动态Web文档 毕业论文外文文献翻译.doc

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1、外文翻译Dynamic Web DocumentsSo far, the model we have used is that of Fig. 6-6: the client sends a file name to the server, which then returns the file. In the early days of the Web, all content was, in fact, static like this (just files). However, in recent years, more and more content has become dyna

2、mic, that is, generated on demand, rather than stored on disk. Content generation can take place either on the server side or on the client side. Let us now examine each of these cases in turn.Server-Side Dynamic Web Page GenerationTo see why server-side content generation is needed, consider the us

3、e of forms, as described earlier. When a user fills in a form and clicks on the submit button, a message is sent to the server indicating that it contains the contents of a form, along with the fields the user filled in. This message is not the name of a file to return. What is needed is that the me

4、ssage is given to a program or script to process. Usually, the processing involves using the user-supplied information to look up a record in a database on the servers disk and generate a custom HTML page to send back to the client. For example, in an e-commerce application, after the user clicks on

5、 PROCEED TO CHECKOUT, the browser returns the cookie containing the contents of the shopping cart, but some program or script on the server has to be invoked to process the cookie and generate an HTML page in response. The HTML page might display a form containing the list of items in the cart and t

6、he users last-known shipping address along with a request to verify the information and to specify the method of payment. The steps required to process the information from an HTML form are illustrated in Fig. 7-33.Figure 7-33. Steps in processing the information from an HTML form.The traditional wa

7、y to handle forms and other interactive Web pages is a system called the CGI (Common Gateway Interface). It is a standardized interface to allow Web servers to talk to back-end programs and scripts that can accept input (e.g., from forms) and generate HTML pages in response. Usually, these back-ends

8、 are scripts written in the Perl scripting language because Perl scripts are easier and faster to write than programs (at least, if you know how to program in Perl). By convention, they live in a directory called cgi-bin, which is visible in the URL. Sometimes another scripting language, Python, is

9、used instead of Perl.As an example of how CGI often works, consider the case of a product from the Truly Great Products Company that comes without a warranty registration card. Instead, the customer is told to go to to register on-line. On that page, there is a hyperlink that says:Click here to regi

10、ster your product This link points to a Perl script, say, When this script is invoked with no parameters, it sends back an HTML page containing the registration form. When the user fills in the form and clicks on submit, a message is sent back to this script containing the values filled in using the

11、 style of Fig. 7-30. The Perl script then parses the parameters, makes an entry in the database for the new customer, and sends back an HTML page providing a registration number and a telephone number for the help desk. This is not the only way to handle forms, but it is a common way. There are many

12、 books about making CGI scripts and programming in Perl. A few examples are (Hanegan, 2001; Lash, 2002; and Meltzer and Michalski, 2001).CGI scripts are not the only way to generate dynamic content on the server side. Another common way is to embed little scripts inside HTML pages and have them be e

13、xecuted by the server itself to generate the page. A popular language for writing these scripts is PHP (PHP: Hypertext Preprocessor). To use it, the server has to understand PHP (just as a browser has to understand XML to interpret Web pages written in XML). Usually, servers expect Web pages contain

14、ing PHP to have file extension php rather than html or htm.A tiny PHP script is illustrated in Fig. 7-34; it should work with any server that has PHP installed. It contains normal HTML, except for the PHP script inside the tag. What it does is generate a Web page telling what it knows about the brow

15、ser invoking it. Browsers normally send over some information along with their request (and any applicable cookies) and this information is put in the variable HTTP_USER_AGENT. When this listing is put in a file test.php in the WWW directory at the ABCD company, then typing the URL will produce a We

16、b page telling the user what browser, language, and operating system he is using.Figure 7-34. A sample HTML page with embedded PHP.PHP is especially good at handling forms and is simpler than using a CGI script. As an example of how it works with forms, consider the example of Fig. 7-35(a). This fig

17、ure contains a normal HTML page with a form in it. The only unusual thing about it is the first line, which specifies that the file action.php is to be invoked to handle the parameters after the user has filled in and submitted the form. The page displays two text boxes, one with a request for a nam

18、e and one with a request for an age. After the two boxes have been filled in and the form submitted, the server parses the Fig. 7-30-type string sent back, putting the name in the name variable and the age in the age variable. It then starts to process the action.php file, shown in Fig. 7-35(b) as a

19、 reply. During the processing of this file, the PHP commands are executed. If the user filled in Barbara and 24 in the boxes, the HTML file sent back will be the one given in Fig. 7-35(c). Thus, handling forms becomes extremely simple using PHP.Figure 7-35. (a) A Web page containing a form. (b) A PH

20、P script for handling the output of the form. (c) Output from the PHP script when the inputs are Barbara and 24, respectively.Although PHP is easy to use, it is actually a powerful programming language oriented toward interfacing between the Web and a server database. It has variables, strings, arra

21、ys, and most of the control structures found in C, but much more powerful I/O than just printf. PHP is open source code and freely available. It was designed specifically to work well with Apache, which is also open source and is the worlds most widely used Web server. For more information about PHP

22、, see (Valade, 2002).We have now seen two different ways to generate dynamic HTML pages: CGI scripts and embedded PHP. There is also a third technique, called JSP (JavaServer Pages), which is similar to PHP, except that the dynamic part is written in the Java programming language instead of in PHP.

23、Pages using this technique have the file extension jsp. A fourth technique, ASP (Active Server Pages), is Microsofts version of PHP and JavaServer Pages. It uses Microsofts proprietary scripting language, Visual Basic Script, for generating the dynamic content. Pages using this technique have extens

24、ion asp. The choice among PHP, JSP, and ASP usually has more to do with politics (open source vs. Sun vs. Microsoft) than with technology, since the three languages are roughly comparable.The collection of technologies for generating content on the fly is sometimes called dynamic HTML.Client-Side Dy

25、namic Web Page GenerationCGI, PHP, JSP, and ASP scripts solve the problem of handling forms and interactions with databases on the server. They can all accept incoming information from forms, look up information in one or more databases, and generate HTML pages with the results. What none of them ca

26、n do is respond to mouse movements or interact with users directly. For this purpose, it is necessary to have scripts embedded in HTML pages that are executed on the client machine rather than the server machine. Starting with HTML 4.0, such scripts are permitted using the tag . The most popular scr

27、ipting language for the client side is JavaScript, so we will now take a quick look at it.JavaScript is a scripting language, very loosely inspired by some ideas from the Java programming language. It is definitely not Java. Like other scripting languages, it is a very high level language. For examp

28、le, in a single line of JavaScript it is possible to pop up a dialog box, wait for text input, and store the resulting string in a variable. High-level features like this make JavaScript ideal for designing interactive Web pages. On the other hand, the fact that it is not standardized and is mutatin

29、g faster than a fruit fly trapped in an X-ray machine makes it extremely difficult to write JavaScript programs that work on all platforms, but maybe some day it will stabilize.As an example of a program in JavaScript, consider that of Fig. 7-36. Like that of Fig. 7-35(a), it displays a form asking

30、for a name and age, and then predicts how old the person will be next year. The body is almost the same as the PHP example, the main difference being the declaration of the submit button and the assignment statement in it. This assignment statement tells the browser to invoke the response script on

31、a button click and pass it the form as a parameter.Figure 7-36. Use of JavaScript for processing a form.What is completely new here is the declaration of the JavaScript function response in the head of the HTML file, an area normally reserved for titles, background colors, and so on. This function e

32、xtracts the value of the name field from the form and stores it in the variable person as a string. It also extracts the value of the age field, converts it to an integer by using the eval function, adds 1 to it, and stores the result in years. Then it opens a document for output, does four writes t

33、o it using the writeln method, and closes the document. The document is an HTML file, as can be seen from the various HTML tags in it. The browser then displays the document on the screen.It is very important to understand that while Fig. 7-35 and Fig. 7-36 look similar, they are processed totally d

34、ifferently. In Fig. 7-35, after the user has clicked on the submit button, the browser collects the information into a long string of the style of Fig. 7-30 and sends it off to the server that sent the page. The server sees the name of the PHP file and executes it. The PHP script produces a new HTML

35、 page and that page is sent back to the browser for display. With Fig. 7-36, when the submit button is clicked the browser interprets a JavaScript function contained on the page. All the work is done locally, inside the browser. There is no contact with the server. As a consequence, the result is di

36、splayed virtually instantaneously, whereas with PHP, there can be a delay of several seconds before the resulting HTML arrives at the client. The difference between server-side scripting and client-side scripting is illustrated in Fig. 7-37, including the steps involved. In both cases, the numbered

37、steps start after the form has been displayed. Step 1 consists of accepting the user input. Then comes the processing of the input, which differs in the two cases.Figure 7-37. (a) Server-side scripting with PHP. (b) Client-side scripting with JavaScript.This difference does not mean that JavaScript

38、is better than PHP. Their uses are completely different. PHP (and, by implication, JSP and ASP) are used when interaction with a remote database is needed. JavaScript is used when the interaction is with the user at the client computer. It is certainly possible (and common) to have HTML pages that u

39、se both PHP and JavaScript, although they cannot do the same work or own the same button, of course.JavaScript is a full-blown programming language, with all the power of C or Java. It has variables, strings, arrays, objects, functions, and all the usual control structures. It also has a large numbe

40、r of facilities specific for Web pages, including the ability to manage windows and frames, set and get cookies, deal with forms, and handle hyperlinks. An example of a JavaScript program that uses a recursive function is given in Fig. 7-38.Figure 7-38. A JavaScript program for computing and printin

41、g factorials.JavaScript can also track mouse motion over objects on the screen. Many JavaScript Web pages have the property that when the mouse cursor is moved over some text or image, something happens. Often the image changes or a menu suddenly appears. This kind of behavior is easy to program in

42、JavaScript and leads to lively Web pages. An example is given in Fig. 7-39.Figure 7-39. An interactive Web page that responds to mouse movement.JavaScript is not the only way to make Web pages highly interactive. Another popular method is through the use of applets. These are small Java programs tha

43、t have been compiled into machine instructions for a virtual computer called the JVM (Java Virtual Machine). Applets can be embedded in HTML pages (between and ) and interpreted by JVM-capable browsers. Because Java applets are interpreted rather than directly executed, the Java interpreter can prev

44、ent them from doing Bad Things. At least in theory. In practice, applet writers have found a nearly endless stream of bugs in the Java I/O libraries to exploit.Microsofts answer to Suns Java applets was allowing Web pages to hold ActiveX controls, which are programs compiled to Pentium machine langu

45、age and executed on the bare hardware. This feature makes them vastly faster and more flexible than interpreted Java applets because they can do anything a program can do. When Internet Explorer sees an ActiveX control in a Web page, it downloads it, verifies its identity, and executes it. However,

46、downloading and running foreign programs raises security issues, which we will address in Chap. 8.Since nearly all browsers can interpret both Java programs and JavaScript, a designer who wants to make a highly-interactive Web page has a choice of at least two techniques, and if portability to multi

47、ple platforms is not an issue, ActiveX in addition. As a general rule, JavaScript programs are easier to write, Java applets execute faster, and ActiveX controls run fastest of all. Also, since all browers implement exactly the same JVM but no two browsers implement the same version of JavaScript, J

48、ava applets are more portable than JavaScript programs. For more information about JavaScript, there are many books, each with many (often 1000) pages. A few examples are (Easttom, 2001; Harris, 2001; and McFedries, 2001).Before leaving the subject of dynamic Web content, let us briefly summarize wh

49、at we have covered so far. Complete Web pages can be generated on-the-fly by various scripts on the server machine. Once they are received by the browser, they are treated as normal HTML pages and just displayed. The scripts can be written in Perl, PHP, JSP, or ASP, as shown in Fig. 7-40.Figure 7-40. The various ways to generate and dis