Introduction to Applets

1
Introduction to Applets

• Chapter 21

2
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

3
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 computers file system
• Communicate with any computer except originating
  server
• Access many facts about the local computer,
  except name and version of OS

4
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

5
Applet class

• Applet is a subclass of Panel, thus it inherits
  all of Panels 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
  Web-page 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)

6
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 int angle private
  String message private CannonBall
  cannonBall private Scrollbar slider
  // public CannonWorld() public
  void init() // changed the layout
  manager setLayout(new BorderLayout())
  setSize ( FrameWidth, FrameHeight
  ) // setTitle( "Cannon Game" )
  angle 45 message
  "Angle " angle

7
Http and Applets

• ltHTMLgtltHEADgt ltMETA HTTP-EQUIV"CONTENT-TYPE"
  CONTENT"text/html charsetwindows-1252"gt ltTITLE
  gtCannon World Applet Demolt/TITLEgtlt/HEADgtltBODY
  LANG"en-US" DIR"LTR"gtltP ALIGNCENTERgtltFONT
  SIZE5 STYLE"font-size 20pt"gtCS II Web-page to
  Display an Appletlt/FONTgtlt/PgtltP
  ALIGNCENTERgtltBRgtltBRgtlt/PgtltP ALIGNCENTERgtltAPPLET
  CODEBASE"http//www.cs.uni.edu/fienup/cs062s06/
  sessions/session24/CannonWorld_Applet/"
  CODE"CannonWorldApplet"
  ALIGNLEFT WIDTH600 HEIGHT400gtlt/APPLETgtltBR
  CLEARLEFTgtltBRgtltBRgtlt/BODYgtlt/HTMLgt

8
Parameters in ltappletgt tag

• ltAPPLET CODEBASE"http//www.cs.uni.edu"
  CODE"CannonWorldApplet"
• ALIGNLEFT WIDTH600 HEIGHT400gt ltPARAM
  namename1 valuevalue1gt
• You do not have a Java enabled browser
• lt/APPLETgt
• In the Java Applet, getParameter(name1) can be
  used to access the parameter string.

9
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

10
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.