外文文献及翻译-Java-and-the-Internet.doc

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 isn’t immediately obvious if you’re

coming from a traditional programming 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.

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 within 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 that’s 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 all the

gobbledygook there.) These programs can be written in most languages. Perl is 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.

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 the graph.

And you’ve 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, and then sends the

page back to you; you must then back up a page and try again. Not only is this

slow, it’s inelegant.

The problem with discussions of client-side programming is that they aren’t

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 produced by client-side programming.

The rest of this section provides an overview of the issues and approaches in

client-side programming.

2.Plug-ins

One 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

plug-in is not a trivial task, and isn’t something you’d 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

manufacturer. 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).

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, 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 aren’t doing amazingly sophisticated things

with scripting languages so this is not too much of a hardship.

This 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

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 tools automatically create pages using JavaScript.

However, if you’re already fluent in Visual Basic or Tcl/Tk, you’ll be more

productive using those scripting languages rather than learning a new one.

(You’ll have your hands full dealing with the Web issues already.)

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 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 mini-program that will run only under 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 beauty—it provides you with a way to

automatically distribute the client software from the server at the time the user

way Java is designed, the programmer needs to create only a single program,

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