JSP技术概述与应用框架外文翻译(共21页).doc

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1、精选优质文档-倾情为你奉上外文原文Overview of JSP Technology and JSP application frameworksAutor: Zambon Giulio/ Sekler MichaelSource: Springer-Verlag New York Inc1 Benefits of JSP JSP pages are translated into servlets. So, fundamentally, any task JSP pages can perform could also be accomplished by servlets. Howeve

2、r, this underlying equivalence does not mean that servlets and JSP pages are equally appropriate in all scenarios. The issue is not the power of the technology, it is the convenience, productivity, and maintainability of one or the other. After all, anything you can do on a particular computer platf

3、orm in the Java programming language you could also do in assembly language. But it still matters which you choose.JSP provides the following benefits over servlets alone: It is easier to write and maintain the HTML. Your static code is ordinary HTML: no extra backslashes, no double quotes, and no l

4、urking Java syntax. You can use standard Web-site development tools. Even HTML tools that know nothing about JSP can be used because they simply ignore the JSP tags. You can divide up your development team. The Java programmers can work on the dynamic code. The Web developers can concentrate on the

5、presentation layer. On large projects, this division is very important. Depending on the size of your team and the complexity of your project, you can enforce a weaker or stronger separation between the static HTML and the dynamic content. Now, this discussion is not to say that you should stop usin

6、g servlets and use only JSP instead. By no means. Almost all projects will use both. For some requests in your project, you will use servlets. For others, you will use JSP. For still others, you will combine them with the MVC architecture . You want the appropriate tool for the job, and servlets, by

7、 themselves, do not complete your toolkit.2.Advantages of JSP Over Competing Technologies A number of years ago, Marty was invited to attend a small 20-person industry roundtable discussion on software technology. Sitting in the seat next to Marty was James Gosling, inventor of the Java programming

8、language. Sitting several seats away was a high-level manager from a very large software company in Redmond, Washington. During the discussion, the moderator brought up the subject of Jini, which at that time was a new Java technology. The moderator asked the manager what he thought of it, and the m

9、anager responded that it was too early to tell, but that it seemed to be an excellent idea. He went on to say that they would keep an eye on it, and if it seemed to be catching on, they would follow his companys usual embrace and extend strategy. At this point, Gosling lightheartedly interjected You

10、 mean disgrace and distend. Now, the grievance that Gosling was airing was that he felt that this company would take technology from other companies and suborn it for their own purposes. But guess what? The shoe is on the other foot here. The Java community did not invent the idea of designing pages

11、 as a mixture of static HTML and dynamic code marked with special tags. For example, ColdFusion did it years earlier. Even ASP (a product from the very software company of the aforementioned manager) popularized this approach before JSP came along and decided to jump on the bandwagon. In fact, JSP n

12、ot only adopted the general idea, it even used many of the same special tags as ASP did.So, the question becomes: why use JSP instead of one of these other technologies? Our first response is that we are not arguing that everyone should. Several of those other technologies are quite good and are rea

13、sonable options in some situations. In other situations, however, JSP is clearly better. Here are a few of the reasons.2.1 Versus .NET and Active Server Pages (ASP). NET is well-designed technology from Microsoft. ASP.NET is the part that directly competes with servlets and JSP. The advantages of JS

14、P are two fold.First, JSP is portable to multiple operating systems and Web servers; you arent locked into deploying on Windows and IIS. Although the core .NET platform runs on a few non-Windows platforms, the ASP part does not. You cannot expect to deploy serious ASP.NET applications on multiple se

15、rvers and operating systems. For some applications, this difference does not matter. For others, it matters greatly.Second, for some applications the choice of the underlying language matters greatly. For example, although .NETs C# language is very well designed and is similar to Java, fewer program

16、mers are familiar with either the core C# syntax or the many auxiliary libraries. In addition, many developers still use the original version of ASP. With this version, JSP has a clear advantage for the dynamic code. With JSP, the dynamic part is written in Java, not VBScript or another ASP-specific

17、 language, so JSP is more powerful and better suited to complex applications that require reusable components. You could make the same argument when comparing JSP to the previous version of ColdFusion; with JSP you can use Java for the real code and are not tied to a particular server product. Howev

18、er, the current release of ColdFusion is within the context of a J2EE server, allowing developers to easily mix ColdFusion and servlet/JSP code.2.2 Versus PHP PHP (a recursive acronym for PHP: Hypertext Preprocessor) is a free, open-source, HTML-embedded scripting language that is somewhat similar t

19、o both ASP and JSP. One advantage of JSP is that the dynamic part is written in Java, which already has an extensive API for networking, database access, distributed objects, and the like, whereas PHP requires learning an entirely new, less widely used language. A second advantage is that JSP is muc

20、h more widely supported by tool and server vendors than is PHP.2.3 Versus Pure Servlets JSP doesnt provide any capabilities that couldnt, in principle, be accomplished with servlets. In fact, JSP documents are automatically translated into servlets behind the scenes. But it is more convenient to wri

21、te (and to modify!) regular HTML than to use a zillion println statements to generate the HTML. Plus, by separating the presentation from the content, you can put different people on different tasks: your Web page design experts can build the HTML by using familiar tools and either leave places for

22、your servlet programmers to insert the dynamic content or invoke the dynamic content indirectly by means of XML tags.Does this mean that you can just learn JSP and forget about servlets? Absolutely not! JSP developers need to know servlets for four reasons: 1. JSP pages get translated into servlets.

23、 You cant understand how JSP works without understanding servlets. 2. JSP consists of static HTML, special-purpose JSP tags, and Java code. What kind of Java code? Servlet code! You cant write that code if you dont understand servlet programming. 3. Some tasks are better accomplished by servlets tha

24、n by JSP. JSP is good at generating pages that consist of large sections of fairly well structured HTML or other character data. Servlets are better for generating binary data, building pages with highly variable structure, and performing tasks (such as redirection) that involve little or no output.

25、 4. Some tasks are better accomplished by a combination of servlets and JSP than by either servlets or JSP alone. 2.4 Versus JavaScript JavaScript, which is completely distinct from the Java programming language, is normally used to dynamically generate HTML on the client, building parts of the Web

26、page as the browser loads the document. This is a useful capability and does not normally overlap with the capabilities of JSP (which runs only on the server). JSP pages still include SCRIPT tags for JavaScript, just as normal HTML pages do. In fact, JSP can even be used to dynamically generate the

27、JavaScript that will be sent to the client. So, JavaScript is not a competing technology; it is a complementary one. It is also possible to use JavaScript on the server, most notably on Sun ONE (formerly iPlanet), IIS, and BroadVision servers. However, Java is more powerful, flexible, reliable, and

28、portable.2.5 Versus WebMacro or Velocity JSP is by no means perfect. Many people have pointed out features that could be improved. This is a good thing, and one of the advantages of JSP is that the specification is controlled by a community that draws from many different companies. So, the technolog

29、y can incorporate improvements in successive releases. However, some groups have developed alternative Java-based technologies to try to address these deficiencies. This, in our judgment, is a mistake. Using a third-party tool like Apache Struts that augments JSP and servlet technology is a good ide

30、a when that tool adds sufficient benefit to compensate for the additional complexity. But using a nonstandard tool that tries to replace JSP is a bad idea. When choosing a technology, you need to weigh many factors: standardization, portability, integration, industry support, and technical features.

31、 The arguments for JSP alternatives have focused almost exclusively on the technical features part. But portability, standardization, and integration are also very important. For example, the servlet and JSP specifications define a standard directory structure for Web applications and provide standa

32、rd files (.war files) for deploying Web applications. All JSP-compatible servers must support these standards. Filters can be set up to apply to any number of servlets or JSP pages, but not to nonstandard resources. The same goes for Web application security settings. Besides, the tremendous industr

33、y support for JSP and servlet technology results in improvements that mitigate many of the criticisms of JSP. For example, the JSP Standard Tag Library and the JSP 2.0 expression language address two of the most well-founded criticisms: the lack of good iteration constructs and the difficulty of acc

34、essing dynamic results without using either explicit Java code or verbose jsp:useBean elements.3. Misconceptions About JSP Forgetting JSP Is Server-Side Technology Here are some typical questions Marty has received (most of them repeatedly). Our server is running JDK 1.4. So, how do I put a Swing co

35、mponent in a JSP page? How do I put an image into a JSP page? I do not know the proper Java I/O commands to read image files. Since Tomcat does not support JavaScript, how do I make images that are highlighted when the user moves the mouse over them? Our clients use older browsers that do not unders

36、tand JSP. What should we do? When our clients use View Source in a browser, how can I prevent them from seeing the JSP tags? All of these questions are based upon the assumption that browsers know something about the server-side process. But they do not. Thus: For putting applets with Swing componen

37、ts into Web pages, what matters is the browsers Java versionthe servers version is irrelevant. If the browser supports the Java 2 platform, you use the normal APPLET (or Java plug-in) tag and would do so even if you were using non-Java technology on the server. You do not need Java I/O to read image

38、 files; you just put the image in the directory for Web resources (i.e., two levels up from WEB-INF/classes) and output a normal IMG tag. You create images that change under the mouse by using client-side JavaScript, referenced with the SCRIPT tag; this does not change just because the server is usi

39、ng JSP. Browsers do not support JSP at allthey merely see the output of the JSP page. So, make sure your JSP outputs HTML compatible with the browser, just as you would do with static HTML pages. And, of course you need not do anything to prevent clients from seeing JSP tags; those tags are processe

40、d on the server and are not part of the output that is sent to the client.Confusing Translation Time with Request Time A JSP page is converted into a servlet. The servlet is compiled, loaded into the servers memory, initialized, and executed. But which step happens when? To answer that question, rem

41、ember two points: The JSP page is translated into a servlet and compiled only the first time it is accessed after having been modified. Loading into memory, initialization, and execution follow the normal rules for servlets. The most frequently misunderstood entries are highlighted. When referring t

42、o the table, note that servlets resulting from JSP pages use the _jspService method (called for both GET and POST requests), not doGet or doPost. Also, for initialization, they use the jspInit method, not the init method.JSP page translated into servlet Servlet compiled Servlet loaded into servers m

43、emory jspInit called _jspService called.4.What are application frameworks: A framework is a reusable, semi-complete application that can be specialized to produce custom applications Johnson. Like people, software applications are more alike than they are different. They run on the same computers, e

44、xpect input from the same devices, output to the same displays, and save data to the same hard disks. Developers working on conventional desktop applications are accustomed to toolkits and development environments that leverage the sameness between applications. Application frameworks build on this

45、common ground to provide developers with a reusable structure that can serve as the foundation for their own products. A framework provides developers with a set of backbone components that have the following characteristics:1.They are known to work well in other applications.2. They are ready to us

46、e with the next project.3. They can also be used by other teams in the organization. Frameworks are the classic build-versus-buy proposition. If you build it, you will understand it when you are donebut how long will it be before you can roll your own? If you buy it, you will have to climb the learn

47、ing curveand how long is that going to take? There is no right answer here, but most observers would agree that frameworks such as Struts provide a significant return on investment compared to starting from scratch, especially for larger projects.Other types of frameworks: The idea of a framework ap

48、plies not only to applications but to application componentsas well. Throughout this article, we introduce other types of frameworks that you can use with Struts. These include the Lucene search engine, the Scaffold toolkit, the Struts validator, and the Tiles tag library. Like application framework

49、s, these tools provide semi-complete versions of a subsystem that can be specialized to provide a custom component.Some frameworks have been linked to a proprietary development environment. This is not the case with Struts or any of the other frameworks shown in this book. You can use any development environment with Struts: Vi