CSC 224 Lecture Notes

• Java program running inside a browser
  • .class files downloaded from the Web
  • Embedded in an HTML file
• Told by browser when to start and stop
  • Does not have its own frame (window)
  • Applet can be out of view but still accessible
• Severely limited access to environment
  • Applets are untrusted by default

• Applets have been upgraded to Swing by Sun
• In this class we'll focus on the original Applet model, Why?
  • The browsers (Netscape and IE) do not support the new versions
• To use new Swing GUI components:
  • Extend JApplet not Applet
  • Use Java Plug-In and HTML converter
  • Or include swing.jar archive
  • Getting everything right is difficult
  • Only use the new version for a company Intranet where you have control of everything
• For now, we'll stick with Java 1.1 applets
  • Here's a tutorial I would recommend: Sun's Applet Tutorial

• All applets must subclass java.applet.Applet
• 
• User customizes java.applet.Applet by overriding some methods
• public void init(){...}
  invoked when applet is initially loaded by browser
• public void start(){...}
  invoked when entering the web page that contains the applet
• public void stop(){...}
  invoked when leaving the web page that contains the applet
• public void destory(){...}
  invoked when the applet is going to be destroyed by the browser

class OurApplet extends Applet {
    . . .
    public void paint(Graphics g) { . . . }
    . . .
}

paint

The basic display method. Many applets implement the paint method to draw the applet's representation within a browser page.

update

A method you can use along with paint to improve drawing performance.

Applets inherit their paint and update methods from the Applet class, which inherits them from the Abstract Window Toolkit (AWT) Component class.

• Communication only with originating host
• Reading and writing files
  • Only on server from which the applet comes
  • i.e. cannot read or write files on local machine
• No access to user information
  • Only system information (version, OS, etc.)
• Pop-up windows tagged with warning

• Browser has security manager
  • Checks all potential security hazards
  • Can be checked while loading code
• Java bytecode is interpreted by JVM
  • Never given direct access to processor
  • Can be checked at run-time
• Signed applets can be given more access

• Abstact out the commonalities into a base class AnimationApplet
  • animationThread field
  • delay field
  • start() method
  • stop() method
  • run() method
• You just implement what is different
  • paint() method
  • init() method
  • destroy() method
  • Any other class specific stuff that you need for your functionality.
• java.applet.Applet ----> AnimationApplet ----> Your applet class

• Notice that sometimes the screen flickers
• Caused by the mechanics of how applets redraw themselves
  repaint() -----------> update() ----------------> paint()
• The defualt implementation of update() (which comes from Container class) does the following:
  • clears the background by filling it in with the background color
  • sets the pen color to the foreground color
  • calls the paint() method
• Sometimes this happens slow enough for the human eye to catch the flicker
• How do we fix this problem??? Use an off screen image (double buffering)!!!
• Drawing on an off screen image is just like drawing on an on screen image.
• Once the off screen image is done, you display it like any other image (ie .gif, .jpeg, etc)

• Each applet file means separate request
  • .class files, images, audio files, etc.
• Java ARchive file can hold all applet files
  • Use jar tool to archive necessary files
  • Use jarsigner tool to digitally sign JAR file
• Use ARCHIVE attribute in APPLET tag
  • <APPLET... ARCHIVE="foo.jar" ...>
  • Archive searched before requesting server file

Operation	Command
To create a JAR file	jar cf jar-file input-file(s)
To view the contents of a JAR file	jar tf jar-file
To extract the contents of a JAR file	jar xf jar-file
To extract specific files from a JAR file	jar xf jar-file archived-file(s)
To run an application packaged as a JAR file (version 1.1)	jre -cp app.jar MainClass
To run an application packaged as a JAR file (version 1.2 -- requires Main-Class manifest header)	java -jar app.jar
To invoke an applet packaged as a JAR file	<applet code=AppletClassName.class archive="JarFileName.jar" width=width height=height> </applet>