APPLETS - JAVA

                        Applets
OBJECTIVES :
* To describe applets and their purpose           

* To discuss embedding applets in HTML pages
What is an applet  :
*An applet is a subclass of Panel
*It is a container which can hold GUI components
*It has a graphics context which can be used to draw images
*An applet embedded within an HTML page
*Applets are defined using the <applet> tag
*Its size and location are defined within the tag
*The browser contains a Java Virtual Machine which executes the applet
*The applet .class file is downloaded, through the net, into the Virtual machine.
•Unfortunately, most browsers have a very old version of the JVM
Applications and Applets :
*Java applications are executed from the command line
*A Java VM must be installed
*The VM is given the name of a class which contains a main() method
*The main method instantiates the objects necessary to start the application
*Applets are executed by a browser
*The browser either contains a VM or loads the Java plugin
*The programmer must implement a class which is a subclass of applet
*There is no main method.  Instead, the applet contains an init() method which is invoked by the Browser when the applet starts
HTML and Applets :
*The HTML applet tag contains the following parameters:
<Applet  code="name of .class file"
  codebase="URL where code is loaded from"
name="applet identifier"
   align="LEFT|RIGHT|CENTER
width="size in pixels"
height="size in pixels“>
<param name=“aName1” value=“aValue”>
<param name=“aName2” value=“aValue”>
</Applet>
<HTML>
<HEAD>
<TITLE> Sample Applet</TITLE>
</HEAD>
<BODY>
<APPLET code="Sample.class" WIDTH=100 HEIGHT=200>
This text will display if the browser does not support applets
</APPLET>
</BODY>
</HTML>
import java.applet.*;
import java.awt.*;
public class Sample extends Applet implements ActionListener
{
  private Button okButton = new Button("OK");
  private Button cancelButton = new Button("Cancel");
  public void init()
  {
  okButton.addActionListener(this);
  cancelButton.addActionListener(this);
  add(okButton);
  add(cancelButton);
  }
  public void actionPerformed(ActionEvent x)
  {
  // ...
  }
...
}
<Applet code="Menu.class“>
  <param name="MenuName" value="My Web Site“>
  <param name="Font" value="Serif">
</Applet>
public class Menu extends Applet
{
  public void init()
  {
  String menuName = getParameter("MenuName");
  String theFont = getParameter("Font");
  // do something with parameters
  }

Applet Security :
*Java applets execute within a Sandbox
*Applets cannot access the local file system
*Applets cannot connect to systems other than the server from which they were downloaded
*Applets cannot listen for inbound network connections
*Applets cannot spawn processes or load local jar files
*Applets cannot terminate the Java VM
*Applets cannot change the security policy
*Signed applets can be granted more access rights
*These rights are controlled by the SecurityManager
Browser Issues :
*Unfortunately, Applets are not heavily used
*Browsers support is limited
*Browsers often contain outdated or buggy Java virtual machines
*Each browser has its own compatibility issues.  This usually means that the programmer has to implement workarounds for the various browsers
*Microsoft has not been very cooperative in terms of ensuring that IE correctly implements applets.  Even the Java plugin has issues under IE
*Because the AWT and Swing are not highly regarded, use of Java in the client is minimal.

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