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Introduction to Applets
Chapter 21
Applets
• An applet is a Java application that is
intended to be invoked and executed
through a Web browser.
• Click Here To View Cannon World Applet
Security Issues
• Applets are loaded from remote computer
and executed locally. Since an “untrusted”
stranger wrote the applet, applets are
restricted. Applets CANNOT:
– run any local executable program
– read or write to local computer’s file system
– Communicate with any computer except
originating server
– Access many facts about the local computer,
except name and version of OS
Applets vs. Applications
• Much of the code for Applets is similar to
Applications
• Differences:
– Applets are created by subclassing Applet,
instead of Frame
– Applets begin execution with the method
init, instead of main
– Applets are halted and restarted as the Web
browser moves to a new page and returns
Applet class
• Applet is a subclass of Panel, thus it inherits all
of Panel’s graphical component attributes.
• Applets have four methods available for
overriding by clients:
– init() called only once when the applet is first loaded
(use like the constructor)
– start() called to begin execution each time the Webpage containing the applet is exposed
– stop() called each time the Web-page containing the
applet is hidden
– Destroy() called when the applet is about to be
terminated (should free any resource being used)
CannonWorld as an Applet
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class CannonWorld extends Applet // was Frame
{
public static final int FrameWidth = 600;
public static final int FrameHeight = 400;
private
private
private
private
//
int
String
CannonBall
Scrollbar
angle;
message;
cannonBall;
slider;
public CannonWorld() {
public void init() {
// changed the layout manager
setLayout(new BorderLayout());
setSize ( FrameWidth, FrameHeight );
//
setTitle( "Cannon Game" );
angle
message
…
= 45;
= "Angle: " + angle;
Http and Applets
•
<HTML>
<HEAD>
<META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html;
charset=windows-1252">
<TITLE>Cannon World Applet Demo</TITLE>
</HEAD>
<BODY LANG="en-US" DIR="LTR">
<P ALIGN=CENTER><FONT SIZE=5 STYLE="font-size: 20pt">CS II Webpage to Display an Applet</FONT></P>
<P ALIGN=CENTER><BR><BR>
</P>
<P ALIGN=CENTER><APPLET
CODEBASE="http://www.cs.uni.edu/~fienup/cs062s06/sessions/session24/
CannonWorld_Applet/"
CODE="CannonWorldApplet" ALIGN=LEFT WIDTH=600
HEIGHT=400>
</APPLET><BR CLEAR=LEFT><BR><BR>
</BODY>
</HTML>
Parameters in <applet> tag
<APPLET CODEBASE="http://www.cs.uni.edu"
CODE="CannonWorldApplet"
ALIGN=LEFT WIDTH=600 HEIGHT=400>
<PARAM name=name1 value=“value1”>
You do not have a Java enabled browser
</APPLET>
In the Java Applet, getParameter(“name1”) can be
used to access the parameter string.
Loading Resources from the Server
• getImage(URL) – retrieve the image
specified by the URL (jpeg and gif)
• getAudioClip(URL) – return audioClip
object, then play it (or play(URL) short cut)
• getCodeBase() returns URL for codebase
• appletContext.showDocument(URL)
instructs the Web browser to display the a
new page from the URL
URL objects
• Constructed from a string or previous URL
and a string
• URL have an openStream() method which
returns an inputStream object, so you can
read from a URL just like reading from a
file.
