毕业论文外文翻译-Java和因特网.doc

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1、Java and the InternetIf 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 perspective. A

2、lthough Java is very useful for solving traditional standalone programming problems, it is also important because it will solve programming problems on the World Wide Web. What is the Web?The Web can seem a bit of a mystery at first, with all this talk of “surfing,” “presence,” and “home pages.” Its

3、 helpful to step back and see what it really is, but to do this you must understand client/server systems, another aspect of computing thats full of confusing issues. Client/Server computingThe primary idea of a client/server system is that you have a central repository of informationsome kind of da

4、ta, often in a databasethat you want to distribute on demand to some set of people or machines. A key to the client/server concept is that the repository of information is centrally located so that it can be changed and so that those changes will propagate out to the information consumers. Taken tog

5、ether, the information repository, the software that distributes the information, and the machine(s) where the information and software reside is called the server. The software that resides on the remote machine, communicates with the server, fetches the information, processes it, and then displays

6、 it on the remote machine is called the client. The basic concept of client/server computing, then, is not so complicated. The problems arise because you have a single server trying to serve many clients at once. Generally, a database management system is involved, so the designer “balances” the lay

7、out of data into tables for optimal use. In addition, systems often allow a client to insert new information into a server. This means you must ensure that one clients new data doesnt walk over another clients new data, or that data isnt lost in the process of adding it to the database (this is call

8、ed transaction processing). As client software changes, it must be built, debugged, and installed on the client machines, which turns out to be more complicated and expensive than you might think. Its especially problematic to support multiple types of computers and operating systems. Finally, there

9、s the all-important performance issue: You might have hundreds of clients making requests of your server at any one time, so any small delay is crucial. To minimize latency, programmers work hard to offload processing tasks, often to the client machine, but sometimes to other machines at the server

10、site, using so-called middleware. (Middleware is also used to improve maintainability.) The simple idea of distributing information has so many layers of complexity that the whole problem can seem hopelessly enigmatic. And yet its crucial: Client/server computing accounts for roughly half of all pro

11、gramming activities. Its responsible for everything from taking orders and credit-card transactions to the distribution of any kind of datastock market, scientific, government, you name it. What weve come up with in the past is individual solutions to individual problems, inventing a new solution ea

12、ch time. These were hard to create and hard to use, and the user had to learn a new interface for each one. The entire client/server problem needs to be solved in a big way. The Web as a giant serverThe Web is actually one giant client/server system. Its a bit worse than that, since you have all the

13、 servers and clients coexisting on a single network at once. You dont need to know that, because all you care about is connecting to and interacting with one server at a time (even though you might be hopping around the world in your search for the correct server). Initially it was a simple one-way

14、process. You made a request of a server and it handed you a file, which your machines browser software (i.e., the client) would interpret by formatting onto your local machine. But in short order people began wanting to do more than just deliver pages from a server. They wanted full client/server ca

15、pability so that the client could feed information back to the server, for example, to do database lookups on the server, to add new information to the server, or to place an order (which required more security than the original systems offered). These are the changes weve been seeing in the develop

16、ment of the Web. The Web browser was a big step forward: the concept that one piece of information could be displayed on any type of computer without change. However, browsers were still rather primitive and rapidly bogged down by the demands placed on them. They werent particularly interactive, and

17、 tended to clog up both the server and the Internet because any time you needed to do something that required programming you had to send information back to the server to be processed. It could take many seconds or minutes to find out you had misspelled something in your request. Since the browser

18、was just a viewer it couldnt perform even the simplest computing tasks. (On the other hand, it was safe, because it couldnt execute any programs on your local machine that might contain bugs or viruses.) To solve this problem, different approaches have been taken. To begin with, graphics standards h

19、ave been enhanced to allow better animation and video within browsers. The remainder of the problem can be solved only by incorporating the ability to run programs on the client end, under the browser. This is called client-side programming. Client-side programmingThe Webs initial server-browser des

20、ign provided for interactive content, but the interactivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HyperText Markup Language (HTML) contains simple mechanisms for data gathering: text-entry

21、boxes, check boxes, radio boxes, lists and drop-down lists, as well as a button 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 passes through the Common Gateway Interface (CGI) provided on all Web servers. The text withi

22、n the submission tells CGI what to do with it. The most common action 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 when you push a button on a Web page, you can sometimes see “cgi-bin” within al

23、l the gobbledygook there.) These programs can be written in most languages. Perl has been a common choice because it is designed for text manipulation and is interpreted, so it can be installed on any server regardless of processor or operating system. However, Python (my favoritesee www.Python.org)

24、 has been making inroads because of its greater power and simplicity. Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with CGI. However, Web sites built on CGI programs can rapidly become overly complicated to maintain, and there is also the problem of

25、 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 the Web did not foresee how rapidly this bandwidth would be exhausted for

26、the kinds of applications people developed. For example, any sort of dynamic graphing is nearly impossible to perform with consistency because a Graphics Interchange Format (GIF) file must be created and moved from the server to the client for each version of the graph. And youve no doubt had direct

27、 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, and then sends the page back to you;

28、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 there, just idly waiting for the

29、 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 client-side programming is that they arent ve

30、ry 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 produced by client-side

31、 programming. The rest of this section provides an overview of the issues and approaches in client-side programming. Plug-insOne of the most significant steps forward in client-side programming is the development of the plug-in. This is a way for a programmer to add new functionality to the browser

32、by downloading 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.” (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 n

33、ot 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 client-side programming is that it allows an expert programmer to develop a new language and add that language to a browser without the permission of the browser m

34、anufacturer. Thus, plug-ins provide a “back door” that allows the creation of new client-side programming languages (although not all languages are implemented as plug-ins). Scripting languagesPlug-ins resulted in an explosion of scripting languages. With a scripting language, you embed the source c

35、ode for your client-side 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 HTML page,

36、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. This

37、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 client

38、-side 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 co

39、mmonly discussed browser scripting languages are JavaScript (which has nothing to do with Java; its named that way just 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 language. There are others

40、 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). Unfortunately, the flavor of JavaScript on the two browsers can vary widely (the Mozilla browser, freely downloadabl

41、e from www.Mozilla.org, supports the ECMAScript standard, which may one day become universally supported). In addition, there are probably more JavaScript books available than there are for the other browser languages, and some tools automatically create pages using JavaScript. However, if youre alr

42、eady 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.) JavaIf a scripting language can solve 80 percent of the client-side programming problems, what about the o

43、ther 20 percentthe “really hard stuff?” Java is a popular solution for this. Not only is it a powerful programming language built to be secure, cross-platform, and international, but Java is being continually extended to provide language features and libraries that elegantly handle problems that are

44、 difficult in traditional programming languages, such as multithreading, database access, network programming, and distributed computing. Java allows client-side programming via the applet and with Java Web Start. An applet is a mini-program that will run only under a Web browser. The applet is down

45、loaded 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 server at the time the user nee

46、ds 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 with all computers that ha

47、ve 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 much work as possible on the client before and after making requests of the server. For example, you wont need to send a request form ac

48、ross 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 immediate win of speed and responsive

49、ness, 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 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 require extr