Servlet和JSP技术简述外文文献翻译.doc

1、An Overview of Servlet and JSP TechnologyNagle ,WiegleyAbstract: Servlet program running in the server-side, 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 port

2、ability, more savings to invest. .Keywords: JSP Technology;Servlet;HTTP server1. A Servlets JobServlets are Java programs that run on Web or application servers, acting as a middle layer between requests coming from Web browsers or other HTTP clients and databases or applications on the HTTP server.

3、 Their job is to perform the following tasks, as illustrated in Figure 1-1.Figure 1-1 Web middleware role1.1 Read 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 HTTP client progra

4、m.1.2 Read the implicit HTTP 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

5、 HTTP information. Both varieties are critical. The HTTP information includes cookies, information about media types and compression schemes the browser understands, and so on.1.3 Generate the results.This process may require talking to a database, executing an RMI or EJB call, invoking a Web servic

6、e, or computing the response directly. Your real data may be in a relational database. Fine. But your database probably doesnt speak HTTP or return results in HTML, so the Web browser cant talk directly to the database. Even if it could, for security reasons, you probably would not want it to. The s

7、ame argument applies to most other applications.You need the Web middle layer to extract the results inside a document.1.4 Send 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 compres

8、sed 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.1.5 Send the implicit HTTP response data.Figure 1-1 shows a single arrow going from the Web middle layer (th

9、e servlet or JSP 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 data involves telling the browser or other client what type of do

10、cument is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks. 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 many cases, however, a static r

11、esult 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:2.1 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 ar

12、e specific to particular user requests. You dont 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., HTTP request headers). Either kind of input can be used to build the output

13、page. In particular, it is quite common to build a user-specific page based on a cookie value.2.2 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,

14、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

15、 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.2.3 The Web page uses information from corporate databases or other server-side sources.If the information is in a databas

16、e, 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. Going from the client to the Web t

17、ier 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 going through the Web server does n

18、ot 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 HTTP requests but can be used for other types of servers as well. For example, se

19、rvlets 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 http:/jcp.org/en/jsr/detail?id=116). In practice, however, this use of servlets has not caught on, and well only be discussin

20、g HTTP servlets.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.3.1 EfficientWith traditional CGI, a new process is started for each HTTP

21、 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 system process. Similarly, in trad

22、itional 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 requirements and saves time by

23、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, however, remain in memory even

24、 after they complete a response, so it is straightforward to store arbitrarily complex data between client requests.3.2 ConvenientServlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, an

25、d 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? Youre already convinced that Java technology makes for more reliable and reusable code than does Visual Basic, VBScript, or C+. Why go

26、 back to those languages for server-side programming?3.3 PowerfulServlets 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 using a server-specific API.

27、 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 maintain information from

28、 request to request, simplifying techniques like session tracking and caching of previous computations.3.4 PortableServlets 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, servle

29、ts 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 (J2EE; see so indust

30、ry support for servlets is becoming even more pervasive.3.5 InexpensiveA 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 inexpensive server and migrate to more expen

31、sive 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 proprietary package.Price and portabilit

32、y 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 behind the U.S. Marty also taught one of his JSP and servlet training courses (see in Manila, and there was great interest in se

33、rvlet 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 (or had, at that time) highly unfavorable currency exchange rates

34、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, embedded in the regular Apache Web server, or embedded in Microsoft IIS

35、). 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. If their project becomes even larger, they might want to move to a

36、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 becomes quite large and complex, they might want to use Enterprise Ja

37、vaBeans (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 it off of their Linux box and onto an IBM mainframe running IBM Web

38、Sphere. But once again, none of their servlets or JSP pages have to be rewritten.3.6 SecureOne 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 filt

39、er 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 used CGI libraries.A second source of problems is the fact that

40、 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, which is really some random part of program memory. So, programmers who forget t

41、o 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 invoke a program on the local operating system, it does not use a shell to do so.

42、 And, of course, array bounds checking and other memory protection features are a central part of the Java programming language.3.7 MainstreamThere are a lot of good technologies out there. But if vendors dont support them and developers dont know how to use them, what good are they? Servlet and JSP

43、 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. Several low-cost plugins add support to Microsoft IIS and Zeus as well. They run on Windo

44、ws, 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 Web applications. They are used by the airline industry (most United Airlines and D

45、elta Airlines Web sites), e-commerce (), online banking (First USA Bank, Banco Popular de Puerto Rico), Web search engines/portals (), 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

46、. 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.References：1Clifton G.M. Branham, Arthur Jonathan .Servlets and JSP in an undergraduate database courseJ.Proceedings of the International Confere

47、nce on Parallel and Distributed Processing Techniques and Applications,2003(3):1490-1496.2Kirkegaard, Christian.Static analysis for Java servlets and JSPJ.Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics),2006(4):336-

48、352.3Nakaike,Takuya.JSP Splitting for improving execution performanceJ.Proceedings - International Symposium on Applications and the Internet,20048:117-126.4Hassan, Doaa .Developing a security typed java servletJ.Proceedings - The 4th International Symposium on Information Assurance and Security,200

49、8(10):215-220.Servlet和JSP技术简述Nagle ，Wiegley摘要：Servlet程序在服务器端运行，动态地生成Web页面与传统的CGI和许多其他类似CGI的技术相比，Java Servlet具有更高的效率，更容易使用，功能更强大，具有更好的可移植性，更节省投资。关键字：JSP技术；Servlet；HTTP服务1Servlet的功能Servlet是运行在Web或应用服务器上的Java程序，它是一个中间层，负责连接来自Web浏览器或其他HTTP客户程序的请求和HTTP服务器上的数据库或应用程序。Servlet的工作是执行西门的任务，如图1.1所示 。图1.1Web中间件的作用1.1读取客户发送的显式数据最终用户一般在页面的