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1、外文文献:Java and the InternetIf Java is, in fact, yet ano ther computer program ming Ian guage, you may question why it is so important and why it is being promoted as a revolutionary step in computer programming. The answer isnt immediately obvious if youre coming from a traditional programming perspe。

2、ctive. Although Java is very useful for sol ving traditi onal sta nd-al one program ming problems, it is also importa nt because it will solve program ming problems on the World Wide Web.1. Client-side programmingThe Webs in itial server-browser desig n provided for in teractive content, but the in 。

3、teractivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HTML contains simple mechanisms for data gatheri ng: text-e ntry boxes, check boxes, radio boxes, lists and drop-dow n lists, as well as a 。

4、butt on that can only be programmed to reset the data on the form or submit” the data on the form back to the server. This submission passesthrough the Com mon Gateway In terface (CGI) provided on all Web servers. The text with in the submissi on tells CGI what to do with it. The most com mon acti o。

5、n is to run a program located on the server in a directory thats typically called cgi-bin. ” (If you watch the address window at the top of your browser whe n you push a butt on on a Web page, you can sometimes see cgi-b in ” with in all the gobbledygook there.) These programs can be writte n in mos。

6、t Ian guages. Perl is a com mon choice becauseit is desig ned for text mani pulati on and is in terpreted, so it can be in stalled on any server regardless of processor or operat ing system.Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with it. Howev。

7、er, Web sites built on CGI programs can rapidly become overly complicated to maintain, and there is also the problem of resp onse time. The resp onse of a CGI program depe nds on how much data must be sent, as well as the load on both the server and the In ternet. (On top of this, start ing a CGI pr。

8、ogram tends to be slow.) The in itial desig ners of the Web did not foresee how rapidly this ban dwidth would be exhausted for the kinds of applicati ons people developed. For example, any sort of dyn amic graph ing is n early impossible to perform with con siste ncy because a GIF file must be creat。

9、ed and moved from the server to the clie nt for each vers ion of the graph. And you e no doubt had direct experienee with something as simple as validati ng the data on an in put form. You press the submit butt on on a page; the data is shipped back to the server; the server starts a CGI program tha。

10、t discovers an error, formats an HTML page informing you of the error, and the n sends the page back to you; you must the n back up a page and try again. Not only is this slow, its inelegant.The soluti on is clie nt-side program ming. Most mach ines that run Web browsers are powerful engines capable。

11、 of doing vast work, and with the origi nal static HTML approach they are sitti ng there, just idly wait ing for the server to dish up the n ext page. Clie nt-side program ming means that the Web browser is har nessed to do whatever work it can, and the result for the user is a much speedier and mor。

12、e in teractive experie nee at your Web site.The problem with discussions of client-side programming is that they arent very different from discussions of programming in general. The parameters are almost the same, but the platform is differe nt: a Web browser is like a limited operati ng system .In 。

13、the end, you must still program, and this accounts for the dizzying array of problems and solutions produced by clie nt-side program ming. The rest of this secti on provides an overview of the issues and approaches in clie nt-side program ming.2. Plug-insOne of the most sig ni fica nt steps forward 。

14、in clie nt-side program ming is the developme nt of the plug-i n. This is a way for a programmer to add new fun cti on ality to the browser by dow nl oadi ng a piece of code that plugs itself into the appropriate spot in the browser. It tells the browser from now on you can perform this new activity。

15、.”(You need to download the plug-in only once.) Some fast and powerful behavior is added to browsers via plug-ins, but writing a plug-in is not a trivial task, and isn something youd want to do as part of the process of buildi ng a particular site. The value of the plug-i n for clie nt-side program 。

16、ming is that it allows an expert programmer to develop a new Ian guage and add that Ian guage to a browser without the permissi on of the browser manufacturer. Thus, plug-ins provide a back door” that allows the creation of new client-side programming Ianguages (although not all Ian guages are imple。

17、me nted as plug-i ns).3. Scripting languagesPlug-i ns resulted in an explosi on of scripti ng Ian guages. With a script ing Ianguage you embed the source code for your client-side program directly into the HTML page, and the plug-in that interprets that Ianguage is automatically activated while the 。

18、HTML page is being displayed. Scripting Ian guages tend to be reas on ably easy to un dersta nd and, because they are simply text that is part of an HTML page, they load very quickly as part of the sin gle server hit required to procure that page. The trade-off is that your code is exposed for every。

19、 one to see (and steal). Gen erally, however, you arent doing amaz in gly sophisticated things with scripti ng Ian guages so this is not too much of a hardship.This poi nts out that the script ing Ian guages used in side Web browsers are really inten ded to solve specific types of problems, primaril。

20、y the creatio n of richer and more in teractive graphical user in terfaces (GUIs). However, a scripting Ianguage might solve 80 percent of the problems encountered in client-side programming. Your problems might very well fit completely with in that 80 perce nt, and since scripti ng Ian guages can a。

21、llow easier and faster developme nt, you should probably con sider a scripti ng Ian guage before look ing at a more invo Ived solutio n such as Java or ActiveX program ming.The most commonly discussed browser scripting Ianguages are JavaScript (which has nothing to do with Java; it named that way ju。

22、st to grab some of Javas marketing momentum), VBScript (which looks like Visual Basic), and Tcl/Tk, which comes from the popular cross-platform GUI-building Ianguage. There are others out there, and no doubt more in developme nt.JavaScript is probably the most com monly supported. It comes built int。

23、o both Netscape Navigator and the Microsoft Internet Explorer (IE). I n additi on, there are probably more JavaScript books available tha n there are for the other browser Ianguages, and some tools automatically create pages using JavaScript. However, if youre already flue nt in Visual Basic or Tcl/。

24、Tk, you be more productive using those scripti ng Ian guages rather tha n lear ning a new one. (Yo uhave your hands full deali ng with the Web issues already.)4. JavaIf a scripting Ianguage can solve 80 percent of the client-side programming problems, what about the other 20 percen the really hard s。

25、tuff?” The most popular solution today is Java. Not only is it a powerful program ming Ian guage built to be secure, cross-platform, and intern ati on al, but Java is being continually extended to provide Ianguage features and libraries that elegantly handle problems that are difficult in traditiona。

26、l program ming Ian guages, such as multithread ing, database access ,n etwork program ming, and distributed comput ing. Java allows clie nt-side program ming via the applet.An applet is a mi ni-program that will run on ly un der a Web browser. The applet is downloaded automatically as part of a Web 。

27、page (just as, for example, a graphic is automatically downloaded). When the applet is activated it executes a program. This is part of its beauty it provides you with a way to automatically distribute the clie nt software from the server at the time the user n eeds the clie nt software, and no soon。

28、 er. The user gets the latest version of the client software without fail and without difficult rei nstallatio n. Because of the way Java is desig ned, the programmer n eeds to create on ly a sin gle program, and that program automatically works with all computers that have browsers with built-in Ja。

29、va interpreters. (This safely includes the vast majority of machines.) Since Java is a full-fledged program ming Ian guage, you can do as much work as possible on the clie nt before and after making requests of the server. For example, you won need to send a request form across the Internet to disco。

30、ver that yove gotte n a date or some other parameter wrong, and your clie nt computer can quickly do the work of plott ing data in stead of wait ing for the server to make a plot and ship a graphic image back to you. Not only do you get the immediate win of speed and resp on sive ness, but the gen e。

31、ral n etwork traffic and load on servers can be reduced, preve nting the en tire In ter net from slow ing dow n.One adva ntage a Java applet has over a scripted program is that it in compiled form, so the source code isn available to the clie nt. On the otherhand, a Java applet can be decompiled wit。

32、hout too much trouble, but hidi ng your code is ofte n n ot an importa nt issue. Two other factors can be importa nt. As you will see later in this book, a compiled Java applet can comprise many modules and take multiple server hits” (accesses) to download. (In Java 1.1 and higher this is mi ni mize。

33、d by Java archives, called JAR files, that allow all the required modules to be packaged together and compressedfor a single dow nl oad.) A scripted program will just be in tegrated into the Web page as part of its text (and will gen erally be smaller and reduce server hits). This could be importa n。

34、t to the resp on sive ness of your Web site. Ano ther factor is the all-importa nt lear ning curve. Regardless of what youve heard, Java is not a trivial language to learn. If you re a Visual Basic programmer, moving to VBScript will be your fastest soluti on, and si nee it will probably solve most 。

35、typical client/server problems you might be hard pressedto justify learning Java. If you re experieneed with a scripting Ianguage you will certainly ben efit from look ing at JavaScript or VBScript before committ ing to Java, since they might fit your needs handily and you be more productive soon er。

36、.to run its applets withi5. ActiveXTo some degree, the competitor to Java is MicrosoftsActiveX, although it takes a completely different approach. ActiveX was originally a Windows-only solution, although it is now being developed via an in depe ndent con sortium to become cross-platform. Effectively。

37、, ActiveX says if your program conn ects to its en vir onment just so, it can be dropped into a Web page and run under a browser that supports ActiveX. ” (IE directly supports ActiveX and Netscape does so using a plug-i n.) Thus, ActiveX does not constrain you to a particular Ianguage. If, for examp。

38、le, you e already an experie need Win dows programmer using a Ian guage such as C+, Visual Basic, or Borla nd Delphi, you can create ActiveX comp onents with almost no cha nges to your program ming kno wledge. ActiveX also provides a path for the use of legacy code in your Web pages.6. SecurityAutom。

39、atically downloading and running programs across the Internet can sound like a virus-builder dream. ActiveX especially brings up the thorny issue of security in clie nt-side program min g. If you click on a Web site, you might automatically download any number of things along with the HTML page: GIF。

40、 files, script code, compiled Java code, and ActiveX components. Some of these are benign; GIF files cant do any harm, and scripti ng Ian guages are gen erally limited in what they can do. Java was also desig ned to run its applets with in a an dboX of safety, which preve nts it from writi ng to dis。

41、k or access ing memory outside the san dbox.ActiveX is at the opposite end of the spectrum. Programming with ActiveX is like program ming Win dows you can do anything you want. So if you click on a page that dow nl oads an ActiveX comp onent, that comp onent might cause damage to the files on your d。

42、isk. Of course, programs that you load onto your computer that are not restricted to running in side a Web browser can do the same thing. Viruses downloaded from Bulletin-Board Systems (BBSs) have long been a problem, but the speed of the Internet amplifies the difficulty.The soluti on seems to be d。

43、igital sig natures” whereby code is verified to show who the author is. This is based on the idea that a virus works because its creator can be anonymous, so if you remove the anonymity individuals will be forced to be resp on sible for their acti ons. This seems like a good pla n because it allows 。

44、programs to be much more fun cti on al, and I suspect it will elimi nate malicious mischief. If, however, a program has an uninten ti onal destructive bug it will still cause problems.The Java approach is to preve nt these problems from occurri ng, via the san dbox. The Java in terpreter that lives 。

45、on your local Web browser exam ines the applet for any untoward instructions as the applet is being loaded. In particular, the applet cannot write files to disk or erase files (one of the main stays of viruses). Applets are gen erally con sidered to be safe, and since this is essential for reliable 。

46、client/server systems, any bugs in the Java Ianguage that allow viruses are rapidly repaired. (It worth noting that the browser software actually enforces these security restrictions, and some browsers allow you to select different security levels to provide varying degrees of access to your system.。

47、)You might be skeptical of this rather drac onian restrictio n aga inst writ ing files to your local disk. For example, you may want to build a local database or save data for later use offline. The initial vision seemed to be that eve ntually every one would get on li ne to do any thi ng importa nt。

48、, but that was soon seen to be impractical (although low-cost “hternet appliance might someday satisfy the n eeds of a sig ni fica nt segme nt of users). The soluti on is the signed applet that uses public-key encryption to verify that an applet does in deed come from where it claims it does. A sig 。

49、ned applet can still trash your disk, but the theory is that since you can now hold the applet creator accountable they won do vicious things. Java provides a framework for digital sig natures so that you will eve ntually be able to allow an applet to step outside the san dbox if n ecessary.Digital 。

50、sig natures have missed an importa nt issue, which is the speed that people move around on the In ter net. If you dow nl oad a buggy program and it does somethi ng un toward, how long will it be before you discover the damage? It could be days or eve n weeks. By the n, how will you track dow n the p。

51、rogram thatsd one it? And what good will it do you at that point?7ln ternet vs. i ntranetThe Web is the most gen eral soluti on to the clie nt/server problem, so it makes sen sethat you can use the same tech no logy to solve a subset of the problem, in particular the classic client/server problem wi。

52、thin a company. With traditional client/server approachesyou have the problem of multiple types of client computers, as well as the difficulty of installing new client software, both of which are han dily solved with Web browsers and clie nt-side program ming. When Web tech no logy is used for an in。

53、 formati on n etwork that is restricted to a particular compa ny, it is referred to as an intran et. I ntra nets provide much greater security tha n the Intern et, since you can physically control accessto the servers within your company. In terms of training, it seems that once people un dersta nd 。

54、the gen eral con cept of a browsersitmuch easier for them to deal with differe nces in the way pages and applets look, so the lear ning curve for new kinds of systems seems to be reduced.The security problem brings us to one of the divisi ons that seems to be automatically forming in the world of cl。

55、ient-side programming. If your program is running on the In ternet, you don k now what platform it will be worki ng un der, and you want to be extra careful that you don dissem in ate buggy code. You n eed somethi ng cross-platform and secure, like a scripti ng Ian guage or Java.If youre running on 。

56、an intranet, you might have a different set of con stra in ts. Its not un com mon that your mach ines could all be In tel/Wi ndows platforms. On an intran et, youre resp on sible for the quality of your own code and can repair bugs whe n theyre discovered. In additi on, you might already have a body。

57、 of legacy code that youve been using in a more traditional clie nt/server approach, whereby you must physically in stall clie nt programs every time you do an upgrade. The time wasted in in stalli ng upgrades is the most compelli ng reas on to move to browsers, because upgrades are in visible and a。

58、utomatic. If you are invoIved in such an intranet, the most sensible approach to take is the shortest path that allows you to use your existi ng code base, rather tha n trying to recode your programs in a new Ian guage.When faced with this bewildering array of solutions to the client-side programmin。

59、g problem, the best plan of attack is a cost-benefit analysis. Con sider the con stra ints of your problem and what would be the shortest path to your soluti on. Si nee clie nt-side program ming is still program min g, it s always a good idea to take the fastest development approach for your particu。

60、lar situati on. This is an aggressive sta nee to prepare for in evitable encoun ters with the problems of program developme nt.8. Server-side programmingThis whole discussi on has ignored the issue of server-side program ming. What happenswhen you make a request of a server? Most of the time the req。

61、uest is simply end me this file. ” Your browser then interprets the file in some appropriate fashi on: as an HTML page, a graphic image, a Java applet, a script program, etc. A more complicated request to a server gen erally invo Ives a database tran sacti on. A com mon sce nario invo Ives a request。

62、 for a complex database search, which the server the n formats into an HTML page and sends to you as the result. (Of course, if the clie nt has more in tellige nee via Java or a scripti ng Ian guage, the raw data can be sent and formatted at the clie nt end, which will be faster and less load on the。

63、 server.) Or you might want to register your n ame in a database whe n you join a group or place an order, which will invo Ive cha nges to that database. These database requests must be processed via some code on the server side, which is gen erally referred to as server-side program ming. Traditi o。

64、n ally, server-side program ming has bee n performed using Perl and CGI scripts, but more sophisticated systems have been appeari ng. These in clude Java-based Web servers that allow you to perform all your server-side program ming in Java by writ ing what are called servlets. Servlets and their off。

65、spri ng, JSPs, are two of the most compelli ng reas ons that compa nies who develop Web sites are moving to Java, especially because they elimi nate the problems of deali ng with differe ntly abled browsers.9. separate arena: applicationsMuch of the brouhaha over Java has bee n over applets. Java is。

66、 actually a general-purpose programming Ianguage that can solve any type of problemat least in theory. And as pointed out previously, there might be more effective ways to solve most clie nt/server problems. Whe n you move out of the applet arena (and simulta neously release the restricti ons, such as the one against writing to disk) you enter the world of general-purpose ap。