JSP+MySQL网上教学系统的设计与实现
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附录A 英文原文
JSP technology -- friend or foe?
A bit of history
Before diving into an explanation of presentation technology, it's helpful to fill in some details on the situation that led to the birth of the technology. Just 10 short years ago, the term thin client was a novelty. We still lived in a world of desktop applications, powered by wimpy 286 microprocessors with 14-inch monitors that we squinted at. Boy, have times changed! Now my desktop does nothing but power a Web browser, while servers from Sun, IBM, HP, Compaq, and the rest churn out computations, business logic, and content. And that little monitor? Replaced by flat-screen, plasma, whopping 21- and 25-inch beauties. Why? So we can see the intricate and complex HTML displays that serve as a front-end to these powerful applications. No suffice; now we expect flashy graphics, moving images, color-coordinated presentations that would look good in any room in the house, and speedy rendering to boot.
The premise
Today, a decade beyond those fledgling Windows applications, we are still dealing with this huge shift in the presentation paradigm. The woeful Visual Basic and C programmers who remain now find themselves working either on back-end systems or Windows-only applications, or they have added a Web-capable language such as the Java language to their toolbox. An application that doesn't support at least three of four ML-isms -- such as HTML, XML, and WML -- is considered shabby, if not an outright failure. And, of course, that means we all care very deeply about the ability to easily develop a Web presentation layer.
As it turns out, using the new Internet, and all the languages we have at our disposal -- Java, C, Perl, Pascal, and Ada, among others -- hasn't been as easy as we might have hoped. A number of issues creep up when it comes to taking the programming languages everyone used for back-end systems and leveraging them to generate markup language suitable for a client. With the arrival of more options on the browser (DHTML and JavaScript coding, for example), the increase in graphic artist talent in the Web domain, and tools that could create complex interfaces using standard HTML, the demand for fancy user interfaces has grown faster than our ability to develop these front ends to our applications. And this has given rise to presentation technology.
Presentation technology was designed to perform a single task: convert content, namely data without display details, into presentation -- meaning the various user interfaces you see on your phone, PalmPilot, or Web browser. What are the problems that these presentation technologies claimed to solve? Let's take a look.
Work vs. rework
Besides the separation of content and presentation, another measure of a presentation technology's usefulness is the amount of rework that it eliminates. The divergence of presentation and content enforces a divergence in the roles of those developing the content. A programmer can focus on the raw content presented in the examples above, and a graphic artist or webmaster can attend to the presentation. A slight overlap of roles remains, however, in the process of taking the presentation -- or markup -- designed by the artist and applying it to the content the programmer's code delivers.
In the simplest case, the artist su into the presentation technology. The application is "started up," and the content magically becomes a user interface. Of course, as we all know, development rarely ends there. Next come revisions and changes to the interface and new business rules that must be coded. This is where the true test of the presentation technology's flexibility comes into play. While it is usually simple to update the raw content being fed into the presentation layer, rarely can the graphic artists easily edit their original work. Changes to the presentation layer are common (we've all been victim to marketing departments changing this or that). So now a problem arises: what do the designers change to tweak their work? The original markup language page they gave to the developer? Probably not, as that page has most likely had custom tags or code inserted (JSP pages, template engines), converted to a Java servlet, or changed into something totally unrecognizable.
Often the designer must rework the original page and resubmit this page to the developer. Then the developer has to reconvert this page to the specific format needed for use in the presentation technology. Alternatively, the designer has to learn a scripting language or at least know that which areas of the page's source code from the developer are off limits. Of course, this is an error-prone, dangerous way to operate. Once you've determined that a presentation technology allows a clean split between content and presentation, you should try to ensure that a minimum amount of rework is necessary in order to make presentation changes.
The promise of JSP technology
Now, on to the specifics of JSP coding. The promise of JSP technology is to supply the designer and developer the only presentation technology they will ever need. JSP technology is part of the J2EE platform, which is the strongest show of support Sun can give one of its Java products. To a search on 'JSP' at amazon.com; you'll find more books devoted to JSP technology than about almost any other single Java API. Before I dive into the specific problems that JSP technology presents, you need a clear understanding of what it claims to do.
Content vs. presentation
Above all, JSP technology is about separating content from presentation, foremost in Sun's published set of goals for JSP pages. In fact, JSP design stemmed directly from the complaints of developers who were tired of typing out.println("<HTML><HEAD><TITLE>" + pageInfo.getTitle() + "</TITLE></HEAD>"); into their servlet code. This mixing of hard-coded content with runtime variables presented a horrible burden on servlet developers. It also made making even minor changes to the presentation layer difficult for the developer.
JSP technology addresses this situation by allowing normal HTML pages (and later, WML or other markup language pages) to be compiled at runtime into a Java servlet, essentially mimicking the out.println() paradigm, without requiring the developer to write this code. And it allows you to insert variables into the page that are not interpreted until runtime.
Code vs. markup
Second on the JSP technology's list of features is something that might raise a bit of concern. JSP coding lets you insert Java code directly into a page of markup. To understand why this decision was made, recall that when the JSP specification was being developed, Sun's competition from Microsoft was at an all-time high, primarily due to the success of Microsoft Active Server Pages (ASP). The similarity of the name JavaServer Pages to Active Server Pages was not merely coincidental. And the ability to mimic many of ASP's features was also 需要完整内容的请联系QQ752018766，本文免费，转发请注明源于www.751com.cn
and developer. The designer creates markup, using only standard HTML, WML, or whatever language is appropriate, and the developer writes code. Of course, many designers today have learned JavaScript, so it should come as no surprise that many of these same designers have begun to learn JSP coding. Often, instead of just doing pure markup, they encode a complete JSP page and hand it over to the developer. Then the usual tweaking takes place, and the developer puts the JSP page into place as a front-end for some portion of the overall application. The key, though, is that many designers do not learn JSP coding, so it must also be workable in that environment.
HTML vs. XML
One of the most significant disadvantages of JSP technology, and one of the most overlooked, is its incompatibility with XML. More precisely, and particularly in the HTML realm, JSP pages are not required to be XHTML-compatible. XHTML is a World Wide Web Consortium (W3C) specification that is now replacing HTML 4.0. XHTML defines the HTML tagset in terms of a well-formed XML document. For example, the <br> tag must be converted to
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