Servlet和JSP技術(shù)簡述(英文翻譯)

1、An Overview of Servlet and JSP TechnologyGildas Avoine and Philippe OechslinEPFL, Lausanne, Switzerland1.1 A Servlet's JobServlets are Java 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 applic

2、ations 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 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

3、 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 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 information. B

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

5、 directly. Your real data may be in a relational database. Fine. But your database probably doesn't speak or return 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

6、 most other applications. You need the Web middle layer to extract the incoming data from the stream, talk to the application, and embed the results 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

7、 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 servlet/JSP task is to wrap the results inside of HTML.5Send the implicit response data.Figure 1-1 shows a single arro

8、w 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 behind-the-scenes information. Again, both varieties are critical to effective development. Sending response data involves telling the browser or o

9、ther client what type of document is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks. 1.2 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 ma

10、ny 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 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-confirmatio

11、n 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. 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 b

12、e 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 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

13、 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

14、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.3The Web page uses information from corporate databases or other server-side sources.If the inform

15、ation 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 applet by itself for a search engine site:"Downloading 50 terabyte applet, please wait!" Obviously, that is silly; you need to talk to the database. G

16、oing 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 security with little or no performance penalty. After all, the database call is usually the rate-limiting step, so go

17、ing through the Web server does not slow things down. In fact, a three-tier 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 requests but can be used for other types of serv

18、ers 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 ://en/jsr/detail?id=116). In practice, however, this use of servlets has not caught on, an

19、d we'll only be discussing servlets.1.3 The Advantages of Servlets Over "Traditional" CGIJava servlets are more efficient, easier to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies.1EfficientWith traditional CGI, a n

20、ew process is started for each 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 lightweight Java thread, not a heavyweight operating syst

21、em 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, however, there would be N threads, but only a single copy of the servlet class would be loaded. This approach reduces server memory r

22、equirements and saves time by instantiating fewer objects. Finally, when a CGI program finishes handling a request, the program terminates. This approach makes it difficult to cache computations, keep database connections open, and perform other optimizations that rely on persistent data. Servlets,

23、however, remain in memory even after they complete a response, so it is straightforward to store arbitrarily complex data between client requests.2ConvenientServlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting headers, handling cookies,

24、 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 Java programming language, why learn Perl too? You're already convinced that Java technology makes for more reliable and reusable code than does Visual Bas

25、ic, VBScript, or C+. Why go back to those languages for server-side programming?3PowerfulServlets support several capabilities 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 usin

26、g a server-specific API. Communicating with the 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 optimizations. Servlets can also m

27、aintain information from request to request, simplifying techniques like session tracking and caching of previous computations.4PortableServlets 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.

28、 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 Enterprise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition

29、 (J2EE; see :/java.sun /j2ee/), so industry support for servlets is becoming even more pervasive.5InexpensiveA 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 and JSP you can start with a free or inexpe

30、nsive server and migrate to more expensive servers with high-performance 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 the purchase of a pr

31、oprietary package.Price and portability are somewhat connected. For example, Marty tries 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 :/co

32、urses.coreservlets /) in Manila, and there was great interest in servlet and JSP technology there.Now, why are India and the Philippines both so interested? We surmise that the answer is twofold. First, both countries have large pools of well-educated software developers. Second, both countries have

33、 (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. 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, embe

34、dded in the 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 not free. But none of their servlets or JSP pages have to be rewritten.

35、If their project becomes even larger, they might want to move to a distributed (clustered) environment. No problem: they could move to Macromedia JRun Professional, which supports distributed applications (Web farms). Again, none of their servlets or JSP pages have to be rewritten. If the project be

36、comes quite large and complex, they might want to use Enterprise JavaBeans (EJB) to encapsulate their business logic. So, they might switch to 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

37、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 rewritten.6SecureOne of the main sources of vulnerabilities in traditional CGI stems from the fact that the programs are often executed by general-purpose operati

38、ng system shells. So, the CGI programmer must be careful 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 being uncovered in widely

39、 used CGI libraries.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 example, in C and C+ it is perfectly legal to allocate a 100-element array and then write into the 999th "element," whic

40、h is really some random part of program memory. So, programmers who forget to perform this check open up their system to deliberate or accidental 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 invok

41、e 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.7MainstreamThere are a lot of good technologies out there. But if vendors don't support them

42、 and developers don't know how to use them, what good are they? Servlet and 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

43、l low-cost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM mainframe operating systems. They are the single most popular application of the Java programming language. They are arguably the most popular choice for developing medium to large

44、Web applications. They are used by the airline industry (most United Airlines and Delta Airlines Web sites), e-commerce (ofoto ), online banking (First USA Bank, Banco Popular de Puerto Rico), Web search engines/portals (excite ), large financial sites (American Century Investments), and hundreds of

45、 other sites that you visit every day.Of course, popularity alone is no proof of good technology. Numerous counter-examples abound. But our point is that you are not experimenting with a new and unproven technology when you work with server-side Java.Servlet和JSP技術(shù)簡述Gildas Avoine and Philippe Oechsli

46、nEPFL, Lausanne, Switzerland1.1 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 客戶程序的請求和 服務(wù)器上的數(shù)據(jù)庫或應(yīng)用程序。Servlet的工作是執(zhí)行西門的任務(wù)，如圖所示 。圖中間件的作用（1） 讀取客戶發(fā)送的顯式數(shù)據(jù)。最終用戶一般在頁面的HTML表單中輸入這些數(shù)據(jù)。然而，數(shù)據(jù)還有可能來自applet或定制的 客戶程序。（2） 讀取由瀏覽器發(fā)送的隱式請求數(shù)據(jù)。圖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

47、式數(shù)據(jù)，以及后臺的 信息。兩種數(shù)據(jù)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(jié)果。這個(gè)過程可能需要訪問數(shù)據(jù)庫、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)庫中。該數(shù)據(jù)庫可能不理解 ，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(huì)話。即使它能夠做到這一點(diǎn)，為了安全上的考慮，我們也不希望讓它這么做。對應(yīng)大多數(shù)其他應(yīng)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(shù)據(jù)，與應(yīng)用程序會(huì)話，并將結(jié)果嵌入到文檔中。（4） 向客戶發(fā)送顯式數(shù)據(jù)（即文檔）。這個(gè)文檔可以用各種

48、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，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。（5） 發(fā)送隱式的 響應(yīng)數(shù)據(jù)。圖中顯示了一條從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，以及后臺的 信息。同樣，兩種數(shù)據(jù)對開發(fā)來說都式至關(guān)重要的。 響應(yīng)數(shù)據(jù)的發(fā)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(shè)置cookie和緩存參數(shù)，以及其他類似的任務(wù)。1.2 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求，服務(wù)器無需

49、調(diào)用servlet就可以處理這些請求。然而，許多情況下靜態(tài)的結(jié)果不能滿足要求，我們需要針對每個(gè)請求生成一個(gè)頁面。實(shí)時(shí)構(gòu)建頁面的理由有很多種：1、網(wǎng)頁基于客戶發(fā)送的數(shù)據(jù)。例如，搜索引擎生成的頁面，以及在線商店的訂單確認(rèn)頁面，都要針對特定的用戶請求而產(chǎn)生。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。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 請求的報(bào)頭）。兩種輸入都可用來構(gòu)建輸出頁面?；赾ookie值針對具體用戶構(gòu)建頁面的情況尤其普遍。2、頁面由頻繁改變的數(shù)據(jù)導(dǎo)出。如果頁面需要根據(jù)每個(gè)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(shí)構(gòu)建響應(yīng)。但是，如果頁面周

50、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(wù)器上構(gòu)建新的頁面（和客戶請求無關(guān)），或者僅僅在用戶請求該頁面時(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恍韬唵蔚氐却脩舻恼埱?。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)建頁面，也有可能會(huì)返回之前構(gòu)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(shù)據(jù)庫或其他數(shù)據(jù)庫斷數(shù)據(jù)源的信息。如果數(shù)據(jù)存儲(chǔ)在數(shù)據(jù)庫中，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，請等待！”

51、。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。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。畢竟數(shù)據(jù)庫調(diào)用通常是對速度影響最大的步驟，因而，經(jīng)過中間層可以執(zhí)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(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）。但在實(shí)踐中，servelt的這種用法尚不流行，在此，我們只論述 Servle

52、t。 Servlet相對于“傳統(tǒng)”CGI的優(yōu)點(diǎn)和傳統(tǒng)CGI及許多類CGI技術(shù)相比，Java servelt效率更高、更易用、更強(qiáng)大、更容易移植、更安全、也更廉價(jià)。1、效率 應(yīng)用傳統(tǒng)的CGI，針對每個(gè) 請求都用啟動(dòng)一個(gè)新的進(jìn)程。如果CGI程序自身相對比較簡短，那么啟動(dòng)進(jìn)程的開銷會(huì)占用大部分執(zhí)行時(shí)間。而使用servelt，Java虛擬機(jī)會(huì)一直運(yùn)行，并用輕量級的Java線程處理每個(gè)請求，而非重量級的操作系統(tǒng)進(jìn)程。類似地，應(yīng)用傳統(tǒng)的CGI技術(shù)，如果存在對同一CGI程序的N個(gè)請求，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。

53、這種方式減少了服務(wù)器的內(nèi)存需求，通過實(shí)例化更少的對象從而節(jié)省了時(shí)間。最后，當(dāng)CGI程序結(jié)束對請求的處理之后，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)庫連接打開，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（即使請求處理完畢），因而可以直接存儲(chǔ)客戶請求之間的任意復(fù)雜數(shù)據(jù)。2、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 報(bào)頭，處理cookie，跟蹤會(huì)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(jīng)了解了Java編程語言，為什么還有學(xué)校Perl呢？您已經(jīng)承認(rèn)應(yīng)用Java技術(shù)編寫的代碼要比

54、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(fā)服務(wù)器端的程序呢？3、強(qiáng)大 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)器對話，而常規(guī)的CGI程序做不到這一點(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)庫連接共享和類似的資源共享優(yōu)化。Servelt還能維護(hù)請求之間的信息，使得諸如會(huì)話跟蹤和計(jì)算結(jié)果緩存等技術(shù)變得更為簡單。4、可移植性Servelt使用Java編程語言，并且遵循標(biāo)準(zhǔn)的API。所有主要的Web服務(wù)器。實(shí)際上都直接或通過插件支持servlet。因此。為Macromedia