1、 畢業設計英文資料翻譯TOPIC An Overview of Servlet and JSP Technology 題 目 Servlet和JSP技術簡述 作 者 相 鳳 珍 屆 別 2009 屆 系 別 計算機與信息工程系 專 業 通信工程 指導教師 胡 虛 懷 職 稱 教 授 完成時間 2009年5月20日 An Overview of Servlet and JSP TechnologyGildas Avoine and Philippe OechslinEPFL, Lausanne, Switzerland1.1 A Servlet's JobServlets are Jav

2、a programs that run on Web or application servers, acting as a middle layer between requests coming from Web browsers or other clients and databases or applications on the server. Their job is to perform the following tasks, as illustrated in Figure 1-1.Figure 1-11Read the explicit data sent by the

3、client.The end user normally enters this data in an HTML form on a Web page. However, the data could also come from an applet or a custom client program.2Read the implicit request data sent by the browser.Figure 1-1 shows a single arrow going from the client to the Web server (the layer where servle

4、ts 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 information. Both varieties are critical. The information includes cookies, information about media types and compression schemes the browser understands, a

5、nd so on.3Generate the results.This process may require talking 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 or return results in HTML, so th

6、e 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 Web middle layer to extract the incoming data from the stream, talk to the application, and embed the r

7、esults inside a document.4Send the explicit data (i.e., 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 compressed format like gzip that is layered on top of some other underlying format. But, HTML is by far

8、the most common format, so an important servlet/JSP task is to wrap the results inside of HTML.5Send the implicit response data.Figure 1-1 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 itse

9、lf and the behind-the-scenes information. Again, both varieties are critical to effective development. Sending response 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. 1.2 Why Build

10、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

11、 pages need to be built on-the-fly:1 The Web page is based on data sent by the client.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 su

12、bmits. Just remember that the user submits two kinds of data: explicit (i.e., HTML form data) and implicit (i.e., request headers). Either kind of input can be used to build the output page. In particular, it is quite common to build a user-specific page based on a cookie value.2The Web page is deri

13、ved 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)

14、, 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 returni

15、ng a previously built page if that page is still up to date.3The Web page uses information from corporate databases or other server-side sources.If the information is in a database, you need server-side processing even if the client is using dynamic Web content such as an applet. Imagine using an ap

16、plet by itself for a search engine site:"Downloading 50 terabyte applet, please wait!" Obviously, that is silly; you 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 appr

17、oach) provides increased flexibility and security 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 often faster because the middle tier can perform cachi

18、ng and connection pooling.In principle, servlets are not restricted to Web or application servers that handle 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

19、 Initiation Protocol) servers was recently standardized (see ://en/jsr/detail?id=116). In practice, however, this use of servlets has not caught on, and we'll only be discussing servlets.1.3 The Advantages of Servlets Over "Traditional" CGIJava servlets are more efficient, easie

20、r to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies.1EfficientWith traditional CGI, a new process is started for each request. If the CGI program itself is relatively short, the overhead of starting the process can dominate the e

21、xecution 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

22、 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 program terminates. Th

23、is approach makes it difficult to cache computations, keep database connections open, and perform other optimizations that rely on persistent data. Servlets, however, remain in memory even after they complete a response, so it is straightforward to store arbitrarily complex data between client reque

24、sts.2ConvenientServlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting headers, handling cookies, tracking sessions, and many other such high-level utilities. In CGI, you have to do much of this yourself. Besides, if you already know the J

25、ava programming language, why learn Perl too? You're already convinced that Java technology makes for more reliable and reusable code than does Visual Basic, VBScript, or C+. Why go back to those languages for server-side programming?3PowerfulServlets support several capabilities that are diffic

26、ult 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 Web server makes it easier to translate relative URLs into concrete path names, for instance.

27、Multiple servlets can also share data, making it easy to implement database connection pooling and similar resource-sharing optimizations. Servlets can also maintain information from request to request, simplifying techniques like session tracking and caching of previous computations.4PortableServle

28、ts 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 Internet Information Server

29、(with a separate plugin), IBM WebSphere, iPlanet Enterprise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition (J2EE; see :/java.sun /j2ee/), so industry support for servlets is becoming even more pervasive.5InexpensiveA number of free or very inexpens

30、ive Web servers are good for development use or deployment of low- or medium-volume Web sites. Thus, with servlets and 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 pro

31、ject meets initial success. This is in contrast to many of the other CGI alternatives, which require a significant initial investment for 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 h

32、im 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 :/courses.coreservlets /) in Manila, and there was great interest in servlet and JSP technology there.Now, why are India and the Philippines both

33、so interested? We surmise that the answer is twofold. 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-purpose Web server from a U.S

34、. company consumed a large part of early project funds.But, with servlets and JSP, they could start with a free server: Apache Tomcat (either standalone, embedded in the regular Apache Web server, or embedded in Microsoft IIS). Once the project starts to become successful, they could move to a serve

35、r like Caucho Resin that had higher performance and easier administration but that is not 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) environment. No problem: they could move to Macrome

36、dia JRun Professional, which supports distributed applications (Web farms). Again, none of their servlets or JSP pages have to be rewritten. If the project becomes quite large and complex, they might want to use Enterprise JavaBeans (EJB) to encapsulate their business logic. So, they might switch to

37、 BEA WebLogic or Oracle9i AS. Again, none of their servlets or JSP pages have to be rewritten. Finally, if their project becomes even bigger, they might move it off of their Linux box and onto an IBM mainframe running IBM WebSphere. But once again, none of their servlets or JSP pages have to be rewr

38、itten.6SecureOne 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 careful to filter out characters such as backquotes and semicolons that are treated specially

39、 by the shell. Implementing this precaution is harder than one might think, and weaknesses stemming from this problem are constantly being uncovered in widely used CGI libraries.A second source of problems is the fact that some CGI programs are processed by languages that do not automatically check

40、array or string bounds. For example, 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 check open up their system to deliberate or acciden

41、tal buffer overflow attacks.Servlets suffer from neither of these problems. Even if a servlet executes a system call (e.g., with Runtime.exec or JNI) 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

42、 features are a central part of the Java programming language.7MainstreamThere 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 JSP technology is supported by servers from Apache, Oracle, IB

43、M, Sybase, BEA, Macromedia, Caucho, Sun/iPlanet, New Atlanta, ATG, Fujitsu, Lutris, Silverstream, the World Wide Web Consortium (W3C), and many others. Several low-cost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM mainframe operating sys

44、tems. 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), e-commerce (ofoto ), online banki

45、ng (First USA Bank, Banco Popular de Puerto Rico), Web search engines/portals (excite ), 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 counter-examples abound. But our po

46、int is that you are not experimenting with a new and unproven technology when you work with server-side Java.Servlet和JSP技術簡述Gildas Avoine and Philippe OechslinEPFL, Lausanne, Switzerland1.1 Servlet的功能Servlets是運行在Web或應用服務器上的Java程序，它是一個中間層，負責連接來自Web瀏覽器或其他 客戶程序的請求和 服務器上的數據庫或應用程序。Servlet的工作是執行西門的任務，如圖所示

47、 。終端客戶Web 服務器數據庫Legcay 應用程序Java 應用程序Web 服務器圖中間件的作用（1） 讀取客戶發送的顯式數據。最終用戶一般在頁面的HTML表單中輸入這些數據。然而，數據還有可能來自applet或定制的 客戶程序。（2） 讀取由瀏覽器發送的隱式請求數據。圖中顯示了一條從客戶端到Web服務器的單箭頭，但實際上從客戶端傳送到Web服務器的數據有兩種，它們分別為用戶在表單中輸入的顯式數據，以及后臺的 信息。兩種數據都很重要。 信息包括cookie、瀏覽器所能識別的媒體類型和壓縮模式等。（3） 生成結果。這個過程可能需要訪問數據庫、執行RMI或EJB調用、調用Web服務，或者直接計

48、算得出對應的響應。實際的數據可能存儲在關系型數據庫中。該數據庫可能不理解 ，或者不能返回HTML形式的結果，所有Web瀏覽器不能直接與數據庫進行會話。即使它能夠做到這一點，為了安全上的考慮，我們也不希望讓它這么做。對應大多數其他應用程序，也存在類似的問題。因此，我們需要Web中間層從 流中提取輸入數據，與應用程序會話，并將結果嵌入到文檔中。（4） 向客戶發送顯式數據（即文檔）。這個文檔可以用各種格式發送，包括文本（HTML或XML），二進制（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，HTML式最常用的格式，故而servelt和JSP的重要任務之一就式

49、將結果包裝到HTML中。（5） 發送隱式的 響應數據。圖中顯示了一條從Web中間層到客戶端的單箭頭。但是，實際發送的數據有兩種：文檔本身，以及后臺的 信息。同樣，兩種數據對開發來說都式至關重要的。 響應數據的發送過程涉及告知瀏覽器或其他客戶程序所返回文檔的類型（如HTML），設置cookie和緩存參數，以及其他類似的任務。1.2 動態構建網頁的原因預先建立的文檔可以滿足客戶的許多請求，服務器無需調用servlet就可以處理這些請求。然而，許多情況下靜態的結果不能滿足要求，我們需要針對每個請求生成一個頁面。實時構建頁面的理由有很多種：1、網頁基于客戶發送的數據。例如，搜索引擎生成的頁面，以及在線

50、商店的訂單確認頁面，都要針對特定的用戶請求而產生。在沒有讀取到用戶提交的數據之前，我們不知道應該顯示什么。要記住，用戶提交兩種類型的數據：顯示（即HTML表單的數據）和隱式（即 請求的報頭）。兩種輸入都可用來構建輸出頁面。基于cookie值針對具體用戶構建頁面的情況尤其普遍。2、頁面由頻繁改變的數據導出。如果頁面需要根據每個具體的請求做出相應的改變，當然需要在請求發生時構建響應。但是，如果頁面周期性地改變，我們可以用兩種方式來處理它：周期性地在服務器上構建新的頁面（和客戶請求無關），或者僅僅在用戶請求該頁面時再構建。具體應該采用哪種方式要根據具體情況而定，但后一種方式常常更為方便，因為它只需簡

51、單地等待用戶的請求。例如，天氣預報或新聞網站可能會動態地構建頁面，也有可能會返回之前構建的頁面（如果它還是最新的話）。3、頁面中使用了來自公司數據庫或其他數據庫斷數據源的信息。如果數據存儲在數據庫中，那么，即使客戶端使用動態Web內容，比如applet，我們依舊需要執行服務器端處理。想象以下，如果一個搜索引擎網站完全使用applet，那么用戶將會看到：“正在下載50TB的applet，請等待！”。顯然，這樣很愚蠢；這種情況下，我們需要與數據庫進行會話。從客戶端到Web層再到數據庫（三層結構），要比從applet直接到數據庫（二層結構）更靈活，也更安全，而性能上的損失很少甚至沒有。畢竟數據庫調用

52、通常是對速度影響最大的步驟，因而，經過中間層可以執行高速緩存和連接共享。理論上講，servelt并非只用于處理 請求的Web服務器或應用服務器，它同樣可以用于其他類型的服務器。例如，servlet能夠嵌入到FTP或郵件服務器中，擴展他們的功能。而且，用于會話啟動協議服務器的servlet API最近已經被標準化（參見）。但在實踐中，servelt的這種用法尚不流行，在此，我們只論述 Servlet。 Servlet相對于“傳統”CGI的優點和傳統CGI及許多類CGI技術相比，Java servelt效率更高、更易用、更強大、更容易移植、更安全、也更廉價。1、效率 應用傳統的CGI，針對每個 請

53、求都用啟動一個新的進程。如果CGI程序自身相對比較簡短，那么啟動進程的開銷會占用大部分執行時間。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進程。類似地，應用傳統的CGI技術，如果存在對同一CGI程序的N個請求，那么CGI程序的代碼會載入內存N次。同樣的情況，如果使用servlet則啟動N個線程，單僅僅載入servlet類的單一副本。這種方式減少了服務器的內存需求，通過實例化更少的對象從而節省了時間。最后，當CGI程序結束對請求的處理之后，程序結束。這種方式難以緩存計算結果，保持數據庫連接打開，或是執行依靠持續性數據的其他優化。然而

54、，servelt會一直停留在內存中（即使請求處理完畢），因而可以直接存儲客戶請求之間的任意復雜數據。2、便利Servelt提供大量的基礎構造，可以自動分析和解碼HTML的表單數據，讀取和設置 報頭，處理cookie，跟蹤會話，以及其他次類高級功能。而在CGI中，大部分工作都需要我們資金完成。另外，如果您已經了解了Java編程語言，為什么還有學校Perl呢？您已經承認應用Java技術編寫的代碼要比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言來開發服務器端的程序呢？3、強大 Servlet支持常規CGI難以實現或根本不能實現的幾項功能。Servlet能夠直接于Web服務器對話，而常規的CGI程序做不到這一點，至少在不使用服務器專有API的情況下是這樣。例如，與Web服務器的通信使得講相對URL轉換成具體的路徑名變得更為容易。多個servelt還可以共享數據，從而易于實現數據庫連接共享和類似的資源共享優化。Servelt還能維護請求之間的信息，使得諸如會話跟蹤和計算結果緩存等技術變得更為簡單。4、可移植性Servelt使用Java編程語言，并且遵循標準的API。所有主要的Web服務器。實際上都直接或通過插件支持ser