ch3.htm

Java_by_Example,初级经典例子哦,珍藏版本

<P>

If a required applet is not located on your system, it can be

downloaded automatically to your system and then run. To the user,

this exchange of applets over the Internet is mostly transparent.

All the user knows is that she's looking at a page that contains

a game of TicTacToe, an animated image of Duke, or some other

Java-based content. In this way, the Internet becomes almost an

extension of the user's basic system, sort of a gigantic hard

drive that contains a practically infinite number of accessible

applets and applications.

<P>

Currently, there's a client/server relationship between a browser

that wants to display an applet and the system that can supply

the applet. The client is a computer that requires services from

another system; the server is the computer that provides those

services. In the case of a Java applet, the client is the computer

that's trying to display an HTML document that contains a reference

to an applet. The server is the computer system that uploads the

applet to the client, thereby allowing the client to use the applet.

In Figure 3.2, you could call the local computer the client and

the remote computer the server.

<P>

It won't be long, however, before the difference between a client

and a server begins to get muddy. When Java browsers can send

as well as receive applets, computers will constantly switch between

being a client and a server. For example, suppose a user loads

up his favorite Java-compatible browser and connects to a Web

site. The home page on the Web site contains an animated title,

so your system downloads the applet that displays this title.

For the time being, your system is the client and the remote system

is the server.

<P>

Now, however, you decide that you want to search the remote system's

public databases for a particular file. Because you've just written

a handy search application that can do the job for you, your system

transmits the application to the remote computer, where it sets

to work finding the file you specified. Suddenly, your computer

is the server and the remote computer is the client.

<P>

This sort of switching between client and server tasks is a step

toward  making the Internet a huge extension of your computer.

That is, more and more, the Internet will seem to be a part of

your own local system, rather than a collection of computers located

all over the world. You'll be able to access the Internet almost

as easily as your own hard drive. In fact, you might not even

need a hard drive at all! You can just run applications located

somewhere else on the Internet and store your data in any number

of special storage sites.

<H2><A NAME="Security"><FONT SIZE=5 COLOR=#Ff0000>

Security</FONT></A></H2>

<P>

You may have heard horror stories about people who have downloaded

programs from the Internet only to find, after running the program,

that it infected their system with a virus or otherwise wreaked

havoc with their computer. Therefore, you may be reluctant to

jump on the applet bandwagon. After all, with so many applets

flying around the Internet, trouble could rear its ugly head like

a demon from a Clive Barker movie.

<P>

The truth, however, is that Java applets are a secure way to transmit

programs on the Internet. This is because the Java interpreter

will not allow an applet to run until the interpreter has confirmed

that the applet's byte-code has not been corrupted or changed

in some way (Figure 3.3). Moreover, the interpreter determines

whether the byte-code representation of the applet sticks to all

of Java's rules. For example, a Java applet can never use a pointer

to gain access to portions of computer memory for which it doesn't

have access. The bottom line is that, not only are Java applets

secure, they are virtually guaranteed not to crash the system.

<P>

<A HREF="f3-3.gif"><B> Figure 3.3 : </B><I>Applets are verified before they are run, so they are virtually guaranteed to be safe and secure.

</I></A><P>



<H2><A NAME="ExampleYourPagesontheWeb"><FONT SIZE=5 COLOR=#Ff0000>

Example: Your Pages on the Web</FONT></A></H2>

<P>

Because you bought this book to learn how to write applets, you're

probably also interested in setting up your own Web pages. (Of

course, you may be interested only in creating applets that other

people can use. That's okay, too.) To set up your own Java-compatible

Web site, you need to create publicly accessible folders on your

hard drive. You also need to gather all the applets you need so

you can store them together in one place on your hard drive. That

is, you'll want your Web pages to contain local applets. The instructions

in this section will get you started on organizing a public folder.

<P>

First, create a folder on your hard drive's root directory. Name

this folder something like PUBLIC. The PUBLIC folder will contain

all of the files that are accessible to Web users who connect

to your pages. Inside the PUBLIC folder, create a folder called

something like APPLETS. The APPLETS folder will contain the applets

that are referenced in your Web pages.

<P>

Now that you have your folders created, copy all the applets you

need into the APPLETS folder. The applets' .CLASS files should

be in the APPLETS directory, with their support files (such as

graphics and sounds) in appropriately named folders within the

APPLETS directory. For example, if you wanted to use the BouncingHeads

applet in one of your pages, you'd copy the contents of the JAVA\DEMO\BOUncINGHEADS

folder to your APPLETS folder, ending up with the directory structure

shown in Figure 3.4.

<P>

<A HREF="f3-4.gif"><B> Figure 3.4 : </B><I>You need to set up your public directories properly.

</I></A><P>

<P>

The next step is to create the HTML files for your pages. When

you've written these pages, they should be placed in the PUBLIC

folder. Listing 3.3 shows the HTML file for a simple home page

that displays the BouncingHeads applet and enables the user to

view the applet's source code. The corresponding Web page is shown

in Figure 3.5. Notice that the user can view the applet's source

code by clicking on the link at the bottom of the page. When she

does, she sees a window similar to Figure 3.6.

<P>

<A HREF="f3-5.gif"><B> Figure 3.5 : </B><I>A simple, Java-powered home page.



</I></A><P>

<P>

<A HREF="f3-6.gif"><B> Figure 3.6 : </B><I>The user can view the applet's source code by clicking the link.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 3.3&nbsp;&nbsp;HOMEPAGE.htmL: A Home Page Displaying

a Java Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Big Benny&lt;/title&gt;

&lt;h1&gt;Welcome to Big Benny's Home Page&lt;/h1&gt;

&lt;b&gt;We're hot wired for Java!&lt;/b&gt;

&lt;p&gt;

&lt;applet 

    codebase=&quot;applets&quot;

    code=&quot;BounceItem.class&quot;

    width=400

    height=150&gt;

&lt;/applet&gt;

&lt;p&gt;

&lt;a href=&quot;applets/BounceItem.java&quot;&gt;The source code&lt;/a&gt;

for this applet is available for your viewing pleasure.

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Generally, you do not need to copy Java source code files to your APPLETS folder. Source code is not required in order to load and run applets.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

By placing all the publicly accessible files in one root directory,

you can more easily set up your system's security to ensure that

the rest of your system is kept safe from prying eyes. Moreover,

you know exactly where you need to store any additional files

that you may add to your site. Exactly how you create the folders

for your Web pages will depend upon the applets and files you

need to use, but the previous example should get you going fairly

easily.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Of course, to create a Web site, you need to be connected to the Internet either directly or through an Internet provider that enables you to set up your own Web pages. If you are currently unable to have your own Web site, you can still set one up on your hard drive and then use your Java-compatible Web browser to load and view your Web pages. Then, when you get your Internet access, you'll be all ready to go. To find more information on setting up your own Web site, crank up your browser and log onto the handy Lycos directory at <TT><A TARGET="resource window" HREF="http://www.lycos.com">http://www.lycos.com</A></TT>; then fish around under the Computers, Web Publishing &amp; HTML subdirectory.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#Ff0000>

Summary</FONT></A></H2>

<P>

Thanks to a Java-compatible browser's ability to download applets,

users who log onto Java-powered Web pages can enjoy the Java experience

without even realizing what's going on behind the scenes. This

is unlike other types of applications on the Internet that the

user must explicitly download before they can be run. When more

and more applets start running rampant on the Information Superhighway,

the Internet will become a virtual extension of your own computer

system, one that's almost as easily accessed as your local hard

drive. When this happens, the Internet will start living up to

its hype.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#Ff0000>

Review Questions</FONT></A></H2>

<OL>

<LI>What is a local applet?

<LI>What is a remote applet?

<LI>Explain the client/server relationship as it applies to Java

applets.

<LI>How will the client/server focus of the Internet change as

applications start to flow two ways, both from and to a remote

computer?

<LI>Explain why Java applets are secure and guaranteed not to

crash the system on which they're executing.

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#Ff0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Add additional applets to the simple Web site you set up in

this chapter. You can copy the applets from the JAVA\DEMO folder.

<LI>Modify the HOMEPAGE.htmL file to display the additional applets

you added in exercise 1.

<LI>Log onto the Internet and then find and download additional

applets that you can use in your own pages. Start off at this

URL: <TT><A TARGET="resource window" HREF="http://www.javasoft.com/applets/appletsites.html">http://www.javasoft.com/applets/appletsites.html</A></TT>

</OL>

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