JAVA外文文献+翻译.pdf

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1、燕山大学本科生毕业设计（翻译）1 Java and the Internet If Java is,in fact，yet another computer programming language，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

2、perspective。Although Java is very useful for solving traditional stand-alone programming problems，it is also important because it will solve programming problems on the World Wide Web。1.Client-side programming The Webs initial serverbrowser design provided for interactive content,but the interactivi

3、ty 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 gathering:textentry boxes,check boxes,radio boxes,lists and drop-down lists，as well as a button that can only

4、be programmed to reset the data on the form or“submit”the data on the form back to the server。This submission passes through the Common Gateway Interface（CGI)provided on all Web servers.The text within the submission tells CGI what to do with it.The most common action is to run a program located on

5、the server in a directory thats typically called“cgi-bin。”(If you watch the address window at the top of your browser when you push a button on a Web page,you can sometimes see“cgibin within all the gobbledygook there.)These programs can be written in most languages.Perl is a common choice because i

6、t is designed for text manipulation and is interpreted,so it can be installed on any server regardless of processor or operating system.Many powerful Web sites today are built strictly on CGI，and you can in fact do nearly anything with it。However,Web sites built on CGI programs can rapidly become ov

7、erly complicated to maintain,and there is also the problem of 燕山大学本科生毕业设计（翻译）2 response time。The response of a CGI program depends on how much data must be sent,as well as the load on both the server and the Internet.(On top of this，starting a CGI program tends to be slow.)The initial designers of t

8、he Web did not foresee how rapidly this bandwidth would be exhausted for the kinds of applications people developed.For example，any sort of dynamic graphing is nearly impossible to perform with consistency because a GIF file must be created and moved from the server to the client for each version of

9、 the graph。And youve no doubt had direct experience with something as simple as validating the data on an input form.You press the submit button on a page；the data is shipped back to the server;the server starts a CGI program that discovers an error,formats an HTML page informing you of the error，an

10、d then sends the page back to you；you must then back up a page and try again.Not only is this slow，its inelegant。The solution is client-side programming.Most machines that run Web browsers are powerful engines capable of doing vast work,and with the original static HTML approach they are sitting the

11、re,just idly waiting for the server to dish up the next page.Client-side programming means that the Web browser is harnessed to do whatever work it can,and the result for the user is a much speedier and more interactive experience at your Web site。The problem with discussions of clientside programmi

12、ng is that they arent very different from discussions of programming in general。The parameters are almost the same，but the platform is different：a Web browser is like a limited operating system。In the end，you must still program，and this accounts for the dizzying array of problems and solutions produ

13、ced by client-side programming。The rest of this section provides an overview of the issues and approaches in client-side programming.2。Plugins One of the most significant steps forward in clientside programming is the development of the plugin.This is a way for a programmer to add new functionality

14、to the browser by downloading a piece of code that plugs itself into 燕山大学本科生毕业设计（翻译）3 the appropriate spot in the browser。It tells the browser“from now on you can perform this new activity.”（You need to download the plugin only once。）Some fast and powerful behavior is added to browsers via plugins,b

15、ut writing a plugin is not a trivial task，and isnt something youd want to do as part of the process of building a particular site。The value of the plug-in for clientside programming is that it allows an expert programmer to develop a new language and add that language to a browser without the permis

16、sion of the browser manufacturer。Thus,plugins provide a“back door”that allows the creation of new client-side programming languages（although not all languages are implemented as plug-ins）.3.Scripting languages Plug-ins resulted in an explosion of scripting languages.With a scripting language you emb

17、ed the source code for your clientside program directly into the HTML page，and the plug-in that interprets that language is automatically activated while the HTML page is being displayed.Scripting languages tend to be reasonably easy to understand and,because they are simply text that is part of an

18、HTML page,they load very quickly as part of the single server hit required to procure that page。The trade-off is that your code is exposed for everyone to see(and steal）.Generally，however,you arent doing amazingly sophisticated things with scripting languages so this is not too much of a hardship.Th

19、is points out that the scripting languages used inside Web browsers are really intended to solve specific types of problems，primarily the creation of richer and more interactive graphical user interfaces(GUIs).However,a scripting language might solve 80 percent of the problems encountered in clients

20、ide programming。Your problems might very well fit completely within that 80 percent，and since scripting languages can allow easier and faster development，you should probably consider a scripting language before looking at a more involved solution such as Java or ActiveX programming.The most commonly

21、 discussed browser scripting languages are JavaScript（which has nothing to do with Java；its named that way just to grab some of 燕山大学本科生毕业设计（翻译）4 Javas marketing momentum），VBScript（which looks like Visual Basic）,and Tcl/Tk,which comes from the popular cross-platform GUIbuilding language.There are oth

22、ers out there,and no doubt more in development.JavaScript is probably the most commonly supported.It comes built into both Netscape Navigator and the Microsoft Internet Explorer（IE）.In addition，there are probably more JavaScript books available than there are for the other browser languages,and some

23、 tools automatically create pages using JavaScript。However，if youre already fluent in Visual Basic or Tcl/Tk,youll be more productive using those scripting languages rather than learning a new one。（Youll have your hands full dealing with the Web issues already。)4.Java If a scripting language can sol

24、ve 80 percent of the clientside programming problems,what about the other 20 percentthe“really hard stuff？”The most popular solution today is Java.Not only is it a powerful programming language built to be secure,cross-platform,and international，but Java is being continually extended to provide lang

25、uage features and libraries that elegantly handle problems that are difficult in traditional programming languages，such as multithreading，database access，network programming,and distributed computing.Java allows client-side programming via the applet.An applet is a miniprogram that will run only und

26、er a Web browser。The applet is downloaded automatically as part of a Web page(just as,for example,a graphic is automatically downloaded).When the applet is activated it executes a program.This is part of its beautyit provides you with a way to automatically distribute the client software from the se

27、rver at the time the user needs the client software，and no sooner.The user gets the latest version of the client software without fail and without difficult reinstallation.Because of the way Java is designed,the programmer needs to create only a single program，and that program automatically works wi

28、th all computers that have browsers with built-in Java interpreters.(This safely includes the vast majority of machines。)Since Java is a full-fledged programming language,you can do as 燕山大学本科生毕业设计（翻译）5 much work as possible on the client before and after making requests of the server。For example,you

29、 wont need to send a request form across the Internet to discover that youve gotten a date or some other parameter wrong,and your client computer can quickly do the work of plotting data instead of waiting for the server to make a plot and ship a graphic image back to you.Not only do you get the imm

30、ediate win of speed and responsiveness,but the general network traffic and load on servers can be reduced，preventing the entire Internet from slowing down.One advantage a Java applet has over a scripted program is that its in compiled form，so the source code isnt available to the client。On the other

31、 hand,a Java applet can be decompiled without too much trouble，but hiding your code is often not an important issue.Two other factors can be important.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

32、and higher this is minimized by Java archives，called JAR files，that allow all the required modules to be packaged together and compressed for a single download。）A scripted program will just be integrated into the Web page as part of its text（and will generally be smaller and reduce server hits）.This

33、 could be important to the responsiveness of your Web site.Another factor is the allimportant learning curve。Regardless of what youve heard,Java is not a trivial language to learn。If youre a Visual Basic programmer，moving to VBScript will be your fastest solution,and since it will probably solve mos

34、t typical client/server problems you might be hard pressed to justify learning Java.If youre experienced with a scripting language you will certainly benefit from looking at JavaScript or VBScript before committing to Java,since they might fit your needs handily and youll be more productive sooner.t

35、o run its applets withi 5.ActiveX To some degree,the competitor to Java is Microsofts ActiveX，although it takes a completely different approach.ActiveX was originally a Windows-only 燕山大学本科生毕业设计（翻译）6 solution，although it is now being developed via an independent consortium to become crossplatform。Eff

36、ectively，ActiveX says“if your program connects to its environment 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 plugin.)Thus，ActiveX does not constrain you to a particular language.If，for example,yo

37、ure already an experienced Windows programmer using a language such as C+,Visual Basic，or Borlands Delphi,you can create ActiveX components with almost no changes to your programming knowledge.ActiveX also provides a path for the use of legacy code in your Web pages。6.Security Automatically download

38、ing and running programs across the Internet can sound like a virusbuilders dream.ActiveX especially brings up the thorny issue of security in client-side programming.If you click on a Web site，you might automatically download any number of things along with the HTML page:GIF files,script code,compi

39、led Java code,and ActiveX components.Some of these are benign；GIF files cant do any harm，and scripting languages are generally limited in what they can do.Java was also designed to run its applets within a“sandbox of safety，which prevents it from writing to disk or accessing memory outside the sandb

40、ox.ActiveX is at the opposite end of the spectrum。Programming with ActiveX is like programming Windowsyou can do anything you want。So if you click on a page that downloads an ActiveX component,that component might cause damage to the files on your disk。Of course，programs that you load onto your comp

41、uter that are not restricted to running inside 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 solution seems to be“digital signatures,whereby code is verified to show who the

42、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 燕山大学本科生毕业设计（翻译）7 forced to be responsible for their actions.This seems like a good plan because it allows programs to be much more functional,and I suspect i

43、t will eliminate malicious mischief.If，however，a program has an unintentional destructive bug it will still cause problems。The Java approach is to prevent these problems from occurring，via the sandbox.The Java interpreter that lives on your local Web browser examines the applet for any untoward inst

44、ructions as the applet is being loaded。In particular,the applet cannot write files to disk or erase files(one of the mainstays of viruses).Applets are generally considered to be safe,and since this is essential for reliable client/server systems，any bugs in the Java language that allow viruses are r

45、apidly repaired。(Its 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。）You might be skeptical of this rather draconian restriction against writing f

46、iles 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 eventually everyone would get online to do anything important，but that was soon seen to be impractical(although lowcost“Internet appliances might someday

47、 satisfy the needs of a significant segment of users).The solution is the“signed applet”that uses public-key encryption to verify that an applet does indeed come from where it claims it does。A signed applet can still trash your disk,but the theory is that since you can now hold the applet creator ac

48、countable they wont do vicious things.Java provides a framework for digital signatures so that you will eventually be able to allow an applet to step outside the sandbox if necessary。Digital signatures have missed an important issue,which is the speed that people move around on the Internet。If you d

49、ownload a buggy program and it does something untoward，how long will it be before you discover the damage？It could be days or even weeks.By then,how will you track down the program thats done it?And what good will it do you at that point？燕山大学本科生毕业设计（翻译）8 7.Internet vs.intranet The Web is the most ge

50、neral solution to the client/server problem，so it makes sense that you can use the same technology to solve a subset of the problem,in particular the classic client/server problem within a company。With traditional client/server approaches you have the problem of multiple types of client computers，as