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Download Java Applets and CGI Scripts - Washington and Lee University
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Applets
Computer Science 101
Applets and CGI Scripts
Look and Feel
• An applet is a Java application that can run in
a Web browser.
• With some restrictions, an applet can have
the same look, feel, and functionality of any
Java application.
• Applets run in a “protected” environment in
the client's web browser, so that hackers can't
write destructive programs (e.g., erase client's
disk!)
How Java Applications Are Run
Inputs from me
Anyfile.class
Disk on my
computer
Java virtual
machine
Outputs to me
How Java Applets Are Run
Inputs from me
The Applet HTML Tag
General form:
<APPLET CODE="Anyfile.class" WIDTH=anInt HEIGHT=anInt>
Anyfile.class
Disk on any
computer
Anyfile.html
Java virtual
machine
Outputs to me
Web browser
WIDTH and HEIGHT give the number of pixels in the
dimensions of the applet’s “window” on the page
Example:
<APPLET CODE="CircleAreaAndRadius.class" WIDTH=300 HEIGHT=150>
An HTML code asks the browser to load a Java program
HTML Code for the Web Page
Changes to a Java Program
• The program extends GBApplet instead of
GBFrame
• The main method is omitted
• The size of the window is specified in the
HTML file, not in the Java file
From Application to Applet
From Application to Applet
import javax.swing.*;
import BreezySwing.*;
import javax.swing.*;
import BreezySwing.*;
public class CircleAreaAndRadius extends GBFrame{
public class CircleAreaAndRadius extends GBApplet{
// Set up window objects
// Set up window objects
// Declare data variables
// Declare data variables
// Implement the buttonClicked method
// Implement the buttonClicked method
public static void main(String[] args){
CircleAreaAndRadius theGUI = new CircleAreaAndRadius();
theGUI.setSize (300, 150);
theGUI.setVisible(true);
}
// Main method goes away
}
}
Some W&L Science Applets to Try
http://www.cs.wlu.edu/chatbots/george
http://www.cs.wlu.edu/chatbots/robert
Interactive chat with “George Washington” &
“Robert E. Lee”
http://chaos.wlu.edu/106/programs.html
The Wonderful World of Chaos
http://www.cs.wlu.edu/~levy/bw
Evolve beautiful fractal images
CGI Scripts: A Different Approach
to Interaction
• Recall client/server model from last time: Server
stores and “serves up” complete web pages to
client, over network.
• What if we want to change the appearance of the
pages dynamically, based on user input?
• Java applets may be more powerful than we
need, and too much work to program.
• Solution: a program that “automagically”
generates the right HTML code for us...
CGI Scripts: A Different Approach
to Web Interaction
• CGI: Common Gateway Interface – another
protocol; specifies how pages are presented
over the Web
• Script – any “lightweight” program that is
relatively small and solves a particular,
uninteresting problem quickly.
• E-commerce, Search Engines: the “killer
apps” for CGI scripts
CGI Scripts: Client/Server View
<HTML>
_____
_____
</HTML>
Network
connection
Client’s browser
CGI Script
Server’s disk
CGI Scripts: Specifying
Parameters
• Need, e.g., to tell Annie the subject you want
to search for.
• Look at top of browser:
http://annie.wlu.edu/search/X?SEARCH=robots&l=&m=&b=&p=&Da=&Db=&SORT=
parameters
• Then Annie generates a web page containing
links to these books.
Popular CGI Scripting Languages
• Perl : The “duct tape” of the Internet
• Python: Object-oriented, pseudocode-like
language
• C: Low-level language for old-school hackers
• Javascript: Lightweight Java, used mainly (?)
for annoying pop-ups
• JSP, ASP – commerical packages that
simplify script-writing.
