hù)升級(jí)的工作量，所有的操作只需要針對(duì)服務(wù)器進(jìn)行；如果是異地，只需要把服務(wù)器連接

96、專網(wǎng)即可，實(shí)現(xiàn)遠(yuǎn)程維護(hù)、升級(jí)和共享。所以客戶機(jī)越來(lái)越“瘦”，而服務(wù)器越來(lái)越“胖”是將來(lái)信息化發(fā)展的主流方向。今后，軟件升級(jí)和維護(hù)會(huì)越來(lái)越容易，而使用起來(lái)會(huì)越來(lái)越簡(jiǎn)單，這對(duì)用戶人力、物力、時(shí)間、費(fèi)用的節(jié)省是顯而易見(jiàn)的，驚人的。因此，維護(hù)和升級(jí)革命的方式是“瘦”客戶機(jī)，“胖”服務(wù)器。</p><p>　　（2）成本降低，選擇更多。</p><p>　　大家都知道windows在桌面電腦上幾乎一

97、統(tǒng)天下，瀏覽器成為了標(biāo)準(zhǔn)配置，但在服務(wù)器操作系統(tǒng)上windows并不是處于絕對(duì)的統(tǒng)治地位。 現(xiàn)在的趨勢(shì)是凡使用B/S架構(gòu)的應(yīng)用管理軟件，只需安裝在Linux服務(wù)器上即可，而且安全性高。所以服務(wù)器操作系統(tǒng)的選擇是很多的，不管選用那種操作系統(tǒng)都可以讓大部分人使用windows作為桌面操作系統(tǒng)電腦不受影響，這就使的最流行免費(fèi)的Linux操作系統(tǒng)快速發(fā)展起來(lái)，Linux除了操作系統(tǒng)是免費(fèi)的以外，連數(shù)據(jù)庫(kù)也是免費(fèi)的，這種選擇非常盛行。</p

98、><p>　　比如說(shuō)很多人每天上“新浪”網(wǎng)，只要安裝了瀏覽器就可以了，并不需要了解“新浪”的服務(wù)器用的是什么操作系統(tǒng)，而事實(shí)上大部分網(wǎng)站確實(shí)沒(méi)有使用windows操作系統(tǒng)，但用戶的電腦本身安裝的大部分是windows操作系統(tǒng)。</p><p>　?。?） 應(yīng)用服務(wù)器運(yùn)行數(shù)據(jù)負(fù)荷較重。</p><p>　　由于B/S架構(gòu)管理軟件只安裝在服務(wù)器端（Server）上，網(wǎng)絡(luò)管理

99、人員只需要管理服務(wù)器就行了，用戶界面主要事務(wù)邏輯在服務(wù)器（Server）端完全通過(guò)WWW瀏覽器實(shí)現(xiàn)，極少部分事務(wù)邏輯在前端（Browser）實(shí)現(xiàn)，所有的客戶端只有瀏覽器，網(wǎng)絡(luò)管理人員只需要做硬件維護(hù)。但是，應(yīng)用服務(wù)器運(yùn)行數(shù)據(jù)負(fù)荷較重，一旦發(fā)生服務(wù)器“崩潰”等問(wèn)題，后果不堪設(shè)想。因此，許多單位都備有數(shù)據(jù)庫(kù)存儲(chǔ)服務(wù)器，以防萬(wàn)一。</p><p><b>　　5、廉價(jià)</b></p>

100、<p>　　對(duì)于開(kāi)發(fā)用的網(wǎng)站、低容量或中等容量網(wǎng)站的部署，有大量免費(fèi)或極為廉價(jià)的Web服務(wù)器可供選擇。因此，通過(guò)使用servelt和jsp，我們可以從免費(fèi)或廉價(jià)的服務(wù)器開(kāi)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。這與其他CGI方案形成鮮明的對(duì)比，這些CGI方案在初期都需要為購(gòu)買專利軟件包投入大量的資金。</p><p>　　價(jià)格和可移植性在某種程度上是相互關(guān)聯(lián)的。例如，