Java applets

1
Java applets

• M. L. Liu

2
Introduction

• Java applets are one of three kinds of Java
  programs
• An application is a standalone program that can
  be invoked from the command line.
• An applet is a program that runs in the context
  of a browser session.
• A servlet is a program that is invoked on demand
  on a server program and that runs in the context
  of a web server process.

3
Applets, web page, client, server

• Applets are programs stored on a web server,
  similar to web pages.
• When an applet is referred to in a web page that
  has been fetched and processed by a browser, the
  browser generates a request to fetch (or
  download) the applet program, then executes the
  program in the browsers execution context, on
  the client host.

4
Applet Execution - 1

• An applet program is written as a subclass of the
  java.Applet class or the javax.swing.Japplet
  class. There is no main method you must
  override the start method. Applet objects uses
  AWT for graphics. JApplet uses SWING.
• It is a Graphics object that runs in a Thread
  object, so every applet can perform graphics, and
  runs in parallel to the browser process.

5
Applet Execution

• When the applet is loaded, these methods are
  automatically invoked in order
• the init( ) method is invoked by the Java Virtual
  Machine.
• The start( ) method
• The paint( ) method.
• The applet is now running and rendered on the web
  page.
• You program the start( ) method and the paint( )
  method for your application, and invoke a
  repaint() call to re-render the graphics, if
  necessary.
• At the end of the execution, the stop( ) method
  is invoked, followed by the destroy( ) method to
  deallocate the applets resources.

6
Applet Security http//java.sun.com/docs/books/tut
orial/applet/overview/

• For security reasons, applets that are loaded
  over the network have several restrictions
  (Trojan Horse problem).
• an applet cannot ordinarily read or write files
  on the computer that it's executing on.
• an applet cannot make network connections except
  to the host (server) that it came from.

7
HTML tags for applets - 1

• ltAPPLET specifies the beginning of the HTML
  applet code
• CODE"demoxx.class" is the actual name of the
  applet (usually a 'class' file)
• CODEBASE"demos/" is the location of the applet
  (relative as here, or a full URL)
• NAME"smily" the name you want to give to this
  instance of the applet on this page
• WIDTH"100" the physical width of the applet on
  your page
• HEIGHT"50"  the physical height of the applet on
  your page
• ALIGN"Top" where to align the applet within its
  page space (top, bottom, center)

8
HTML tags for applets - 2

• ltPARAM  specifies a parameter that can be passed
  to the applet
• NAMEname1" the name known internally by the
  applet in order to receive this parameter
• VALUE"000000" the value you want to pass for
  this parameter
• gt end of this parameter
• ltPARAM  specifies a parameter that can be passed
  to the applet (applet specific)
• NAMEname2" the name known internally by the
  applet in order to receive this parameter
• VALUE"ffffff" the value you want to pass for
  this parameter
• gt end of this parameter
• lt/APPLETgt specifies the end of the HTML applet
  code

9
The HelloWorld Applet

• ltHTMLgt
• ltBODYgt
• ltAPPLET codehello.class width900 height300gt
• lt/APPLETgt
• lt/BODYgt
• lt/HTMLgt
• // applet to display a message in a window
• import java.awt.
• import java.applet.
• public class hello extends Applet
• public void init( )
• setBackground(Color.yellow)

• public void paint(Graphics g)
• final int FONT_SIZE 42
• Font font new Font("Serif", Font.BOLD,
  FONT_SIZE)
• // set font, and color and display message on
• // the screen at position 250,150
• g.setFont(font)
• g.setColor(Color.blue)
• // The message in the next line is the
  one you will see
• g.drawString("Hello, world!",250,150)

10
Advanced Applets

• You can use threads in an applet embedding
  threads in Applets.
• You can make socket calls in an applet, subject
  to the security constraints.

11
Proxy server

• A proxy server can be used to circumvent the
  security constraints.

12
Summary

• An applet is a Java class
• Its code is downloaded from a web server
• It is run in the browsers environment on the
  client host
• It is invoked by a browser when it scans a web
  page and encounters a class specified with the
  APPLET tag
• For security reason, the execution of an applet
  is normally subject to restrictions
• applets cannot access files in the file system on
  the client host
• Applets cannot make network connection exception
  to the server host from which it originated